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Annealing , normalizing , quenching , martensitic transformation (1)

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  • Critical Temperature of Steel is 724°C
  • Last -> Because of this, stress-relief annealing means a through-heating process at a correspondingly high temperature
  • … A high level of residual stress is generally to be expected with workpieces that have a large cross section, are quenched at a high cooling rate, and are made of a steel of low hardenability..… i.e., on the reduction of residual stressesSoaking time -> The hold time at maximum temperature for a heat-treatment process
  • Hypoeutectoid -> At L.H.S of Eutectoid pointFor hypereutectoid steels, normalizing is a special process

Transcript

  • 1. THIRUNAVUKARASU.H 13MY12 PSG TECH KOVAI Annealing , normalizing , quenching , martensitic transformation .
  • 2. Annealing  heat treatment that alters the microstructure of a material causing changes in properties such as strength, hardness, and ductility  It the process of heating solid metal to high temperatures and cooling it slowly so that its particles arrange into a defined lattice
  • 3. Stages in annealing  Heating to the desired temperature ,  Holding or soaking at that temperature,  Cooling or quenching ,usually to room temperature .  In practice annealing concept is most widely used in heat treatment of iron and steals
  • 4. Purpose of annealing  It is used to achieve one or more of the following purpose . 1. To relive or remove stresses 2. To include softness 3. To alter ductility , toughness, electrical, magnetic. 4. To Refine grain size 5. To remove gases 6. To produce a definite microstructure .
  • 5. Application  Annealing process is employed in following application  Casting  Forging  Rolled stock  Press work ….
  • 6. Types of annealing  Full annealing  Process annealing  Stress relief annealing  Re crystallization annealing , and  Spheroidise annealing.
  • 7. Full annealing  Heating the steal to a temperature at or near the critical point , holding there for a time period and then allowing it to cool slowly in the furnace itself . Example In full annealing of hypoeutectoid steels less than 0.77% is heated to 723 to 910 C above A3 line convert to single phase austenite cooled slowly in room temperature . Resulting structure is coarse pearlite with excess of ferrite it is quite soft and more ductile  cooling rate of full annealing is 30-40 C
  • 8. Full annealing
  • 9. Process annealing  Process annealing is a heat treatment that is often used to soften and increase the ductility of a previously strain hardened metal . Ductility is important in shaping and creating a more refined piece of work through processes such as rolling, drawing, forging, spinning, extruding and he ading.  Example it is extensively employed for steel wires and sheet products (especially low carbon steels) A1 temperature and cooled at any desired rate  The temperature range for process annealing ranges from 260 °C (500 °F) to 760 °C (1400 °F), depending on the alloy in question.
  • 10. Process annealing
  • 11. Stress-Relief Annealing  It is an annealing process below the transformation temperature Ac1, with subsequent slow cooling, the aim of which is to reduce the internal residual stresses in a workpiece without intentionally changing its structure and mechanical properties
  • 12. Causes of Residual Stresses 1. Thermal factors (e.g., thermal stresses caused by temperature gradients within the workpiece during heating or cooling) 2. Mechanical factors (e.g., cold-working) 3. Metallurgical factors (e.g., transformation of the microstructure)
  • 13. How to Remove Residual Stresses?  R.S. can be reduced only by a plastic deformation in the microstructure.  This requires that the yield strength of the material be lowered below the value of the residual stresses.  The more the yield strength is lowered, the greater the plastic deformation and correspondingly the greater the possibility or reducing the residual stresses  The yield strength and the ultimate tensile strength of the steel both decrease with increasing temperature
  • 14. Stress-Relief Annealing Process  For plain carbon and low-alloy steels the temperature to which the specimen is heated is usually between 450 and 650˚C, whereas for hot-working tool steels and high-speed steels it is between 600 and 750˚C  This treatment will not cause any phase changes, but recrystallization may take place.  Machining allowance sufficient to compensate for any warping resulting from stress relieving should be provided
  • 15. Stress-Relief Annealing – R.S.  In the heat treatment of metals, quenching or rapid cooling is the cause of the greatest residual stresses  To activate plastic deformations, the local residual stresses must be above the yield strength of the material.  Because of this fact, steels that have a high yield strength at elevated temperatures can withstand higher levels of residual stress than those that have a low yield strength at elevated temperatures  Soaking time also has an influence on the effect of stress-relief annealing
  • 16. Spheroidise annealing  The process is limited to steels in excess of 0.5% carbon and consists of heating the steel to temperature about A1 (727°C). At this temperature any cold worked ferrite will recrystallise and the iron carbide present in pearlite will form as spheroids or “ball up”. As a result of change of carbides shape the strength and hardness are reduced.  To remove coarse pearlite and making machining process easy .  It forms spherodite structure of maximum soft and ductility easy to machining and deforming. Objectives  To soften steels  To increase ductility and toughness  To improve machinablity and formability
  • 17. Materials  Spheroidzing is extensively employed for  Medium carbon steel  High carbon (tool steel)
  • 18. 2. Normalizing  A heat treatment process consisting of austenitizing at temperatures of 30–80˚C above the AC3 transformation temperature followed by slow cooling (usually in air)  The aim of which is to obtain a fine- grained, uniformly distributed, ferrite–pearlite structure  Normalizing is applied mainly to unalloyed and low-alloy hypoeutectoid steels  For hypereutectoid steels the austenitizing temperature is 30– 80˚C above the AC1 or ACm
  • 19. Quenching  Quenching is the rapid cooling of metal or an alloy from an elevated temperature.  This is usually done with water, brine, oil, polymer, or even forced or still air.  There are two types of quenching – the first is cooling to obtain an acceptable microstructure and mechanical properties that will meet minimum specs after tempering.  The second consists of rapid cooling of iron-base alloys and nonferrous metals to retain uniformity in the material. Quenching is performed to control the transformation of austentite and to form the microstructure. When only selected areas of the material are quenched, the process is called selective quenching
  • 20. Quenching  Soaking temperature 30-50°C above A3 or A1, then fast cooling (in water or oil) with cooling rate exceeding a critical value. The critical cooling rate is required to obtain non-equilibrium structure called martensite. During fast cooling austenite cannot transform to ferrite and pearlite by atomic diffusion.  Martensite is supersaturated solid solution of carbon in α-iron (greatly supersaturated ferrite) with tetragonal body centered structure. Martensite is very hard and brittle. Martensite has a “needle-like” structure.  Kinetics of martensite transformation is presented by TTT diagrams (Time-Temperature-Transformation). With the quenching-hardening process the speed of quenching can affect the amount of marteniste formed. This severe cooling rate will be affected by the component size and quenching medium type
  • 21. Martensite, “Martensitic Transformation”  In an alloy, martensite is a metastable transitional structure between two allotropic modifications whose abilities to dissolve a solute differ, the high temperature phase having the greater solubility.  The amount of high temperature phase transformed to martensite depends upon the temperature attained in cooling. Martensite is also a metastable phase of steel, formed by the transformation of austentite below a specified temperature.  Martensite is characterized by an interstitial supersaturated solid solution of carbon in iron having a body-centered tetragonal lattice that resembles an acicular, needlelike pattern that can be observed in laboratory testing.  Martensitic transformation is a reaction that takes place in some metals during the cooling phase causing the formation of the acircular structures called “martensite
  • 22. End