Microstructure and mechanical properties of a multiphase 17%Cr stainless steel

The mechanical Properties of an experimental 17%Cr multiphase alloy were studied as function of heat treatment parameters. Containing majoritary ferrite and martensite, this type of steel is applied in tubullars and casings in the off shore oil and gas production, where high corrosion and mechanical resistance are required. Quenching and tempering heat treatments produced different combinations of ferrite, martensite, austenite and precipitates. The as quenched material has 35% of ferrite, martensite and 3.8% of retained austenite. The material quenched and tempered at 500 deg C has lower austenite (1.2%), and tempered martensite with Nb carbides. The tensile properties were excellent, but the low temperature (-46 deg C) impact toughness was very small. A high toughness was obtained in the material quenched and tempered at 650 deg C, with a microstructure of ferrite, tempered martensite and 7.2% of reversed austenite, but the mechanical resistance was under 621 Mpa (90 ksi), which is considered too low for the application envisioned. The material tempered at 600 deg C showed an interesting combination of mechanical resistance and low temperature toughness. The flow stress curves of the material were modelled by Hollomon's equation and Voce's equation with correlation coefficients (R²) higher than 0.98. (author)