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Heat treatment

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Heat treatment

  1. 1. 1
  2. 2. Heat-Treatment  Heat treatment is a method used to alter the physical, and sometimes chemical properties of a material. The most common application is metallurgical  It involves the use of heating or chilling, normally to extreme temperatures, to achieve a desired result such as hardening or softening of a material  It applies only to processes where the heating and cooling are done for the specific purpose of altering properties intentionally 2
  3. 3. 3 Metal Fabrication  How do we fabricate metals?  Blacksmith - hammer (forged)  Molding - cast  Forming Operations  Rough stock formed to final shape Hot working vs. Cold working • T high enough for • well below Tm recrystallization • work hardening • Larger deformations • smaller deformations
  4. 4. 4 FORMING roll Ao Ad roll • Rolling (Hot or Cold Rolling) (I-beams, rails, sheet & plate) Ao Ad force die blank force • Forging (Hammering; Stamping) (wrenches, crankshafts) often at elev. T Adapted from Fig. 11.8, Callister 7e. Metal Fabrication Methods - I ram billet container container force die holder die Ao Adextrusion • Extrusion (rods, tubing) ductile metals, e.g. Cu, Al (hot) tensile force Ao Addie die • Drawing (rods, wire, tubing) die must be well lubricated & clean CASTING JOINING
  5. 5. 5 plaster die formed around wax prototype • Sand Casting (large parts, e.g., auto engine blocks) • Investment Casting (low volume, complex shapes e.g., jewelry, turbine blades) Metal Fabrication Methods - II Investment Casting • pattern is made from paraffin. • mold made by encasing in plaster of paris • melt the wax & the hollow mold is left • pour in metal wax FORMING CASTING JOINING Sand Sand molten metal
  6. 6. 6 CASTING JOINING Metal Fabrication Methods - III • Powder Metallurgy (materials w/low ductility) pressure heat point contact at low T densification by diffusion at higher T area contact densify • Welding (when one large part is impractical) • Heat affected zone: (region in which the microstructure has been changed). Adapted from Fig. 11.9, Callister 7e. (Fig. 11.9 from Iron Castings Handbook, C.F. Walton and T.J. Opar (Ed.), 1981.) piece 1 piece 2 fused base metal filler metal (melted) base metal (melted) unaffectedunaffected heat affected zone FORMING
  7. 7. 7 Heat Treatment
  8. 8. Steel Crystal Structures: •Ferrite – BCC iron w/ carbon in solid solution (soft, ductile, magnetic) •Austenite – FCC iron with carbon in solid solution (soft, moderate strength, non-magnetic) •Cementite – Compound of carbon and iron FE3C (Hard and brittle) •Pearlite – alternate layers of ferrite and cementite. •Martensite – iron – carbon w/ body centered tetragonal – result of heat treat and quench HT: ferrite then austentite then martensite
  9. 9. Review on Time-Temperature-Transformation (TTT)Curve  TTT diagram is a plot of temperature versus the logarithm of time for a steel alloy of definite composition.  It is used to determine when transformations begin and end for an isothermal heat treatment of a previously austenitized alloy  TTT diagram indicates when a specific transformation starts and ends and it also shows what percentage of transformation of austenite at a particular temperature is achieved. 9
  10. 10. Time-Temperature-Transformation (TTT)Curve The TTT diagram for AISI 1080 steel (0.79%C, 0.76%Mn) austenitised at 900°C 10
  11. 11. 11
  12. 12. Types of Heat-Treatment (Steel)  Annealing / Normalizing,  Case hardening,  Precipitation hardening,  Tempering, and Quenching 12
  13. 13. Designer Alloys:  Utilize heat treatments to design optimum microstructures and mechanical properties (strength, ductility, ardness….)  Strength in steels correlates with how much martensite remains in the final structure  Hardenability: The ability of a structure to transform to martensite  Martensite  Has the Strongest microstructure.  Can be made more ductile by tempering.  Therefore, the optimum properties of quenched And tempered steel are realized if a high content of martensite is produced. 13
  14. 14. Problem: It is difficult to maintain the same conditions throughout the entire volume of steel during cooling: The surface cools more quickly than interior, producing a range of microstructures throughout. The martensitic content, and the hardness, will drop from a high value at the surface to a lower value in the interior of the specimen. 14
  15. 15. Heat treatment of Steels Heat Treatment:-  Controlled heating and cooling of metals to alter their physical and mechanical properties without changing the product shape,  associated with increasing the strength of material,  alter certain manufacturability;  Improve machining,  improve formability, and  restore ductility after a cold working operation. 15
  16. 16. 16 Annealing: Heat to Tanneal, then cool slowly. Based on discussion in Section 11.7, Callister 7e. Thermal Processing of Metals Types of Annealing • Process Anneal: Negate effect of cold working by (recovery/ recrystallization) • Stress Relief: Reduce stress caused by: -plastic deformation -nonuniform cooling -phase transform. • Normalize (steels): Deform steel with large grains, then normalize to make grains small. • Full Anneal (steels): Make soft steels for good forming by heating to get g, then cool in furnace to get coarse P. • Spheroidize (steels): Make very soft steels for good machining. Heat just below TE & hold for 15-25h.
