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Heat treatment : the best one


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Heat treatment : the best one

  2. 2. Metal Heat Treating Topics of Presentation • What Is Metal Heat Treating? • Where Is It Used? • Why and How It Is Done? • What Processes & Equipment Are Used for Heat Treating? Arvind Thekdi - E3M, Inc. Sales
  3. 3. What is Heat Treating ? Controlled Heating And Cooling of Metal to Change Its Properties and Performance. Through: • Change in Microstructure • Change in Chemistry or Composition Temperature Holding (soak) Time Arvind Thekdi - E3M, Inc. Sales
  4. 4. A Few Heat Treating Facts • Heat Treating of Metals Represents Approximately 100 BCF Gas Load Nationwide. • Heat Treaters Use Natural Gas to Supply About 2/3 of the Energy Used for Heat Treating (induction, vacuum & commercial atmospheres main competition). • Current Share of Gas Decisions is about 50 / 50 Between Gas & Electric. Arvind Thekdi - E3M, Inc. Sales
  5. 5. Why Use Heat Treating ? In simple Terms…. • Soften a Part That Is Too Hard. • Harden a Part That Is Not Hard Enough. • Put Hard Skin on Parts That Are Soft. • Make Good Magnets Out of Ordinary Material. • Make Selective Property Changes Within Parts. Arvind Thekdi - E3M, Inc. Sales
  6. 6. Who uses Heat Treating ? • Aircraft Industry • Automobile Manufacturing • Defense Sector • Forging • Foundry • Heavy Machinery Manufacturing • Powder Metal Industries Arvind Thekdi - E3M, Inc. Sales
  7. 7. What Industrial Sectors Use Heat Treating ? SIC Industry 331 332 34 Steel Mills Iron and Steel Foundries Metal Fabrication Machinery and Electrical/Electronic Equipment Transportation Equipment Commercial Heat Treating Steel Service Centers 35 & 36 37 3398 5051 Arvind Thekdi - E3M, Inc. Sales
  8. 8. Types of Heat Treaters • Commercial Heat Treaters – Heat Treating of Parts As ―Job-shop‖. – Reported Under SIC Code 3398. – Approx. 10% of All Heat Treating Production Is by Commercial Heat Treaters. – Usually There Are 4 to 5 Captive Heat Treaters for Each Commercial Heat Treater Shop. • Captive Heat Treaters – Usually a Part of Large Manufacturing Business. – They Usually Produce ―Products‖ Rather Than Parts. – Captive Heat Treating Is Scattered Through All Manufacturing SIC Codes (DEO has over 100 individual SIC‘s for Heat Treaters). Arvind Thekdi - E3M, Inc. Sales
  9. 9. Commonly Heat Treated Metals • Ferrous Metals • Non-ferrous Metals – – – – – Steel Cast Iron Alloys Stainless Steel Tool Steel – – – – Aluminum Copper Brass Titanium Steel Is the Primary Metal Being Heat Treated. More Than 80% of Heat Treating Is Done for Steel Arvind Thekdi - E3M, Inc. Sales
  10. 10. Heat Treating Processes Arvind Thekdi - E3M, Inc. Sales
  11. 11. Steps in Heat Treating Operation • Loading • Cleaning • Pre-wash with coalescer • De-phosphate system • Spray rinse •Tempering • Surface coating • Unloading Arvind Thekdi - E3M, Inc. Sales • Heating • Preheating • Heating • Soak & diffusion • Pre-cooling • Quenching (Cooling) • Post-wash
  12. 12. Commonly Used Equipment for Heat Treating Operations • • Metal Cleaning (Wash-Rinse) Equipment Gas fired furnaces – – – – • Direct fired using burners fired directly into a furnace Indirect fired furnaces: radiant tube, muffle, retort etc. Molten salt (or lead) bath Fluidized bed Electrically heated Furnaces – Induction heating – Electrical resistance heating – Other (i.e. Laser, electron-beam etc.) • • • Quench or cooling equipment Material handling system Testing and quality control laboratory equipment Arvind Thekdi - E3M, Inc. Sales
  13. 13. Gas Fired Metal Heat Treating Furnaces Arvind Thekdi - E3M, Inc. Sales
  14. 14. Electrically Heated Equipment for Metal Heating Electric Atmosphere Furnace Vacuum Furnace Induction Equipment Arvind Thekdi - E3M, Inc. Sales
  15. 15. Types of Heat Treating Furnaces Box 4560 Induction Carbottom Bell, Hood, tipup 100 150 150 Vertical Pit 60 Vacuum 13425 Box 100 7700 Salt Bath Lead pot (cont.) Rolelr hearth Barre-roller Cont. stripline Cont. induction 200 Conveyor 4395 Car Bottom 6000 Pusher Pusher Fluidized bed Salt-bath 3195 Conveyor 4890 Bell, hood, tip-up 50 330 5 55 130 4765 Vertical Pit Rotary hearth, shaker hearth Plasma Induction Laser 2510 Vacuum Electric beam Flame heads Arvind Thekdi - E3M, Inc. Sales
  16. 16. Heat Treating Furnaces Two Primary Types • Atmospheric – Operated at ambient (atmosphere) pressure. – Load is heated and cooled in presence of air or special gases (process atmospheres), in liquid baths or in a fluidized bed. • Vacuum – Operated at vacuum or sub-atmospheric pressure. – May involve high pressure gas cooling using special gases. – Includes ion or plasma processing equipment. Arvind Thekdi - E3M, Inc. Sales
  17. 17. Heat Source for Gas Fired Furnaces • Direct Fired Burners * • Radiant Tubes * • Muffle or Retort Heated by Outside Burners/Electrical Elements • Hot Oil or Steam Heating Muffle Burners Direct fired muffle furnace Arvind Thekdi - E3M, Inc. Sales * These could be directly exposed to the work or can be outside a muffler a retort.
  18. 18. Nonferrous Heat Treating Furnaces Types of Furnaces Coil/foil Annealing Furnaces Rod/wire Annealing Furnaces Log Homogenizing Furnaces Ingot Preheating Furnaces Aging Furnaces Indirect Heating (Radiant Tubes or Electrical Resistance) Temperature Range 350°F to 1150°F Atmosphere With Dew Point Control May Includes Water Quench or Controlled Cooling Arvind Thekdi - E3M, Inc. Sales
  19. 19. Why Use Protective Atmospheres? • To Prevent Oxidation, Loss of Carbon (Decarburizing), and Avoid Corrosion. – Most Gases Containing Oxygen (i.e. Air, Water Vapor [H2O], Carbon Dioxide [CO2] React With Iron, Carbon and Other Elements Present in Steel and Other Metals. – Reactivity Depends on Temperature and Mixture of Gases in Contact With Steel. • To Avoid and Eliminate Formation of Flammable or Explosive Mixtures Arvind Thekdi - E3M, Inc. Sales
  20. 20. Types of Process Atmospheres • Protective – To Protect Metal Parts From Oxidation or Loss of Carbon and Other Elements From the Metal Surfaces. • Reactive – To Add Non-metallic (i.e., Carbon, Oxygen, Nitrogen) or Metallic (i.e., Chromium, Boron, Vanadium) Elements to the Base Metal. • Purging – To Remove Air or Flammable Gases From Furnaces or Vessels. Arvind Thekdi - E3M, Inc. Sales
  21. 21. Importance of Protective Atmospheres in Heat Treating • Proper composition and concentration in a furnace is required to give the required surface properties for the heat treated parts. • Loss of atmosphere ―control‖ can result in unacceptable parts and result in major economic penalty - it can cost a lot! • Atmospheres contain potentially dangerous (explosive, life threatening) gases and must be treated with ―respect‖. • New advances in measurement and control of atmospheres in heat treating allow precise control of atmospheres to produce quality parts. Arvind Thekdi - E3M, Inc. Sales
  22. 22. Commonly Used Atmospheres in Heat Treating Protective and Purging – Endothermic gases • Lean – high and low dew point • Rich - high and low dew point – Nitrogen – Mixture of N2 and small amount of CO Arvind Thekdi - E3M, Inc. Sales Reactive – Exothermic gases – Mixture (or individual) of gases: Hydrogen, CO, CH4, Nitrogen and other hydrocarbons – Dissociated Ammonia (H2 + N2)
  23. 23. Source of Atmospheres Requirement: A Mixture of Gases (CO, H2, CO2, H2O and N2) That Give the Required Composition for the Processing Atmosphere. • Natural Gas (Hydrocarbon) Air Reaction • Natural Gas - Steam Reaction • Ammonia Dissociation or Ammonia-air Reaction Or: • Mixture of Commercial Gases (N2, H2 and Hydrocarbons) Arvind Thekdi - E3M, Inc. Sales
  24. 24. Use of Atmospheres in a Plant Requirement: A Mixture of Gases (CO, H2, CO2, H2O and N2) That Give the Required Composition for the Processing Atmosphere. • Most plants have an in-house, centrally located, atmosphere gas generators for different types of atmospheres required in the plant • In some cases one or more generators may be located for each ―shop‖ or production area • In many cases other gases (i.e. N2, H2, NH3) are piped from storage tanks located within the plant premises and distributed by a piping system to furnaces. • Gas flow is mixed, measured and controlled prior to its injection in the furnace. Arvind Thekdi - E3M, Inc. Sales
  25. 25. ANNEALING • Annealing, involves heating to a predetermined temperature, holding at this temperature, and finally cooling at a very slow rate. • The temperature, to which steel is heated, and the holding time are determined by various factors such as the chemical composition of steel, size and shape of steel component and final properties desired.
  26. 26. Purpose of Annealing i. Relieve internal stresses developed during solidification, machining, forging, rolling, or welding; ii. Improve or restore ductility & toughness; iii. Enhance machinability iv. Eliminate chemical non-uniformity; v. Refine grain size; and vi. Reduce the gaseous contents in steel.
  27. 27. CLASSIFICATION Annealing treatment can be classified into groups based on the following: 1. Heat treatment temperature • Full annealing • Partial annealing • Sub-critical annealing 2. Phase transformation • First-order annealing • Second-order annealing 3. Specific purpose • Full annealing • Isothermal annealing • Diffusion annealing • Partial annealing • Recrystallization annealing • Spheroidisation annealing
  28. 28. 1.1 In full annealing the steel is heated above the critical temperature(A3) and then cooled very slowly. 1.2 Partial annealing, also known as incomplete annealing or intercritical annealing, involves heating of steel to a temp. lying between lower critical temperature(A1) and upper critical temperature (A3 or Acm). 1.3 Subcritical annealing is a process in which the maximum temp. to which is heated is always less than the lower critical temperature(A1).
