Q690C粒状贝氏体钢MAG焊接头中M-A组元的演变及其对热影响区冲击韧度的影响

为了研究Q690C低碳粒状贝氏体钢热影响区M-A组元演变及其对各亚区韧性的影响，采用全自动熔化极活性气体保护焊(Metal active-gas welding, MAG)施焊并在接头各个区域开V型缺口进行冲击试验，分析各亚区韧性与各亚区组织、断口、M-A组元统计结果之间的对应关系。结果表明，焊缝为针状铁素体，断口上韧窝细小密集，呈韧性断裂，虽然M-A组元数量很多，且存在于晶界，但是由于尺寸小，大都为块状，所以对冲击韧度影响不大；熔合区与粗晶区均为上贝氏体，且M-A组元大多以长条状分布于上贝氏体板条束间；其中熔合区的冲击韧度最差，系粗大基体引发的解理断裂；其余各亚区韧性接近母材，无明显恶化，M-A组元多呈较小块状，存在于晶界。由上述结果可知，M-A的分布、尺寸与形态的演变受控于为领先相的基体组织；细小的贝氏体基体及针状铁素体基体可弥补M-A组元对韧性的危害；组织控制仍应以基体组织控制为重点。

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

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.