  17. 17. Decarburization during Heat Treatment  Decrease in content of carbon in metals is called Decarburization  It is based on the oxidation at the surface of carbon that is dissolved in the metal lattice  In heat treatment processes iron and carbon usually oxidize simultaneously  During the oxidation of carbon, gaseous products (CO and CO2) develop  In the case of a scale layer, substantial decarburization is possible only when the gaseous products can escape 17
  18. 18. Decarburization Effects  The strength of a steel depends on the presence of carbides in its structure  In such a case the wear resistance is obviously decreased  In many circumstances, there can be a serious drop in fatigue resistance  To avoid the real risk of failure of engineering components, it is essential to minimize decarburization at all stages in the processing of steel 18
  19. 19. Annealing  Annealing: a heat treatment in which a material is exposed to an elevated temperature for an extended time period and then slowly/controlled cooled.  Annealing temperature and the control cooling rate depend on the alloy composition and the type of the annealing treatment. Three stages of annealing 1. Heating to the desired temperature (austenite or Austenite-Cementite) 2. Holding or “soaking” at that temperature 3. Cooling, usually to room temperature 50 - 20 ºC/hr 19
  20. 20. Types of Annealing 1. Stress-Relief Annealing (or Stress-relieving) 2. Normalizing 3. Isothermal Annealing 4. Spheroidizing Annealing (or Spheroidizing ) 20
  21. 21. 1. 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 21
  22. 22. 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) 22
  23. 23. 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 23
  24. 24. 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 24
  25. 25. 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 25
  26. 26. Relation between heating temperature and Reduction in Residual Stresses  Higher temperatures and longer times of annealing may reduce residual stresses to lower levels 26
  27. 27. Stress Relief Annealing - Cooling  The residual stress level after stress-relief annealing will be maintained only if the cool down from the annealing temperature is controlled and slow enough that no new internal stresses arise.  New stresses that may be induced during cooling depend on the (1) cooling rate, (2) on the cross-sectional size of the workpiece, and (3)on the composition of the steel 27
  28. 28. 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 transformation temperature 28
  29. 29. Normalizing – Heating and Cooling 29
  30. 30. Normalizing – Austenitizing Temperature Range 30
  31. 31. Effect of Normalizing on Grain Size  Normalizing refines the grain of a steel that has become coarse-grained as a result of heating to a high temperature, e.g., for forging or welding Carbon steel of 0.5% C. (a) As-rolled or forged; (b) normalized. Magnification 500 31
  32. 32. Need for Normalizing  Grain refinement or homogenization of the structure by normalizing is usually performed either to improve the mechanical properties of the workpiece or (previous to hardening) to obtain better and more uniform results after hardening  Normalizing is also applied for better machinability of low-carbon steels 32
  33. 33. Normalizing after Rolling  After hot rolling, the structure of steel is usually oriented in the rolling direction  To remove the oriented structure and obtain the same mechanical properties in all directions, a normalizing annealing has to be performed 33
  34. 34. Normalizing after Forging  After forging at high temperatures, especially with workpieces that vary widely in crosssectional size, because of the different rates of cooling from the forging temperature, a heterogeneous structure is obtained that can be made uniform by normalizing 34