  29. 29. Classification based on phase transformation features. 2.1 First-order annealing is performed on steel with the sole aim of achieving some properties. Any change in the characteristics of steel achieved by this type of annealing is not correlated to phase can be performed at a wide range of temperatures above or below the critical temperatures. 2.2 The second-order annealing differs from the former in the sense that the end results in the former are essentially due to phase transformation which takes place during the treatment.
  30. 30. Types of annealing based on specific purpose 3.1 Full annealing • • • In this, steel is heated to its 50°C above the austenitic temperature and held for sufficient time to allow the material to fully form austenite or austenite-cementite grain structure. The material is then allowed to cool slowly so that the equilibrium microstructure is obtained. The austenitising temp is a function of carbon content of the steel and can be generalized as: • For hypoeutectoid steels and eutectoid steel » Ac3+(20-40oC) [to obtain single phase austenite] • For hypereutectoid steels » Ac1+(20-40oC) [to obtain austenite+ cementite]
  31. 31. Purpose of full annealing • • • • • To relieve internal stresses To reduce hardness and increase ductility For refining of grain size To make isotropic in nature in mechanical aspects For making the material having homogeneous chemical composition • For making the material suitable for high machining processes • To make steel suitable for undergoing other heat treatment processes like hardening, normalizing etc.
  32. 32. • The grain structure has coarse Pearlite with ferrite or Cementite (depending on whether hypo or hyper eutectoid). The steel becomes soft and ductile.
  33. 33. • The formation of austenite destroys all structures that have existed before heating. Slow cooling yields the original phases of ferrite and pearlite in hypoeuetectoid steels and that of cementite and pearlite in hypereutectoid steels.
  34. 34. 3.2 Isothermal Annealing • It is a process in which hypoeutectoid steel is heated above the upper critical temperature and this temperature is maintained for a time and then the temperature is brought down below lower critical temperature and is again maintained. Then finally it is cooled at room temperature. This method rids any temperature gradient. • The prefix ―isothermal‖ associated with annealing implies that transformation of austenite takes place at constant temperature.
  35. 35. The closer the temp of isothermal holding is to A1, coarser is the pearlite, softer is the steel, but longer is the time of isothermal transformation.
  36. 36. Advantages: • • • • Improved machinability. Homogeneous structure and better surface finish. Time required for complete cycle is comparably less. The process is of great use for alloy steels as the steels have to be cooled slowly. Limitation: It is suitable only for small-sized components. Heavy components cannot be subjected to this treatment because it is not possible to cool them rapidly and uniformly to the holding temperature at which transformation occurs. Thus structure wont be homogeneous mechanical properties will vary across the cross-section.
  37. 37. 3.3 Diffusion Annealing • This process, also known as homogenizing annealing, is employed to remove any structural non-uniformity like dendrites, columnar grains and chemical inhomogeneity which promote brittleness and reduce ductility and toughness of steel. • Process:  Steel is heated sufficiently above the upper critical temperature (say, 1000-2000oC), and held at this temperature for 10-20 hours, followed by slow cooling.  Segregated zones are eliminated, and a chemically homogeneous steel is obtained by this treatment as a result of diffusion.  Heating to such a high temp. results in considerable coarsening of austenitic grains & heavy scale formation. The coarse austenite thus obtained further transforms to coarse pearlite on cooling, which is not a desirable structure as mechanical properties are impaired.
  38. 38.  The main aim of homogenising annealing is to make the composition uniform, i.e to remove chemical heterogeneity.  The impact energy and ductility of the steel increase as the homogenizing temperature increases and the hardness, yield strength and tensile strength decrease with an increase in the homogenizing temperature.  Homogenizing annealing has a few shortcomings as well. It results in:    Grain coarsening of austenite, thereby impairing the properties Thick scales on the surface of steels It is an expensive process
  39. 39. 3.4 Partial Annealing • Partial annealing, also known as inter-critical annealing or incomplete annealing, is a process in which steel is heated between A1 and Acm and is followed by slow cooling. • Generally, hypereutectoid steels are subjected to this treatment. The resultant microstructure consists of fine pearlite and cementite instead of coarse pearlite and a network of cementite at grain boundaries. • As low temperatures are involved in this process, it is less expensive than full annealing.
  40. 40. • Hupoeutectoid steels are subjected to this treatment to improve machinability. However, steels with coarse structure of ferrite and pearlite or with widmanstätten structure are not suitable for this treatment. This is because only a considerable amount of ferrite remains untransformed, and only a part of it along with pearlite transforms to austenite. • This coarse or accicular untransformed ferrite results in poor mechanical properties.
  41. 41. 3.5 Recrystallization Annealing • The process consists of heating steel above the recrystallization temperature, holding at this temperature and cooling thereafter. • It is used to treat work-hardened parts made out of low-Carbon steels (< 0.25% Carbon). This allows the parts to be soft enough to undergo further cold working without fracturing.
  42. 42. • Recrystallization temp(Tr) is given by: • Tr= (0.3-0.5)Tmp • As little scaling and decarburization occurs in recrystallization annealing, it is preferred over full annealing. • No phase change takes place and the final structure consists of strain-free, equiaxed grains of fine ferrite produced at the expense of deformed elongated ferrite grains. • However It would produce very coarse grains if the steel has undergone critical amount of deformation. In such cases, full annealing is preferred.
  43. 43. Aims of Recrystallization Annealing    To restore ductility To refine coarse grains To improve electrical and magnetic properties in grain-oriented Si steels.
  44. 44. 3.5 Spheroidization annealing • Spheroidization annealing consists of heating, soaking and cooling, invariably very slowly to produce spheroidal pearlite or globular form of carbides in steels. • To improve the machinability of the annealed hypereutectoid steel spheroidize annealing is applied. • Hypereutectoid steels consist of pearlite and cementite. The cementite forms a brittle network around the pearlite. This presents difficulty in machining the hypereutectoid steels. • This process will produce a spheroidal or globular form of a carbide in a ferritic matrix which makes the machining easy. • Prolonged time at the elevated temperature will completely break up the pearlitic structure and cementite network. The structure is called spheroidite.
  45. 45. Spheroidising Process: • Heat the part to a temperature just below the FerriteAustenite line, line A1 or below the AusteniteCementite line, essentially below the 727 ºC (1340 ºF) line. Hold the temperature for a prolonged time and follow by fairly slow cooling. Or • Cycle multiple times between temperatures slightly above and slightly below the 727 ºC (1340 ºF) line, say for example between 700 and 750 ºC (1292 1382 ºF), and slow cool. Or • For tool and alloy steels heat to 750 to 800 ºC (13821472 ºF) and hold for several hours followed by slow cooling.
  46. 46. • All these methods result in a structure in which all the Cementite is in the form of small globules (spheroids) dispersed throughout the ferrite matrix. This structure allows for improved machining in continuous cutting operations such as lathes and screw machines. Spheroidization also improves resistance to abrasion.
  47. 47. Spheroidizing process applied at a temperature below and above the LCT.
  48. 48. Spheroidizing process applied at a temperature below and above the LCT.
  49. 49. Aims Of Spheroidization Annealing:     minimum hardness maximum ductility maximum machinability maximum softness
  50. 50. TEMPERING – Martensite is a very strong phase, but it is normally very brittle so it is necessary to modify the mechanical properties by heat, treatment in the range 150—700°C. – Essentially, martensite is a highly Supersaturated solid solution of carbon in iron which, during tempering, rejects carbon in the form of finely divided carbide phases. – The end result of tempering is a fine dispersion of carbides in an α-iron matrix which often bears little structural similarity to the original as-quenched martensite.
  51. 51. NORMALIZING  The normalizing of steel is carried out by heating above the UCT (Upper Critical Temperature) to single phase austenitic region to get homogeneous austenite, soaking there for some time and then cooling it in air to room temperature.  The austenitising temperature range are: For hypoeutectoid steels and eutectoid steel • Ac3 + (40-60oC) For hypereutectoid steels • Acm + (30-50oC)
  52. 52.  During normalising we use grain refinement which is associated with allotropic transformation upon heating γ→α  Parts that require maximum toughness and those subjected to impact are often normalized.  When large cross sections are normalized, they are also tempered to further reduce stress and more closely control mechanical properties.  The microstructure obtained by normalizing depends on the composition of the castings (which dictates its hardenability) and the cooling rate.
  53. 53. Figure below shows the normalizing temperatures for hypoeutectoid and hypereutectoid steels
  54. 54. AIMs OF NORMALIZING • To produce a harder and stronger steel than full annealing • To improve machinability • To modify and/or refine the grain structure • To obtain a relatively good ductility without reducing the hardness and strength • Improve dimensional stability • Produce a homogeneous microstructure • Reduce banding • Provide a more consistent response when hardening or case hardening
  57. 57. COMPARISON OF ANNEALING AND NORMALIZING  The metal is heated to a higher temperature and then removed from the furnace for air cooling in normalizing rather than furnace cooling.  In normalizing, the cooling rate is slower than that of a quench-and-temper operation but faster than that used in annealing.  As a result of this intermediate cooling rate, the parts will possess a hardness and strength somewhat greater than if annealed.  Fully annealed parts are uniform in softness (and machinability) throughout the entire part; since the entire part is exposed to the controlled furnace cooling. In the case of the normalized part, depending on the part geometry, the cooling is non-uniform resulting in non-uniform material properties across the part.  Internal stresses are more in normalizing as compared to annealing.  Grain size obtained in normalizing is finer than in annealing.  Normalizing is a cheaper and less time-consuming process.
  58. 58. Comparison of temperature ranges in annealing and normalizing
  59. 59. Comparison of timetemperature cycles for normalizing and full annealing The slower cooling of annealing results in higher temperature transformation to ferrite and pearlite and coarser microstructures than does normalizing.
  60. 60. effect of annealing and normalizing on ductility of steels Annealing and normalizing do not present a significant difference on the ductility of low carbon steels. As the carbon content increases, annealing maintains the % elongation around 20%. On the other hand, the ductility of the normalized high carbon steels drop to 1 to 2 % level.
