The effect of the initial microstructure of the X70 low-carbon microalloyed steel on the heat affected zone formation and the mechanical properties of laser welded joints

In this paper, the heat affected zone (HAZ) of laser welded joints of the X70 steel were studied by the transmission electron microscopy method. The effect of the initial microstructure (coarse-grained hot-rolled and fine-grained after cross-helical rolling) on the HAZ formation and the mechanical characteristics of the welded joints were shown. It was found that the microstructure in the inter-critical HAZ of the steel after cross-helical rolling was more dispersed, homogeneous, and uniform compared to that of the coarse-grained hot-rolled one due to the initial fine-grained ferrite-bainitic-pearlite microstructure and the absence of pronounced ferrite-pearlite banding in the base metal. The character of the microhardness value distribution in the HAZ of the steel after cross-helical rolling was smooth with the gradual decrease from 370 down to 185 HV as shifted towards the base metal. In the HAZ of the coarse-grained hot-rolled steel, the heterogeneous microhardness value (up to 640–670 НV) distribution was revealed. The reason was the upper degenerate bainite microstructure with high residual stresses, characterized by laths up to 2.0–2.5 μm long and a high martensitic-austenitic constituent fraction (10–16%) of a slender shape along the boundaries of bainite laths. The conclusion was drawn that one of the ways to reduce the brittleness of the laser welded joints could be using the initially fine-grained steels possessing the homogeneous (mainly bainitic) microstructure.