Effect of Cr on the Microstructure and Strength-Toughness of High-Strength and Heat-Resistant Stainless Steel

The effect of Cr content on the microstructure and mechanical properties of CSS-42L steel is investigated by X-ray diffractometer, scanning electron microscopy, and transmission electron microscopy. The results show that increasing Cr from 8% to 13.5% significantly improves toughness and ductility while moderately decreasing the strength. The tensile strength, fracture toughness (K_IC), and impact absorbing energy of 13.5% Cr steel are 1.8 GPa, 88.6 MPa√m, and 58.5 J, respectively. 13.5%Cr steel possesses larger grain size and fewer undissolved M₆C carbides than 8%Cr and10%Cr steels, which is attributed to that Cr addition increases Cr content in the (Mo,Cr)₆C, reducing the dissolution temperature and ability to inhibit grain growth. Cr significantly decreases the Martensite start (Ms) temperature from 263 to 53.1 °C and increases the retained austenite from 0.3 to 13.19 vol%. Cr increases the number density and diameter of nanoscale M₂C, which is attributed to Cr promoting the dissolution of Mo and increasing the nucleation rate. Meanwhile, the higher Cr content also increases the growth rate of the carbides along the diameter direction. Cr addition reduces the contribution from coherency strengthening caused by decreased lattice misfit and increased the contribution of Orowan dislocation looping resulted from higher volume fraction and size of M₂C.