Significant improvement in CGHAZ toughness of HSLA steel via welding with trailing mechanical treatment

Abstract

The Coarse-grained heat-affected zone (CGHAZ) is well known as the local brittle zone in the welding joints of HSLA steels. In the present work, compressive strain is applied to the specimen during the CGHAZ welding thermal simulation process to achieve microstructure refinement and modification for toughness improvement. The strain is applied to the specimen when the temperature of the simulated thermal cycle decreased to 1100 °C, with the purpose of achieving dynamic recrystallization in the CGHAZ. Parallel experiments without the compressive strain are also conducted as a reference. The microstructure of the CGHAZ with and without the compressive strain was observed by optical microscopy (OM) and scanning electron microscopy (SEM). Transmission electron microscopy (TEM) was used for observation of the matrix and microstructure of martensite-austenite (M-A) constituents. Electron back-scattered diffraction (EBSD) techniques were used to get the information of boundary misorientation distribution. The toughness of the specimens under each condition was assessed through instrumented Charpy impact test at −20 °C. Results showed that the compressive strain refined prior austenite grain (PAG) size and packet size, while the block width and lath width were almost unaffected. Refinement of the PAG also modified the microstructure of M-A constituents. The microstructure refinement and modification of M-A constituents improved crack initiation energy and crack propagation energy, resulting in significant promotion of toughness from 60.20 J to 163.10 J.