  61. 61. effect of annealing and normalizing on the tensile strength AND YIELD POINT of steels  The tensile strength and the yield point of the normalized steels are higher than the annealed steels.  Normalizing and annealing do not show a significant difference on the tensile strength and yield point of the low carbon steels.  However, normalized high carbon steels present much higher tensile strength and yield point than those that are annealed. This can be illustrated from the figures.
  62. 62. effect of annealing and normalizing on the hardness of steels Low and medium carbon steels can maintain similar hardness levels when normalized or annealed. However, when high carbon steels are normalized they maintain higher levels of hardness than those that are annealed.
  63. 63. ADVANTAGES OF NORMALIZING OVER ANNEALING  Better mechanical properties  Lesser time-consuming  Lower cost of fuel and operation ADVANTAGES OF ANNEALING OVER NORMALIZING  Greater softness  Complete absence of internal stresses which is a necessity in complex and intricate parts
  64. 64. HARDENING • It is the process of heating the steel to proper austenitizing temperature , soaking at this temperature to get a fine grained and homogeneous austenite , and then cooling the steel at a rate faster than its critical cooling rate.
  65. 65. OBJECTIVES OF HARDENING The aims of hardening are: 1. Main aim of hardening is to induce high hardness. The cutting ability of a tool is proportional to its hardness. 2. Many machine parts and all tools are hardened to induce high wear resistance higher is the hardness , higher is the wear and the abrasion resistance .For example ,gears, shaft. 3. The main objective of hardening machine components made of structural steel sis to develop high yield strength with good toughness and ductility to bear high working stresses.
  66. 66. Austenising Temperature for Pearlitic Steels • The steel is first heated to proper austenising temperature to obtain a homogeneous and fine grained austenite. This temperature depends on the composition(carbon as well as alloying elements). • The austenitising temperature of plain carbon steels depends on the carbon content of the steel and is generalised as : • For hypo-eutectoid steels :Ac3 + (20 — 40°C) • For hyper-eutectoid steels and eutectoid steel:Ac1 + (20 — 40°C)
  67. 67. Austenising Temperature for Pearlitic Steels
  68. 68. Austenising Temperature for Pearlitic Steels • Hypereutectoid steels, when heated in above temperature range, to obtain homogeneous and finegrained austenite which on quenching transforms to finegrained (very fine needles/plates), and hard martensite as is desired to be obtained. • Heating these steels only up to critical range (between Ac3 and Ac1) is avoided in practice. • Steel then has austenitic and ferrite. • On quenching, only austenite transforms to martensite, and ferrite remains as it is, i.e., incomplete hardening occurs . • The presence of soft ferrite does not permit to achieve high hardness, if that is the objective.
  69. 69. Austenising Temperature for Pearlitic Steels • If the aim is to get high strength by the process of tempering ferrite does not permit this as it has low tensile and yield strengths . • In fact, ferrite forms the easy path to fracture. • Quenching of hypoeutectoid steels from temperatures much above the proper temperatures , when austenite has become coarse, results in coarse acicular form of martensite. • Coarse martensite is more brittle, and a unit or two lower in hardness. It lowers the impact strength even after tempering, and is more prone to quenchcracking.
  70. 70. Austenising Temperature for Pearlitic Steels • Hypereutectoid steels, when heated in the above range, i.e., just above Ac1 have fine grains of austenite and proeutectoid cementite. • On quenching austenite transforms to fine martensite and cememtite remains unchanged. • As the hardness of cementite (≈ 800 BHN) is more than that of martensite (650-750 BHN), its presence increases the hardness, wear and abrasion resistance as compared to only martensitic structure. • If temperature of austenitisation is much higher than Ac1 but still below Acm temperature, a part of proeutectoid cementite gets dissolved to increase the carbon content of austenhlc(> 0.77%)
  71. 71. Austenising Temperature for Pearlitic Steels • On quenching as-quenched hardness is less, because : 1. Lesser amount of proeutectoid cementite is present. 2. Larger amount of soft retained austenite is obtained as the dissolved carbon of cementite has lowered the Ms and Mf temperature. 3. A bit coarser martensite has lesser hardness.
  72. 72. Austenising Temperature for Pearlitic Steels • Heating hypereutectoid steels to a temperature higher than Acm results in 100% austenite . It is very coarse austenite as very rapid grain-growth occurs due to dissolution of restraining proeutectoid cementite . • The as-quenched hardness is low because of: 1) Absence of harder cementite. 2) As more carbon has dissolved in austenite, more retained austenite is obtained. 3) Coarser martensite is a bit less hard and more brittle. • Thus, these temperatures are avoided in carbon steels
  73. 73. PROCESS OF QUENCHING • When a heated steel object (say at 840°C) is plunged into a stationary bath of cold it has three stages as: Stage A -vapour-blanket stage: • Immediately on quenching, coolant gets vapourized as the steel part is at high temperature, and thus, a continuous vapour- blanket envelopes the steel part. • Heat escapes from the hot surface very slowly by radiation and conduction through the blanket of water vapour. • Since the vapour-film is a poor heat conductor, the cooling rate is relatively low (stage A in fig ). This long stage is undesirable in most quenching operations.
  75. 75. PROCESS OF QUENCHING Stage B-Intermittent contact stage (Liquid-boiling stage): • Heat is removed in the form of heat of vaporization in this stage as is indicated by the steep slope of the cooling curve. • During this stage, the vapour-blanket is broken intermittently allowing the coolant to come in contact with the hot surface at one instant, but soon being pushed away by violent boiling action of vapour bubble. • The rapid cooling in this stage soon brings the metal surface below the boiling point of the coolant.
  76. 76. PROCESS OF QUENCHING • The vaporization then stops. Second stage corresponds to temperature range of 500◦ to 100◦c , and this refers to nose of the CCT curve of the steel , when the steel transforms very rapidly ( to non martensite product ). • Thus, the rate of cooling in this stage is of great importance in hardening of steels. Stage C-Direct-Contact stage (Liquid-cooling stage): • This stage begins when the temperature of steel surface Is below the boiling point of coolant. • Vapours do not form. The cooling is due to convection and conduction through the liquid. Cooling is slowest here.
  77. 77. QUENCHING MEDIUMS • As the aim is to get martensite, the coolant should have quenching power to cool austenite to let it transform to martensite. The following factors effect the quenching power of the coolant : • The cooling rate decreases as the temperature of water and brine increases, i.e., it increases stage ‗A‘, i.e., helps in persistence of the vapour blanket stage. • The increased temperature brings it closer to its boiling point, and thus, requires less heat to form vapour, specially above 60°C. • Good range of temperature for water as coolant is 20-40°C. • Oils in general, show increased cooling rates with the rise of temperature, with optimum cooling rates in range 50°—80°C.
  78. 78. QUENCHING MEDIUMS • In oils, the increase of temperature increases the persistence of vapour-blanket, but this resulting decrease in the cooling rate is more than compensated by the decrease of viscosity (with the rise in temperature) to result in increase of rate of heat removal through the oil. • If the boiling point of a coolant is low, vapours form easily to increase the ‗A‘ stage of cooling. ¡t is better to use a coolant with higher boiling point. A coolant with low specific heat gets heated up at a faster rate to form vapours easily. • A coolant with low latent heat of vapourisation changes into vapour easily to promote ‗A‘ stage, i.e., decreases the cooling rate. • A coolant with high thermal conductivity increases the cooling rate. Coolants with low viscoity provide faster cooling rates and decrease the ‗A‘ stage.
  79. 79. QUENCHING MEDIUMS • A coolant should be able to Provide rate of cooling fast enough to avoid transformation of austenite to pearlite and bainite . Plain carbon steel invariably require çooling in water or brine. Whereby alloy steels are quenched normally in oils. • But milder the cooling medium , lesser the internal stresses developed , and thus lesser the danger of distortion , or cracks . An ideal quenching medium is one which is able to provide very fast cooling rate near the nose of the curve ( 650 -550°C)and at the same time it should provide very considerable slower rate if cooling within the range of martensitic transformation( 300 200°C) to minimize internal stresses .
  80. 80. WATER • The oldest and still the most popular quenching medium, water meets the requirements of low cost ,general easy availability, easy handling and safety. • The cooling characteristics change more than oil with the rise of temperature, specially there is a rapid fall in cooling capacity as the temperature rises above 60°C, because of easy formation of vapourblanket. • The optimum cooling pover is when water is 2O-4O°C. • Thc cooling power of water is between brine and oils. • Water provides high cooling power to avoid the transformation of austenite to pearlite/bainite, but the major draw back is that it also provides high cooling rate in the the temperature range of martensite formation. • At this stage, the steel is simultaneously under the influence of structural stresses (non-uniform change in structure) and thermal stresses which increase the risk of crack formation.
  81. 81. BRINE • Sodium chloride aqueous solutions of about 10% by weight are widely used and are called brines. • The cooling power is between 10% NaOH aqueous solution and water. • These are corrosive to appliances. • The greater cooling efficiency of brines, or other aqueous solutions is based as : • In brine heating of the solution at the steel surface causes the deposition of crystal of the salt on hot steel surface . • This layer of solid crystals disrupts with mild explosive violence, und throws off a cloud of crystals. This action destroys the vapour-film from the surface, and thus permits direct contact of the coolant with the steel surface with an accompanying rapid removal of heat. • Brines are used where cooling rates faster than water arc requited.
  82. 82. OILS • Oils have cooling power between water at 40°C to water at 90°C. • In oil-quench, considerable variation can be obtained by the use of animal, vegetable, or mineral oil, or their blends. • Oils should be used at 50- 80°C when these are more fluid, i.e less VISCOUS, which increases the cooling power. • As the oils used generally have high boiling points, moderate increase of temperature of oil does not very much increase the vapour blanket stage. However, oils in contrast to water, or brine, have much lower quenching power . • Its this relatively slow cooling rate in the range of martensitic formation is atlvantageous as it helps in minimsing the danger to crack formation. • Oils with high viscosity are less volatile, and thus have decreased vapour-blanket stage (increase thecooling rate). As lesser volatile matter is lost, their cooling power is not affected much with use.
