What is duplex stainless steel?
What is duplex stainless steel?
Duplex stainless steel is a kind of stainless steel with ferrite phase and austenite phase coexisting. At the same time, it is also a kind of steel with excellent corrosion resistance, high strength, easy to manufacture and many other excellent properties.
Duplex stainless steel has a history of more than 60 years. The first batch of duplex stainless steel in the world was produced in Sweden in 1930 and used in sulfite paper industry.
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 divided 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.
2205 standard duplex stainless steel
2205 is a duplex stainless steel with n addition. The addition of N improved the corrosion resistance of 2205, especially the welding condition. The early duplex stainless steel can resist medium strength uniform corrosion and chloride stress corrosion cracking, but its performance will be greatly reduced when it is used under welding conditions. In order to improve this situation, 2205 duplex stainless steel is added to the N element, which not only improves the corrosion resistance, but also improves the welding performance. The N content of 2205 dual phase steel is required to be 0.08-0.2% in ASTM standard, and the content of Cr, Mo and Ni is also required. Therefore, the pitting equivalent prEN value of 2205 dual phase steel reaches 35.8, further improving the corrosion resistance.
With proper heat treatment, 22% Cr, 3.5% Ni, 3% Mo and 0.16% N in 2205 will produce microstructure including the phase equilibrium of austenite and ferrite. This kind of structure and chemical composition make 2205 stainless steel have better and more extensive corrosion resistance than 316 and 317 stainless steel, and the yield strength is twice higher than that of ordinary austenitic stainless steel.
2205 is the most widely used duplex stainless steel material. All 2205 stainless steel before leaving the factory shall be subject to metallographic inspection to prevent σ phase during processing. The most common application form of 2205 is welded pipe and pipe fittings. Under the condition of strong uniform corrosion and stress corrosion, plate is also widely used.
Chemical composition and product standard of 2205
The chemical composition of 2205 dual phase steel is shown in the table below.
Chemical composition
|
Element |
Typical components |
ASTM standard |
|
C |
0.02 |
0.030max |
|
Mn |
0.07 |
2.0 max |
|
P |
0.025 |
0.030 max |
|
S |
0.001 |
0.020 max |
|
Si |
0.40 |
1.0 max |
|
Cr |
22.4 |
21.0-23.0 |
|
Ni |
5.8 |
4.5-6.5 |
|
Mo |
3.3 |
2.5-3.5 |
|
N |
0.16 |
0.08-0.20 |
|
Fe |
Allowance |
Allowance |
2205 ASTM and ASME standards for materials are shown in the following table
|
Product form |
ASTM standard |
ASME standard |
|
A276 |
— |
|
|
|
A479 |
|
|
A240 |
SA240 |
|
|
Pipe (welded and seamless) |
A790 |
SA790 |
|
Pipe fittings (welded and seamless) |
A789 |
SA789 |
Corrosion resistance of 2205
Chloride stress corrosion cracking
Ferritic steel without nickel has innate immunity to chloride stress corrosion cracking, even in the harsh 42% MgCl2 solution. On the other hand, austenitic stainless steel with nickel is easily affected by chloride stress corrosion cracking. The resistance of austenitic and ferritic stainless steels to chloride stress corrosion cracking depends on the content of nickel in the alloy.
In a sense, dual phase alloy is the synthesis of austenite and ferrite phases, but the composition in dual phase alloy tends to a certain phase. For example, the content of Ni in ferrite phase is much less than that in austenite phase, so the resistance of biphase alloy to chloride stress corrosion cracking is much better than that of traditional 300 series stainless steel.
The following table shows the corrosion test results of 304 and 2205 in several boiling solutions.
|
|
Stress corrosion fracture test |
||
|
Alloy |
Boiling 42% MgCl2 |
Boiling 33% LiCl |
Boiling 26% NaCl |
|
304L(8%Ni) |
Fail(20h) |
Fail(96h) |
Fail(850h) |
|
439 (ferritic type) |
Adopt(2000h) |
Adopt(2000h) |
Adopt(1000h) |
|
2205 |
Fail(89h) |
Adopt(1000h) |
Adopt(1000h) |
|
2205(weld) |
Fail(89h) |
Adopt(1000h) |
Adopt(1000h) |
Corrosion and crevice corrosion
For the evaluation of pitting corrosion and crevice corrosion of chloride ions, ASTM standard g-48 test method (10% fecl3-6h2o) can be used, and the temperature can be gradually increased until crevice corrosion is found. It is found that the temperature at which crevice corrosion occurs is called critical crevice corrosion temperature, which can be used to measure the ability of materials to resist crevice corrosion. However, it is unnecessary to indicate the limited use temperature of alloy in chlorinated solution.
