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17767705 heat-treatment-oct08


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Heat treatment

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17767705 heat-treatment-oct08

  1. 1. Heat Treatment Thirugnanam K SEA Materials Engineering
  2. 2. Fundamentals of Heat Treatment for Metallic Materials Introduction The purpose of this presentation is to provide a basic understanding of the metallurgical processes associated with the heat treatment of metallic materials.
  3. 3. Thermal Processes: 1. Shape change of materials Such as forging, forming, extrusion, rolling, welding and casting (foundry). 2. No shape change of materials Such as heat treatment and coating.
  4. 4. Liquid water and ice are familiar examples of how a material can exist in various forms. Steel also exists in various forms, including several different solid forms. Various forms of material
  5. 5. What is Heat treatment? • Answer: The controlled heating and cooling of materials for the purpose of altering their structures and properties.
  6. 6. Purposes of performing a heat treatment process on a metallic component •Modification of the microstructure for improvement of machining, cold forming processes. •Obtaining the required mechanical properties, such as strength, toughness, hardness, wear resistance and fatigue life based on application. •Reduction in brittleness, residual stress, dimensional instability of components. •Surface protection from environment, such as oxidation, corrosion medium and stress corrosion.
  7. 7. Fundamentals of Heat Treatment Processing Development for Metallic Components When a material is selected per design requirements and application, the next step is how to have the designed parts satisfy these requirements, such as mechanical properties (strength, hardness, toughness, residual stress state and fatigue strength) and microstructure. They are achieved through a proper heat treatment process. How to develop a heat treatment process??? Tool one: Fe-Carbon phase diagram For heating above the austenitizing temperature Tool two: T-T-T or IT curve of selected material Determination of cooling rate (reduction of cracking risk and distortion) to obtain the required microstructure.
  8. 8. Iron-Carbon phase Diagram Since steel is a Fe-C ally, the carbon is a key element in any grades of steel.
  9. 9. Most alloying elements in the steels are Cr, Mo, Ni, Si, Mn, as well as V and W. •All of them change the positions of the A1, A3 and Am boundaries and the eutectoid composition will be changed. •All important alloying elements decrease the eutectoid carbon content. For example, 1080 (0.8% C) steel is called hyper-eutectoid steel. However, H19 (0.3% C) is hyper-eutectoid steel due to addition of Cr (2%), W (8%) and V (1%). Eutectoid point shifts toward to left in the Fe-C diagram. •Austenite-stabilizing elements (Mn and Ni) decrease A1 temperature, i.e., expend γ gamma zone. •Ferrite-stabilizing elements (Cr, Si, Mo , W, V and Ti) increase A1 temperature, i.e., shrink γ gamma zone. •The effect of combination of alloying elements on the Fe-C diagram is very complicated (may expand or shrink gamma zone). Effects of alloying elements on the Fe-C phase diagram
  10. 10. 1. 1035 steel – 1570 F (855 C) 2. 1040 steel – 1555 F (845 C) 3. 4340 steel – 1570 F (855 C) 4. 5130 steel – 1570 F (855 C) 5. 8620 steel – 1600 – 1700 F (870 – 925 C for carburizing) 6. H19 steel – 2005-2200 F (1095 – 1205 C) 7. D2 steel – 1795-1875 F (980-1025 C) 8. T2 steel – 2300-2375 F (1260-1300 C) Examples of austenitizing temperature development for steel hardening
  11. 11. Full TTT Diagram The complete TTT diagram for an iron-carbon alloy of eutectoid composition. A: austenite B: bainite M: martensite P: pearlite
  12. 12. Effect of the carbon content on the T-T-T- curve profile 1. Move the nose of T-T-T curve toward the lower-right direction. 2. Lowering Ms point temperature (martensitic transformation start temperature). 3. Increasing hardening ability or hardenability using same cooling rate.
  13. 13. Effect of the Alloying elements on the T-T-T- curve profile A 0.42C 0.20Mn 0.21Mo B 0.48C 0.94Mn 0.25Mo C 0.36C 0.17Mn 0.82Mo F E 0.23C 0.82Mn 1.22Cr 0.53Mo 0.22V 1. Most of them move the nose of T-T-T curve toward the lower-right direction. 2. Some of them promote the formation of two noses in T-T-T curves 3. Some of them lower Ms temperature point (martensitic transformation start temperature). 4. Most of them increase hardening ability or hardenability using same cooling rate. F 1.5C 12Cr 1Mo 1V D 0.39C 0.56Mn 0.34Ni 0.74Mo
  14. 14. So What’s a CCT Diagram? • Phase Transformations and Production of Microconstituents takes TIME. • Higher Temperature = Less Time. • If you don’t hold at one temperature and allow time to change, you are “Continuously Cooling”. • Therefore, a CCT diagram’s transition lines will be different than a TTT diagram.
