Evolution of M-A Constituents in HAZ and Its Effect on Toughness of MAG Joint of Granular Bainitic Steel Q690C

Abstract

To study the evolution of M-A constituents and their effects on toughness of each HAZ(heat affected zone) sub-zone, a low carbon granular bainitic steel Q690 is welded using automatic metal active-gas welding(MAG) welding and Charpy V-notch impact test is carried out. The microstructures, fractograph and statistical results of M-A constituents is analysed. The results show that the microstructure of weld metal(WM) is acicular ferrite and the dimples on fracture surface of the WM are small and dense, showing ductile fracture feature. Although large amounts of M-A constituents existed in grain boundary in the WM, the toughness is not deteriorated owing to the blocky shape and small size. The microstructure of FZ(fusion zone) and CGHAZ(coarse grain heat affected zone) are upper bainite with string M-A constituents between them; the toughness of FZ is the worst because of the cleavage fracture of coarse grains; the toughness of other sub-zones, in which M-A constituents exist in grain boundary with small blocky shape, are close to base metal(BM). So, the evolution of distribution, size, and shape of M-A constituents is controlled by matrix microstructure; fine bainite and acicular ferrite could remedy the deterioration of toughness derived from M-A constituents; the microstructure control should focus on the matrix microstructure.