Ultra-high strength steel made from AISI 304L using a novel thermo-mechanical processing technique

Abstract

Presented here is a technique of producing an ultra-high strength steel starting with an annealed low strength AISI 304L stainless steel. Annealed AISI 304L stainless steel was severely rolled at room temperature to a thickness reduction of 93%. Microstructure after rolling of SS304L showed subgrain formation with high dislocation density. The resulting strength of this steel after rolling was 1.7 GPa. Subsequent aging of this at 400 °C for 120 h further increased the strength to 2.2 GPa. The ductility of the as-rolled and subsequently aged conditions was similar and in the range of 5 to 7%. Electron back scatter diffraction analysis and vibrating-sample magnetometer showed that in the as-rolled condition the fraction of strain induced martensite was ≈ 72% with the remaining being untransformed austenite in the deformed state. The median spacing between boundaries (low as well as high angle) varied between 700 nm for the as-rolled condition to 900 nm after rolling and subsequent aging at 400 °C for 120 h. Atom probe tomography along with small angle neutron scattering showed that the process of aging resulted in a spinodal phase separation of Cr rich regions with 7 nm wavelength and precipitation of the G-phase having 1.75 nm radius. It was estimated that G-phase precipitation, rather than spinodally decomposed Cr-rich regions, is the dominant hardening phase.