  35. 35. Normalizing – Holding Time  Holding time at austenitizing temperature may be calculated using the empirical formula: t = 60 + D where t is the holding time (min) and D is the maximum diameter of the workpiece (mm). 35
  36. 36. Normalizing - Cooling  Care should be taken to ensure that the cooling rate within the workpiece is in a range corresponding to the transformation behavior of the steel-in-question that results in a pure ferrite–pearlite structure  If, for round bars of different diameters cooled in air, the cooling curves in the core have been experimentally measured and recorded, then by using the appropriate CCT diagram for the steel grade in question, it is possible to predict the structure and hardness after normalizing 36
  37. 37. 3. Isothermal Annealing  Hypoeutectoid low-carbon steels as well as medium-carbon structural steels are often isothermally annealed, for best machinability  An isothermally annealed structure should have the following characteristics: 1. High proportion of ferrite 2. Uniformly distributed pearlite grains 3. Fine lamellar pearlite grains 37
  38. 38. Principle of Isothermal Annealing  Bainite formation can be avoided only by very slow continuous cooling, but with such a slow cooling a textured (elongated ferrite) structure results (hatched area) 38
  39. 39. Process - Isothermal Annealing  Austenitizing followed by a fast cooling to the temperature range of pearlite formation (usually about 650˚C.)  Holding at this temperature until the complete transformation of pearlite  and cooling to room temperature at an arbitrary cooling rate 39
  40. 40. 4. Spheroidizing Annealing  It is also called as Soft Annealing  Any process of heating and cooling steel that produces a rounded or globular form of carbide  It is an annealing process at temperatures close below or close above the AC1 temperature, with subsequent slow cooling 40
  41. 41. Spheroidizing - Purpose  The aim is to produce a soft structure by changing all hard constituents like pearlite, bainite, and martensite (especially in steels with carbon contents above 0.5% and in tool steels) into a structure of spheroidized carbides in a ferritic matrix (a) a medium-carbon low-alloy steel after soft annealing at 720C; (b) a high-speed steel annealed at 820C. 41
  42. 42. Spheroidizing - Process  Process: A  Heat the part to a temperature just below the Ferrite-Austenite line, line A1 727 ºC.  Hold the temperature for a prolonged time,  Fairly slow cooling. Or  Process: B  Cycle multiple times between temperatures slightly above and slightly below the 727 ºC line, say for example between 700 and 750 ºC,  Slow cooling, or  Process: C  For tool and alloy steels heat to 750 to 800 ºC,  Hold for several hours,  Slow cooling. 42
  43. 43. Spheroidizing - Uses  Such a soft structure is required for good machinability of steels having more than 0.6%C and for all cold-working processes that include plastic deformation.  Spheroidite steel is the softest and most ductile form of steel 43
  44. 44. Spheroidizing - Mechanism  The physical mechanism of soft annealing is based on the coagulation of cementite particles within the ferrite matrix, for which the diffusion of carbon is decisive  Globular cementite within the ferritic matrix is the structure having the lowest energy content of all structures in the iron–carbon system  The carbon diffusion depends on temperature and time 44
  45. 45. Annealing - summary 47 •Most heat treating operations begin with heating the alloy into the austenitic phase field to dissolve the carbide in the iron
  46. 46. 48 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license. Annealing - summary Schematic summary of the simple heat treatments for (a) hypoeutectoid steels and (b) hypereutectoid steels.
  47. 47.  Recommend temperatures for the process annealing, annealing, normalizing, and spheroidizing of 1020, 1077, and 10120 steels. 49 Example: Design Heat Treatment Temp.
