Heat treatment is an improtent operation in the manufacturing process of machine parts and tools. Only by the heat treatment is it possible to impart the high mechanical properties to steel required for the normal operation of morden machinery and tools.Heat tretment may be undertaken for the
following purposes:
(1)Relieving internal stresses.
(2)Improvent in ductility .
(3)Refinment of grain size .
(4)Increasing hardness or tensile strength and achieving changes in chemical compositionof metal surface as in the case of case of case hardening.
SOME IMPROTENT HEAT TRETMENT PROCESS
(A)Stress Relieving--The process of stress reliving consist of heating steel unifromly to atemperature below the lower critical temperature , holding at this temperature for suficient time , followed by uifrom cooling. Unifrom cooling is of utmost improtance as non-unifrom cooling will itself result in the development of internal stresses.For plain carbon steels and low aloy alloy sttels,maximum stress releving temperature is generally limited to about 600 degree celcius. Higher temperatures are employed for high alloy steels. Hardened and tempered steels, if service conditions demand ,can be stress relived by heating up to a temperature which is lower thane temperaing temperature by 25-30 degree Celsius .The extent to which stresses can be relieved depend on the temperature employed and holding time.
(B)Annealing--Annealing is one of the most improtent widely-used operations in the heat tretment of steel.The purpose of annealing is to obtain softness, improve machinability increase or restore ductility and toughness , relive internal stress,reduce or eliminate structural inhomogeneity, refine grain size and to prepare steel for subsequent heat tretment. There are several types of annealing.
*Full Annealing
*Isotermal Annealing
*Diffusion Annealing
*Partial Annealing
*Recrystallization Annealling
*Process Annealing
(C)Normalizing--Normalizing is a process of heating steel to about 40-50 degre celcius above uper critical temperature ,holding for proper time ,andthen cooling in still air or slightly agitated air to room temperature.In special casas,cooling rate can be controlled either by changing air temperature orair volume.After nomalizing the resulant microstructure should be pearlite .normalised carbon steel consists of perlite and ferite in hypoeutectoid steelsand of perlite and cementote in hypoeutectoid steels.
(D)Hardening--Hardening is a heat treating process in which steel is heated to a temperature above the critical point ,held at this temperature, and then quenched in water, oil, or molten salt baths.Hypoeutectoid steels are heated from 30 to 50 degree celcius above point Ac3while hypereutectoid steels are heatedabove point Ac1. In ther first case, ferrite+pearlite, and in the second ,pearlite+cementite are transfromed into austenite upon heating .A considerabale part of the cementite is retained.
(E)Tempering--Tempering tretment lowers hardness,strength and ware resistance of the harden steel marginally. Especially steel is br heating to a high temperature through below the melti8ng point , then cooling it usually in air .The process has the effect of toughening by lessenngbrittleness and reducing internal stresses.The first stage of tempering is also referred to as low temperature tempering. The maximumtemperature to which steel ids heated is restricated to about 250 degree celcius at this stage.the second stage of tempering consistsof heatingf steel in the temperature range varying from 350 to500 degree celcius. The third stage of tempering is also popularly known as high temperature tempering. It consist of heating steel within a temperature range of 500 to 680 degree celcius.
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