1. Weldability comparison between 316L and 304 stainless steel
① 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 will be produced during the cooling process. The larger the tensile stress, the larger the welding heat input, and the slower the cooling rate, the more likely to form thermal cracks, corrosion cracking and deformation.
316L stainless steel can be welded by all standard welding methods. According to different applications, 316Cb, 316L or 309Cb stainless steel filler rods or electrodes can be used for welding. Among the commonly used welding methods, the heat input of MIG and TIG welding is relatively high. In addition to protecting the high-temperature metal, the argon flow also has a certain degree of cooling effect, which increases the crack resistance of the weld, thereby reducing the welding deformation.
316L stainless steel is generally used without post-weld annealing treatment, and austenitic stainless steel is generally not subjected to stress-relief annealing treatment after welding. The reason is that the plasticity and toughness of austenitic stainless steel are very good, and its performance does not need to be restored by post-weld stress relief annealing heat treatment; secondly, the temperature range of 450~850℃ is the sensitization temperature of austenitic stainless steel, and austenitic stainless steel has a long-term Heating in this area will reduce its corrosion resistance. If there is ferrite in the weld, brittleness at 475°C may also occur. The post-weld stress relief annealing heat treatment is just in this temperature region (except solution treatment and stabilization treatment).
However, in some special cases, post-weld stress relief annealing heat treatment is also required for 316L stainless steel. One is to stabilize the geometry of the equipment parts and the residual stress of welding needs to be eliminated; environment, the tensile residual stress also needs to be eliminated.
②304 stainless steel 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, there is no need for pre-weld preheating and post-weld heat treatment during welding. 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 compound) occurs, and ferrite generated under the action of ferrite-forming elements causes low-temperature embrittlement, as well as defects such as decreased 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-poor layer, and the chromium-poor layer will lead to the use of the product. 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. To prevent high temperature cracking of the weld metal, it is generally believed that the control of delta ferrite in the austenite must be effective. Generally, it is best to contain more than 5% ferrite at room temperature. The main purpose is corrosion-resistant stainless steel, low carbon and stable steel should be selected, and appropriate post-weld heat treatment should be carried out; while the steel with structural strength as the main purpose should not be subjected to post-weld heat treatment to prevent deformation and precipitation due to precipitation. Carbides and mutual embrittlement occur.
2. Comparison of corrosion resistance between 316L and 304 stainless steel
① 316L stainless steel corrosion resistance
As a molybdenum-containing stainless steel, 316L stainless steel has better corrosion resistance than 304 stainless steel, and the production equipment for pulping and papermaking has excellent corrosion resistance. And 316 stainless steel is also resistant to erosion by marine and aggressive industrial atmospheres. High heat resistance, 316L stainless steel has good oxidation resistance in intermittent use below 1600 degrees and continuous use below 1700 degrees. In the range of 800-1575 degrees, it is best not to continuously operate 316L stainless steel products, but when 316 stainless steel is used continuously outside this temperature range, it has good heat resistance.
The carbide precipitation resistance of 316L stainless steel is better than that of 316 stainless steel, and the above temperature range can be used. As the low C series of 316 steel, 316L has the same characteristics as 316 steel, and its resistance to grain boundary corrosion is excellent. It is a product with special requirements for resistance to grain boundary corrosion in the use of 316 steel.
② 304 stainless steel corrosion resistance
304 stainless steel is a high-alloy steel that can resist corrosion in air or chemically corrosive media. 304 stainless steel has excellent corrosion resistance and intergranular corrosion resistance. For oxidizing acids, the experimental results are: 304 stainless steel has strong corrosion resistance in nitric acid with a concentration of ≤65% below the boiling temperature. It also has good corrosion resistance to alkaline solutions and most organic and inorganic acids.
The main reasons for the rusting of 304 stainless steel are: one is the presence of chloride ions in the use environment; the other is that the stainless steel has not undergone solid solution treatment. , the alloying elements are not dissolved into the matrix, resulting in a low content of basic structure alloy and poor corrosion resistance; the third is that this material without titanium and niobium has a natural tendency to intergranular corrosion. The addition of titanium and niobium, together with stabilization treatment, can reduce intergranular corrosion.
In addition, the main difference between 316L and 304 stainless steel in chemical composition is that 316L stainless steel contains molybdenum. Adding alloying element molybdenum to austenitic stainless steel can enhance the thermal strength and creep strength of stainless steel. Improve its resistance to pitting and intergranular corrosion.
Molybdenum can passivate the surface of stainless steel in reducing and strong oxidizing salt solutions, and can improve corrosion resistance and prevent pitting corrosion of steel in chloride solutions. Adding Mo can improve the resistance to reducing acid and pitting corrosion, and reducing the content of carbon can improve the resistance to intergranular corrosion and improve the welding performance.
Adding molybdenum element can better prevent pitting corrosion, 304 belongs to low carbon stainless steel and 316L belongs to ultra low carbon stainless steel. The lower carbon content can reduce the occurrence of intergranular corrosion, but both 304 and 316L are more sensitive to Cl particles, and the ability of 304 to resist cl- is weaker than that of 306L, so in the environment with relatively high CL- content, usually 316L is used.