Hot cracks: Hot cracks are cracks that occur slightly below the solidification temperature.

Cracks produced at high temperatures above 300°C are called thermal cracks. Most of the thermal cracks occur in the weld, and sometimes also appear in the heat-affected zone. This kind of cracks crack along the grain boundary, and most of the sections have obvious oxidation colors.
1 Cause: Hot cracks are cracks formed by the combined action of tensile stress and low melting point eutectic. No matter which side of the effect is increased, it can promote the formation of hot cracks in the weld.
2 preventive measures
1 Control the chemical composition, limit the content of easy-to-generate low-melting eutectic and harmful impurities, reduce the content of nickel, carbon, sulfur, and phosphorus in the weld metal, and increase the elements such as chromium, molybdenum, silicon and manganese, which can reduce hot cracks The production.
2.2.2 Improve the weld metal structure, refine the crystal grains, reduce or disperse segregation, and reduce the harmful effects of the low melting point eutectic. 2.2.3 Select the appropriate electrode coating type. The use of low-hydrogen type coated electrodes can refine the weld grains, reduce impurity segregation, and improve crack resistance. The acid-coated electrode is highly oxidative, and the alloying elements are burnt more, the crack resistance is reduced, and the product grain is coarse, which makes hot cracks easy to occur.
2.2.4 Control the shape of the weld and try to get a weld with a larger weld forming coefficient.
2.2.5 Adopt multi-layer and multi-pass welding method to control the temperature between layers to avoid the segregation cases from gathering in the center of the weld.
2.2.6 Preheat before welding to reduce cooling rate and stress.
2.2.7 Welding receiving arc molten pool should be filled to reduce arc crater cracks.
2.2.8 Choose a reasonable welding sequence and welding direction to reduce welding stress.
2.2.9 Use low current and fast welding speed to reduce the overheating of the weld pool and rapid cooling to reduce segregation and improve crack resistance.
3. Reheat cracks:
Reheating cracks are the reheating of weldments after welding in a certain temperature range, such as cracks produced by post-weld heat treatment or other heating processes. Post-weld heat treatment cracks occur during the heating process of post-weld stress relief heat treatment. The reheat crack originates from the heat-affected crude product area and the weld root, and has the characteristics of discontinuous pockets of crystals.
3.1 Reasons
3.1 After the weld is heated again, the supersaturated solid solution carbide formed by the first thermal process is precipitated again, that is, precipitated carbides are precipitated, resulting in intragranular strengthening, and the slip strain is concentrated on the original austenite grain boundaries. When the plastic strain capacity of the grain boundary is not enough to withstand the strain produced by the relaxation stress process, reheat cracks will occur.
3.1.2 In the post-weld heat treatment of the joint, it is easy to cause the just brittle elements to gather on the grain boundary, weaken the bonding force of the grain boundary, and produce reheat cracks.
3.2 Preventive measures
3.2.1 Reduce the overheating tendency of the heat-affected zone and refine the austenite grain size.
3.2.2 Select appropriate welding materials to improve the plasticity of the metal at the stress-relieving heat treatment temperature, so as to improve the ability to bear the strain of loose ground.
3.2.3 Increase the preheating temperature, adopt slow cooling after welding, and make the weld shape uniform and flat to reduce welding residual stress and stress concentration.
3.2.4 Use correct heat treatment specifications and processes, and try not to stay too long in the heat sensitive area.