  83. 83. POLYMER QUENCHANTS • polymer quenchants cool rapidly the heated steel to Ms temperature, and then rather slowly when martensite is forming . • Polymer quenchants are water-soluble organic chemicals of high ,molecular weights, and are generally polyalkylene glycol-based, or polyvinyl pyrolidene-based. • Widely different cooling rates can be obtained by varying the concentration of Organic additives in water; higher the additions, slower is the cooling rate of solution. • There are little dangers of distortions and cracks.
  84. 84. SALT BATHS • It is an ideal quenching medium for a steel of not very large section but with good hardenabilty. • Addition of O.3-O.5‘% water almost doubles the cooling capacity. Normally holding time is 2-4 minutes/cm of section thickness. • Salt baths used for austenitising keep the steel clean.
  85. 85. INTERNAL STRESSES S DURING QUENCHING • internal stresses are produced due to non-uniform plastic deformation. In quenching of steels ,this may be caused by thermal stresses, structural stresses, or both, or even premature failure of part in service. • Cooling during quenching lakes place non-uniformly, i.e., causes temperature gradient across the section. • Surface layers contract more than the central portion. • Contraction of surface is resisted by the central portion, and this puts the central portion under the compressive stresses, and the surface layers in tension . • If the magnitude of stress becomes more than the yield stress of steel (at that deformation occurs. • These stresses that develop in a quenched part as a result of unequal cooling are called thermal stresses.
  86. 86. INTERNAL STRESSES DURING QUENCHING • Structural stresses are the stresses which develop due to due to phase change (mainly austenite to martensite), and at different times. • Structural stresses are developed due to two main reasons: 1. Austenite and its transformation products have unequal specific volume i.e. change in volume occurs when transformation occurs. 2. Phase changes occur at different times in the surface and in centre. • Under right conditions, both types of stresses get superimposed to become larger than the yield strength to cause warping. but when the tensile internal stresses become larger than the tensile strength cracks appear. • If an austenitised steel is quenched, it contracts thermally till Ms temperature is attained .
  88. 88. INTERNAL STRESSES DURING QUENCHING • figure(a) illustrates this in stage 1 As surface cools faster than centre, i.e., contracts more than centre distribution of stresses across the section is illustrated in fig (b), i.e, the surface is under tensile nature of stress, while centre is under compressive stresses. • Only thermal Stresses are produced in stage 2 , surface having attained Ms temperature, transforms to martenSite, and thus expands, while the centre is still contracting as it is getting cooled. • In stage Il, centre may get plastically deformed ,as it is still ductile austenite. • In stage 3, martensite of surface and austenite of centre continue contracting leading to slight increase in stress levels.
  89. 89. INTERNAL STRESSES DURING QUENCHING • In stage IV, centre has attained M5 temperature, and begins to expand as it forms martensite, while surface is still contracting. • The centre, as it expands, puts the surface in higher stress levels . • The surface has little deformation as it consists of brittle martensie. • It is during this stage, the greatest danger of cracking exists. • Thus, stress levels are highest not in the beginning of the quench, but when the centre is transforming to martensite.
  90. 90. INTERNAL STRESSES DURING QUENCHING • However, higher is the Ms temperature of the steel, lesser is the expansion, there is reduced danger of quench-cracking. • Increase of carbon and alloying elements lower the Ms temperature making the steel more prone to quench cracking.
  91. 91. RETAINED AUSTENITE • Martensitic transfomiation is essentially an athermal transformation. • Austenite begins to transform to martensite at Ms, and the amount of martensite formed increases as the temperature decreases to complete at Mf temperature. • Less than 1 % of austenite may not transform because of unfavourable stress conditions. • The Ms and Mf temperatures are lowered as the amounts of carbon content and alloying elements(except cobalt and aluminium) increase in the steel. • In a quenched steel, the amount of martensite formed depends on the location of Ms and Mf and the temperature of the coolant (which is normally room temperature. As long as room temperature lies between Ms and Mf temperatures, austenite does not transform completely to martensite as it has not been cooled below Mf temperature. • This untransformed austenite is retained austenite.
  92. 92. ADVANTAGES OF RETAINED AUSTENITE • 10% retained austenite is normally desirable as its ductility relieves some internal stresses developed during hardening. This reduces the danger to distortion or cracks. • The presence of 30-40% retained austenite makes straightening of components possible after hardening. • Non distorting tools owe their existence to retained austenite . It tries to balance transformational volume changes during heating as well as cooling cycles of heat treatment to produce little overall change in size of the tools.
  93. 93. DISADVANTAGES 1) The presence of soft austenite decreases the hardness of hardened steels. 2) As retained austenite may transform to lower bainite, or martensite later in service ,increase in dimensions of the part occurs. 3) This creates problems in precision gauges or dies. 4) Stresses may develop in the part itself as well as in adjacent pans. Grinding-cracks are mainly due to retained austenite transforming to martensite. 5) Austenite is non-magnetic, decreases the magnetic properties of the steels.
  94. 94. ELIMINATION OF RETAINED AUSTENITE • Retained austenite is generally undesirable. It is eliminated by one of the methods: 1. Sub-Zero treatment (cold treatment): • It Consists of cooling the hardened steel (having retained austenite) to a temperature below 0°C or its Mf temperature. • There is no reason to cool a steel much below its Mf temperature. • Sub-zero treatment is more effective if it is done immediately after the quenching operation (normally done to room temperature). • Sub-zero treatment is done in a low temperature-cooling unit, which consists of double-walled vessel. • The interior is made of copper in which the parts to be deepfrozen are kept, and the exterior is made of steel provided with good heat-insulation.
  95. 95. ELIMINATION OF RETAINED AUSTENITE • The space in between the vessels is filled with some coolant. • The sub-zero coolants could be, dry ice (Solid CO2) + acetone (— 78°C); Ice + NaCl (—23°C); liquid air (—183°C); liquid N2 (— 196°C); Freon (— 111°C). • Total time of cooling in this unit is 1/2 to 1 hour. • As this treatment transforms austenite to martensite, steels after sub-zero treatment have high hardness, wear and abrasion resistance, and have no danger of grinding-cracks.
  96. 96. ELIMINATION OF RETAINED AUSTENITE • The stresses are increased further and thus, tempering should be done immediately after sub-zero treatment. • Carburised steels, ball-bearing steels, highly alloy tool steels, are normally given cold treatment. 2. Tempering: • The second stage of tempering eliminates the retained austenite in most steels. • In high alloy steels, multiple tempering is able to eliminate the retained austenite during cooling from the tempering temperature.
  97. 97. DEFECTS IN HARDENING • The main defects produced during hardening are: 1. Mechanical properties not up to specifications: • The common defect in hardened tools, or component is too low a hardness. • One or more of the followings could be the cause of such a defect. • Insufficient fast cooling due to overheated or even polluted coolant could be responsible for a defect. • The presence of scale, or oil, etc. on the surface also decreases the cooling rate. • Circulation of coolant, or agitation of component may also result in such defect.
  98. 98. DEFECTS IN HARDENING • A shorter austenitising time can also cause such a defect. Lower austenitising temperature can also result in such a defect. • Decarburisation can also result in low surface hardness. If too high temperature had been used, which produces larger amount of retained austenite can result in low surface hardness. 2. Soft Spots: • Soft areas on the hardened surface are called ‗soft spots‘. • The adhering scale, or decarburisation of some areas or prolonged vapour-blanket stage due to overheated coolant or insufficient agitation or circulation of coolant, or rough surface could cause presence of soft spots surface.
  99. 99. ELIMINATION OF RETAINED AUSTENITE 3. Quench Cracks: • Quench cracks form as a result of internal stresses developed of tensile nature exceeding the tensile strength of the steel. • Steel with lower Ms temperature due to higher contents of alloying elements are more prone to quench cracks. Higher carbon also results in more brittle martensite. • Quench cracks can form if there is more time lag between the process of quenching and tempering. • Overheating of steel or a wrong coolant which gave much faster rate of cooling, or there is faulty design of the component with sharp corners and sharp transition between sections, or a wrong steel has been chosen. • Presence of large amounts of retained austenite causes grinding cracks. • The other defects could be distortion and warpage; change in dimensions; oxidation and decarburisation
  100. 100. Tempering of plain carbon steels – In the as-quenched martensite structure,the laths or plates are heavily dislocated to an extent that individual dislocations are very difficult to observe in thin-foil electron micrographs. – A typical dislocation density for a 0.2 wt% carbon steel is between 0.3 and 1.0 x 1012 cm cm-3. As the carbon content rises above about 0.3 wt%, fine twins about 5—10 nm wide are also observed. – Often carbide particles, usually rods or small plates, are observed (Fig. 9.1).
  101. 101. Tempering of plain carbon steels – These occur in the first-formed martensite, i.e. the martensite formed near Ms, which has the opportunity of tempering during the remainder of the quench. – This phenomenon, which is referred to as autó-tempering, is clearly more likely to occur in steels with a high Ms.
  102. 102. Stages of Tempering
  103. 103. Stages of Tempering – On reheating as-quenched martensite, the tempering takes place in four distinct but overlapping stages: – Stage 1, up to 250°C — precipitation of E-iron carbide; partial loss of tetragonality in martensite. – Stage 2, between 200 and 300°C — decomposition of retained austenite . – Stage 3, between 200 and 350°C — replacement of &iron carbide by cementite; martensite loses tetragonality. – Stage 4, above 350°C — cementite coarsens and spheroidizes; recrystallization of ferrite.
  104. 104. Tempering — stage 1 – Martensite formed in medium and high carbon steels (0.3—1.5 wt% C) is not stable at room temperature because interstitial carbon atoms can diffuse in the tetragonal martensite lattice at this temperature. – This instability increases between room temperature and 250°C, when €-iron carbide precipitates in the martensite (Fig. 9.2). – This carbide has a close-packed hexagonal structure, and precipitates as narrow laths or rodlets on cube planes of the matrix with a welldefined orientation relationship .
  105. 105. Tempering — stage 1 – At the end of stage 1 the martensite still possesses a tetragonality, indicating a carbon content of around 0.25 wt%. – It follows that steels with lower carbon contents are unlikely to precipitate €-carhide. – This stage of tempering possess an activation energy of between 60 and 80 kJ mo1, which is in the right range for diffusion of carbon in martensite. The activation energy has been shown to increase linearly with the carbon concentration between 0.2 and 1.5 wt% C. – This would be expected as increasing the carbon concentration also increases the occupancy of the preferred interstitial sites, i.e. the octahedral interstices at the midpoints of cell edges, and centres of cell faces, thus reducing the mobility of C atoms.
