What are the common false defects?

Thermal crack (also called crystallization crack): It occurs in the cooling crystallization process after weld formation, mainly at the grain boundary. Metallographically, it is called intergranular crack, which is located in the center of weld metal and arc pit of arc welding, radiating longitudinally or transversely, and can penetrate the surface and heat affected zone in severe cases. The causes of hot cracks are related to segregation during welding, uneven cold and hot, and high sulfur content in covered electrode (filler metal) or base metal. Cold crack: the crack that appears when cooling to low temperature or room temperature after welding, or the crack that appears after a period of time after welding (this kind of cold crack is called delayed crack, especially 14MnMoVg, 18MnMoNbg, 14MnMoNbB and other alloy steels, also known as delayed crack sensitive steel).

Cold cracks mostly appear in the heat affected zone near the fusion line between the weld and the base metal, and their orientation is mostly parallel to the fusion line, but there are also cold cracks longitudinal or transverse to the weld axis. Cold cracks are mostly transgranular cracks, which are caused by the fact that the low plastic structure in the heat-affected zone of weld bead can't bear the stress caused by volume change and microstructure transformation during cooling, or when hydrogen atoms in weld join with each other to form molecular state and enter the fine pores of metal, they will produce great compressive stress together with the welding stress, which will lead to cracking (called hydrogen embrittlement crack), and the covered electrode or the high phosphorus content in the base material. Reheat cracks: cracks that may occur when the welded parts are reheated within a certain temperature range after welding (such as heat treatment or other heating processes to eliminate welding stress, and repair welding, etc.). ) mostly occurs in the welding overheating area, which belongs to intergranular crack, and its cause is related to the strain caused by microstructure change.