Factors affecting welding quality of duplex stainless steel
Duplex stainless steels are a family of grades combining good corrosion resistance with high strength and ease of fabrication.Their physical properties are between those of the austenitic and ferritic stainless steels but tend to be closer to those of the ferritics and to carbon steel.The chloride pitting and crevice corrosion resistance of the duplex stainless steels is a function od chromium,molybdenum,and nireogen content.The factors affecting welding quality of duplex stainless steel are:
Effect of N content
The impact toughness of the joint can be improved with the increase of N content in the weld metal of duplex stainless steel, which is due to the increase of Y phase content in the weld metal, the reduction of Cr2N precipitation and the reduction of hydrogen embrittlement tendency.
S32750 is added with high N (0.24-0.32%). During welding, the weld metal shall also contain corresponding N content to match its metal properties with the base metal. Therefore, the welding material with corresponding N content shall be selected.
Influence of welding heat input
When s32570 duplex stainless steel is welded, different from the weld area, the w (n) of HAZ will not change during welding, which is the w (n) of base metal. Therefore, the main factor affecting the microstructure and properties is the heat input during welding. Maintaining the two-phase ratio of duplex stainless steel welded joints (weld metal and welded HAZ), especially maintaining the necessary amount of austenite in welded HAZ, is particularly important to ensure the performance of welded joints. The welding line energy is too small, the cooling speed of weld metal and heat affected zone is too fast, and austenite is too late to precipitate, resulting in the increase of ferrite phase content in the structure, resulting in hardened structure and cracks, which is also unfavorable to the impact toughness of HAZ; If the heat input is too large during welding, the range of weld heat affected zone will increase. Although sufficient austenite can be formed in the metallographic structure, it will also cause the grain growth of hza and σ The precipitation of equal harmful phases, coarse and disordered metallographic grains, resulting in embrittlement, which is mainly manifested in the decrease of plastic index of welded joints. In addition, pitting corrosion resistance is an important characteristic of duplex stainless steel, which is closely related to its chemical composition and microstructure.
Although the N content of S32750 is higher than that of ordinary duplex stainless steel and the sensitivity of pitting potential is small, it is still necessary to control the welding cooling rate to ensure its corrosion resistance. Therefore, in the welding process, controlling the welding linear energy and ensuring a reasonable cooling rate range is still the key to obtain high-quality duplex stainless steel joints.
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