GB/ T 12770 DIN 17455 Stainless Steel Welded Tube 30 Inch SS ERW Pipe

Role of alloying elements in duplex stainless steel

The effects of some of the most important alloying elements on mechanical, physical and corrosion properties of duplex stainless steels are briefly introduced.

Cr:

The chromium content of steel must not be less than 10.5% to form a stable chromium-containing passivation film to protect steel from atmospheric corrosion. The corrosion resistance of stainless steel increases with increasing chromium content. Chromium is a ferrite forming element, adding chromium to steel can promote the formation of body - centered cubic ferrite. When the chromium content is higher in steel, more nickel is needed to form austenite or biphase (ferrite - austenite) structure. Higher chromium content also promotes the formation of intermetallic phase. The chromium content of austenitic stainless steel should be at least 16%, and that of duplex stainless steel should be at least 20%. Chromium also increases the oxidation resistance of steel at high temperatures. This role of chromium is important because it affects the formation and removal of oxide or tempering colors after heat treatment or welding. Pickling and tempering removal of duplex stainless steel are more difficult than austenitic stainless steel.

Mo:

The synergistic effect of Mo and Cr can improve the pitting resistance of stainless steel. Molybdenum is three times as resistant to pitting and crevice corrosion as chromium in a chloride-containing environment when stainless steel contains at least 18% chromium. Molybdenum is a ferrite forming element and also increases the tendency of stainless steel to form intermetallic phases. Therefore, the molybdenum content of austenitic stainless steel is usually less than about 7.5%, and that of duplex stainless steel is less than 4%.

N:

Nitrogen improves the pitting and crevice corrosion resistance of austenitic and duplex stainless steels, and also significantly improves the strength of steels. In fact, it is the most effective solution strengthening element and low-cost alloying element. The improvement of toughness of nitrogen-containing duplex stainless steel is attributed to its higher austenite content and lower intermetallic phase content. Nitrogen did not prevent the precipitation of intermetallic phase, but delayed the formation of intermetallic phase, allowing enough time for the processing and manufacturing of duplex stainless steel. Nitrogen is added to highly corrosion-resistant austenitic and duplex stainless steels with high chromium and molybdenum content to counteract their tendency to form σ phases.

Nitrogen is a strong austenitic forming element and can partially replace nickel in austenitic stainless steel. Nitrogen can decrease the stacking fault energy and increase the work hardening rate of austenite.