LSAW steel pipe weld transverse cracks cause analysis

LSAW pipe welding when the first pre-welded, then welded inside, and finally outside welding. For different diameters, different wall thickness welded steel pipe welding comparing the situation found: thin-walled pipe welding, the weld was bright red back white; thick-walled pipe welding, the weld back red dark red; intermediate wall pipe welding, the weld on the back red light in between. This phenomenon indicates that different thickness of steel pipe welding seam on the back there is a large temperature difference, this difference will cause stress in the longitudinal direction of the pipe wall thickness distribution are different.

Factors affecting the size of the tensile stress is mainly welding seam weld both sides of the maximum temperature and cooling rate. Since the front of the weld puddle temperature is much higher than the melting point, the temperature of the opposite weld metal is far below the melting point of the front of the cooling heat weld way to pass through the heat conduction to the reverse side, so that the cooling rate of the weld opposite lower the front of the weld cooling rate. When the temperature of the weld opposite high, close to both sides simultaneously stretching the resulting longitudinal tensile stress is small; when the welding temperature is lower sides, close to the elastic deformation in the opposite state to free expansion, the resulting longitudinal tensile stress is also small; when the temperature in the middle of the weld zone opposite, the resulting longitudinal tensile stress is large, it is easier to produce a range of transverse cracks.

Pipe welding stress distribution with different plate welding, pipe, straight seam welded pipe due to be bound by the resulting longitudinal tensile stress than the big flat welding. When the weld is cooled, without considering the ambient temperature in the case, the weld seam is greater than the cooling rate of the front opposite the cooling speed when the weld is cooled to generate compressed reverse plastic deformation temperature range, the opposite will shrinkage bending deformation of the front seam and the front generate additional longitudinal tensile stress, if the temperature of the weld is still in front of the solid - liquid between the brittle temperature range, the tensile stress in the dual role of the weld may have transverse cracks, and due to bending deformation of the weld surface closer to the larger tensile stress, which is the lateral surface or cracks in the weld seam to strengthen the main reason.