Hardening • Purpose: 1.)To harden the steel to the maximum level by austenite to martensite transformation. 2.)To increase wear resistance and cutting ability of steel. Steels can be hardened by following methods: 1.)Conventional hardening:The conventional hardening process consists of heating the steel to above A3 temperature for hypoeutectoid steels and above A1 temperature for hypereutectoid steels by 500C, austenitising for sufficient time and cooling with a rate just exceeding the critical cooling rate so that steel to room temperature or below room temperature. Due to this ,the usual diffusion transformations are stopped and the austenite transforms to martensite by a diffusionless process. All the time hypeutectoid steels are hardened from above A3 temperature.They are not hardened between temperatures between A1 and A3, because the phases which exist at this temperature are austenite and proeutectoid ferrite and only austenite gets transformed to martensite with no change in ferrite. Such steels show free ferrite in their microstructures and since ferrite is a soft phase, the hardness of hardened steel gets reduced. • On the other hand hypereutectoid steels are always hardened from temperatures between A1 and Acm. At this temperature, austenization is not complete and some proeutectoid cementite will exist along austenite at the temperature of heating. Such steels after hardening show free Fe 3C along with martensite in their microstructures. Since Fe 3C being a hard phase the hardness of hardened substance doesnot get reduced. Moreover, this free Fe3C does not increase the britttleness of steels because usually it is fine, well distributed and partially spheroidised. Also, the grain size remains fine because the Fe3C particles do not allow to coarsen the austenite. • However if these steels are hardened above Acm temperature, the following drawbacks are observed: 1.)Since Acm line is steep, higher temperatures are required to cross the Acm line. Due to this and absence of Fe3C above Acm temperature, heavy grain coarsening occurs during austenitization and results in coarse grained martensite which is extremely brittle. 2.)Quenching from such a high temperature results in more distortions and may lead to cracking of the components. 3.)Due to higher temperature oxidation and decarburization are more. 4.)The amount of retained austenite increases because of higher thermal stresses. • Hence it is not heated above Acm and also the free carbides present in the structure increases wear resistance and cutting ability of these steels. • A proper quenching medium should be used such that the component gets cooled at a rate just exceeding the critical cooling rate of that steel.Faster cooling than the above also produces martensite but the tendency of warping and cracking is more and hence should be avoided.The critical cooling rates depend largely on the alloying elements and to a lesser extent on the carbon present in the steel. Alloy steels have less critical cooling rate and hence some of the alloy steels can be hardened by air cooling. • High carbon steels have slightly more critical cooling rate and has to be hardened by oil quenching. Medium cooling rates have still higher critical cooling rates and has to be hardened by water or brine quenching. Low carbon steels are having still high critical cooling rates and cannot be hardened by quenching(even brine). • For high carbon steels containing more than 0.7% carbon and for some alloy steels Mf is below room temperature. If these steels are cooled only upto room temperature all austenite will not tranform to martensite but a part of it will appear as retained austenite.This retained austenite decreases the hardness of the hardened steel.Therefore, the steel should be cooled below room temperature to eliminate part of retained austenite. 2.)The timed quench (Interrupted quench) • For plain carbon steels of low to medium carbon,critical cooling rates are high and therefore,very fast cooling from austenizing temperature is necessary to prevent the formation of pearlite or bainite at temperatures near the nose of the TTTdiagrams. • Once this region of rapid transformation has been passed,the transformation of austenite becomes slow.Therefore it is possible to obtain a completely martensitic structure in a steel of low hardenability by cooling it rapidly to a temperature below the nose of the ttt diagram and then cooling it more slowly through the temperature range martensite is formed. • Since the cooling rate between Ms and Mf is reduced,cracking tendency also gets reduced. • The process consists of heating the steel to the austenization temperature,quenching for a short period in cold water or brine to a temp between the nose and Ms and then cooling in some other medium like oil to room temperature. • For steels of slightly high hardenability (i.e. of slightly less critical cooling rate)like high carbon steels and low alloy steels,the initial quench may be in oil with subsequent cooling in air. 3.)Martempering(marquenching) • In this process, the austenitized steel is cooled rapidly avoiding the nose of the I.T diagram to a temperature between the nose and Mg, soaked at this temperature for a sufficient time for the equlization of temperature but not long enough to permit the formation of bainite and then cooled to room temperature in air or oil. • Since the component has to be held for some time for equalization of temperature, the process will be applicable to steels of slightly high hardenability such as high carbon steels and low alloy steels.The process produces martensitic structures with the following advantages: 1.)It results in less distortions and wraping, since the martensite formation occurs at the same time throughout the cross section of the component. 2.)There is less possibility of quenching cracks appearing in the component. This is a hardening process named so because of martensite temperaing. Austempering • In this process, austenitized steel is cooled with a rate exceeding the critical cooling rate of that steel to a temperature between the nose and Ms,forged or rolled at this temperature and cooled to room temperature in oil.Also, this results is increased dislocation density in martensite and a finer distribution of carbides on tempering.Ausformed structures in tempering at low temperatures show better combination of T.S and ductility.Steels with sufficient hardenability can be only be ausformed.
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