Microstructural Characteristics and Mechanical Properties in the Laser Beam Welded Joints of High-Strength Microalloyed Steel
The laser beam welded joints of the high-strength microalloyed steel consist of the base metal (BM), the partially transformed heat-affected zone I (PTHAZ-I), the partially transformed heat-affected zone II (PTHAZ-II), the fully transformed heat-affected zone (FTHAZ) and the fusion zone (FZ). The martensite/austenite (M/A) islands of the bainites in the PTHAZ-II are dense versus those in the PTHAZ-I, and the martensites exist in the FTHAZ and the FZ together with the bainites under the slow welding speed. The M/A islands of the bainites and/or the martensites in the PTHAZ become denser under the high welding speed, and the martensites in the FTHAZ and the FZ predominate. The low-angle boundary density and the high local misorientation proportion are used to identify the constituents roughly, consistent with the microstructural characteristics in the welded joints. The misorientation angle distributions are employed to evaluate the degree of the phase transformation, reflecting the change law of the degree of the phase transformation in the welded joints. The hardness and the tensile tests indicate that the HAZ and the FZ are stronger than the BM, dependent on the microstructures in the welded joints. The strengthening in the HAZ and the FZ is mainly attributed to the products of non-equilibrium phase transformation with the high dislocation density.