  15. 15. Slow Cooling Time in region indicates amount of microconstituent!
  16. 16. Medium Cooling Cooling Rate, R, is Change in Temp / Time °C/s
  17. 17. Fast Cooling This steel is very hardenable… 100% Martensite in ~ 1 minute of cooling!
  18. 18. Heat Treatment can be considered in terms of three aspects 1. In crystallographic change 2. In microstructure change 3. In mechanical and physical property change
  19. 19. •In crystallographic change From BCC to FCC to BCT BCC FCC BCT BCC – Body centered cubic FCC – Face centered cubic BCT – Body centered tetragonal
  20. 20. •In microstructure change From pearlite to austenite to martensite through quenching (fast cooling)
  21. 21. In mechanical properties, such as hardness SAE 1050 SAE 4147
  22. 22. Heat Treatment Process Tree Heat Treatment (Heating, Holding & Cooling) Annealing Normalizing Through hardening Case hardening •Stress relief annealing •Recrystallization annealing •Spheroidized annealing •Isothermal annealing •Quenching & tempering •Interrupted quenching •Isothermal •Austempering •Martempering •Vacuum •Induction hardening •Flame hardening •Laser hardening •Carburizing •Carbonitriding •Nitrocarburizing
  23. 23. FULL ANNEALING • It consists of heating steel to austenitic region (790-900°C), followed by slow cooling, preferably in the furnace itself or in any good heat- insulating material.
  24. 24. Full annealing 8620 ASR AS Received
  25. 25. Objectives of the Full Annealing • To improve ductility • To facilitate cold working or machining • To remove internal stresses completely • To get enhanced magnetic and electrical properties • To promote dimensional stability • To refine grain structure • Disadvantage: – The prolonged heat treatment cycle, involved in this process, makes it quite expensive.
  26. 26. ISOTHERMAL ANNEALING • In this process, hypoeutectoid steel is heated above the upper critical temperature (A3 -750- 900°C)and held for some time at this temperature. • The steel is then cooled rapidly to a temperature less than the lower critical temperature (i.e. 600 - 700 °C) • After all the austenite is transformed into lamellar pearlite, steel is cooled in air.
  27. 27. Adv. of Isothermal Annealing • The time required is less compare to Full Annealing. • Hence cheaper than full annealing process. • Improves Machinability and also results in a better surface finish by machining. • Widely used for alloy steels.
  28. 28. Disadv. of Isothermal Annealing • Used for hypoeutectoid steels only • 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.
  29. 29. PROCESS ANNEALING • Steel is heated to a temperature below the lower critical temperature (670-720°C), and is held at this temperature for sufficient time and then cooled.
  30. 30. Process Annealing • To reduce hardness and to increase ductility of cold-worked steel so that further working may be carried out easily. • It is an intermediate operation and is sometimes referred to as in-process annealing. • Mostly used in sheet and wire industries
  31. 31. Spheroidise Annealing • Spheroidising is a heat treatment process which results in a structure consisting of globules or spheroids of carbide in a matrix of ferrite.
  32. 32. Spherodised Annealing • Spherodisation can take place by the following methods – Prolonged holding at a temperature just below the lower critical line (727°C) – Heating and cooling alternately between temperature that are just above and just below the lower critical line. – Heating to a temperature above the lower critical line and then either cooling very slowly in the furnace or holding at a temperature just below the lower critical line.
  33. 33. •Spheroidizing annealing 8620 ASR As Received
  34. 34. Purpose of Spheroidising • The majority of all spheroidising activity is performed for improving the cold formability of steels. • The spheroidised structure is desirable when minimum hardness, maximum ductility, or (in high carbon steels ) maximum machinability is important.
  35. 35. Spheroidise Annealing • Low-carbon steels are seldom spheroidised for machining, because in the spheroidised condition they are excessively soft and gummy. • The cutting tool will tend to push the material rather than cut it, causing excessive heat and wear on the cutting tip.
