Sub-Zero Celsius treatment: a promising option for future martensitic stainless steels

A series of samples of (in wt.%) 11.5Cr-0.67C martensiticstainless steel grade were austenitized in Argon for 1 hour attemperatures ranging from 1010°C to 1190°C. Additionally, aseries of samples of (in wt.%) 15.0Cr-5.8Ni-1.0Mo-0.03C (EN1.4418) martensitic stainless steel grade were solution nitridedat 1110°C for 2 hours in N2 at a pressure of 1 atm. Some of thesamples were subjected to a sub-zero Celsius treatment. Fourtypes of sub-zero Celsius treatments were investigated: (i)isothermal holding at -78°C; (ii) immersion in boiling nitrogenfollowed by immersion in water; (iii) immersion in boilingnitrogen followed by isothermal holding at -78°C; (iv)isothermal holding at -78°C followed by immersion in boilingnitrogen and subsequently in water. The effectiveness of thedifferent sub-zero Celsius treatments in promoting anadditional hardening response was investigated by reflectedlightmicroscopy and Vickers micro-hardness indentation.Complementary electron back-scatter diffraction was appliedfor determining the phase fractions of austenite and martensite.Data shows that sub-zero Celsius treatment yields anadditional hardening response when austenite is retained in thematerial. The relevance of the additional response wasobserved to be a function of the sub-zero Celsius treatmentapplied, as well as of the stability of the retained austenite.