  48. 48. Austempering  Material is quenched above the temperature when Martensite forms MS, around 215 ºC ( Eutectoid steel)  Hold longer at this temperature, the Austenite transforms into Bainite  Tendency to crack is severely reduced. 50
  49. 49. Martempering  Martempering is similar to Austempering except that the part is slowly cooled through the martensite transformation.  The structure is martensite, which needs to tempered just as much as martensite that is formed through rapid quenching.  The biggest advantage of Martempering over rapid quenching is that there is less distortion and tendency to crack. 51
  50. 50. Tempering  Process done subsequent to quench hardening  Quench-hardened parts are often too brittle.  Brittleness is caused by a predominance of martensite.  This brittleness is removed by tempering.  Tempering results in a desired combination of hardness, ductility, toughness, strength, and structural stability.  The mechanism of tempering depends on the steel and the tempering temperature. 52
  51. 51. Tempering  Martensite is a somewhat unstable structure.  When heated, the Carbon atoms diffuse from Martensite to form a carbide precipitate and the concurrent formation of Ferrite and Cementite, which is the stable form.  Tool steels for example, lose about 2 to 4 points of hardness on the Rockwell C scale. Even though a little strength is sacrificed, toughness (as measured by impact strength) is increased substantially.  Springs and such parts need to be much tougher — these are tempered to a much lower hardness. 53
  52. 52. Tempering process  Tempering at temperatures 300°C - 400°C. – Soaking time varies (2 to 8 hr)s depending on the parts size. – At these temperatures martensite transforms to trostite (very fine mixture of ferrite and cementite). – Trostite is softer than martensite and more ductile.  Tempering at temperatures higher than 400°C but lower than lower critical point (A1).  Soaking time varies (2 to 8 hrs) depending on the parts size.  At these temperatures martensite transforms to sorbite (fine mixture of ferrite and cementite).  Sorbite and trostite are principally similar structures differing only in the particles size.  Sorbite is more more ductility and toughness, and less strong than trostite. 54
  53. 53. Example: Design of a Quench and Temper Treatment A rotating shaft that delivers power from an electric motor is made from a 1050 steel. Its yield strength should be at least 150,000 psi, yet it should also have at least 15% elongation in order to provide toughness. Design a heat treatment to produce this part. 55
  54. 54. What happens during rapid cooling?  Phase diagrams only show stable phases that are formed during slow cooling  If cooling is rapid, the phase diagram becomes invalid and metastable phases may form  In the case of steel, the formation of ferrite and cementite requires the diffusion of carbon out of the ferrite phase.  What happens if cooling is too rapid to allow this?  The crystal lattice tries to switch from fcc (austenite) to bcc (ferrite). Excess carbon distorted body centred lattice (BCT) MARTENSITE 56
  55. 55. Hardening and Tempering  Steels can be heat treated to high hardness and strength (wear properties) levels. Structural components subjected to high operating stress need the high strength of a hardened structure. Similarly, tools such as dies, knives, cutting devices, and forming devices need a hardened structure to resist wear and deformation  As-quenched hardened steels are so brittle that even slight impacts may cause fracture.  Tempering is a heat treatment that reduces the brittleness of a steel without significantly lowering its hardness and strength. All hardened steels must be tempered before use. 57
  56. 56.  Hardenability: is the ability of the Fe-C alloy to be hardened by forming martensite. Hardenability is not “hardness”.  It is a qualitative measure of the rate at which hardness decreases with distance from the surface because of decreased martensite content.  Hardenability depends on  Carbon content  Alloying elements  Geometry  Cooling media 58 Hardening and Tempering
  57. 57. Hardenability Curve 59 1. Quenched end cools most rapidly and contains most martensite. 2. Cooling rate decreases with distance from quenched end: greater C diffusion, more pearlite/bainite, lower hardness 3. High hardenability means that the hardness curve is relatively flat.
  58. 58. 0.40 wt% C, + different additional alloying elements 60 •Alloying elements delay formation of pearlite, bainite : more martensite •Can also define hardenability in terms of cooling rate (0C/s) Hardenability
  59. 59. Quenching Geometry 61
  60. 60. Effect of quenching media 62
  61. 61. Direct Hardening – Austenitizing and quench:  Austenitizing – again taking a steel with .6% carbon or greater and heating to the austenite region.  Rapid quench to trap the carbon in the crystal structure – called martensite (BCT)  Quench requirements determined from isothermal transformation diagram (IT diagram).  Get “Through” Hardness!!!
