Automatic Submerged Arc Welding Process of SA-240 321 Austenitic Stainless Steel
Duplex stainless steel is stainless steel in which ferrite and austenite each account for about 50%. For duplex stainless steel, the equivalent index of pitting corrosion resistance PRE (PER=%Cr+3.3%Mo+16%N) is a key index, The larger the PRE, the stronger the pitting corrosion resistance of duplex stainless steel. Internationally, duplex stainless steel is generally divided according to the size of PRE.
PRE≤30 is called low nickel molybdenum duplex stainless steel, the typical representative grade is S32304; PRE is 30-39 is called duplex stainless steel, the typical representative grade is S31803, S32205; PRE≥40 is called super duplex stainless steel; typical representative The grade is S32570.
In the past ten years, foreign countries have not only paid attention to the development of super duplex steel but also paid special attention to the expansion of low-nickel-molybdenum duplex stainless steel grades. The main reason is that the raw material cost is low, but the pitting corrosion resistance is moderate. This type of steel currently has There are as many as 10 species, but our country has only 2 species so far. The key to the research and development of such steel grades is:
1. Low nickel molybdenum duplex stainless steel is essentially a branch of high manganese and high nitrogen stainless steel. The key to smelting is to not only control the matching of Cr, Mo, and N content to make PRE=26~28, but also control the matching of Ni, Mn, and N content to ensure the two-phase balance in the supply state. The N content is very important, and the higher Cr, Mn content, and lower Ni content are beneficial to increase the interstitial solid solution content of the N element. Smelting is not difficult.
2. The solution treatment is another important link to control the two-phase equilibrium, which should be controlled reasonably.
3. For low-nickel-molybdenum duplex stainless steels with low Ni content and high Cr content, the upper limit of the hot forging temperature may be lowered. Special attention should be paid when seamless steel pipes are pierced.
4. The strength of low-nickel-molybdenum duplex stainless steel is very high. It is mainly used for small and medium-sized thin-walled pipes. GTAW or PAW without filler metal is the most suitable welding method. In order to ensure that the weld metal has a dual-phase structure, 2% N2 should be added to the shielding gas. In addition, it is the best choice to use double-layer protection to extend the life of the electrode. If you want to add filler metal, you must go through experiments. There are successful precedents for S32003, S32304, and S81921 steels.