Effect of cryogenic treatment on microstructure evolution and mechanical properties of high nitrogen plastic die steel

In this study, the effect of cryogenic treatment on the microstructure, especially the retained austenite (RA) evolution behavior, and mechanical properties of 55Cr17Mo1VN plastic die steel was investigated. The quenching microstructure consisted of martensite, RA, and undissolved carbides. After the cryogenic treatment, the RA volume fraction decreased from 44.7 to 35.4%, while the hardness increased from 39.0 to 54.6 HRC because of the increase in the dislocation density. Besides, the “new martensite” phase broke the blocky RA phase during the cryogenic treatment, and the two phases keep a strict K–S or N–W orientation relationship, which improved the stability of the steel. Interestingly, in the cryogenically treated sample, the RA decomposed completely. On the other hand, the untreated sample retained 20.7% of its RA phase after the tempering process. The cryogenic treatment decreased the thermal stability of the RA phase during the tempering process, which can be attributed to the release of the hydrostatic pressure and the generation of a large number of precipitates from the RA phase. As a result of precipitation strengthening, fine grain strengthening, and dislocation strengthening, the cryogenically treated sample showed excellent tensile strength (∼2241 MPa) and high hardness (∼56.2 HRC).