The following table is the comparison of crevice corrosion temperature test between annealed 2205 steel plate and 316L, 317L and other alloys.
Crevice corrosion data in 10% FeCl3 solutionof 2205
|
Alloy |
Crevice corrosion temperature (℃) |
|
Typical 316 |
-3 |
|
Typical 317 |
2 |
|
2205 |
20 |
|
E-BRITE26-1 |
24 |
|
AL-6XN |
45 |
|
Inconel625 |
45 |
|
AL 29-4C |
52 |
Uniform corrosion of 2205
2205 is resistant to dilute reducing acid, high concentration of oxidizing acid and low concentration of organic acid, but it should be used carefully under high temperature and high concentration. The following table shows the corrosion test comparison between 316 and 2205 in normal state and welding state.
Corrosion Comparison test data
|
Test solution (boiling) |
Corrosion rate (mm/a) |
|||
|
|
Typical 316L |
2205 |
||
|
|
base material |
Welding material |
base material |
Welding material |
|
20% acetic acid |
＜0.01 |
＜0.01 |
＜0.01 |
＜0.01 |
|
45 formic acid |
0.60 |
0.53 |
0.01 |
0.01 |
|
1% hydrochloric acid |
0.02 |
1.61 |
0.02 |
0.02 |
|
65% nitric acid |
0.56 |
0.46 |
0.52 |
0.49 |
|
10% oxalic acid |
1.22 |
1.13 |
0.20 |
0.13 |
|
20% phosphoric acid |
0.02 |
0.03 |
0.02 |
0.03 |
|
10% sodium bisulfate |
1.82 |
1.43 |
0.65 |
0.51 |
|
50% sodium hydroxide |
1.97 |
2.17 |
0.61 |
0.57 |
|
10% sulfamic acid |
3.15 |
3.03 |
0.56 |
0.44 |
|
10% sulphuric acid |
16.1 |
16.7 |
5.23 |
5.08 |
|
Ferric sulfate + 50% sulfuric acid (A262B) |
0.66 |
0.59 |
0.51 |
0.45 |
2205 intergranular corrosion resistance test in welding state can be carried out according to ASTM a262e (16% H2SO4 + CuSO4 solution)
Physical property of 2205
- Density: 7.88g/cm3
- Specific heat: 420J / kg-k
- Thermal conductivity: (20-100 ℃) 19W / m-k
- Coefficient of thermal expansion: (20-100 ℃) 13.7 × 10-6 / ℃
- Average modulus of elasticity: 190 GPA
Mechanical property of 2205
The mechanical properties at typical room temperature are shown in the table below.
Mechanical properties at room temperature
|
Project |
ASTM and ASME minimum performance values |
Plate series ＞4.76mm |
Plate series ＜4.76mm |
|
0.2% yield strength |
450MPa |
515 Mpa |
585 MPa |
|
Tensile strength |
625 MPa |
760 MPa |
860 MPa |
|
Elongation rate δ5(%) |
25 |
35 |
30 |
|
Hardness |
32Rc/290HB(max) |
235HB |
27 Rc |
Tensile properties at high temperature
2205 dual phase steel is allowed to be used below 316 ℃ in ASME Boiler and pressure vessel code. Its strength can be expressed by the allowable stress in ASME Boiler and pressure vessel standard. The following table shows the allowable stress comparison of typical 316 and 2205. The weld coefficient of welded pipe fittings is 0.85.
Such a large allowable stress can be used in process equipment design with great benefit.
Maximum allowable stress (according to ASME Code)
|
Temperature (℃) |
2205（MPa） |
Typical 316（MPa） |
|
38 |
155 |
130 |
|
93 |
155 |
112 |
|
149 |
150 |
101 |
|
204 |
144 |
92 |
|
260 |
141 |
86 |
|
316 |
139 |
81 |
ASME Boiler and Pressure Vessel Code, Section II, Part D, Table 1A.
Impact property
2205 dual phase steel can be transformed from high temperature plastic failure to low temperature brittle fracture, and this plastic brittle transition temperature can be fully increased by holding at 343-538 ° C for a long time. Improper welding processes, such as the use of pure CR stainless steel filler, can improve the sensitivity of the weld to impact brittleness.