  106. 106. Tempering — stage 2 • During stage 2. austenite retained during quenching is decomposed usually in the temperature range 230-300°C. • In martensitiC plain carbon steels below 0.5 carbon. the retained austenite is often below 2%, rising to around 6 % at 0.8 wt C and over 30 % at 1.25 wt C. • The little available evidence suggests that in the range 230-300°C, retained austenite decomposes to bainitic ferrte and cementite, but no detailed comparison between this phase and lower bainite has yet been made.
  107. 107. Tempering — stage 3 – During the third stage of tempering, cementite flrst appears in the microstructure as a Widmanstatten distribution of plates which have a well-defined orientation relationship with the matrix which has now lost its tetragonality and become ferrite. – This reaction commences as low as 100°C and is fully developed at 300°C, with particles up to 200 nm long and 15 nm in thickness. – Similar structures are often observed in lower carbon steels as quenched, as a result of the formation of Fe3C during the quench.
  108. 108. Tempering — stage 3 – During tempering, the most likely sites for the nucleation of the cementite are the €-iron carbide irterfaces with the matrix (Fig 9.2) and as the Fe3C particles grow, the €-iron carbide particles gradually disappear. – The twins occurring in the higher carbon martensites are also site for the nucleation and growth of cementite which tends to grow along the twin boundaries forming colonies of similarly oriented lath shaped particles (Fig. 9.3) which can be readily ditinguished from the normal Widmanstatten habit.
  109. 109. Tempering — stage 3 – A third site for the nucleation of cementite is the grain boundary regions (Fig, 9.4)of both the interlath boundaries of martensite and the original austenite grain b0unjaries. – The cementite can form as very thin films which are difficult to detect but which gradually sp1eroidise to give rise to welI-defined particles of Fe3C in the grain boundary regions. – There is some evidence to show that these. boundary cementite films can adversely affect ductility. However it can be modified by addition of alloying elements.
  110. 110. Tempering — stage 3 – During the third stage of tempering , the tetragonality of thc matrix disappears and it is then, essentially, ferrite, not supersaturated with respect to carbon. – Subsequent changes in the morpriology of cementite particles occur by process where the smaller particles dissolve in the matrix providing carbon for the selective growth of the larger particles.
  111. 111. Tempering — stage 3 – During the third stage of tempering , the tetragonality of thc matrix disappears and it is then, essentially, ferrite, not supersaturated with respect to carbon. – Subsequent changes in the morpriology of cementite particles occur by process where the smaller particles dissolve in the matrix providing carbon for the selective growth of the larger particles.
  112. 112. Tempering — stage 4 – It is useful to define a fourth stage of tempering in which the cementite particles undergo a coarsening process and essentially lose their crystallographic morphology, becoming spheroidized. – It commences between 300 and 400◦C, while spheroidizatiun takes place increasingly up to 700◦C. – At the higher end of this range of tempera. ture the martensite lath boundaries are replaced by more equi-axid fèrrite grain boundaries by a process which is best described as recrystallization. – The final result is an equi-axed array of ferrite grains with coarse spheroidized particles of Fe3C (Fig. 9.5), partly, but not exclusively, by the grain boundaries.
  113. 113. Tempering — stage 4 – The spherodisation of the Fe3C is encouraged by the resulting decrease in surface energy. – The particles which preferentially grew and spheroidize are located mainly at interlath boundaries and prior austenite boundaries, although some particles remain in the matrix. – The boundary sites are preferred because of the greater ease of diffusion in these regions. Also, the growth of cementite into ferrite is associated with a decrease in density so vacancies are required to accommodate the growing cementite. – Vacancies will diffuse away from cementite particles which are redissolving in the ferrite and towards cementite particles which are growing, so that the rate controlling process is likely to be the diffusion of vacancies.
  114. 114. Tempering — stage 4 – The original martensite lath boundaries remain stable up to about 600°C, but in the range 350—600°C. there is considerable rearrangement of the dislocations within the laths and at those lath boundaries which are essentially low angle boundaries. – This leads to a marked reduction in the dislocation density and to lath-shaped ferritic grains closely related to the packets of similarly oriented laths in the original martensite. – This process, which is essentially one of recovery, is replaced between 600 and 700°C by recrystallization which results in the formation of equi-axed ferrite grains with spheroidal Fe3C particles in the boundaries and within the grains.
  115. 115. Tempering — stage 4 – This process occurs most readily in carbon steels. – At higher carbon content, the increased density of cementite particles is much more effective in pinning the ferrite boundaries, so recrystallisation is much more sluggish. – The final process is the continued coarsening of the cementite particles and gradual ferrite grain growth.
  116. 116. Role of carbon content – Firstly, the hardness of the as-quenched martensite is largely influenced by the carbon content, as is the morphology of the martensite laths which have a up to 0.3 wt% C, changing at higher carbon contents. – The Ms temperature is reduced as the carbon content increases, arid thus the probability of the occurrence of auto-tempering is less. – During fast quenching in alloys with less than 0.2 wt % C, the majority (up to 90%) of the carbon segregates to dislocations and lath boundaries, but with slower quenching some precipitation of cementite occurs. – On subsequent tempering of low carbon steels up to 200°C further segregation of carbon takes place. but no precipitation has been observed.
  117. 117. Role of carbon content
  118. 118. Role of carbon content • Under normal circumstances it is difficult to detect any tetragonality in martensitic in steels with less than 0.2 wt% C, a fact which can explained by the rapid segregation of carbon during quenching. • The hardness change; during tempering are also very dependent on carbon content, as shown in figure for steels up to 0.4 wt% C. • Above this concentration, an increase in hardness has been observed in temperature range 50—150°C, as €carbide precipitation strengthens the martensite. • However, the general trend is an overall
  119. 119. Mechanical properties of tempered plain carbon steels – The absence of other alloying elements means that the hardenability of the steels is low, so a fully martensitic structure is only possible in thin sections. – However, this may not be a disadvantage where shallow hardened surface layers are all that is required. – Secondly, at lower carbon levels, the Ms temperature is rather high, so autotempering is likely to take place. – Thirdly, at the higher carbon levels the presence of retained austenite will influence the results. – Added to these factors, plain carbon steels can exhibit quench cracking which makes it difficult to obtain reliable test results. This is particularly the case at higher carbon levels, i.e. above 0.5 wt% carbon.
  120. 120. Tempering of alloy steels – The addition of allying elements to a steel has a substantial effect on the kinetics of the y →α transformation, and also of the pearlite reaction. – Most common alloying elements move the TTT curves to longer times, with the result that it is much easier to miss the nose of the curve during quenching. – This essentially gives higher hardenability, since martensite structures can be achieved at slower cooling rates and, in practical terms, thicker specimens can be made fully martensitic. – Alloying elements have also been shown to have a substantial effect in depressing the Ms temperature.
  121. 121. The effect of alloying elements on the formation of iron carbides • It is clear that certain elements, notably silicon can stabilize the €-iron carbide to such an extent that it is still present in the ‗microstructure after tempering at 400°C in steels with 1-2 wt% Si, and at even higher temperatures if the silicon is further increased. • The evidence suggests that both the nucleation and growth of the carbide is slowed down and that silicon enters into the €-carbide structure. • It is also clear that the transformation of €-iron carbide to cementite is delayed considerably.
  122. 122. The effect of alloying elements on the formation of iron carbides • While the tetragonality of martensite disappears by 300°C in plain carbon steels, in steels containing some alloying elements, e.g. Cr, Mo, W V, Ti, Si, the tetragonal lattice is still observed after tempering at 450°C and even as high as 500°C . • It is clear that these alloying elements increase the stability of the supersaturated iron-carbide solid solution. • In contrast manganese and nickel decrease the stability.
  123. 123. The effect of alloying elements on the formation of iron carbides • Alloying elements also greatly influence the proportion of austenite retained on quenching. • Typically a steel with 4% molybdenum, 0.2%C, in the martensitic state contains less than 2% austenite, and about 5% is detected in a steel with 1% vanadium and 0.2%C. • The austenite can be revealed as a fine network around the martensite
  124. 124. The effect of alloying elements on the formation of iron carbides • On tempering each of the above steels at 300°C, the austenite decomposes to give thin grain boundary films of cementite which, in the case of the higher concentrations of retained austenite, can be fairly continuous along the lath boundaries. • It is likely that this interlath cementite is responsible for tempered embrittlement frequently encountered as a toughness minimum in the range 300—350°C, by leading to easy nucleation of cracks, which then propagate across the tempered martensite laths.
  125. 125. • The effect of alloying elements on the formation also restrain the coarsening Alloying elements canof iron carbides of cementite in the range 400-700°C, a basic process during the fourth stage of tempering. • Several alloying elements, notably silicon, chromium. molybdenum and tungsten, cause the cementite to retain its fine Widmanstatten structure to higher temperatures, either by entering into the cementite structure or by segregating at the carbide-ferrite interfaces. • Whatever the basic cause may be, the effect is to delay significantly the softening process during
  126. 126. The effect of alloying elements on the formation of iron carbides • This influence on the cementite dispersion has other effects, in so far as the carbide particles, by remaining finer, slow down the reorganization of the dislocations inherited from the martensite, with the result that the dislocation substructures refine more slowly. • In plain-carbon Steel cementite particle begin to coarsen in the temperature range 350 -400°C and addition of chromium, silicon, molybdenum or tungsten delays the coarsening to the range 500-550°C. • It should be emphasized that up to 500°C the only carbides to form are those of iron.
  127. 127. The formation of alloy carbides Secondary Hardening • A number of the familiar alloying elements in steels form carbides, nitrides and borides which are thermodynamically more stable than cementite. • It would therefore be expected that when strong carbide elements are present in a steel in sufficient concentration, their carbides would be formed in preference to cementite . • Nevertheless during the tempering of all ahoy steels, alloy carbides do not form until the temperature range 500-600°C , because below this the metallic alloying elements cannot diffuse sufficiently rapidly to allow alloy carbides to nucleate.
