1, The phosphorus impurities in steel exist in the form of Fe2P and Fe3P. Fe3P can form a low-melting eutectic with iron, which gathers at the grain boundary to weaken the bonding force between the grains, making the steel at room temperature or low temperature. Time becomes brittle, causing cold cracking.

2, Due to chemical reactions, decomposition of air or water during the welding process, a certain amount of hydrogen is dissolved in the weld, and hydrogen energy flows to the weld defect and enriches to form hydrogen embrittlement, and even form hydrogen-induced delayed cracks, which are cracks The most serious and dangerous problem in the

3.The third reason is the presence of hydrogen in the weld, and oxygen atoms or compounds can also make the metal brittle and cause cold cracks. Fourth, the steel with high quenching tendency or the thick plate with high rigidity because no preheating or warm-cooling measures are taken, and the cooling rate is fast, resulting in large internal stress and welding residual stress, which is more than the hot material can withstand. In addition, the tack weld of the weldment is too weak and cracks during the welding process, and the crack of the tack weld is not eliminated when the weld is welded, which is also the source of cracks.

During the welding process of the gate valve and gate, the gate is prone to cracks, and the cracks produced by the surfacing welding of the gate are mainly cold cracks.