The sub-zero Celsius treatment of stainless steels: experiments and perspective

The present work is concerned with the effect of austenitization temperature, interstitial content and cryogenic treatment on the microstructure of martensitic stainless steels. The effect of the austenitization temperature on the microstructure was investigated on three martensitic stainless steels: an 11wt-% Cr, 0.7 wt-% C model alloy, AISI 420 and AISI 431. The austenitization temperatures ranged between 990°C and 1190°C in Ar for 1 hour. The interstitial content in bulk specimens of the commercial martensitic stainless steels, AISI 420, AISI 431 and EN 1.4418 grades, and the ferritic stainless steel, AISI 409 grade, was varied by solution nitriding at 1110°C for 2 h in N2 gas, such that a nitrogen concentration gradient was obtained. Both series of specimens were subjected to various sub-zero Celsius treatments: quenching in boiling nitrogen followed by upquenching in water; isothermal treatment at -78 °C with and without a preceding quench in boiling nitrogen. For all the martensitic qualities an additional hardening response is obtained by sub-zero treatment; AISI 409 remained unaffected by sub-zero Celsius treatment. Results can consistently be explained in terms of recently gained insight in the understanding of athermal and thermally activated martensitic transformation in steel. The present work demonstrates that optimization of both austenitization treatment and subsequent sub-zero treatment can result in a hardening response well beyond was regularly achievable for these steels.