During the welding process of stainless steel pipes, cracks sometimes occur in the weld zone. According to the time and temperature of formation, there are mainly three forms of cold cracks, hot cracks and reheat cracks; according to the formation site, it can be divided into vertical cracks, transverse cracks, Weld root cracks, arc pit cracks, fusion line cracks, and heat affected zone cracks. This kind of defect is the most dangerous for the welded structure. Not only is it easy to scrap the stainless steel pipe, it may also cause serious accidents.
In the welding process, the welding crack formed by the stainless steel material in the weld zone and the heat affected zone cooled to the high temperature zone near the solid line is called a hot crack, and the danger is high.
The main reason is that the low-melting eutectic and impurity in the molten pool metal produce severe intragranular and intergranular segregation during the crystallization process. At the same time, it is pulled along the grain boundary under the action of welding stress 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 sulfur, phosphorus and other harmful impurities in stainless steel pipes and welding consumables, reduce the sensitivity of hot cracks, or adjust the chemical composition of the weld metal, optimize the weld microstructure, refine the grain, Strengthen plasticity and reduce segregation; also use alkaline welding materials, reduce the content of impurities in the weld, optimize the degree of segregation; select suitable welding process parameters, properly increase the weld forming factor, use multi-layer multi-channel welding Method: When the arc is broken, use the same lead plate as the parent material, or gradually extinguish the arc and fill the arc pit to prevent the formation of hot cracks in the arc pit.
Cold cracking refers to the formation of cracks when welded joints of stainless steel tubes cool to lower temperatures. This kind of defect may occur immediately after welding, or it may occur within a few hours, so it is also called delayed crack. It has high universality and greater danger.
There are three main reasons for formation, namely hardened structure formed by martensitic transformation, welding residual stress formed by a large degree of binding, and hydrogen remaining in the weld.
The solution is to use low-hydrogen type welding materials; to remove the oil and moisture on the weldment and reduce the hydrogen content in the weld before welding; to select suitable welding process parameters and heat input to reduce the hardenability tendency of the weld; after welding Hydrogen treatment is immediately required to allow hydrogen to escape from the welded joint. For stainless steel welded tubes with a high tendency to harden, preheating before welding and heat treatment in time after welding will improve the microstructure and properties of the joint.
After welding, cracks formed when the stainless steel tube is heated again in the corresponding temperature range are called reheat cracks.
The reason for this is that reheat cracking usually occurs in low-alloy high-strength steels, pearlitic heat-resistant steels, and stainless steels containing vanadium, chromium, molybdenum, boron, and other alloying elements. After being subjected to a welding heat cycle, it is heated to sensitive areas. form. Most cracks originate from the coarse-grained region of the heat-affected zone of the weld. Most of the reheat cracks are formed in the stainless steel welded pipe and in the stress concentration area, and the reheat cracks are sometimes formed in the multilayer welding.
The solution is to meet the design requirements, select the lower strength of the welding material, making the weld strength weaker than the parent material, the stress relaxation in the weld to prevent the formation of cracks in the heat affected zone; minimize the welding residual stress and stress Concentrate; control welding pipe heat input, reasonable choice of preheating and heat treatment temperature, try to avoid sensitive areas.
The degree of risk of cracks is very high, especially the practical application and yield of stainless steel welded pipe products will be seriously affected. Therefore, such defects must be avoided as much as possible, and corresponding compensations should be made after the occurrence. If they cannot be compensated, they cannot be used in practical applications.
Source: China Stainless Steel Welded Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)