1. Quenching The cold cracks formed in the near-seam area or weld seam are related to the rapid changes in the mechanical properties and the complex stress states during the metal phase transition. Cold cracks mainly occur in medium-carbon steels, high-carbon steels, and high-strength steels. The main characteristic of this kind of steel is that it is easy to quench and form a brittle hard martensite structure. Especially in the welding conditions, the heating temperature in the near-seam zone is very high, and near the fusion line is above 1350°C, which causes severe austenite overheating and significant grain growth. From metallurgy, it can be seen that coarse austenite grains are more easily quenched and transformed into coarse martensite structures, which deteriorates the metal properties in the near-seam zone, especially the plasticity and brittleness. At this time, cold cracking will occur under the influence of complicated welding stress. The
2, the role of hydrogen In the welding of high temperatures, some hydrogen-containing compounds distinguish precipitated atomic state of hydrogen, a large number of hydrogen dissolved in the molten pool metal. As the temperature of the bath decreases, the solubility of hydrogen in the metal decreases dramatically. However, the cooling rate of the weld pool is very high, and the hydrogen will not escape and remain in the weld metal. The solubility and diffusion capacity of hydrogen in austenite and ferrite are also significantly different. Generally, the carbon equivalent of the weld metal is always lower than that of the base metal, so the austenite decomposes at a higher temperature at the weld. At this time, the austenite transformation has not yet occurred in the near-seam zone. As the solubility of hydrogen in the weld metal suddenly drops and the diffusion capacity increases, hydrogen diffuses into the austenite in the near seam zone. This causes a large amount of hydrogen to accumulate in the near-seam area. As the temperature drops, the austenite in the near-seam zone of the pressure vessel changes, the temperature is already very low, the solubility of hydrogen is lower, and the diffusion capacity is also very weak. Hydrogen then enters the fine pores of the metal in a gaseous state and causes a great deal of pressure, causing the local metal to exert a great deal of stress, thereby forming cold cracks. In summary, there are two causes of cold cracks in pressure vessels: one is the embrittlement of the metal; the other is the effect of welding stress.