Microstructure, Hardness and Residual Stress Distributions in Butt-Welded Joint

International audienceThis paper investigates the characterization of the microstructure changes and the distribution of hardness and residual stress of MIG-welded high-strength low-alloy steel. Residual stresses are experimentally measured by the contour method and the experimental values are numerically treated by MATLAB to find out a representative function which is used as an input of the finite element model. The microstructure of different regions of the weld joint is also investigated and shows the grains size change in the weld. Micro-hardness distribution shows a strong influence of the bainite and ferrite grain size. Residual stresses distribution shows that high tension ferrite grained weld metal is the most critical zone for cracking growth, mainly near the pre-existing porosity. The correlation between results shows that the hardness and the residual stresses values were proportional to the percentage of bainite and inversely proportional to the ferrite grain size. A particular attention is paid in this paper to the microstructure and the hardness around porosity defect. The results highlighted the interest of respecting proper welding procedures to avoid micro-porosities and to lower tension residual stress. Ultrasonic inspection should be obligatory performed in the weld zone to detect internal defect and identify the reliability of the piece