- Crack is one of the most harmful welding defects. It is an area defect. Its occurrence will significantly reduce the load-bearing area. What is more serious is that a sharp gap is formed at the end of the crack. The stress is highly concentrated and it is easy to expand and cause damage. , cracks are very harmful, especially cold cracks.
- Hot cracks occur at the end of the solidification of the weld metal. The sensitive temperature zone is approximately the high temperature zone near the solidus line. The common hot cracks are crystal cracks. The reason for their formation is that during the solidification process of the weld metal, crystallization segregation causes the impurities to be produced at a low level. The melting point eutectic is enriched in the grain boundaries, forming a so-called "liquid film". In a specific sensitive temperature zone (also called a brittle temperature zone), its strength is very small, and it is subject to tensile stress due to the solidification shrinkage of the weld. Then it cracks to form a crack. The common situation of crystal cracks is that it cracks along the center length of the weld, which is a longitudinal crack. Sometimes it occurs between two columnar crystals inside the weld, which is a transverse crack.
- Factors affecting crystal cracks:
The influence of alloying elements and impurities. The increase of impurity elements such as carbon, sulfur, and phosphorus will expand the sensitive temperature zone and increase the chance of crystallization cracks. The influence of cooling rate. Increased cooling rate will aggravate crystallization segregation. The second is to increase the crystallization temperature range, both of which will increase the probability of crystallization cracks; the influence of crystallization stress and restraint stress. In the brittle temperature zone, the strength of the metal is very low, and the welding stress causes this part of the metal to be stretched. When the tensile stress When it reaches a certain level, crystal cracks will appear.
- Precaution:
Reduce the content of harmful elements such as sulfur and phosphorus, and use materials with lower carbon content for welding; add certain alloying elements to reduce columnar crystals and segregation, such as molybdenum, vanadium, titanium, niobium, etc., which can refine the grains and use penetration depth Shallow welds improve heat dissipation conditions so that low melting point substances float on the surface of the weld and do not exist in the weld; rationally select welding specifications and use preheating and postheating to reduce the cooling rate; adopt a reasonable assembly sequence , reduce welding stress.
- Characteristics of reheat cracks:
It occurs during the reheating process such as post-weld heat treatment, and the welding heat-affected zone produces an overheated coarse-grained area; the generation temperature is 550~650°C, intergranular cracking, and precipitation-strengthened steel types are prone to occur, which is related to welding residual stress.
- Precaution:
Properly preheat or use post-heating, control the cooling rate, reduce residual stress, avoid stress concentration, and avoid or shorten sensitive temperature areas.