  128. 128. The formation of alloy carbides Secondary Hardening • The metallic elements diffuse substitutionally, in contrast to carbon and nitrogen which move through the iron interstitially. • With the result that the diffusivities of carbon and nitrogen are of several orders of magnitude greater in iron than those of the metallic alloying elements. • Consequently higher temperatures are needed for the necessary diffusion of the alloying elements. • It is this ability of certain alloying elements to form fine alloy carbide dispersions in the range 500—600°C, which remain very fine even after prolonged tempering, that allows the development of high strength levels in many alloy steels.
  129. 129. The formation of alloy carbides Secondary Hardening • Indeed, the formation of alloy carbides between 500 and 600°C is accompanied by a marked increase in strength, often in excess to that of the as-quenched martensite. • This phenomenon, which is referred to as secondary hardening, is best shown in steels containing molybdenum, vanadium, tungsten, titanium, and also in chromium steels at higher alloy concentrations.
  130. 130. The formation of alloy carbides Secondary Hardening • This secondary hardening process is a type of age-hardening reaction, in which a relatively coarse cementite dispersion is replaced by a new and much finer alloy carbide dispersion. • On attaining a critical dispersion parameter, the strength of the steel reaches a maximum, and as the carbide dispersion slowly coarsens, the strength drops.
  131. 131. The formation of alloy carbides Secondary Hardening • The process is both time and temperature dependent, so both variables are often combined in a parameter: P = T(k + log t) where: T is the absolute temperature and t the tempering time in hours, while k is a constant which is about 20 for alloy steels, usually referred to as the Holloman-Jaffe parameter.
  132. 132. Martempering Interrupted quench from the A1  Delay cooling just above martensitic transformation for a length of time to equalize T throughout the piece  Reduce thermal gradient btw surface & center  Minimize distortion, cracking, and residual stress.  137
  133. 133. CASE HARDENING • Low carbon steels cannot be hardened by heating due to the small amounts of carbon present. • Case hardening seeks to give a hard outer skin over a softer core on the metal. • The addition of carbon to the outer skin is known as carburising.
  134. 134. Different stages of Case Hardening
  135. 135. • Case hardening or surface hardening is the process of hardening the surface of a metal object while allowing the metal deeper underneath to remain soft, thus forming a thin layer of harder metal (called the "case") at the surface. For steel or iron with low carbon content, which has poor to no hardenability of its own, the case hardening process involves infusing additional carbon into the case • Case hardening is usually done after the part has been formed into its final shape, but can also be done to increase the hardening element content of bars to be used in a pattern welding or similar process. The term face hardening is also used to describe this technique, when discussing modern armour • Because hardened metal is usually brittler than softer metal, through-hardening (that is, hardening the metal uniformly throughout the piece) is not always a suitable choice for applications where the metal part is subject to certain kinds of stress. In such applications, case hardening can provide a part that will not fracture (because of the soft core that can absorb stresses without cracking) but also provides adequate wear resistance on the surface
  136. 136. Case Hardened Chisel Chisel: - cutting edge is hard and wear-resistant - tang is tough and elastic If the chisel would be hard throughout, it could break when the hammer is striked onto it! Figure - Cut through a hardened chisel - 1 cutting edge (hard), 2 twig (tough)
  138. 138. CARBURIZING The highest hardness of a steel is obtained when its carbon content ishigh, around 0.8 weight % C (Figure 1). Steels with such high carbon content are hard, but also brittle, and therefore cannot be used in machine parts such as gears, sleeves and shafts that are exposed to dynamic bending and tensile stresses during operation. A carbon content as high as 1% C also makes the steel difficult to machine by cutting operations such as turning or drilling. These shortcomings can be eliminated by using a low carbon content steel to machine a part to its final form and dimensions prior to carburizing and hardening. The low carbon content in the steel ensures good machinability before carburizing. After carburizing and quenching the part will have a hard case but a softer core that will assure wear and fatigue resistance. The martensitic case attains a hardness corresponding to its carbon content. The case is typically 0.1–1.5 mm (0.004- 0.060 inches) thick. The core of the part maintains its low carbon concentration and corresponding lower hardness.
  139. 139. A carburizing atmosphere must be able to transfer carbon – and also nitrogen in the case of carbonitriding – to the steel surface to provide the required surface hardness. To meet hardness tolerance requirements this transfer must result in closely controlled carbon or nitrogen concentrations in the steel surface. The carbon concentration, as indicated in Figure 3, can be controlled by the ratio (vol% CO)2/(vol% CO2) in the furnace atmosphere. The atmosphere nitrogen activity, which plays an important role in carbonitriding
  140. 140. PROPERTIES OF CARBURISED STEEL The gas-carburized (carbonitrided) part can be said to consist of a composite material, where the carburized surface is hard but the unaffected core is softer and ductile. Compressive residual stresses are formed in the surface layer upon quenching from the carburizing temperature. The combination of high hardness and compressive stresses results in high fatigue strength, wear resistance, and toughness.
  141. 141. Maximum hardness for unalloyed steels is obtained when the carbon concentration is about 0.8%C, as was shown. Above that carbon concentration the hardness decreases as the result of an increased amount of retained austenite. The hardness curve therefore often exhibits a drop in hardness close to the surface, where the carbon concentration is highest. Carbon, nitrogen and almost all alloying elements lower the Ms-temperature (see reference [2] for the definition of Ms temperature). This leads to a retained austenite concentration gradient that increases towards the surface after carburizing and quenching.
  142. 142. • To compensate for this effect, the surface carbon concentration after carburizing that provides maximum surface hardness has to be lowered as the alloy content of the steel increases. Carbide forming elements, such as chromium and molybdenum, can counteract this effect and raise the surface carbon concentration that provides maximum hardness. This is because the formation of carbides leads to a lowered carbon concentration in the austenite, although the average carbon concentration is high. Table 1 gives some examples of the relation between maximum hardness and carbon concentration for different types of steels. Mo-alloyed steels obtain the highest surface hardness and Ni-alloyed steels the lowest. MnCr steels obtain an intermediate surface hardness
  143. 143. Case and Carburizing depth According to European standards [6], the case depth is abbreviated to CHD (case hardened depth) and defined as the depth from the surface to the point where the hardness is 550HV, as shown . Sometimes a hardness other than 550HV is used to define the case depth
  144. 144. Attained case depth depends not only on carburizing depth, but also on the hardening temperature, the quench rate, the hardenability of the steel and the dimensions of the part. This is illustrated in the schematic CCT diagrams. The hyperbolic temperature/ time-dependent parts of the transformation curves depict the transformation from austenite to ferrite/pearlite. For a high hardenability steel these curves are located far to the right in the diagram, ensuring that the cooling curves do not cross the ferrite/pearlite transformation curve
  145. 145. Hardenability increases not only with base steel alloy content but also with increased carbon and nitrogen concentrations. The carburized or carbonitrided case therefore has higher hardenability than the base steel
  146. 146. In Figure ―a‖ the cooling curves for both ―surface‖ and ―center‖ cross the transformation line for the base steel, the core. This means that the core will transform to ferrite/pearlite upon cooling from hardening temperature. If the cooling curves are related to the ―case‖ instead, it can be seen that the cooling line for the surface passes to the left of the ferrite/pearlite transformation curve. Thus the―surface‖ cooling line first crosses the Ms (case) line, meaning that the austenite will transform to martensite, as is the intention in case hardening. The hardenability of steel number 1 in Figure ―b ―is too low to result in martensite transformation even for the carburized case
  147. 147. – In Figure ―c‖ carbonitriding is a method for achieving high enough hardenability to form a martensitic case. (The ―surface‖ cooling line passes to the left of the carbonitrided transformation curve.) Carbonitriding is a way to make water-quench steels become oil hardening steels. – In Figure ―d‖ schematically shows the effect of part dimensions on cooling rate. The bigger the dimensions, the slower the cooling rate. Therefore there is a certain maximum diameter for a certain steel grade that can be hardened to form a martensitic case. When a martensitic case is formed the case depth will decrease with
  148. 148. Carburizing depth is not standardized but is nevertheless used in practice, and is defined as the depth from the surface to the point corresponding to a specified carbon concentration. As a guideline, the case depth (CHD) for common steels and part dimensions is approximately equal to the carburizing depth to the point where the carbon concentration is about 0.35%. The carburizing depth depends on treatment time and temperature. With prolonged carburizing time carbon can diffuse to a greater depth into the steel. Increasing the temperature increases the rate of diffusion and thus increases the carburizing depth
  149. 149. CHEMISTRY OF CASE HARDENING Carbon itself is solid at case-hardening temperatures and so is immobile. Transport to the surface of the steel was as gaseous carbon monoxide, generated by the breakdown of the carburising compound and the oxygen packed into the sealed box. This takes place with pure carbon, but unworkably slowly. Although oxygen is required for this process it is re-circulated through the CO cycle and so can be carried out inside a sealed box
  150. 150. – The sealing is necessary to stop the CO either leaking out, or being oxidised to CO2 by excess outside air. – Adding an easily decomposed carbonate "energiser" such as barium carbonate breaks down to BaO + CO2 and this encourages the reaction – C (from the donor) + CO2 <—> 2 CO
  151. 151. Increasing the overall abundance of CO and the activity of the carburising compound.[1] It is a common knowledge fallacy that case-hardening was done with bone, but this is misleading. Although bone was used, the main carbon donor was hoof and horn. Bone contains some carbonates, but is mainly calcium phosphate (as hydroxylapatite). This does not have the beneficial effect of encouraging CO production and it can also introduce phosphorus as an impurity into the steel alloy.
  152. 152. Carbon transfer from gas to surface Possible carbon transfer reactions are 2CO → C+CO2 CH4 → C + 2H2 CO+H2→ C+H2O It has been shown that the last of these reactions,is by far the fastest and is therefore the rate-determining reaction in carburizing atmospheres with CO and H2 as major gas components . The slowest carburizing reaction is from methane, with a rate that is only about 1% of the rate of carburizing from CO+H2 .
  153. 153. Interaction between Furnace Atmosphere and Steel In the above reaction, carbon monoxide (CO) and hydrogen(H2) react so that carbon (C) is deposited on the steel surface and water vapor (H2O) is formed. The furnace atmosphere must contain enough carbon monoxide and hydrogen to allow the carburizing process to proceed in a uniform and reproducible fashion. The supply of fresh gas must compensate for the consumption of CO and H2. A higher gas flow is required in cases where the furnace charge area is high, resulting in a high rate of carbon transfer from gas to surface. In the initial part of a carburizing cycle, there is also a high carbon transfer rate, which may be compensated for by increasing the gas supply.
