Stainless steel Tube welding process and precautions after welding
When welding stainless steel tubes, it is mainly because of its strong dendrite direction, large linear expansion coefficient, large shrinkage stress during welding and cooling, hot cracking easily, and large deformation tendency. Measures to prevent hot cracking of stainless steel tubes in production include: welding austenitic stainless steel tubes with electrodes whose weld metal is austenite-ferrite duplex structure; using low-hydrogen electrodes to promote the refinement of weld metal crystals and reduce Harmful impurities in the small welds can improve the crack resistance of the welds; use the fastest welding speed possible, wait for the stainless steel tubes weld layer to cool before welding the next one to reduce the overheating of the weld; when the stainless steel tubes welding ends or is interrupted, the The arc should be slow to fill the crater to prevent crater cracks; use a smaller welding current.
When the stainless steel tubes butt weld and heat-affected zone are kept at a temperature of 450-850℃ for a certain period of time, Cr carbides may be precipitated at the grain boundaries and intergranular corrosion tends to occur. In the process of welding stainless steel tubes, when the local area of the base metal and weld metal stays in this dangerous temperature range, it will cause intergranular corrosion to the welded joint. Sometimes heat treatment of stainless steel pipe after welding can also cause intergranular corrosion. Measures to avoid grain boundary corrosion in production include: adopt the fastest possible welding speed; do not swing the electrode horizontally; when there are multiple passes, wait until the previous pass has cooled to about 60°C before welding the next pass; if it is in contact with corrosive media The weld is finally welded; minimize the time that the stainless steel tubes welded joint stays in the dangerous temperature range.
Ferritic stainless steel tubes will not undergo phase change during heating and cooling, and will not produce quench hardening. The part heated to above 950℃ (weld seam and heat-affected zone) has a serious tendency to grow grains, and post-weld heat treatment cannot be used to refine the coarse grains of stainless steel pipes, reduce joint toughness, and increase cold cracking tendency. If the weld and heat-affected zone stay in the temperature range of 400～600℃, brittleness at 475℃ is likely to occur. Staying in the temperature range of 650 to 850°C will easily cause precipitation and embrittlement of the σ phase. When welding stainless steel pipes, attention should be paid to the heating and cooling rates in the above two temperature ranges. Air cooling after heating above 600℃ for a short time can eliminate embrittlement at 475℃; heating to 930～980℃ for rapid cooling can eliminate precipitation embrittlement of δ phase. The welded stainless steel pipe can be preheated to prevent cracks.
