• Document: HEAT TREATMENT OF STEEL
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HEAT TREATMENT OF STEEL Heat Treatment of Steel  Most heat treating operations begin with heating the alloy into the austenitic phase field to dissolve the carbide in the iron.  Steel heat treating practice rarely involves the use of temperatures above 1040 C  Classification o Heating and rapid cooling(quenching) o Heating and slow cooling Purpose of heat treatment  Improvement in ductility  Relieving internal stresses  Grain size refinement  Increase of strength and hardness  Improvement in machinability and toughness Factors involved  Temperature up to which material is heated  Length of time that the material is held at the elevated temperature  Rate of cooling  The surrounding atmosphere under the thermal treatment Types of Heat Treatment  Annealing  Normalizing  Hardening  Tempering  Surface Hardening (These differ mainly in the way material is cooled from an elevated temperature) Effects of Heat Treatment Annealing & Normalizing Hardening or Quenching Furnace Cooling Air Oil Water Cooling Quenching Quenching Softer, less strong Harder and stronger More Ductile More brittle Less internal stress More internal stress Less distortion, cracking More distortion, cracking 1 The Iron-Iron Carbide Phase Diagram Heat Treatment of Steel Heat Treatment 2 珠光体 Annealing Process  Material is exposed to an elevated temperature for an extended time period and then slowly cooled, allowing phase changes.  Utilized for low-carbon and medium-carbon steels.  Full Annealing  Process Annealing or Stress Relief Annealing  Spheroidising Purposes of Annealing 1. Relieve Internal Stresses  Internal stresses can build up in metal as a result of processing. Such as welding, cold working, casting, forging, or machining.  If internal stresses are allowed to remain in a metal, the part may eventually distort or crack.  Annealing helps relieve internal stresses and reduce the chances for distortion and cracking. Process Annealing (Intermediate Annealing)  A heat treatment used to negate the effects of cold work, i.e., to soften and increase the ductility of a previously strain-hardened metal.  In process annealing, parts are not as completely softened as they are in full annealing, but the time required is considerably lessened.  Process annealing or stress-relief annealing is frequently used as an intermediate heat-treating step during the manufacture of a part.  Recovery and recrystallization processes occur during the process 3 Stress relief Annealing  Internal residual stresses may develop in metal pieces due to 1. Plastic deformation processes(machining and grinding) 2. Non-uniform cooling of a piece that was processed or fabricated at an elevated temperature(welding or casting)  Distortion and warpage may result if these residual stresses are not removed.  The material is heated to the recommended temperature, held long enough to attain a uniform temperature, and finally cooled to room temperature slowly.  The annealing temperature is ordinarily a relatively low one such that effects resulting from cold work and other heat treatments are not affected. Normalising  Heating the alloy to 55 to 85 degree Celsius above the A3 or Acm and holding for sufficient time so that the alloy completely transforms to austenite, followed by air cooling  To refine the grains and produce a more uniform and desirable size distribution for steels that have been plastically deformed  Normalising does not soften the material as much as full annealing does  The cooling process does not leave the material as ductile or as internally stress- free  A normalized part will usually be a little stronger, harder, and more brittle than a full-annealed part. Hardening 4  Hardening of steels done to increase the strength and wear resistance  Heated to 30-50 C above the upper critical temperature and then quenched  The quicker the steel is cooled , the harder it would be Metals Ferrous Metals Non-Ferrous Metals Steels Cast Irons Plain Carbon Steels Grey Iron Low Carbon Steels White Iron Medium Carbon Steels

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