Microstructural Evolution Investigations Of Welding Transition Zone On Low Alloy Steels Using Low Transformation Temperature Consumables

Residual tensile stress is the common concern in designing and manufacturing high-strength welded structures due to its detrimental effect on fatigue life of welding joints.The application of low transformation temperature?LTT?flux-cored welding consumables are mainly used to improve the fatigue performance by changing the stress state of the welding joint.In Practice,its welding joint with low alloy steel belongs to dissimilar metal weld?DMW?for the LTT welding metal contains high Cr and Ni alloying elements.Due to the inherent physical and chemical heterogeneity exist in the transition zone of DMW,it is the weakest area in the whole joint.Therefore,in this study,the intrinsic factors of strain localization were studied in the perspective of microstructural evolution in the welding transition zone.In this study,the gradient of the chemical composition,the law of the microstructural evolution,the microhardness distribution and the strain localization of the welding transition zone between clad-layer and interlayer were studied using the scanning electron microscope equipped with an energy-dispersive X-ray spectrometer,Vickers-microhardness test and electron backscatter diffraction characterization techniques.It was found that the welding metal was mainly composed of martensite in the vicinity of fusion boundary,the content of?-ferrite was related to dilution rate and Cr_eq/Ni_eq value for both the clad-layer and interlayer of the welding transition zone.The major alloying elements of Cr and Ni had large compositional gradient variations in the transition zone.Moreover,the Ni-content in the transition zone was higher than the average Ni-content of the welding metal for the Ni elemental segregation together with carbon element at fusion boundary.Therefore,the Ni-content increased in the transition zone and formed the carbon-increased region adjust fusion boundary.In addition,the Cr element played an important role in accelerating the segregation of Ni element.From the fusion boundary to the welding metal along the transition zone,the Ms temperature declined significantly as dilution rate decreased.The strain localization and dislocation density of the martensite microstructure were more than that of the mixed microstructures of martensite+?-ferrite near the fusion boundary in the welding metal.