Abstract：Aiming at the transverse welding crack problem of Q355B steel strip (thickness of 7.5 mm) during the preparation of square pipe by welding, the composition analysis, mechanical property inspection and metallographic structure inspection were conducted. The results showed that the structure of base metal steel strip was composed of ferrite + pearlite + very small amount of bainite, which could meet the standard requirements. The crack initiation position of the square tube was in the heat-affected zone of the weld. The metal grains were distorted at the end of crack, which belonged to the extension morphology after stress crack tearing. The microstructure was the Widmannstatten structure with extremely poor plastic toughness, and there were large-size cluster inclusions at the weld which severely cut the continuity of the matrix. After comprehensive analysis, it was believed that the high welding power and too fast cooling speed led to the formation of coarse Widmannstatten structure at the weld, which seriously reduced the ductility and toughness. Combined with the impurity discharge of inclusions such as Si and Mn oxides in the weld pool, the continuity of the weld was destroyed. Under the action of tensile stress in the subsequent correction process, the crack extension was formed, which was the main reason to cause transverse cracks after welding. Such kind of problems could be avoided by optimizing the flash butt welding process and reasonably controlling the austenite grain size in the weld.
2024, Vol. 42 