  62. 62. Heat to austenite range. Want to be close to transformation temperature to get fine grain structure. Austenitizing:
  63. 63. For this particular steel want to cool from about 1400 F to <400 F in about 1 second!
  64. 64. Quenching:  Depending on how fast steel must be quenched (from IT diagram), the heat treater will determine type of quenching required:  Water (most severe)  Oil  Molten Salt  Gas/ Air (least severe)  Many phases in between!!! Ex: add water/polymer to water reduces quench time! Adding 10% sodium hydroxide or salt will have twice the cooling rate!
  65. 65.  Same requirements as austenitizing:  Must have sufficient carbon levels (>0.4%)  Heat to austenite region and quench  Why do?  When only desire a select region to be hardened: Knives, gears, etc.  Object to big to heat in furnace! Large casting w/ wear surface  Types:  Flame hardening, induction hardening, laser beam hardening Direct Hardening - Selective Hardening :
  66. 66. Flame Hardening:
  67. 67. Induction Hardening
  68. 68. Diffusion Hardening (aka Case Hardening):  Why do?  Carbon content to low to through harden with previous processes.  Desire hardness only in select area  More controlled versus flame hardening and induction hardening.  Can get VERY hard local areas (i.e. HRC of 60 or greater)  Interstitial diffusion when tiny solute atoms diffuce into spaces of host atoms  Substitiutional diffusion when diffusion atoms to big to occupy interstitial sites – then must occupy vacancies
  69. 69. Diffusion Hardening:  Requirements:  High temp (> 900 F)  Host metal must have low concentration of the diffusing species  Must be atomic suitability between diffusing species and host metal
  70. 70. CASE HARDENING  Case hardening or surface hardening is the process of hardening the surface of a metal, often a low carbon steel, by infusing elements into the material's surface, forming a thin layer of a harder alloy.  Case hardening is usually done after the part in question has been formed into its final shape 73
  71. 71. Case-Hardening - Processes  Flame/Induction Hardening  Carburizing  Nitriding  Cyaniding  Carbonitriding 74
  72. 72. Flame and induction hardening  Flame or induction hardening are processes in which the surface of the steel is heated to high temperatures (by direct application of a flame, or by induction heating) then cooled rapidly, generally using water  This creates a case of martensite on the surface.  A carbon content of 0.4–0.6 wt% C is needed for this type of hardening  Application Examples -> Lock shackle and Gears 75
  73. 73. Carburizing  Carburizing is a process used to case harden steel with a carbon content between 0.1 and 0.3 wt% C.  Steel is introduced to a carbon rich environment and elevated temperatures for a certain amount of time, and then quenched so that the carbon is locked in the structure  Example -> Heat a part with an acetylene torch set with a fuel-rich flame and quench it in a carbon-rich fluid such as oil 76
  74. 74. Carburizing  Carburization is a diffusion-controlled process, so the longer the steel is held in the carbon-rich environment the greater the carbon penetration will be and the higher the carbon content.  The carburized section will have a carbon content high enough that it can be hardened again through flame or induction hardening 77
  75. 75. Carburizing  The carbon can come from a solid, liquid or gaseous source  Solid source -> pack carburizing. Packing low carbon steel parts with a carbonaceous material and heating for some time diffuses carbon into the outer layers.  A heating period of a few hours might form a high- carbon layer about one millimeter thick  Liquid Source -> involves placing parts in a bath of a molten carbon-containing material, often a metal cyanide  Gaseous Source -> involves placing the parts in a furnace maintained with a methane-rich interior 78
  76. 76. Nitriding  Nitriding heats the steel part to 482–621°C in an atmosphere of NH3 gas and broken NH3.  The time the part spends in this environment dictates the depth of the case.  The hardness is achieved by the formation of nitrides.  Nitride forming elements must be present in the workpiece for this method to work.  Advantage -> it causes little distortion, so the part can be case hardened after being quenched, tempered and machined 81
  77. 77. Cyaniding  Cyaniding is mainly used on low carbon steels.  The part is heated to 870-950°C in a bath of sodium cyanide (NaCN)and then is quenched and rinsed, in water or oil, to remove any residual cyanide.  The process produces a thin, hard shell (0.5- 0.75mm) that is harder than the one produced by carburizing, and can be completed in 20 to 30 minutes compared to several hours.  It is typically used on small parts.  The major drawback of cyaniding is that cyanide salts are poisonous 82