  154. 154. According to the fundamental principles of chemistry, the equilibrium condition for the carburizing reaction 1 is described by an equilibrium constant expressed by: K1 = (ac· PH2O)/(PCO · PH2) The value of K1 is dependent on the temperatureand can be calculated from the relationship: log K1 = –7.494 + 7130/T where T is the absolute temperature in Kelvin. ac is termed carbon activity and is a measure of the ―carbon content‖ of the gas. We see that ac can be calculated if K1 and the gas composition are known. When the carbon activity of the gas, acg, is greater than that of the steel surface, acs, there is a driving force to transfer carbon as expressed by the following equation: dm/dt = k · (acg – acs) dm/dt = k‘ · (ccg – ccs) where: m designates mass, c concentration per unit volume, t time, dm/dt expresses a carbon flow in units of kg/cm2 · s or mol/m2 · s, and
  155. 155. The gradient dc/dx has its highest value at the beginning of the cycle when carbon has only diffused to a thin depth. This results in a high driving force for carbon flux by diffusion into the steel. The rate of the carbon transfer from gas to surface will therefore initially be the limiting step. At the start of a carburizing cycle, the term ccg – ccs has its highest value, and accordingly the driving force for carbon transfer from gas to steel has its highest value. The surface carbon Concentration ccs will increase with increasing carburizing time. The driving force for carbon transfer, ccg – ccs, will thus decrease. The carbon concentration gradient, dc/dx, will decrease concurrently as carbon diffuses into the steel. In conclusion, these limitations will lead to a continuous reduction of carbon flux into the steel
  156. 156. Carburizing Atmospheres Endogas A carburizing atmosphere can be achieved by means of incomplete combustion of propane or methane with air in accordance with one of the reactions: C3H 8 + 7.2 air → 5.7 N2 + 3CO + 4H2 CH4 + 2.4 air → 1.9 N2 + CO + 2H2 The mixing and combustion of fuel and air takes place in special endothermicgas generators
  157. 157. Nitrogen/Methanol Atmospheres Introducing nitrogen and methanol directly into the furnace chamber is a common way of creating the furnace atmosphere. Upon entering the furnace, methanol cracks to form carbon monoxide and hydrogen in accordance with the following reaction: CH 3 OH → CO + 2H 2 complete cracking of methanol into CO and H 2 only occurs if the temperature is above 700-800°C (12921472°F),which is why methanol should not be introduced into a furnace at a lower temperature. The cracking of methanol into CO and H2 requires energy. This energy is taken from the area surrounding the point of methanol injection. There must therefore be sufficient heat flux towards the injection point to ensure proper dissociation
  158. 158. A high gas flow is desirable in the following cases: – At the beginning of a cycle when the furnace is originally airfilled or has been contaminated with air after a door opening. The higher the gas flow is, the faster the correct gas composition will be obtained. – When carbon demand is great, i.e. at the beginning of a process or in cases with a large charge surface area.
  159. 159. Low gas flow can be used in the following cases: – When the furnace is empty. – When the carbon demand is low, i.e. at the end of a process or in cases with a small charge surface area
  160. 160. Purging of a furnace with inert gas.
  161. 161. DIFFERENT TYPES OFCARBURISING – Solid or pack carburising – Liquid carburising – Gas carburising
  162. 162. Pack carburising – The component is packed surrounded by a carbon-rich compound and placed in the furnace at 900 degrees. – Over a period of time carbon will diffuse into the surface of the metal. – The longer left in the furnace, the greater the depth of hard carbon skin. Grain refining is necessary in order to prevent cracking.
  163. 163. The gas-carburizing process In gas carburizing the workpieces are heated in contact with carbon containing gases such as the hydrocarbons, methane, ethane, and propane. The carburizing gases are diluted with an endothermic carrier gas which consists mainly of nitrogen (N2) and carbon monoxide (CO) along with smaller amounts of carbon dioxide (CO2), hydrogen (H2), and water (H2O). Of these gases, N2 is inert and acts only as a dilutent. The carrier gas serves to control the amount of carbon supplied to the steel surface and prevents the formation of soot residue.
  164. 164. The reactions involved in carburizing are as below. First, the methane or propane enrichment of the carburizing-gas mixtures provides the primary source for the carbon for carburizing by slow reactions such as CH4 + CO2 →2CO + 2H2 (14-1) CH4 + H2O →CO + 3H2 (14-2) These reactions decrease the concentrations of CO2 and H2O and increase the amounts of CO and H2. Then the CO breaks down to deposit and allow the carbon to diffuse into the steel surface by the following overall reversible reactions: 2CO ↔C (in Fe) + CO2 (14-3) CO + H2 ↔C (in Fe) + H2O (14-4) The carbon-potential control during carburization is attained by maintaining a steady flow of the carrier gas and varying the flow of the hydrocarbon enrichment gas.
  165. 165. Induction: Why Induction Heat Treatment? Advantages Greatly shortened Highly Highly energy Less-pollution heat treatment cycle selective efficiency process Practical Problems • Lack of systematic heating time and temperature distribution control inside WP. • Nonlinear effect of material properties. • Lack phase transformation data inside WP for hardness and residual stress determination. • Evaluate combination effect of AC power density, frequency and gap on final hardness pattern. • Trial and error, cost and design period. Research content: FEM based electromagnetic/thermal analysis Numerical modeling may provide better prediction + quenching analysis + hardening analysis Research objective: (1) Provide T field, time history inside WP (2) Determine formed content of martensite, pearlite and bainite. (3) Determine hardness distribution in WP. (4) Guidance for induction system design.
  166. 166. Introduction: Induction Hardening Process • Induction heating: metal parts heated to austenite Phase •Fast quenching process transforms austenite to martensite phase •Martensite content determines the hardness •Martensitic structure is the most hardest microstructure workpiece Inductor/coil Heating process Joule heat by eddy current Electromagneti c field Induction coil High freq. AC
  167. 167. Principle: Electromagnetic and Thermal Analysis Electromagnetic Analysis WP Coil Thermal Analysis with finite element model Input AC power to coil Calculation of magnetic vector potential (A) Calculation of magnetic flux density (B) Calculation of magnetic field intensity (H) I 0 A 4 B= C dl r (Gauss’ Law for magnetic field) A (b) FEA model (a) WP geometry H=B/ QN Calculation of electric field intensity (E) B t E QN (Faraday’s Law) QW QEt QC QE QB QR+ QCV (Outside) Calculation of electric field density (D) D= E QS QS (c) Interior element Calculation of current density (J) Calculation of Inducting heat (Qinduction) Output: Heat generation Qinduction in WP H D J t (Ampere’s Circuital Law) (d) Surface element Heat conduction T t c Qinduction = E J = J2/ c T t 2 k k 2 T Qinduction T Qinduction A Induced Joule heat 4 F T 4 Tair Heat radiation A h T Tair Heat convection
  168. 168. Case Study: Complex Surface Hardening Material: Carbon Steel, AISI 1070 Automotive parts from Delphi Inc., Sandusky,Ohio •Concave and convex on surface of workpiece make the heating process not easy to control. concave convex Real spindle to be hardened Geometry Model •ANSYS system is employed for the analysis. •Mesh should be much finer at locations of convex and concave in both coil and workpiece. FEA model and B.C. Mesh generated by ANSYS
  169. 169. Case Study: Material Properties -- AISI 1070 (a) Electromagnetic Properties conductivity WP relative permeability Electrical Resistivity (b) Thermal Properties Specific heat Emissivity Convection coefficient
  170. 170. Case Study: Magnetic Field Intensity Distribution
  171. 171. Effect of current density distribution • Constant current distribution in coil can not result in good heating pattern, especially at concaves of workpiece • Better hardened pattern resulted from modification of Finer coil mesh and enhanced coil current density at area neighboring to surface concaves of workpiece. (a1) Constant current distribution in coil (a2) heated pattern (b1) Adjusted current distribution in coil (b2) heated pattern • Enhanced coil current density suggests utilization of magnetic controller at those area in coil design process. Physically this can be fulfilled by magnetic controller.
  172. 172. Case Study:Temperature Variation with Time in Induction Heating Process t=0.5s Total heating time th = 7.05s f=9600Hz s=1.27mm J=1.256e6 A/m2 t=4s t=2s
  173. 173. Case Study: Heating Curves Summary • A finite element method based modeling system is developed to analyze the coupled electromagnetic/thermal process in induction heating and implemented in ANSYS package, with following capabilities. • Provide electrical and magnetic field strength distribution. • Provide instantaneous temperature field data in workpiece. • Provide Temperature history at any location in heating process. • Provide guidance for inductor/coil design based on adjustment of current density distribution and desired heating patterns.
  174. 174. Quenching of carburized parts Carburized parts are usually quenched from the austenitic condition to produce a hard case with a martensitic structure, as shown
  175. 175. Most gas-carburized parts are directly quenched from the carburizing temperature of about 925°C or from about 845°C without being cooled to room temperature. The decrease in temperature from about 925 to 845°C can be accomplished by allowing the temperature of the carburizing to decrease, moving the workpiece to a lower temperature zone of the furnace, or by transferring the workpiece to another furnace
  176. 176. Tempering of carburized parts Many carburized and hardened parts are placed into service without tempering, especially if the applications for the parts are not critical with respect to cracking and chipping. On the other hand, many hardened carburized parts are given a low-temperature temper treatment, usually in the 150 to 190°C range, since in this temperature range, hardening is not greatly reduced and toughness and resistance to cracking is slightly increased
  177. 177. The diffusion of nitrogen into the surface layers of low carbon steels at elevated temperature. The formation of nitrides in the surface layer creates increased mechanical properties.
