Welding is an important part of metal processing and has a wide range of applications in many fields. Only by ensuring the quality of the product can the safety of the product be improved. However, welding defects are extremely unfavorable factors in production, and stainless steel pipe cracks are a common and very dangerous defect. It not only makes the product scrapped, but also may cause major accidents. Therefore, it is necessary to find out the cause of the welding crack of the stainless steel pipe.
The 304 or 316L stainless steel welded pipes in daily life will have cracks. The more common is that the welded joints of stainless steel welded pipes are often damaged due to cracks and leaks, and most of the cracked areas occur in welds or heat-affected zones. Do you know the five reasons that affect the cracks in stainless steel pipes?
1. Hot crack
Hot cracks are cracks generated during the high temperature stage of the welding cooling process, and mainly exist in the weld metal. It also exists in a small amount near the fracture, and is divided into crystallization (solidification) cracks, liquefaction cracks and polygonal cracks. Among them, crystal cracks are common cracks, which mainly occur in the welding of carbon steel with many impurity elements.
2. Reheat crack
During the stress relief treatment of thick plate welded structures, when there are different degrees of stress concentration in the coarse-grained zone of the heat-affected zone, the additional deformation due to stress relaxation is greater than the creep plasticity of this part, and reheat cracks will occur; the temperature is usually At 550°C to 650°C; the cracks of 316l stainless steel water pipes are usually located in the coarse grain area of the heat-affected zone; the crack form is cracking along the grain boundary.
3. Cold crack
The cracks produced when the welded joint is cooled to a lower temperature (for steel below M. temperature) are called cold cracks. Cold cracks may appear immediately after welding, or may appear after a period of time (hours, days or even longer). This kind of crack is also called delayed crack. It is a common form of cold crack and has a more great danger.
4. Stress corrosion cracks
Some welded structures (such as containers and pipelines, etc.), delayed cracking under the combined action of corrosive media and stress; can occur at any temperature; the location where the crack occurs is usually located in the weld and heat-affected zone; the crack shape is along the Crystalline or transcrystalline.
5. Lamellar tearing
The main reason is that there are layered inclusions (along the rolling direction) inside the steel plate, and the stress perpendicular to the rolling direction generated during welding results in a "step" layer in the heat-affected zone or a little farther away. cracking; the temperature is usually below about 400 °C; the location of the crack is usually located near the heat-affected zone; the crack morphology is transgranular or intergranular.
