The causes of hydrogen crack formation in welding arc welding of stainless steel are mainly caused by three factors, namely, the brittle and brittle hard and brittle structure of hydrogen produced by the welding process, the tensile stress acting on the welded joint, and often when the temperature is up to the normal environment.
In practice, for a given situation (material composition, material thickness, joint type, electrode composition and heat input), the risk of hydrogen cracking can be reduced by heating the joint.
Reducing the cooling rate by preheating can diffuse some hydrogen and reduce the hardness of the stainless steel plate, that is, the hardness of the microstructural area sensitive to the crack, which causes the plate to break easily. When thick walled steel with high carbon equivalent (IIW CE) value is welded, the preheating level can be as high as 200 degrees.
Because there are few cracks above the ambient temperature, it is also important to keep the temperature of the weldment of the stainless steel plate in the manufacturing process. For susceptible steels, it is appropriate to keep the preheating temperature at a given time, usually between 2 and 3 hours, so that hydrogen diffuses from the welding area. Under the condition of sensitive crack, such as high IIW CE steel welded or under high constraint conditions, temperature and heating time should be increased, usually at 250-300 C three to four hours.
For many kinds of steel, post weld heat treatment (PWHT) can be used immediately after welding, i.e., no preheating temperature is allowed. But in fact, since inspection can only be carried out at ambient temperature, it is only after PWHT that the “disposable” defect can be found. In addition, for high hardness steel, second heat treatments may be needed to adjust the hard microstructure after the first PWHT.
In some cases, in order to avoid cracking, more stringent procedures are required (higher preheating temperature and / or lower hydrogen content in welding metal). It includes the following: height constraints, including the weld of section thickness above 50mm, and the operation of the root in the double inclined joint; the thick part (more than 50mm); the low carbon equivalent steel (C < 0.1% and IIWCE < 0.42 C-Mn steel); “clean” or low sulfur steel (S less than 0.008%) because of low sulfur and oxygen content It will increase the hardenability of steel.
The alloyed weld metal, where the preheating level to avoid HAZ cracking may not be enough to protect welding metal. Low hydrogen technology and consumables should be used. For predicting the preheating requirements to avoid welding metal cracking, the diffusion hydrogen level of welded metal and tensile strength of welded metal are usually required as input. The use of austenitic stainless steel and nickel alloy welding metal can also effectively prevent cracking.
Austenitic consumables should be used in case of poor preheating or no cracking prevention. Austenitic stainless steel and nickel alloy electrodes will produce a welding metal, and the solubility of hydrogen to iron at ambient temperature is higher than that of ferritic stainless steel. Therefore, any hydrogen formed during welding is locked in the welding metal and rarely diffuses to HAZ when cooled to ambient temperature.
The commonly used austenitic MMA electrode is 23Cr:12Ni, such as EN 1600:1997. However, because nickel alloy has lower thermal expansion coefficient compared with stainless steel, nickel alloy electrode is preferred to reduce shrinkage strain. When welding up to 0.2%C steel, it is usually not necessary to preheat. However, at a temperature above 0.4%, the minimum temperature of 150 C will be needed to prevent HAZ cracking.
The best way to avoid hydrogen cracking is to reduce the hydrogen content of weld metal, that is, to reduce the amount of hydrogen generated by consumables, that is, using low hydrogen technology or low hydrogen electrode.
According to the amount of welding metal hydrogen produced in the standard test block, the welding process can be divided into high, medium, low, extremely low and ultra-low. MMA is likely to produce more hydrogen content. Therefore, in order to achieve a lower value, the base electrode must be used and roasted according to the manufacturer’s advice, or removed from the special package immediately before use, and the time of exposure to the environment is not more than the specified time by the manufacturer. For the MIG process, cleaner wires are needed to achieve very low hydrogen content.
The following general guidelines are recommended for all types of steel, but the requirements for specific steels should be inspected according to EN 1011-2:2001.
Low carbon steel is easy to weld. If low hydrogen technology or electrode is used, it usually does not need preheating. When welding thick cross section material, it may need preheating, high binding force and more hydrogen.
The thin part of medium carbon and low alloy steel can be welded without preheating, but the thicker part needs lower preheating level. Low hydrogen technology or electrode should be used.
Higher carbon and alloy steels require preheating, low hydrogen processes or electrodes, post weld heating and required slow cooling. It is recommended that the following practical technologies be used to prevent hydrogen cracking, clean joint surface, remove paint, cutting oil, grease and other pollutants. If possible, use the low hydrogen process, roast electrode (MMA) or flux (submerged arc), then store them, or limit the time exposed to environmental conditions.
The stress on the weld is reduced by avoiding large root gap and height constraints. If preheating is specified in the welding procedure, it should also be applied when the temporary attachment is glued or used, and the joint is preheated to at least 75mm from the seam line to ensure that the thickness of the material is heated evenly and the preheating temperature of the measurement and the reverse side of the heating phase is measured.
In acceptance, because the hydrogen crack is a linear defect with sharp edges, it is not allowed to conform to the weld of the quality grade B, C and D according to the requirements of EN ISO 5817. There are several main aspects of detection and remedial measures, because hydrogen cracks are usually very fine and may be subsurface.
Source: China Stainless Steel Plates Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)