  36. 36. Stress Relieving • It is used to relieve stresses that remain locked in a structure as a consequence of a manufacturing sequence. • No microstructural changes occur during the process.
  37. 37. Stress Relieving • The process of stress relieving consists of heating steel uniformly to a temperature below the lower critical temperature (less than 600°C), holding at this temperature for sufficient time, followed by uniform cooling.
  38. 38. Stress Relieving • Sources of internal stresses – solidification of castings – welding – machining – grinding – shot peening – surface hammering – cold working, bending – electroplated coatings
  39. 39. Stress Relieving • Adverse effect of internal stresses – steels with residual stresses under corrosive environment fail by stress- corrosion cracking. – Residual stresses will enhance the tendency of steels towards warpage and dimensional instability. – Fatigue strength is reduced considerably when residual tensile stresses are present.
  40. 40. NORMALISING • WHAT IS NORMALISING? – Normalising is an austenitising heating cycle followed by cooling in still air or slightly agitated air. – Typically, the job is heated to a temperature about 50°C above the upper critical line of the iron-iron carbide phase diagram prior to cooling. (830 - 925°C)
  42. 42. PURPOSE OF NORMALISING • To improve Machinability • To refine the grain structure • To homogenise the microstructure in order to improve the response in hardening operation. • To modify and refine cast dendritic structure • To reduce banded grain structure due to hot rolling.
  43. 43. NORMALISING Vs ANNEALING • Normalised steels are harder than annealed one. • Prolonged heat treatment time and higher energy consumption make the annealing treatment more expensive than normalising. • Cooling rates are not critical for normalising as in the case of annealing. • Annealing improves the machinability of medium carbon steels, whereas normalising improves machinability of low carbon steels.
  44. 44. HARDENING • Certain applications demand high hardness values so that the components may be successfully used for heavy duty purposes. • High hardness values can be obtained by a process known as Hardening.
  45. 45. HARDENING • Hardening treatment consists of heating to austenitising temperature(815 - 870°C), holding at that temperature, followed by rapid cooling such as quenching in water, oil, or salt baths. • The high hardness developed by this process is due to the phase transformation accompanying rapid cooling. • The product of low temperature transformation of austenite is martensite, which is a hard microconstituent of steel.
  46. 46. •Conventional quenching & tempering
  47. 47. HARDENING • Successful hardening usually means achieving the required microstructure, hardness, strength, or toughness while minimising residual stress, distortion, and the possibility of cracking.
  48. 48. Selection of Quenching Medium • Selection of a quenching medium depends on the hardenability of the particular alloy, the section thickness and shape involved and the cooling rates needed to achieve the desired microstructure. – Hardenability: It is the ability of the steel to be transformed partially or completely from austenite to martensite while quenching.
  49. 49. Various Quenching Mediums • Gaseous Quenchants – Helium, Argon and Nitrogen • Liquid Quenchants – Oil – Oil with some additives – Polymer Quenchants – Water – Brine Water
  50. 50. Factors affecting Hardening • Chemical composition of steel • Size and shape of the steel part • Hardening cycle (heating rate, hardening temperature, holding time and cooling rate) • Homogeneity and Grain size of austenite • Quenching Media • Surface condition of steel part.
  51. 51. Hardening • High hardness developed by hardening enables tool steel to cut other metals. • It also improves wear resistance. • Tensile strength and Yield Strength are improved by hardening. • This process is frequently used for chisels, sledge, hand hammers, centre punches, shafts, collars and gears.
  52. 52. Tempering • Tempering consists of heating hardened steel below the lower critical temperature, followed by cooling in air or at any other desired rate.
  53. 53. Tempering • In the as-quenched martensitic condition, the steel is too brittle for most applications. • The formation of martensite also leaves high residual stresses in the steel. • Therefore, hardening is almost always followed by tempering.
  54. 54. Tempering • The purpose of tempering is to relieve residual stresses and to improve the ductility and toughness of the steel. • This increase in ductility is usually attained at the sacrifice of the hardness or strength. • Hardness decreases and toughness increases as the tempering temperature is increased.
  55. 55. Tempering • Dimensional Changes – Martensite transformation is associated with an increase in volume. – During tempering, martensite decomposes into a mixture of ferrite and cementite with a resultant decrease in volume as tempering temperature increases.
  56. 56. Sub-Zero Treatment • Retained Austenite: – In practice, it is very difficult to have a completely martensitic structure by hardening treatment. – Some amount of austenite is present in the hardened steel. – This austenite existing along with martensite is referred to as Retained Austenite.