  178. 178. •Benefits of Nitriding •Types of Nitriding •Future of Nitriding •Process Determination •CVD Reaction •Deposition Process •Diffusion Depth •Process Results
  179. 179. •Principal Reasons for Nitriding are: •Obtain High Surface Hardness •Obtain a Resistant Surface •Increase Wear Resistance •Increase Tensile Strength and Yield Point •Improve Fatigue Life •Improve Corrosion Resistance (Except for Stainless Steels)
  180. 180. •Improves Mechanical Properties •Surface Hardness •Corrosion Resistance •Chemical Reaction •Nitrogen & Iron •Core Properties Not Effected •Temperature Range •495 - 565 ºC •Below Tempering Temperature • White Layer By-Product •Thin •Hard Iron Nitride
  181. 181. •Process methods for nitriding include: •Gas •Liquid •Plasma •Bright •Pack ***Lots of more nitriding methods for specific applications***
  182. 182. •Gas methods: •Case-Hardening Process •Nitrogen Introduction •Surface of a Solid Ferrous Alloy •Suitable Temperature •Between 495 and 565°C (for Steels) •Nitrogenous Gas •Ammonia
  183. 183. Liquid nitriding: •Thermo-chemical Diffusion Treatment •Hardening Components With Repeatability. •Salt Bath, at Less Critical Temperatures. •Preserves Dimensional Stability •Corrosion Protection •Exhibit Long-Term Resistance to Wear, Seizure, Scuffing, Adhesion and Fatigue.
  184. 184. •Vacuum Chamber •Pressure = 0.64 Pa •Pre-Heat Cycle •Surface Cleaning •Ion Bombardment •Control Gas Flow •N, H, CH4 •Ionization by Voltage •Blue-Violet Glow •Wear Resistant Layer
  185. 185. Mechanism of Nitriding • Fe- N equilibrium diagram can be used to study nitriding process. • The solubility limit of nitrogen in iron is temperature dependent, and at 450 °C the iron-base alloy will absorb up to 5.7 to 6.1% of N. • Beyond this, the surface phase formation on alloy steels tends to be predominantly ε-phase. • This is strongly influenced by the C-content of the steel; the greater the carbon content, the more potential for the ε phase to form • As the temperature is further increased to the gamma prime (γ′-nitride) phase temperature at 490 °C , the ―window‖ or limit of solubility begins to decrease at a temperature of approximately 680 °C.
  186. 186. Effect of alloying elements • Plain carbon steel form Iron nitride (brittle & low hardness) on nitriding due to absence of nitriding forming element. • Strong nitriding elements- Al, Mo, Cr, Ti, V etc form nitrides causes internal precipitation of nitrides resulting high surface hardness. • Steel containing several alloying elements have higher hardness than by a single element
  187. 187. Effect of alloying element on hardness after nitriding
  188. 188. •Replacing Liquid Nitriding •Environmental Effects •Ease of Control •More Complex Substrates •Performed at Lower Temperatures •Creates Higher Residual Stress
  189. 189. Gas Nitriding •How do the variables of nitriding steel affect the process and the mechanical properties of the surface? •The following variables were investigated: •Time •Temperature •Gas Velocity •Develop Process Model
  190. 190. CVD Equations CVD Process
  191. 191. CVD Reaction
  192. 192. CVD Process
  193. 193. Surface Composition The Following Variables Were Used in The Calculation of C surface η(T) ρ(T) D(T) δ(T,v) hmass(T,v)
  194. 194. Case Depth
  195. 195. Case Depth
  196. 196. Case Depth
  197. 197. •Time Effects •Increase Diffusion Depth •Temperature Effects •Surface Composition •Deposition Efficiency •Diffusion Rate •Diffusion Depth •Gas Velocity Effects •Surface Composition •Replenishes Nitrogen Gas •Minimizes Stagnant Layer Thickness
  198. 198. •Microstructural Effects •Processing Temperature •Surface Microstructure •Mechanical Property Effects •Improves •Surface Hardness •Wear Resistance •Corrosion Resistance •Fatigue Life •Yield Strength •Lowers •Ductility •Fracture Toughness
  199. 199. Disadvantages – It is an expensive process. – Much more time is required to develop the requisite case depth (due to low temp) – Expensive gas ammonia is used in nitriding. – Expensive alloy steels can only be nitrided and are used. – Nitriding is more expensive than carburising and carbonitriding
  200. 200. Carbonitriding of Steels Carbonitriding is a modified form of carburizing and is not a form of nitriding. The modification in carbonitriding consists of adding ammonia (NH3) to the carburizing gas so that nitrogen diffuses in the steel case along with carbon. Carbonitriding is usually carried out at a lower temperature and for a shorter time than gas carburizing, and so a thinner case is usually produced than by carburizing. Carbonitriding is principally used to produce a hard, wear-resistant case in steels, normally from 0.075 to 0.75 mm thick. Nitrogen increases the hardenability of steel, and so a carbonitrided case has higher hardenability than a carburized case on the same steel. Also, since nitrogen is an austenite stabilizer, high nitrogen levels can result in retained austenite, particularly in alloy steels. Maximum hardness and less distortion can be attained by carbonitriding since less drastic oil quenching than for carburizing can be used.
  201. 201. Laser Hardening
  202. 202. Introduction • LASER beams are invisible electromagnetic radiation in infra-red portion of spectrum. • Used for surface hardening of ferrous material • Laser used for hardening: – YAG- Solid state type – CO2 Gas type • CO2 Laser is commonly used for surface hardening when the power required is more than 500W.
  203. 203. Hardening MechanismHypoeutectoid • Consider the microstructure of a hypoeutectoid steel containing 0.35% carbon. It consists of pearlite colonies surrounded by proeutectoid ferrite. • On heating, – Pearlite  Austenite (dissolution of the cementite), – Growth of the Austenite transformation front into regions of high carbon concentration, at a rate controlled by carbon diffusion between the lamellae. – Ferrite transforms by nucleation and growth of austenite at internal ferrite grain boundaries, (rate controlled by carbon diffusion
  204. 204. • The phase diagram shows that under equilibrium conditions, pearlite begins to transform to austenite at 723◦C(Ac1), and that transformation of ferrite is complete at about (Ac3) temperature. • The high heating rate experienced during laser heating (on the order of 1000 K s−1) results in superheating of Ac1 and Ac3, typically by about 30 and 100◦C, respectively. • The heat is conducted to the bulk at a very fast rate which results in surface quenching and martensitic hardening.
  205. 205. • By selecting power density and the speed of the laser spot a desired case depth can be hardened. • Case depth also depends on hardening response of ferrous material (not more than 2.5mm)
  206. 206. Why LASER Hardening? • Local surface hardening exactly where required • Low distortion and no rework • Short wavelength enabling superior absorption • Closed-loop temperature control • Highest process efficiency of all laser types • Extremely reliable for production processes • Highest level of process safety and
  207. 207. Advantages of LH vs other hardening techniques • Selective areas hardened without affecting surrounding material • Possible automation and integration with other in-line production processes • Quick turn-around time • Treatment depth accurately controlled and highly reproducible due to direct temperature control • Superior hardness can be obtained compared to conventional processes • (typically 20% higher hardness) • No external quenching required → eliminates complex quench equipment • Minimal heat input → limited distortion → no need for post treatment machining → final machined components for laser hardening • Phase transformation + volume expansion → residual compressive stresses into surface → improves mechanical properties, e.g. wear and fatigue resistance, lowers crack sensitivity
  208. 208. Laser hardening applications • Steering gear assemblies • Turbine blades • Cutting edges and edges of dies for sheet metal forming • Cam followers, Gear teeth and Shafts • Rim geometries, e.g. Piston rings • Plastic injection moulds at highly loaded areas on the surface • Cylinder liners in diesel engines • Heavy duty and ball bearing steels • Tool steels
  209. 209. Disadvantages • • • • • High initial cost Laser use 10% of the input energy Depth of case is very limited Working cost is high Difficult to surface harden high alloy steel • Extra care is needed to avoid fusion
  210. 210. – TOOL STEEL are high quality steels made to controlled chemical composition and processed to develop properties useful for working and shaping of other materials. – The Carbon content in them is between 0.1 -1.6% . Tool steel also contain alloying elements like, Chromium, Molybdenum and Vanadium. – Tool steel offers better durability, strength, corrosion resistance and temperature stability, as compared to the Construction & Engg. Steel. – These are used in applications such as Blanking, die forging, forming, extrusion and plastic molding etc..
  211. 211. OBJECTIVES OF HEAT TREATMENT OF TOOL STEELS – To obtain a desired microstructure and properties suitable for machining or cold deformation. – To release residual stresses accumulated during previous thermal and mechanical treatments. – To homogenize the microstructure with globular carbides by a spheroidization treatment. – To dissolve by a normalizing treatment the intergranular carbides that are detrimental to the mechanical properties of tool steels.
  212. 212. CHARACTERISTICS OF TOOL STEELS – Require special heat treatment process. – Higher cost than alloy steels. – Better hardenability than most carbon and alloy steels. – Higher heat resistance. – Easies to heat treat.
  213. 213. Carbides in Tool steels • The
  217. 217. SHOCK RESISTING TOOL STEELS – Carbon content = 0.5-0.6%. Alloying elements – Cr, W , Mo. – These are characterized by good toughness, hardness and improved hardenability. These steels are generally, water or oil- hardened. – ―Low temperature Tempering‖ is carried out where, toughness and hardness of the tool steel are of prime importance, otherwise ―High temperature Tempering‖ is preferred. – Silicon-manganese steels (0.55% C, 2.0% Si, 1.0 % Mn) are included in this group. Due to their high Si-content, decarburization and grain coarsening takes place in these type of steels.
  218. 218. • HEAT TREATMENT PROCEDURE (in general) :– Annealing : Slow & uniform heating in the range of 790-800°C followed by furnace cooling at rate of 8-15°C/hr. – Stress relieving : Heat to 650- 675°C and furnace cooling. – Hardening : Preheating – warming to about 650°C & holding for 20 minutes/ 25mm. Austenitizing – heating to 900-950°C & holding again for 20minutes/25mm. – Tempering : Heating to 205-650°C, holding for 30 minutes/25mm and then, air cooling.
  219. 219. • Applications: – – – – – – – Chisels Pneumatic chisels Punches Shear blades Scarring Tools River sets Driver bits.
  220. 220. HOT WORKED TOOL STEELS – Carbon content = 0.3-0.5% . These steels are used for high temperature metal forming operation (except cutting), where the temperature is around 200-800°C. – These are characterized by high hot yield strength, high red hardness , wear resistance, toughness, erosion resistance, resistance to softening at elevated temperatures, good thermal conductivity – These are divided into 3 groups depending on the principle alloying elements: – Chromium based [H11- H19] – Tungsten based [H20- H26] – Molybdenum based [H41- H43]