Tool Steel Case Study

For tool steel materials there exist the problem of retained austenite. The retained austenite is soft and unstable at lower temperature and it gets converted into martensite at certain favourable conditions. The main objective of the cryogenic treatment when done for tool steels is to convert the retained austenite into fine carbides that precipitate and mix into the martensite matrix.[39-41].While cryogenic treatment is done for tool materials, there are two schools of thought. One states that cryogenic treatment to be done after quenching and before tempering. Lal D. M. et al., 2001 [27] stated the effectiveness of cryogenic treatment soon after quenching. Zurecki Z.,2006 [42] mentioned that cryogenic treatment is only effective after martensitic quenching is over from austenitic temperatures and before tempering.

J., 1999 [45] proposed a new Nu-Bit process. In this process the oarts are rapidly cooled and are cooled for a considerable amount of time. The parts are quenched to cooler temprature immediately after conventional quenching and before tempering.

Yun D. et al.,1998 [44] studied the microstructure changes in the steel when it was subjected to different changes in temperature which is as specified below.

Table 2 Different cycles applied to M2 high-speed steel [44]

Route Cycle

A Quenching from 1,250°C + triple tempering at 560°C

B Quenching from 1,250°C + one cycle 24 h subzero at −196°C + triple tempering at 560°C

C Quenching from 1,250°C + one cycle 48 h subzero at −196°C + triple tempering at 560°C

D Quenching from 1,250°C + three cycles totaling 48 h subzero at −196°C + triple tempering at 560°C

E Quenching from 1,250°C + triple tempering at 560°C + one cycle 48 h subzero at −196°C

Alexandru G. et al.,1990 [43] studied different routes of varying the cryogenic cycles (Table 1) onto the material properties of steels. The routes are given in the tabular form