  57. 57. Sub-zero Treatment • Retained austenite is converted into martensite by this treatment. • The process consists of cooling steel to sub-zero temperature which should be lower than the Mf temperature of the steel (-30 to -70°C) • Tempering is done immediately to remove the internal stresses developed by Sub-zero treatment.
  58. 58. Sub-zero Treatment • Increase in hardness • Increase in wear resistance • Increase in dimensional stability
  59. 59. Martempering • In the conventional hardening process, the surface and centre cool at different rates and transform to martensite at different times. • In Martempering, the steel is quenched into a bath kept just above Ms. After allowing sufficient time for the temperature to become uniform throughout the cross- section, it is air-cooled through the martensitic range. • The transformation to martensite occurs more or less simultaneously across the section.
  60. 60. Adv. Of Martempering • Residual stresses developed during martempering is lower. • It also reduces or eliminates susceptibility to cracking.
  61. 61. Austempering • This is a heat-treating process developed to obtain a structure which is 100 percent bainite. • It is accomplished by first heating the part of the proper austenitising temperature (790 - 915°C) followed by cooling rapidly in a salt bath, held in the bainite range (250-400°C). • The piece is left in the bath until the transformation to bainite is complete.
  62. 62. Bainite • Upper (550-350°C) – Rods of Fe3C • Lower (350-250°C) – Fe3C Precipitates in Plates of Ferrite • It is still Ferrite and Cementite! It’s just acicular.
  63. 63. Austempering • Increased ductility, toughness and strength • Reduced distortion, which lessens subsequent machining time, stock removal, sorting, inspection and scrap. • The shortest overall time cycle to through harden within the hardness range of 35 - 55 HRc, which results in savings in energy and capital investment.
  64. 64. Austempering • Limitation – Limitation on size is necessary since the part is required to attain uniform temperature of the quenching bath rapidly. – Therefore, only comparatively thin sections can be austempered successfully.
  65. 65. •Austempered Ductile Iron (ADI) Aus-ferrite Actually, the definition is Isothermal temperature heat treatment of ductile iron (spheroidized iron)
  66. 66. ADI Microstructures (a) As-received ductile iron. (B) Aus-ferrite at high T. (C) Aus-ferrite at low T. B A C
  67. 67. •ADI grade and mechanical properties
  68. 68. CASE HARDENING • There are situations in which the requirement is such that the outer surface should be hard and wear resistant and the inner core more ductile and tougher. • Such a combination of properties ensures that the component has sufficient wear resistance to give long service life and at the same time has sufficient toughness to withstand shock loads.
  70. 70. CARBURISING • This is the oldest and one of the cheapest methods of case hardening. • It is carried out on low carbon steels which contain from 0.10 - 0.25% carbon. • Carburising is carried out in the temperature range of 900 - 930 °C • The surface layer is enriched with carbon upto 0.7 - 0.9 %
  71. 71. CARBURISING • In this process, carbon is diffused into steel by heating above the transformation temperature and holding the steel for sufficient time in contact with a carbonaceous material which may be a solid medium, a liquid or a gas. • Followed by Quenching and Tempering.
  72. 72. GAS CARBURISING • The Steel is heated in contact with carbon monoxide and/or a hydrocarbon which is readily decomposed at the carburising temperature. • Temperature : 870-950°C • Gas carburising may be either batch or continuous type.
  73. 73. Air Natural gas Mixer Retort Endogas Natural gas Endo-gas Generator Carburizing furnace N2
  74. 74. GAS CARBURISING • Gas atmosphere for carburising is produced from liquid (methanol, iso-propanol) or gaseous hydrocarbons (propane and methane) • An endo-thermic gas generator is used to supply endothermic gas.
