Microstructure and mechanical properties of 9Cr18Mo martensitic stainless steel fabricated by strengthening-toughening treatment

Abstract

Microstructure and mechanical properties of strengthening-toughening treated (1020–1060 °C argon quenching + −73 °C isothermal quenching + 140–240 °C tempering) 9Cr18Mo martensitic stainless steel (9Cr18Mo steel for short) were investigated by using Thermo-Calc software, microstructure characterization and mechanical tests. It was found that the size range of carbides in the 9Cr18Mo steel was wide and approximately 0.2–15.0 μm. Most of micron-carbides (>1 μm) and submicron-carbides (<1 μm) were located at the grain boundary and in the grain interior of original austenite, respectively; and the 9Cr18Mo steel with higher volume fraction of submicron-carbides showed a better combination of strength and impact energy. It was detected that the carbides in the 9Cr18Mo steel mainly consisted of M23C6 and M7C3 with primary element of Cr, Fe, and secondary element of Mo. There were core-shell (M7C3-M23C6) structured carbides with two morphologies of coarse irregular (>10 μm) and micron near-spherical, as well as individual micron-M7C3, micron- and submicron-M23C6 with near-spherical morphology. Quasi-equiaxed grains with body-centered cubic (bcc) structure formed via recovery and recrystallization, {112}<111> bcc twins accompanied with hexagonal ω-Fe phase at the twin boundaries, retained lath- and blocky-austenite were observed in the tempered martensitic matrix of 9Cr18Mo steel. Moreover, the twins and retained austenite were prone to decomposition at higher tempering temperature. It was indicated that the yield strength of 9Cr18Mo steel was mainly correlated with subgrain block size of tempered martensite and dispersion parameter of carbides via the analysis of statistical data, and the subgrain block contributed more to the yield strength of 9Cr18Mo steel. The M7C3-M23C6 core-shell structured carbides and {112}<111> bcc twins associated with hexagonal ω-Fe phase found in this study were also helpful to understand the correlation between microstructure and mechanical properties of other high carbon chromium martensitic steels.