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The four major factors affecting weldability are materials, design, process and service environment. Material factors include the chemical composition of steel, smelting and rolling state, heat treatment, organizational state and mechanical properties. The design factor refers to the safety of the welded structure design. It is not only affected by the material, but also by the structural form to a large extent. Process factors include the welding method used during construction, welding process specifications (such as welding heat input, welding materials, preheating, welding sequence, etc.) and post-weld heat treatment. Service environmental factors refer to the working temperature, load conditions (dynamic load, static load, impact, etc.) and working environment (chemical industry area, coastal area and corrosive medium, etc.) of the welded structure.
1. Material factors
Material factors include the base metal itself and the welding materials used, such as the welding rod for arc welding, the welding wire and flux for submerged arc, the welding wire and shielding gas for gas shielded welding, etc. The base metal and welding material directly participate in the metallurgical reaction in the molten pool or the fusion zone during the welding process, which has an important influence on the weldability and welding quality. Improper selection of base metal or welding materials will result in unqualified weld composition, deterioration of mechanical properties and other service properties, and even welding defects such as cracks, pores, and slag inclusions, which is to make the process weldability worse. Therefore, the correct selection of base metal and welding material is an important factor to ensure good connection.
2. Design factors
The structural design of the welded joint will affect the stress state, which will affect the weldability. When designing the structure, the stress at the joints should be kept in a small state and can shrink freely, which is beneficial to reduce stress concentration and prevent welding cracks, gaps at the joints, cut and sudden changes, excessive pile height, cross welds, etc. It is easy to cause stress concentration and should be avoided as much as possible. Unnecessarily increasing the thickness of the base metal or the volume of the weld will cause multi-directional stress and should be avoided.
3. Process factors
For the same kind of base metal, using different welding methods and technological measures, the weldability exhibited is very different. For example, aluminum and its alloys are difficult to weld with gas welding, but oxygen arc welding can achieve good results; titanium alloys are extremely sensitive to oxygen, nitrogen, and hydrogen, and it is impossible to weld well with gas welding and electrode arc welding, but argon Arc welding or electron beam welding is easier to weld. Therefore, the development of new welding methods and new process measures is an important way to improve process weldability.
The influence of the welding method on the weldability is firstly manifested in the energy density, temperature and heat input of the welding heat source, and secondly manifested in the way to protect the molten pool and the area near the joint, such as slag protection, gas protection, slag-gas joint protection and in vacuum Wait. For high-strength steels with overheating sensitivity, to prevent overheating, gap gas shielded welding, pulse electric low welding, plasma arc welding, etc. can be selected, which is beneficial to improve its weldability.
Process measures play an important role in preventing welding defects and improving the performance of joints. The most common process measures are preheating before welding, slow cooling and post-weld heat treatment. These process measures are effective measures to prevent hardening and brittleness of the heat-affected zone, reduce welding stress, and avoid hydrogen-induced cooling cracks. Reasonable arrangement of the welding sequence can also reduce stress and deformation. In principle, the welded workpiece should be kept in a state of free expansion and contraction as far as possible during the entire welding process. Post-weld heat treatment can eliminate residual stress, and can also allow hydrogen to escape to prevent delayed cracks.
4. Service environment
The service environment of the welded structure is diverse, such as the working temperature, the type of working medium, and the nature of the load are all conditions of use. When the working temperature is high, creep may occur; when the working temperature is low or the load is an impact load, brittle failure is prone to occur; when the working medium is corrosive, the joint is required to have corrosion resistance. The more unfavorable the conditions of use, the less guaranteed the weldability.
Weldability is closely related to factors such as materials, design, process, and service environment. It is impossible for people to deviate from these factors and simply think that the weldability of a certain material is good or not, nor can it be summarized by a certain index. The weldability of the material. In order to analyze and solve the problem of weldability, it is necessary to correctly select the base metal, welding method and welding material according to the requirements of the welding structure's use conditions, and take appropriate process measures to avoid various welding defects.
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Basic knowledge of welding materials (2021-11-01)
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