  75. 75. GAS CARBURISING • Approximate composition of the gas inflow into the furnace is – Nitrogen 40% – Hydrogen 40% – Carbon Monoxide 20% – Carbon Dioxide 0.3% – Methane 0.5% – Water vapour 0.8% – Oxygen in traces
  76. 76. GAS CARBURISING • CHEMICAL REACTIONS – C3H8 ---> 2CH4 + C (Cracking of hyd.carbon) – CH4 + Fe ---> Fe(C) + 2H2 – CH4 + CO2 ---> 2 CO + 2H2 – 2CO + Fe ---> Fe(C) + CO2
  77. 77. 1. Heat and soak at carburizing temperature to ensure temperature uniformity throughout steel. 2. Boost step to increase carbon content of austenite. 3. Diffusion step to provide gradual case/core transition. 4. Gas pressure or oil quench CH4 + Fe=Fe(C) +2H2
  78. 78. Atmosphere carburized surface profile, showing the IGO Vacuum carburized surface profile, showing a clear surface (no IGO)
  79. 79. LIQUID CARBURISING • Popularly known as Salt bath carburising. • In this process, carburising occurs through molten cyanide (CN) in low carbon steel cast pot type furnace heated by oil or gas. • Bath temperature : 815 - 900°C • Salt mixture consists of – Sodium or Potassium Cyanide – Barium chloride
  80. 80. LIQUID CARBURISING • CHEMICAL REACTION – BaCl2 + 2NaCN ---> Ba(CN)2 + NaCl – Ba(CN)2 + Fe ---> Fe(C) + BaCN2
  81. 81. SOLID CARBURISING • This method of carburising is also known as pack carburising. • In this process, steel components to be heat treated are packed with 80% granular coal and 20% BaCO3 as energizer in heat resistant boxes and heated at 930°C in electric chamber furnace for a specific period of time depends on case depth.
  82. 82. SOLID CARBURISING • CHEMICAL REACTION – Energizer decomposes to give CO gas to the steel furnace – BaCO3 ---> BaO + CO2 – CO2 + C ---> 2CO – Carbon monoxide reacts with the surface of steel – 2CO + Fe ---> Fe(C) + CO2
  83. 83. CARBONITRIDING • The surface layer of the steel is hardened by addition of both carbon and nitrogen. • This process is carried out a lower temperatures (in the range 800 - 870°C) in a gas mixture consisting of a carburising gas and ammonia. • A typical gas mixture contains about 15% NH3, 5% CH4 and 80% neutral carrier gas.
  84. 84. Air Natural gas Mixer Retort Endogas Natural gas Endo-gas Generator Carbonitriding furnace NH3 N2 Air Naturalga Mixer Retort Endogas
  85. 85. NITRIDING • Nitriding is most effective for those alloy steels which contain stable nitride forming elements such as Aluminium, Chromium, Molybdenum, Vanadium and Tungsten.
  86. 86. NITRIDING • Nitriding is carried out in a ferritic region below 590°C. • So there is no phase change after nitriding. • Before nitriding, proper heat treatment should be given to steel components. • All machining and finishing operations are finished before nitriding. • The portions which are not to be nitriding are covered by thin coating of tin deposited by electrolysis.
  87. 87. NITRIDING • Anhydrous ammonia gas is passed into the furnace at about 550°C, where it dissociates into nascent nitrogen and hydrogen. • Thus, 2NH3 ----> 2[N]Fe + 3H2 • The surface hardness achieved varies from 900 to 1100 HV.
  88. 88. PLASMA NITRIDING • Plasma nitriding is also known as ion nitriding process. • In this process, the steel component to be nitrided is kept at 450°C in vacuum at a negative potential of the order of 1000 volts with respect to chamber. • Then an appropriate mixture of N2 and H2 is passed at a pressure of 0.2-0.8 m bar. • As a result, plasma formation of these gases takes place.
  89. 89. Ion (Plasma) Nitriding Equipment (Photos Courtesy of Surface Combustion)
  90. 90. Applications: T/C stamped components
  91. 91. FLAME HARDENING • Flame hardening is done by means of oxyacetylene torch. • Heating should be done rapidly by the torch and the surface quenched before appreciable heat transfer to the core occurs. • Application – For large work pieces – Only a small segment requires heat treatment – When the part requires dimensional accuracy
  92. 92. Induction Hardening • Here, an alternating current of high frequency passes through an induction coil enclosing the steel part to be heat treated. • The induced emf heats the steel. • Immediately after heating, water jets are activated to quench the surface.
  93. 93. Induction hardening of camshaft
  94. 94. 20 25 30 35 40 45 50 55 60 65 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 Distance from surface, inch Hardness,HRC(convertedbymicro) Induction hardening Induction hardening of crankshaft
  95. 95. Adv. of Induction Hardening • Provides energy savings • Provides much higher heating rates • Ease of automation and control • Reduced floor space requirements • Quiet and clean working conditions • Suitability for integration in a production line
  97. 97. Q & A Thanks for your patient