What are the causes of welding cracks in stainless steel pipes

In the welding operation of stainless steel pipes, welding cracks are the key issues affecting welding quality and structural safety. The generation of these cracks is affected by a combination of multiple factors, and their specific causes and characteristics are as follows:

1. Overheating cracks: Thermal cracks are cracks generated in the high-temperature stage of the cooling process of stainless steel pipe welding, and they mainly exist in welded metal. There are also a small number of them near the joints of stainless steel pipes, which are divided into crystallization (solidification) cracks, liquefaction cracks, and multilayer cracks. Among them, crystallization cracks are common cracks, which mainly appear in carbon steel welding with more impurity elements.

2. Reheating cracks: During the stress relief treatment of thick plate welding structures, when there are different degrees of stress concentration in the coarse grain area of the heat-affected zone, the additional deformation caused by stress relaxation is greater than the creep plasticity of this part, and then reheating cracks are generated. The production temperature is usually 550℃~650℃; the cracks of 36L stainless steel water pipes are usually located in the coarse grain area of the heat-affected zone. The crack morphology is along the grain boundary.

3. Cold cracks: The welded joint is cooled to a lower temperature (the unit of steel is m. The cracks caused by the temperature being lower than 100°C are called cold cracks. Cold cracks can appear immediately after welding or after a period of time (several hours, days, or even longer). This type of crack is also called delayed crack, which is a common form of cold crack and is more dangerous.

4. Stress corrosion cracking: Some welded structures (such as containers and pipelines, etc.) have delayed cracking. Under the combined action of corrosive media and stress; it can occur at any temperature; cracks usually appear in welds and heat-affected zones. The crack morphology is intergranular or transgranular.

5. Delamination tearing: It is mainly due to the presence of layered inclusions in the steel plate (along the rolling direction), and the presence of stress perpendicular to the rolling direction at the welding point, resulting in "step"-shaped layered cracking in the heat-affected zone or slightly further away; the generation temperature is usually below about 400°C; the cracks are usually located near the heat-affected zone. The crack morphology is transgranular or intergranular.