Comparison of K340 Steel Microstructure and Mechanical Properties Using Shallow and Deep Cryogenic Treatment

In this research, Bohler K340 cold work tool steel was subjected to three different heat treatment protocols, conventional heat treatment (CHT), shallow cryogenic treatment (SCT), and deep cryogenic treatment (DCT). The study compares the effect of SCT and DCT on the microstructure and consequently on the selected mechanical properties (micro- and macroscale hardness and impact toughness). The study shows no significant difference in macroscale hardness after the different heat treatments. However, the microhardness values indicate a slightly lower hardness in the case of SCT and DCT. Microstructure analysis with light (LM) and scanning electron microscopy (SEM) indicated a finer and more homogenous microstructure with smaller lath size and preferential orientation of the martensitic matrix in SCT and DCT samples compared to CHT. In addition, the uniform precipitation of more spherical and finer carbides is determined for both cryogenic treatments. Moreover, the precipitation of small dispersed secondary carbides is observed in SCT and DCT, whereas in the CHT counterparts, these carbide types were not detected. X-ray diffraction (XRD) and electron backscatter diffraction (EBSD) confirms that SCT and DCT are very effective in minimizing the amount of retained austenite down to 1.8 vol.% for SCT and even below 1 vol.% for the DCT variant.