In-situ martensite formation in nitrogen-stabilized precipitation hardenable steel at cryogenic temperatures

Precipitation hardenable (PH) 17-4 stainless steels are used in a wide range of structural applications across industries including additive manufacturing. In this work, targeted nitrogen alloying of 17-4 PH stainless steels was performed to stabilize the austenite phase below room temperature. The transformation to martensite was investigated in-situ at cryogenic temperatures, in the interval 290-80K, using vibrating sample magnetometry (VSM). The investigations show that nitrogen addition can be used to tailor (reduce) the martensite start temperature, M_s, and that martensite formation occurs during cooling, isothermally as well as during (re)heating. Cooling/heating cycles revealed that undercooling below M_s induces multiple burst-type martensite units, which continue to grow on continued cooling as well as during reheating to room temperature. It is demonstrated that, once nucleated, martensite can grow at temperatures more than 100 K above M_s. The findings provide new understanding of how these metastable systems undergo time-dependent, thermally activated martensitic transformations.