Welding is an important part of metal processing and is widely used in many fields. Only by ensuring the quality of products can the safety of products be improved. However, the problem of cracking of stainless steel welded pipes indicates that there is a problem with the quality of the stainless steel welded pipes. The quality of stainless steel welded pipes is determined by the raw material steel strip, so it is necessary to find out the cause and solution of the welding cracks of stainless steel pipes.
Causes of cracking of stainless steel welded pipes
Cracks can be divided into thermal cracks, cold cracks, stress corrosion cracks and lamellar tearing according to the generation temperature and generation time. The locations where cracks form during welding are different. Cracks are visible on the welding surface and can be observed with the naked eye. Some can be hidden inside the weld and can be found by flaw detection; Some are on the weld and some are in the heat affected zone. It should be noted that cracks in stainless steel welded pipes are sometimes generated during the welding process, and sometimes appear after being placed in place or running for a period of time after welding. The latter is called delayed cracks, which are more harmful.
1. Poor raw material steel strip
The quality of stainless steel strips is the basis for determining the quality of stainless steel welded pipes. According to the copper content, it can be divided into low copper material, medium copper material and high copper material. The ductility of the stainless steel pipes produced in this way will be enhanced successively, and the price will also increase accordingly. Unscrupulous manufacturers choose to use medium-frequency furnace materials, which are actually steel strips forged by reprocessing stainless steel scrap. This material has many impurities, high carbon content and high hardness, so it is easy to crack after long-term processing and use.
2. Hardening during processing
The influence of work hardening: Steel pipes produce a large amount of plastic deformation during cold drawing, causing obvious lattice distortion, thereby increasing the lattice energy and the internal energy of the metal, resulting in uneven internal stress of the metal and residual internal stress. This increases the hardness of the metal and reduces its toughness. The higher the hardness of the metal, the greater the residual internal stress during cold drawing and the more obvious the work hardening phenomenon. When the residual stress reaches a certain value, the metal will tear along a certain column of grain boundaries, forming cracks in the steel pipe.
3. Excessive grinding of stainless steel welds
If cracks appear on the weld of the stainless steel welded pipe, it may be because the stainless steel weld has been over-polished, causing the weld to be too thin, so cracks occur during processing and use. This is a product quality issue. Good stainless steel pipe manufacturers often have strict control over their product quality and have corresponding product certificates.
Solutions to cracking of stainless steel welded pipes
1. Using an appropriate amount of lubricant can significantly improve the drawing and bending cracking of 304 stainless steel pipes. The lubricant can form a film with certain toughness and elongation between the male and female dies, which is beneficial to the stretching and bending of the stainless steel welded pipe.
2. For stainless steel tube drawing parts with large drawing deformation and difficult bending forming, polyvinyl fluoride film can be used as a lubricant in actual production. Polyvinyl fluoride film has excellent tear strength, certain toughness and elongation and is easy to clean. After the dry film is applied, it can deform along with the blank during the drawing process and can separate the blank from the mold.
3. Other simple measures to prevent cracking of stainless steel welded pipes include using white cast iron as a pressure ring grinder. Because white cast iron has good oil storage performance and is easy to form a lubricating oil film; For example, using a conical pressure ring, smoothing the edges of the workpiece, and removing micro-cracks and notches can also effectively prevent the stainless steel welded pipe from cracking.
Read more: Implementation Standards for Stainless Steel Welded Pipes
