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.