Effect of high temperature on mechanical properties
According to ASME Boiler and pressure vessel code, the upper limit of service temperature of 2205 dual phase steel is 316 ℃, because there is a problem of “475 ℃ embrittlement” of dual phase steel, which is mainly due to the embrittlement of ferrite when heated between 343-538 ℃. However, this embrittlement is reversible and can be reduced by heating above 593 ℃. However, another embrittlement temperature range is 538-1000 ℃, because of the precipitation of mesophase harmful to impact and corrosion. The whole annealing and rapid cooling treatment can eliminate the brittle phase, as well as the forming stress and “475 ℃ embrittlement”.
Forming and heat treatment of 2205
2205 can be successfully cold bent and stretched. Compared with ordinary austenitic stainless steel, 2205 has high strength and high requirements for forming equipment. The elongation of ferrite phase is smaller than that of austenitic phase, so the bending radius of 2205 is larger than that of austenitic stainless steel.
In addition, the expansion degree of 2205 tube and tube sheet is limited because of its low elastic elongation. 2205 has high strength compared with many tubesheet materials. Therefore, great care should be taken when expanding 2205 tubesheet with other materials.
After severe bending deformation, it is necessary to conduct overall annealing (not just stress relief annealing) to prevent stress corrosion cracking in the service environment. The temperature of stress relief heat treatment is generally between 316-927 ℃, and the influence on material performance is not considered.
Heat treatment of 2205
2205 is usually annealed at 1020-1100 ℃ and cooled rapidly. When it is heat treated near 1100 ℃, the ferrite content will increase greatly.
Welding of 2205
The content of ferrite phase and austenite phase in 2205 dual phase steel is basically the same. Oxyacetylene welding can increase the content of ferrite in the weld and heat affected zone of the base material, and parallel annealing can restore the balance of the two phases. However, the ferrite content of the weld after annealing is a little higher. The ferrite content in the weld shall not be too high.
The matching filler metal is economical for 2205 welding. Filler metal such as AWS 2209 has higher Ni content than base metal, which is mainly for the purpose of making the welding area produce phase balance and making the chemical composition of useful samples of weld area and base metal. When 2205 and dissimilar steel are welded, the filler metal should contain some austenite forming elements so as to form an austenite weld. If there are a lot of ferrite grains in the weld area, the room temperature impact toughness of the material will be greatly reduced.
255 super duplex stainless steel
Alloy 255 is a duplex stainless steel containing 25% Cr, which has high strength and corrosion resistance. There is a good balance between austenite phase and ferrite phase in this alloy, that is, this balance makes alloy 255 have high strength at the same time. It also has good resistance to chloride ion stress corrosion cracking.
It is mainly used in marine corrosive environment, phosphoric acid and chemical fertilizer industry, environmental protection, pulp and paper industry, petrochemical industry and other industries.
Chemical composition of 255
The chemical composition of alloy 255 is shown in the table below (according to ASTM a240).
Chemical composition
|
Element |
Wt% |
|
C |
≤0.040 |
|
Mn |
≤1.5 |
|
P |
≤0.040 |
|
S |
≤0.030 |
|
Si |
≤1.0 |
|
Cr |
24.0-27.0 |
|
Ni |
4.50-6.50 |
|
Mo |
2.90-3.90 |
|
Cu |
1.5-2.5 |
|
N |
0.10-0.25 |
|
Fe |
Allowance |
Physical property of 255
Density: 7.73g/cm3
Specific heat
|
Temperature (℃) |
Specific heat (J/g–k) |
|
52 |
0.481 |
|
102 |
0.496 |
|
202 |
0.525 |
|
302 |
0.554 |
|
402 |
0.583 |
|
502 |
0.665 |
Thermal conductivity
|
Temperature (℃) |
Thermal conductivity (W/cm-k) |
|
23 |
0.133 |
|
100 |
0.147 |
|
200 |
0.163 |
|
300 |
0.182 |
|
400 |
0.198 |
|
500 |
0.229 |
|
600 |
0.233 |
Coefficient of linear thermal expansion
|
Temperature (℃) |
Coefficient of linear thermal expansion (10-6/k) |
|
23-100 |
12.1 |
|
23-150 |
12.5 |
|
23-200 |
12.7 |
|
23-250 |
12.9 |
|
23-300 |
13.2 |
|
23-350 |
13.3 |
|
23-400 |
13.5 |
|
23-500 |
13.8 |
Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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