Reasons For Welding Cracking Of 304 Stainless Steel Pipe

Welding is an important link in metal processing and is widely used in many fields. Only by ensuring product quality can product safety be improved. However, welding defects are an extremely unfavorable factor in production. Cracks are a common and dangerous defect that not only makes the product scrapped, but may also cause major accidents. Therefore, it is necessary to find out the causes of welding cracks in 304 stainless steel pipes.
Depending on the temperature and time of occurrence, cracks can be divided into hot cracks, cold cracks, stress corrosion cracks and delamination tears. The locations of crack formation during the welding process are different. Cracks are visible on the welding surface and are visible to the naked eye; some can be hidden inside the weld and can be found through flaw detection; some are on the weld and some are in the heat-affected zone. It should be noted that cracks sometimes appear during the welding process, and sometimes cracks appear after being put in or running for a period of time after welding. The latter type is called delayed cracking and is harmful.
Thermal Cracks
Because it crystallizes at high temperatures and breaks along grain boundaries, it is also called crystallization cracks. Microscopically, cracks are characterized by intergranular fracture, and oxidation color dominates the crack section. This is due to the segregation phenomenon of the welding pool during the crystallization process, and the precipitates are mainly low-melting-point eutectics and impurities. It is a liquid phase layer when crystallized, and the high-temperature strength is also very low when the crystal solidifies. Under certain conditions, when the welding tensile stress is high, the melt will separate or break in layers, causing the melt to melt.
Cold Crack
Most occur in the heat affected zone and sometimes in the weld metal. Cold cracking mainly manifests as cracking within the grains, the fracture surface has no obvious oxidation color, and the fracture surface is shiny.
Pressure Corrosion Cracking
The delayed fracture of 304 stainless steel pipe under the action of some special media and tensile stress does not have obvious uniform corrosion traces, and the observed stress corrosion cracks are cracks and discontinuities. On the welding surface, most of the cracks are transverse. When stress corrosion cracks occur inside the metal, their shape is similar to that of tree roots, and the fracture shape is a typical brittle fracture. For austenitic stainless steel, due to different corrosion media, the nature of the cracks will also change, either intergranular cracks, or a mixture of transgranular and intergranular cracks. In chlorinated media, stress corrosion cracking of austenitic stainless steel is transgranular cracking.
With the development of welding technology, many unsafe factors will appear in actual production. Therefore, welders should be familiar with the characteristics of 304 stainless steel pipes, welding equipment, welding processes and operating procedures, have a deep understanding of safety technical measures, strictly implement operating procedures, and correctly implement protective measures.