During hardness as cementite precipitates at the expense

During tempering, if the temperature range are 200-300°C for 1 hour duration, austenite inside the steel will decompose into a mixture of cementite and ferrite. Further tempering leads to coarsening of cementite particle and this dislocation structure tends to recover. This recovery is not very obvious when in the steels containing alloying elements such as molybdenum and chromium. The dislocation structure and migration of dislocation cell and martensite boundaries causes the recovery not successful and causes increase in the crystallographic misorientation between adjacent plates. Precipitation of alloy carbides is also a procedure in secondary hardening, these carbides in order to precipitate, it requires a sufficient time and temperature to proceed because these carbides require the long-range diffusion of substitutional atoms. In order to eliminate the cementite completely, the concentration of strong carbide forming elements such as Mo,Cr,Ti,V,Nb must be large because these alloy carbides relatively grow at the expense of the less stable cementite. Carbides like cementite therefore have a kinetic advantage even though they may be metastable. Tempering at first causes a decrease in hardness as cementite precipitates at the expense of carbon in solid solution, but the hardness begins to increase again as the alloy carbides form. Hence the term secondary hardening. Coarsening eventually causes a decrease in hardness at high tempering temperatures or long times, so that the net hardness versus time curve shows a secondary hardening peak. (Tempered Martensite, 2018)

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