  78. 78. Carbonitriding  Carbonitriding is similar to cyaniding except a gaseous atmosphere of ammonia and hydrocarbons (e.g. CH4)is used instead of sodium cyanide.  If the part is to be quenched then the part is heated to 775–885°C; if not then the part is heated to 649–788°C 83
  79. 79. Example Design of Surface-Hardening Treatments for a Drive Train  Design the materials and heat treatments for an automobile axle and drive gear. 84
  80. 80. PRECIPITATION HARDENING  Precipitation hardening (or age hardening), is a heat treatment technique used to increase the yield strength of malleable materials  Malleable materials are those, which are capable of deforming under compressive stress  It relies on changes in solid solubility with temperature to produce fine particles of an impurity phase, which blocks the movement of dislocations in a crystal's lattice 85
  81. 81. Precipitation Hardening  Since dislocations are often the dominant carriers of plasticity, this serves to harden the material  The impurities play the same role as the particle substances in particle-reinforced composite materials.  Alloys must be kept at elevated temperature for hours to allow precipitation to take place. This time delay is called aging 86
  82. 82. Precipitation Hardening  Two different heat treatments involving precipitates can change the strength of a material: 1. solution heat treating 2. precipitation heat treating  Solution treatment involves formation of a single-phase solid solution via quenching and leaves a material softer  Precipitation treating involves the addition of impurity particles to increase a material's strength 87
  83. 83. Precipitation Mechanism – Aluminum Alloy 88
  84. 84. Effect of Aging Time on Precipitates 89
  85. 85. QUENCHING and TEMPERING  In quench hardening, fast cooling rates, depending on the chemical composition of the steel and its section size, are applied to prevent diffusion-controlled trans formations in the pearlite range and to obtain a structure consisting mainly of martensite and bainite  However, the reduction of undesirable thermal and transformational stresses usually requires slower cooling rates 90
  86. 86. Quenching  To harden by quenching, a metal must be heated into the austenitic crystal phase and then quickly cooled  Cooling may be done with forced air, oil, polymer dissolved in water, or brine  Upon being rapidly cooled, a portion of austenite (dependent on alloy composition) will transform to martensite 91
  87. 87. Quenching  Cooling speeds, from fastest to slowest, go from polymer, brine, fresh water, oil, and forced air  However, quenching a certain steel too fast can result in cracking, which is why high-tensile steels such as AISI 4140 should be quenched in oil, tool steels such as H13 should be quenched in forced air, and low alloy such as AISI 1040 should be quenched in brine  Metals such as austenitic stainless steel (304, 316), and copper, produce an opposite effect when these are quenched: they anneal 92
  88. 88. Tempering  Untempered martensite, while very hard, is too brittle to be useful for most applications.  In tempering, it is required that quenched parts be tempered (heat treated at a low temperature, often 150˚C) to impart some toughness.  Higher tempering temperatures (may be up to 700˚C, depending on alloy and application) are sometimes used to impart further ductility, although some yield strength is lost 93
  89. 89. Tempering  Tempering is done to toughen the metal by transforming brittle martensite or bainite into a combination of ferrite and cementite or sometimes Tempered martensite  Tempered martensite is much finer-grained than just-quenched martensite  The brittle martensite becomes tough and ductile after it is tempered.  Carbon atoms were trapped in the austenite when it was rapidly cooled, typically by oil or water quenching, forming the martensite 94
  90. 90. Tempering  The martensite becomes tough after being tempered because when reheated, the microstructure can rearrange and the carbon atoms can diffuse out of the distorted body- centred-tetragonal (BCT) structure.  After the carbon diffuses out, the result is nearly pure ferrite with body-centred structure. 95
  91. 91. 96
  92. 92. Example Design of a Quench and Temper Treatment  A rotating shaft that delivers power from an electric motor is made from a 1050 steel. Its yield strength should be at least 145,000 psi, yet it should also have at least 15% elongation in order to provide toughness. Design a heat treatment to produce this part. 97

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