316L stainless steel

Comparison of weldability between 316L and 304 stainless steel
316L stainless steel welding performance

316L stainless steel belongs to ultra-low carbon pure austenitic stainless steel, which has excellent weldability and a low possibility of intergranular corrosion. Larger tensile stress, larger welding heat input, and slower cooling rate are easy to form thermal cracks, corrosion cracking, and deformation.

316L stainless steel can be welded by all standard welding methods. Depending on the application, 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. Small, and the argon flow not only protects the high-temperature metal but also has a certain degree of cooling effect, which increases the crack resistance of the weld, thereby reducing welding deformation.

The use of 316L stainless steel generally does not require post-weld annealing treatment, and austenitic stainless steel generally does not require stress-relief annealing heat treatment after welding. The reason is that the plasticity and toughness of austenitic stainless steel are very good, and it is not necessary to restore its performance through post-welding stress relief annealing heat treatment; secondly, the temperature range of 450~850°C is the sensitization temperature of austenitic stainless steel, and austenitic stainless steel is used for a long time. Heating in this area will reduce its corrosion resistance. If there is ferrite in the weld, it may also cause brittleness at 475°C. The post-weld stress relief annealing heat treatment is just in this temperature range (except solution treatment and stabilization treatment).

However, sometimes in special circumstances, post-weld stress relief annealing heat treatment is also required for 316L stainless steel. One is to eliminate welding residual stress in order to stabilize the geometry of equipment parts; 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-weld preheating and post-weld heat treatment are not required during welding. Usually have good welding performance. 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) will occur, and the ferrite formed under the action of ferrite-forming elements will cause 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-depleted layer, and the chromium-deficient layer will cause the product to be used. prone to intergranular corrosion. 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 cracking of weld metal, it is generally believed that the control of delta ferrite in 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 grades should be selected, and appropriate post-weld heat treatment should be carried out; steel with structural strength as the main purpose should not be subjected to post-weld heat treatment to prevent deformation and corrosion caused by precipitation. Carbide and mutual embrittlement occur.

Corrosion resistance comparison between 316L and 304 stainless steel
316L corrosion resistance

As a molybdenum-containing stainless steel, 316L stainless steel has better corrosion resistance than 304 stainless steel, and the production equipment for pulp 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 a low C series of 316 steel, 316L has the same characteristics as 316 steel, and its resistance to intergranular corrosion is excellent. It is a product with special requirements for grain boundary corrosion resistance in the application of 316 steel.

304 corrosion resistance

304 stainless steel is a high-alloy steel that can resist corrosion in the air or in 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 for nitric acid below the boiling temperature with a concentration of ≤65%. It also has good corrosion resistance to alkaline solutions and most organic and inorganic acids.

There are several main reasons for the rust of 304 stainless steel. One is the presence of chloride ions in the use environment; the other is that the stainless steel has not undergone solid solution treatment, and the alloying elements have not dissolved into the matrix, resulting in low alloy content in the basic structure and poor corrosion resistance; It is this titanium- and niobium-free material that has an inherent 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 hot 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 to prevent pitting corrosion of steel in chloride solutions. Adding Mo can improve the ability to resist reducing acid and pitting corrosion, and reducing the carbon content can improve the ability to resist intergranular corrosion and improve 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 environments with relatively high Cl- content, usually Is the use of 316L.

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