Low-temperature impact toughness of laser–arc hybrid welded low-carbon bainitic steel

A low-carbon bainitic steel serving for the high-speed train was jointed with the laser–arc hybrid welding, and the low-temperature (−40 °C) impact toughness of the joint was evaluated. The results showed that the microstructure of each micro-zone of the welded joint was mainly bainite, but the morphology and size were quite different. The average grain size of the weld material (WM) was 1.69 μm, which was 19% finer than the base material (BM). At the same time, there was a small amount of martensite-austenite (M-A) constituent at the grain boundary, which led to the microhardness of the WM being higher than that of the BM, up to 293.6 HV, about 1.13 times that of the BM. The low-temperature impact toughness of each micro-zone of the welded joint from high to low was 64 J in the inter-critical heat-affected zone, 50 J in the BM, 49 J in the WM, and 43 J in the coarse grain heat-affected zone. The difference in impact toughness was due to the difference in the content of high-angle grain boundaries and the size of the M-A constituent in each region. The larger the size of the M-A constituent, the easier it was to cause stress concentration and produced micro-cracks, which reduced the impact performance. The high-angle grain boundary holding a low surface energy resulted in the crack propagating along the grain boundary. Therefore, the higher the content of high-angle grain boundaries, the more tortuous the crack propagating path and the better the impact performance.