Thermal cycling to improve the impact toughness of 1Cr11Ni2W2MoV steel with banded microstructure

Abstract

Microscopic segregation of chemical elements results in the banded microstructure of finished steel products, and the harmful effect of banded microstructure on toughness is well recognized. However, typical industrial hot work processing, such as hot rolling and forging, cannot erase the banding, and eliminating it with homogenization heat treatment is also challenging and uneconomical. In this study, the impact energy of a 12%Cr tempered martensitic steel with the banded microstructure was significantly improved from 74 J to 154 J by employing thermal cycling and tempering as the final heat treatment to meet the service performance requirement. Compared to conventional quenching and tempering (Q + T), thermal cycling and tempering (TC + T) simultaneously refined the effective grain size of tempered martensite matrix and coarse M23C6 carbides that were continuously distributed along the δ-ferrite/matrix phase boundaries. As a result, more high angle grain boundaries (HAGBs) were introduced and acted as obstacles in retarding the propagation of microcracks. Additionally, finer carbides precipitated at the banding boundary during tempering, owing to the increasing triple junctions between δ-ferrite and tempered martensite. These finer carbides prevent the initiation of microcracks. The results demonstrate the potential of TC + T as a final heat treatment to remove the detrimental effect of banded microstructure on impact toughness.