Enhanced Mechanical Properties through Reversion in Metastable Austenitic Stainless Steels

A novel processing route of cold rolling and reversion annealing for enhanced mechanical properties has been investigated in metastable 17Cr-7Ni-type austenitic stainless steels, i.e., commercial grades AISI 301LN and AISI 301, and in some experimental heats. The investigation was essentially aimed at studying the possibility of processing nano/submicron-grained structure in these steels and to rationalize the possible effects of alloying elements on the reversion mechanisms. The steels were cold rolled to various reductions between 45 and 78 pct to induce the formation of martensite, and subsequently annealed between 600 °C to 1000 °C for short annealing times (mostly 1 to 100 seconds). Microstructure examinations of the reversion-annealed 301LN steel revealed that an ultrafine-grained austenitic structure was formed by the diffusional transformation mechanism within a short holding time above 700 °C, even after the lowest cold-rolling reduction. In contrast, in 301 steel and experimental heats, the shear type of transformation occurred at temperatures above 650 °C, but fine austenite grains were only formed by recrystallization at higher temperatures or longer holding times, e.g., at 900 °C/100 s. An attempt has been made to determine the reversion mechanisms in various steels by modifying the criteria governing the Gibbs free energy change during the martensite-austenite reversion in Cr-Ni alloys. The room temperature (RT)-tensile property evaluation showed that excellent combinations of yield or tensile strength and elongation are possible to achieve, depending mainly on annealing conditions both in the 301LN and 301 steels, but the experimental heats were too unstable for high ductility. Ultrafine grain size of austenite contributed to this in 301LN and shear-transformed high-dislocated austenite in 301. Upon reversion annealing, the reversion mechanism did not affect the texture. The texture of the reverted fine-grained austenite is very strong compared to the typical texture of commercially cold-rolled and annealed 301LN steel.