Types and solutions of crack defects during stainless steel pipe welding
When stainless steel pipes are welded, cracks sometimes occur in the weld area. Depending on the time and temperature of their formation, there are three main forms: cold cracks, hot cracks and reheat cracks. According to the formation location, they can be divided into vertical cracks, transverse cracks, Weld root cracks, arc crater cracks, fusion line cracks and heat affected zone cracks, etc. This kind of defect is the most dangerous for welded structures. It not only easily causes the stainless steel pipe to be scrapped, but may also cause serious accidents.
hot cracks
During the welding process, the welding cracks formed when the stainless steel material in the heat-affected zone cools to a high temperature range near the solidus line are called hot cracks, which are very dangerous.
The main reason for the formation is that the low-melting-point eutectic and impurities in the molten pool metal produce severe intra- and inter-granular segregation during the crystallization process. At the same time, under the action of welding stress, they are pulled apart along the grain boundaries to generate hot cracks. Hot cracks usually occur in austenitic stainless steels, nickel alloys and aluminum alloys.
The solution is to strictly control the content of harmful impurities such as sulfur and phosphorus in stainless steel pipes and welding materials to reduce the sensitivity of hot cracks; or to adjust the chemical composition of the weld metal, optimize the weld structure, and refine the grains. Enhance plasticity and reduce the degree of segregation; alkaline welding materials can also be used to reduce the content of impurities in the weld and optimize the degree of segregation; select suitable welding process parameters, appropriately enhance the weld forming coefficient, and use multi-layer and multi-pass welding Method; when breaking the arc, use the same lead-out plate as the base material, or gradually extinguish the arc, and fill the arc crater to prevent the formation of thermal cracks in the arc crater.
cold crack
Cold cracking refers to the formation of cracks in the welded joints of stainless steel pipes when they cool to lower temperatures. This kind of defect may appear immediately after welding, or it may not appear until a few hours later, so it is also called delayed crack. It is highly common and more dangerous.
There are three main reasons for the formation, namely the hardened structure formed by martensite transformation, the welding residual stress caused by high restraint and the hydrogen remaining in the weld.
The solution is to use low-hydrogen welding materials; remove oil stains and moisture on the weldment before welding to reduce the hydrogen content in the weld; select appropriate welding process parameters and heat input to reduce the hardening tendency of the weld; after welding Hydrogen elimination treatment should be carried out immediately to make hydrogen escape from the welded joint; for stainless steel welded pipes with high hardening tendency, preheat before welding and conduct heat treatment in time after welding to improve the structure and performance of the joint.
reheat crack
After welding, the cracks formed by heating the stainless steel pipe again within the corresponding temperature range are called reheat cracks.
The reason for its formation is that reheat cracks usually occur in low-alloy high-strength steels, pearlitic heat-resistant steels and stainless steels containing alloy elements such as vanadium, chromium, molybdenum, boron and other alloying elements. After being subjected to a welding heat cycle, they are then heated to sensitive areas. form. Most cracks originate from the coarse-grained area of the welding heat-affected zone. Reheat cracks are mostly formed in stainless steel welded pipes and stress concentrations. Reheat cracks sometimes also form during multi-layer welding.
The solution is to select lower-strength welding materials on the premise of meeting the design requirements, so that the strength of the weld is weaker than the base metal, and the stress is relaxed in the weld to prevent cracks from forming in the heat-affected zone; and to reduce the welding residual stress and stress as much as possible. Concentrate; control the heat input of welded pipe welding, rationally select preheating and heat treatment temperatures, and try to avoid sensitive areas.
The danger of cracks is very high, and it will especially seriously affect the practical application and yield of stainless steel pipe products. Therefore, this defect must be avoided as much as possible, and it must be compensated accordingly after it occurs. If it cannot be compensated, it cannot be used in actual applications.
