Principles for selecting welding materials

Thread Content

Principles for selecting welding materials. Welding materials: The design drawings of pressure equipment specify the requirements for the performance of welded joints; generally, it is required that the performance of these welded joints be no lower than that of the base material, or that they meet the technical requirements specified in the drawings. A welded joint includes the weld zone, fusion zone, and heat-affected zone. The weld zone is composed of the metal deposited by the welding material and the melted base metal. The properties of welding materials refer to those of the weld metal produced by the welding material (excluding the base metal), and they do not indicate the properties of the weld itself. Between the weld metal and the heat-affected zone, apart from carbon migration, no welding material can act on the heat-affected zone during welding to alter its properties. When selecting welding materials, the metallic properties of the weld are the main considerations; to ensure the performance of the welded joint, welding procedures (especially post-weld heat treatment and wire energy) must be employed in conjunction. Basic principles: The basic principle for selecting welding materials is that the properties of the weld metal should be higher than or equal to the lower limit specified in the standards for the corresponding base material, or meet the technical requirements outlined in the drawings. The welding material should be selected based on a comprehensive consideration of the chemical composition, mechanical properties, and weldability of the base metal, as well as the structural characteristics, operating conditions, and welding methods of the pressure-bearing equipment; when necessary, it should be determined through testing. Weld metal formed by welding steel of the same grade: ① The mechanical properties of the weld metal in carbon steel and low-alloy steel must be ensured, and its tensile strength shall not exceed the upper limit specified in the base material standards plus 30 MPa. The weld metal of heat-resistant low-alloy steel must also ensure the chemical composition. ②The weld metal of high-alloy steel shall ensure mechanical properties and corrosion resistance. ③The weld metal of the stainless steel composite steel base layer shall meet the required mechanical properties, and its tensile strength shall not exceed the upper limit specified in the base material standards plus 30 MPa; the weld metal of the cladding layer shall possess corrosion resistance, and mechanical properties shall also be ensured when such requirements exist. Transition welds should be used at the junctions of the clad weld and the base weld, as well as between the clad weld and the base steel plate. Weld metal formed by welding different steel grades: The mechanical properties of the weld metal resulting from the welding of carbon steels and low-alloy steels with different strength grades must be ensured, and its tensile strength shall not exceed the upper limit specified in the standards for the parent material with higher strength. The weld metal between austenitic high-alloy steel and carbon steel or low-alloy steel shall ensure crack resistance and mechanical properties. Welding materials with a higher chromium and nickel content than that of the austenitic alloy steel base material should be used. There are two issues: the strength matching between the weld metal and the base material. For pressure-bearing equipment, the mechanical properties of the welded joint are essential characteristics. For carbon steel and low-alloy steel, the matching of weld metal strength with base material strength is a focal point in both the pressure-bearing equipment industry and the welding industry, and there is much research and debate on this topic. The mechanical property matching between the weld metal and the base material should take into account strength matching, plasticity matching, and toughness matching as a whole. For structural steel, welding materials are selected according to the \"equal strength\" principle, which ensures that the welded joints have sufficient toughness; meanwhile, a moderate degree of \"super-strength\" does indeed help improve the resistance of these joints to brittle fracture. Using high-strength steel with a strength level of 700 MPa to 800 MPa as the base material and welding rods of different strength levels, fall-weight tests and deep-notch wide-plate tensile tests showed that either excessively high or excessively low strength in the weld metal can easily lead to brittle fracture; joints with nearly equal strengths are the most ideal. Low-strength matching of welds can also reduce the preheating temperature from a process perspective, thereby decreasing susceptibility to cold cracks. Welding materials are usually selected based on the nominal guaranteed value of the deposited metal, yet the actual strength of this deposited metal often exceeds that nominal value by a significant margin. If metallurgical factors or the effect of the fusion ratio are also taken into account, the actual strength of the weld metal will be even higher than the nominal guaranteed value of the deposited metal in the welding material. The desire is a “low-strength” match, while reality might be “equal strength”” ； The desire is for \"moderate strength\", while reality might be \"superior strength\"; therefore, the matching issue should be analyzed based on the actual strength level of the weld metal. For dissimilar weld metals, medium and high-alloy heat-resistant steels, martensitic stainless steels, and ferritic stainless steels are used; their weldability is poor. To prevent the formation of cold cracks, a high preheating temperature is required, interpass temperatures must be strictly controlled, and heat treatment is necessary after welding. The requirements for welding process conditions are stringent. In actual welding operations, to avoid preheating and postheating, austenitic steel welding materials are also used to enhance the plasticity of the welded joint and prevent the formation of welding cracks. At this point, since the weld metal differs significantly from the base material in terms of chemical composition, microstructure, physical properties, and mechanical properties, welding residual stresses are inevitable, and these stresses may have an adverse effect on the performance of the welded joint.