Unlike other welded structures, pressure vessels are a special all-welded structure that bears the same physical and chemical effects of various loads, temperatures and working media as the container casing. The weld metal is required not only to have a static load strength substantially equal to that of the casing material, but also to have sufficient ductility and toughness to prevent the welded joint of the pressed part during processing and during operation due to various stresses. In combination with temperature, it fails prematurely or produces brittle fracture. In addition, in some applications, weld metal is also required to resist corrosion from working media. Therefore, the performance requirements for the weld metal of the pressure vessel should follow the principles of equal strength, isoplasticity, toughness and corrosion resistance.
1. Equal strength principle: The equal strength of the welded joint of the pressure vessel should be understood as the strength performance of the pressure vessel not lower than the lower limit value specified by the parent metal standard. Strength properties include normal temperature and high temperature short-term strength. In fact, the absolute equivalent of the strength value of the welded joint to the corresponding strength value of the base metal is not possible, and it is not necessary. In addition, due to the difference in metallographic structure between the base metal and the weld metal, the yield ratio is not the same, and it is difficult to make the tensile strength and yield strength of the weld metal reach the specified values of the parent metal standard at the same time. For high temperature pressure vessels, the strength index of the welded joint should be the high temperature short-term tensile strength at the highest working temperature, and it is not necessary to simultaneously achieve the specified index of normal temperature strength.
2. The principle of isoplasticity and equal toughness: plasticity and iso-toughness of welded joints of pressure vessels means that the ductility and toughness are not lower than the lower limit of the plasticity and toughness index specified by the parent metal standard, or not lower than the technical conditions of the container manufacturing. Specified value. According to the current relevant standards, the impact toughness of pressure vessel welded joints or weld metal is usually measured by Charpy V-notch impact test and characterized by impact absorption work (J). When inspecting the solder material and the product to witness the test panel, the notch of the impact specimen is opened at the center of the weld metal. In the weldability test, the welding process test and the welding process qualification test of the new steel grade, the notch of the impact test piece is respectively opened in the weld metal, the fusion zone and the heat affected zone. The notched impact toughness of each zone shall not be lower than the lower limit specified in the standard.
3. Corrosion resistance principle: Corrosion resistance of welded joints of pressure vessels should be understood as corrosion resistance, hydrogen resistance and oxidation resistance are not lower than the specified values of the parent metal standard or the corresponding technical requirements of the product manufacturing technical conditions. . In order to meet this requirement, the alloy composition of the weld metal is not lower than the standard value of the base metal. Considering that the welding heat process may have an adverse effect on the corrosion resistance of the joint, the main alloy composition should be selected to be slightly higher than the base metal. The carbon content is lower than that of the base material.