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The selection of the welding rod shall be based on the chemical composition, mechanical properties, thickness and joint form of the material to be welded, the characteristics of the welded structure, the state of the force, and the conditions of use of the structure to ensure the weld performance. After comprehensive examinations such as welding construction conditions and technical and economic benefits, the welding rods are selected in a targeted manner, and if necessary, weldability tests are required.
(1) Selection points of welding rods for welding of the same kind of steel
1 Consider the mechanical properties and chemical composition of weld metal
For ordinary structural steel, the strength of the weld metal and the base metal is usually required. The welding rod with the tensile strength equal to or slightly higher than the base metal should be used. For alloy structural steels, it is sometimes required that the alloy composition be the same as or close to the parent metal. In the unfavorable case of high rigidity of the welded structure, high joint stress and easy cracking of the weld, it is considered to select an electrode with lower strength than the base material. When the content of carbon, sulfur, phosphorus and other elements in the base metal is high, cracks are likely to occur in the weld, and an alkaline low-hydrogen electrode with good crack resistance should be selected.
2 consider the use of welding components and working conditions
For weldments subjected to load and impact loads, in addition to meeting the strength requirements, the weld metal should be mainly guaranteed to have high impact toughness and plasticity, and low-hydrogen welding rods with high plastic and toughness indexes can be selected. For weldments in contact with corrosive media, stainless steel electrodes or other corrosion-resistant electrodes shall be selected depending on the nature of the media and the corrosion characteristics. For welding parts working under high temperature, low temperature, wear resistance or other special conditions, the corresponding heat-resistant steel, low-temperature steel, surfacing or other special-purpose welding rods should be used.
3 consider the characteristics of the welded structure and the stress conditions
For thick welded parts with complex structure and large rigidity, due to the large internal stress generated during the welding process, cracks in the weld are easy to occur, and an alkaline low-hydrogen welding rod with good crack resistance should be selected. For weldments with little force and difficult to clean the welded parts, acid electrodes that are not sensitive to rust, scale and oil should be used. For weldments that are subject to conditions that cannot be reversed, electrodes suitable for all-position welding shall be used.
4 consider construction conditions and economic benefits
In the case of meeting the performance requirements of the product, a good-quality acid electrode should be selected. In the case of small or poor ventilation, acid electrodes or low-dust electrodes should be used. For structures with large welding workload, high-efficiency welding rods, such as iron powder electrodes, high-efficiency gravity welding rods, etc., or special welding rods such as bottom-layer welding rods and vertical welding rods should be used as much as possible to improve welding productivity.
(2) Selection points of welding rods for dissimilar steel welding
1 Carbon steel + low alloy steel (or low alloy steel + low alloy high strength steel) with different strength levels
Generally, the strength of the weld metal or joint is not lower than the minimum strength of the two weld metals. The strength of the selected weld metal should ensure that the strength of the weld and the joint is not lower than the strength of the base material. At the same time, the plasticity and impact toughness of the weld metal should not be lower than the performance of the base material with higher strength and poorer plasticity. Therefore, the electrode can be selected from steels having a lower strength level. However, in order to prevent welding cracks, the welding process should be determined according to the steel grade with higher strength grade and poor weldability, including welding specifications, preheating temperature and post-weld heat treatment.
2 low alloy steel + austenitic stainless steel
The electrode should be selected according to the value defined by the chemical composition of the deposited metal. Generally, the Cr25-Ni13 austenitic steel electrode with high content of chromium and nickel and good plasticity and crack resistance is used to avoid brittle hardening. Crack caused by organization. However, the welding process and specifications should be determined according to stainless steel with poor weldability.
3 stainless composite steel plate
Three different performance electrodes should be selected for the welding requirements of the base layer, the composite layer and the transition layer. For the welding of the base layer (carbon steel or low alloy steel), the structural steel electrode of the corresponding strength grade is selected; if the composite layer is directly in contact with the corrosive medium, the austenitic stainless steel electrode of the corresponding composition should be selected. The key is the welding of the transition layer (ie, the interface between the complex layer and the base layer). The dilution effect of the matrix material must be considered. The Cr25-Ni13 austenitic steel electrode with high chromium and nickel content, good plasticity and crack resistance should be selected.
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