Straight Seam Welded Steel Pipe Weld Transverse Crack Reason Analysis

Metallographic sample through the detection of cracks in the surface of the weld cracks visible copper plaque, the study found that that there are a lot of crack surface presence of copper, copper is The main reason causing the weld crack generation. According to the conclusions of the crack detection and analysis, in order to minimize copper content of the weld, welding materials and welding equipment has been improved: with wire copper wire factory cooperation will not be changed to copper wire; periodic replacement of the conductive rod poly PTFE hose to prevent conductive rod wear; instead carbide tip to reduce tip generated due to wear copper scrap; flux and cleaned regularly with new welding head and so on. After the adoption of these measures, the number of transverse cracks welds significantly reduced, but there is still a small amount of crack. The reason is analyzed theoretically, causing cracks in the weld heat two things: First, a low melting point impurities, the second is the tensile stress in the welding process. The source of the low melting point of copper impurities still cracks after a minimum, you need to find a solution from the welding stress areas. Steel production statistics that occur during welding transverse cracks show that the distribution of crack is: less thin-walled tubes and thick-walled tube, medium thickness (12 ~ 16mm) more; fewer large diameter and small pipe diameter greater. According to this distribution, welded steel pipe stress analysis. First pre-welded straight seam submerged arc welded pipe, then the welding, and finally outside welding. Different diameter, wall thickness welded steel pipe welding situation is different when comparing discovery: When the thin-walled pipe weld, the weld on the back of the red line was bright white; within the thick-walled pipe weld seam on the back of the red, dark red; intermediate wall thickness When the inner tube, weld back in between the red luminance. A greater temperature difference between the back of the seam weld this phenomenon within the description of the different thickness of the pipe, the difference will cause stress in the longitudinal direction of the pipe wall thickness distribution is different. Pipe welding stress distribution is different with the flat welding, due to binding of the tube longitudinal tensile stress generated, when the ratio is large flat steel welded longitudinally welded. When the weld cools, without considering the ambient temperature, the cooling rate of the weld seam opposite the front of the cooling rate is greater than when the weld is cooled to reverse compressive plastic deformation temperature range, shrinkage will reverse bending deformation of the front seam and the front to generate additional longitudinal tensile stress, if the temperature is still in front of the weld solid - liquid between the brittle temperature range, the effect of tensile stress in the double transverse weld cracks may, and due to bending deformation of the closer larger weld surface tensile stress, which is a surface transverse cracks in the weld seam mainly within or strengthening. Tensile stress factor is the size of the weld both sides of the main maximum temperature and cooling rate of the weld seam. Since the front of the weld pool temperature is much higher than the melting point of the metal, the weld is much lower than the melting point temperature of the back metal, a positive cooling of the weld heat transfer by heat conduction to the reverse manner, so that a low cooling rate of the opposite weld in front of the weld cooling rate. At higher temperatures the back seam, both sides close to the same time stretching, longitudinal tensile stress is smaller; negative when the temperature is low weld, the opposite is closer to the elastic deformation, which can free expansion, resulting longitudinal tensile Stress is also smaller; negative when the temperature in the middle of the weld area, resulting in a larger longitudinal tensile stress is relatively easy to produce the range of transverse crack. Opposite the weld cooling temperature curve through the shaded area that is positive brittle weld seam on the back of the plastic temperature zones and temperature zones intersect the front seam may produce transverse cracks.