Microstructure and mechanical performance of AISI D2 tool steel after standard and modified deep cryogenic treatment

This research investigates the potential benefits of cyclic deep cryogenic treatment (CDCT) on AISI D2 cold-work tool steel, focusing on hardness and fracture toughness enhancement and their correlation with structural changes. While conventional deep cryogenic treatment (DCT) at −196 °C has demonstrated positive effects on tool steel properties, the specific long durations of DCT remain a limitation for industrial applications. AISI D2 steel specimens underwent conventional heat treatment and deep cryogenic treatment (DCT) at −196 °C for 24 hours, alongside CDCT with 2, 5, and 10 cycles within the same temperature range. The microstructural evolution, phase transformations, residual stresses, hardness, and fracture toughness induced by these treatments were thoroughly analyzed using advanced characterization techniques, including SEM-EDS, EBSD, XRD, and TEM. Macro- and micro-hardness measurements were conducted using Rockwell and Vickers methods, while fracture toughness was assessed using circumferentially notched and fatigue-pre-cracked tensile bar specimens. The results indicate that all samples exhibited comparable bulk hardness values. However, fracture toughness showed significant enhancement with CDCT, with the CDCT2 treatment achieving the highest improvement—68 % greater than that of the conventional treatment. Microstructural analysis reveals refined martensitic structures with nanotwins, stacking faults, dislocations, an enhanced count of small secondary carbides, increased precipitation of nanosized carbides, and lower residual stresses as key contributors to the enhanced mechanical properties.