Strain-induced martensite formation in cryorolled AISI 317 L stainless steel

Abstract

AISI 317 L stainless steel replaces 316 L grade in some applications due to its superior mechanical strength and corrosion resistance. Aiming at expanding its applicability to structural applications, ongoing studies are dedicated to overcoming the trade-off between strength and ductility. The stacking fault energy decreases with deformation temperature and favors stacking faulting, (nano)twinning and strain-induced martensite (SIM) formation, resulting in severe microstructural fragmentation. The effect of temperature on deformation behavior of AISI 317 L steel was investigated in samples rolled at room temperature to thickness reductions of 50% and 85% and at 77 K to reductions in thickness of 10% and 50%. The microstructural evolution was followed by scanning electron microscopy, Vickers microhardness, X-ray diffraction, magnetization, electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI). The nucleation sites in the early stages of the transformation sequence γ → ε → α’ were identified in the 10% cryorolled sample. The highest volume fraction of α’-martensite reached 45.8% in the cryorolled steel to 50% rolling reduction. Much lower fractions were obtained for samples rolled to 10% reduction at 77 K (2%) and at room temperature to 50% (0.3%) and 85% reductions (1.6%). The texture components after cryorolling were Goss and Brass for austenite; rotated cube, α- and γ-fibers for δ-ferrite and α’-martensite. The ε-martensite presents the typical texture of hcp metals with a c/a ratio above the ideal value and 〈0001〉 − oriented tilted about 21° from the normal direction towards the rolling direction. The results show cryorolling as an effective method for enhancing SIM formation and promoting severe microstructural refinement in AISI 317 L stainless steel.