In 1968, the invention of argon oxygen decarburization (AOD), a stainless steel refining process, made a series of new stainless steel possible. One of the progress brought by AOD process is the addition of alloy element n. The toughness and corrosion resistance of the HAZ are close to that of the base metal, and the formation rate of harmful intermetallic phase can be reduced by adding N element to the duplex stainless steel.
Like austenitic stainless steel, duplex stainless steel is a kind of steel in order of corrosion performance, which depends on their alloy composition. Duplex stainless steel has been developing continuously. Modern duplex stainless steel can be pided into four types:
1. Low grade duplex stainless steel 2304 without Mo;
2. Standard duplex stainless steel 2205, accounting for more than 80% of the total duplex steel;
3. 25% Cr duplex stainless steel, typical of alloy 255, can be classified as super duplex stainless steel;
4. Super duplex stainless steel, containing 25-26% Cr, has more Mo and N than 255 alloy. Typical steel grade 2507.
The alloy elements in duplex stainless steel are mainly Cr, Mo, N and Ni. Their functions in duplex steel are as follows:
Cr
At least 10.5% Cr in the steel can form a stable passivation film to protect the steel from atmospheric corrosion. The corrosion resistance of stainless steel increases with the increase of Cr content. Cr is a ferrite element, which can stabilize the iron structure with BCC lattice and improve the oxidation resistance of steel at high temperature.
Mo
The synergistic effect of Mo and Cr can improve the chloride corrosion resistance of stainless steel. Mo is three times more resistant to pitting and crevice corrosion than Cr in chloride environment (see CPT formula). Mo is a ferrite forming element, which can also promote the formation of intermetallic phase. Therefore, the Mo content in austenitic stainless steel is less than 7.5% and that in duplex steel is less than 4%.
N
N element can increase the resistance to pitting and crevice corrosion of austenite and duplex stainless steel, and can significantly improve the strength of steel. It is the most effective element for solution strengthening. At the same time of improving the steel strength, N element can also increase the toughness of austenitic stainless steel and duplex stainless steel, delay the formation of intermetallic phase, make duplex stainless steel have enough time for processing and manufacturing, and also offset the inclination of σ phase which is easy to form due to high Cr and mo,
N is a strong austenitic element, which can partially replace Ni in austenitic stainless steel. In general, N and Ni, which are close to the solubility limit, are added to the duplex stainless steel to adjust the phase equilibrium. It is necessary to reach a balance between the ferrite elements Cr and Ni and the austenite forming elements Ni and N in order to obtain the desired duplex structure.
Ni
Ni is an element that stabilizes austenite structure. The addition of Ni to Fe based alloy can promote the transformation of stainless steel from Bcc (ferrite) to FCC (austenite).
Ni can delay the formation of intermetallic phase, but the effect is far less effective than n.
Here are two kinds of duplex stainless steel to help understand its performance.
Post time: May-10-2022