Z. Zurecki, AIR PRODUCTS & CHEMICALS, INC, ALLENTOWN, PA

Summary: Subject to a continuing debate, cryogenic treatments of alloy steels have been claimed to significantly increase wear resistance and toughness through the interplay of three effects: completing martensitic transformation, promoting uniform precipitation of fine carbides and imparting residual stresses. This study reexamines effects of various heat-treatment schedules including liquid nitrogen (-196oC) and liquid helium (-269oC) quenching on microstructure and selected properties of A2-grade tool steel. Examination methods include SEM, EDS, microhardness, Charpy impact and wear resistance measured using the standard pin-on-disk as well as a diamond stylus micro-scratching technique adopted from the field of thin-film technologies. Results confirm the cryo-treatment enhanced precipitation in the subsequent tempering step of what turns out to be 100-250 nm alloy-depleted carbides, and moderate improvements in wear resistance and hardness, both scaling with the cryogenic treatment time and at the cost of reduced impact resistance. Reported results and correlations provide a basis for optimizing cryo-quench heat-treatment cycle of tool steels and indicate directions for future work.