Mechanisms of Strengthening and Fracture in 17%Cr Martensite/Ferrite Dual-Phase Stainless Steels

Abstract

To clarify the strengthening mechanisms of medium-chromium stainless steels (SSs) with carbides, ferrite, and martensite, 17%Cr SSs with varying martensite contents have been prepared, and the influence of martensite on microstructure, mechanical properties, and fracture has been investigated. According to THERMOCALC calculations, 17%Cr SS undergo a reversible phase transition between austenite and ferrite + M₂₃C₆ through the diffusion of carbon in austenite and M₂₃C₆ in the temperature range of 850–1220 °C. As the martensite content increases, M₂₃C₆ decreases, the martensite grain size increases, and the ferrite grain size initially increases and then decreases. Meanwhile, the yield strength and ultimate tensile strength increase, while both the uniform and post-uniform elongation decrease. It is also found a decrease in work-hardening index and an increase in work-hardening rate with increasing martensite content. Under tensile loading, three types of voids are present in 17%Cr SS: type A only in the grain boundary (GB) area with M₂₃C₆ in martensite-free material and type B and C in ferrite grains close to martensite grains and at ferrite/martensite GBs, respectively. An increase in martensite content leads to more voids, indicating a reduction in material plasticity caused by martensite.