Effect of Hot-Rolling on the Microstructure and Impact Toughness of an Advanced 9%Cr Steel

A 9%Cr martensitic steel with Ta and B additions was subjected to thermo-mechanical treatment (TMT) including rolling in the range of metastable austenite at 900–700 °C followed by water quenching and tempering at different temperatures. Applied TMT with tempering at T ≥ 700 °C substantially improved the impact toughness. The application of the TMT with subsequent tempering at 780 °C decreased the ductile–brittle transition temperature from 40 to 15 °C and increased the upper shelf energy from 300 to 380 J/cm2 as compared to the normalized and tempered (NT) condition. The microstructural observations with scanning and transmission electron microscopes showed the precipitation of fine Ta-rich MX carbonitride and M23C6 carbide during TMT and subsequent tempering. The analysis of the cleavage facets and the secondary cracks with electron back-scattered diffraction (EBSD) revealed that the brittle fracture occurred via cleavage cracking along {100} planes across the laths, while the high-angle boundaries of martensite blocks and packets were effective barriers to the crack propagation. The increased impact toughness of the tempered TMT steel sample was attributed to enhanced ductile fracture owing to the uniform dispersion of the precipitates and favorable {332}⟨113⟩ crystallographic texture.