316L stainless steel welding performance
316L stainless steel is an ultra-low carbon pure austenitic stainless steel with excellent weldability and low possibility of intergranular corrosion. However, because of its small thermal conductivity and large linear expansion coefficient, the welded joints of this steel type will produce during the cooling process. Large tensile stress results in large welding heat input, and when the cooling rate is slow, it is easy to form thermal cracks, corrosion cracking and deformation.
316L stainless steel can be welded by all standard welding methods. Depending on the purpose, 316Cb, 316L or 309Cb stainless steel filler rods or welding rods can be used for welding. Among the commonly used welding methods, the heat input of MIG and TIG welding is relatively large. Small, and in addition to protecting high-temperature metals, the argon gas flow also has a certain degree of cooling effect, increasing the crack resistance of the weld, thereby reducing welding deformation.
When using 316L stainless steel, post-weld annealing treatment is generally not required, while austenitic stainless steel generally does not require stress relief annealing heat treatment after welding. The reason is that austenitic stainless steel has very good plasticity and toughness, and does not need to undergo post-weld stress relief annealing heat treatment to restore its properties; secondly, the temperature range of 450~850°C is the sensitization temperature of austenitic stainless steel, and austenitic stainless steel remains in the Heating in this area will reduce its corrosion resistance. If there are ferrite components in the weld, brittleness at 475°C may occur. The post-weld stress relief annealing heat treatment is just within this temperature range (except solution treatment and stabilization treatment).
However, sometimes under special circumstances, 316L stainless steel must be subjected to post-weld stress relief annealing heat treatment. One is to stabilize the geometry of equipment parts and eliminate welding residual stress; the other is when the equipment is working in areas prone to stress corrosion. environment, it is also necessary to eliminate tensile residual stress.
304 welding performance
Austenitic stainless steel is represented by 18%Cr-8%Ni stainless steel, which is often referred to as 304 stainless steel. In principle, pre-welding preheating and post-welding heat treatment are not required during welding processing. Usually have good welding properties. However, the content of nickel and molybdenum is high, so high-temperature cracks are prone to occur during welding. In addition, mutual embrittlement (Fe-Cr intermetallic compounds) will occur. The ferrite generated under the action of ferrite-generating elements causes low-temperature embrittlement, as well as defects such as reduced corrosion resistance and stress corrosion cracking.
After welding, the mechanical properties of 304 stainless steel welded joints are good, but when there are chromium carbides on the grain boundaries in the heat-affected zone, it is very easy to form a chromium-depleted layer, and the chromium-depleted layer will cause the product to Intergranular corrosion is prone to occur. In order to avoid problems, it is best to use low-carbon (C≤0.03%) grades or grades with titanium and niobium added. In order to prevent high-temperature cracks in weld metals, it is generally believed that controlling delta ferrite in austenite must be effective. It is generally best to contain more than 5% ferrite at room temperature. Stainless steel whose main purpose is corrosion resistance should use low-carbon and stable steel types, and should undergo appropriate post-weld heat treatment; while steel whose main purpose is structural strength should not undergo post-weld heat treatment to prevent deformation and precipitation. Carbides and carbides embrittle each other.