Preventive measures for common defects in welding of dissimilar steels in pressure vessels

Thread Content

Proper selection of materials: The choice of welding materials for dissimilar steels in pressure vessels is crucial for welding processes. The quality and performance of the welded joints are closely related to the welding materials used. In accordance with the principles of reasonable, scientific, and effective material selection, the following guidelines should be observed: (1) When welding carbon steel and low-alloy steel, it is generally required that the strength of the weld metal or joint is not lower than the minimum strength of the two metals being welded. The strength of the deposited metal from the selected electrode must be no less than that of the weaker base metal. Meanwhile, the plasticity and impact toughness of the weld metal should be inferior to those of the base metal with higher strength but poorer plasticity. Therefore, the welding material with the lower strength between the two can be selected. Meanwhile, to prevent welding cracks, the welding procedure should be determined based on steel grades with high strength but poor weldability; this includes welding parameters, preheating temperature, and post-weld heat treatment. (2) When welding low-alloy steel and austenitic stainless steel, electrodes should be selected based on the allowable limits for the chemical composition of the deposited metal. Generally, austenitic steel electrodes with high chromium and nickel contents, as well as good plasticity and crack resistance, are chosen; the welding process and specifications are determined according to the poorer weldability of the stainless steel. (3) For welding stainless steel composite plates, different types of electrodes should be selected according to the requirements for the base layer, transition layer, and overlay layer. For welding of base materials (carbon steel or low-alloy steel), use corresponding structural steel electrodes ； Appropriate austenitic steel electrodes should be used for the double-layer welding ； The transition layer must take into account the dilution effect of the base material; therefore, austenitic steel electrodes with high chromium and nickel content, as well as good plasticity and crack resistance, should be used. A properly designed groove shape should be used for welding dissimilar steels; this involves reducing the dilution rate of the weld metal, preferring butt joints whenever possible, and avoiding sharp corners and gaps. Increasing the groove angle can increase the amount of welding wire that melts into the weld, thereby reducing the fusion ratio. The groove forms are generally selected according to those specified in GB/T 985; for clad plate materials, the forms outlined in NB/T 47015—2011 “Welding procedures for pressure vessels” are used. Both of these can employ V-shaped grooves, combined V- and U-shaped grooves, or X-shaped grooves. Furthermore, when welding composite steel plates, the base metal on the joint side can be thinned through machining, which ensures that during the welding of the clad layer and the transition layer, the weld metal of the transition layer blends into the base metal, thereby guaranteeing the quality of the clad layer welding and the performance of the final product. Select appropriate welding process parameters. The setting of welding process parameters for dissimilar steels is closely related to the fusion ratio. The required fusion zone for welding dissimilar steels is small; therefore, welding should be carried out with low wire energy, typically using a low current, a high welding speed, and multi-layer multi-pass welding. In selecting the preheating temperature, the principle of using a higher temperature should be adopted. For steels with poor weldability, preheating should be employed to prevent the occurrence of cold cracks. One of the two steels requires preheating while the other does not; preheating measures should be taken during welding. For example, since pearlitic heat-resistant steel requires preheating while low-carbon steel does not, it should be preheated at the preheating temperature appropriate for pearlitic heat-resistant steel. If one type of steel requires a higher preheating temperature while another requires a lower one, welding should be carried out with preheating at the higher temperature. The welding procedure specification must be followed correctly. The welding procedure specification for welding dissimilar steels in pressure vessels serves as a reference document for weldment processing and practical operations; it ensures quality consistency when carried out by skilled welders, and it also acts as a standard for welding technicians to conduct quality inspections on the products. For the specific welds of certain products, it is necessary to use theoretical and practical expertise to develop reasonable and scientific welding procedures. These procedures must be provided to the welders, who are required to follow the parameters specified in them strictly; they must not rely on their habitual working methods in order to ensure the quality of the welds.