Duplex stainless steel combines the characteristics of austenitic and ferritic stainless steel. Compared with ferritic stainless steel, it has higher plasticity and toughness, no room temperature brittleness, significantly improved intergranular corrosion resistance and welding performance. At the same time, it maintains the 475 brittleness and high thermal conductivity of ferritic stainless steel, and has characteristics such as superplasticity. Compared with austenitic stainless steel, it has higher strength and significantly improved resistance to intergranular corrosion and chloride stress corrosion. Dual phase stainless steel has excellent resistance to pitting corrosion and is also a nickel saving stainless steel.
1. Historical development
Since its birth in the United States in the 1940s, duplex stainless steel has developed into its third generation. Its main feature is that its yield strength can reach 400-550MPa, which is twice that of ordinary stainless steel, thus saving materials and reducing equipment manufacturing costs. In terms of corrosion resistance, especially under harsh medium environments such as seawater with high chloride ion content, the resistance to pitting corrosion, crevice corrosion, stress corrosion, and corrosion fatigue of duplex stainless steel is significantly better than that of ordinary austenitic stainless steel, and can be comparable to high alloy austenitic stainless steel.
2. Performance characteristics
Due to the characteristics of the two-phase structure, by correctly controlling the chemical composition and heat treatment process, duplex stainless steel combines the advantages of ferritic stainless steel and austenitic stainless steel. It combines the excellent toughness and weldability of austenitic stainless steel with the high strength and chloride stress corrosion resistance of ferritic stainless steel. It is these superior properties that have led to the rapid development of duplex stainless steel as a weldable structural material. Since the 1980s, it has become a steel class on par with martensitic, austenitic, and ferritic stainless steels. Dual phase stainless steel has the following performance characteristics:
(1) Molybdenum containing duplex stainless steel exhibits excellent resistance to chloride stress corrosion under low stress. Generally, 18-8 austenitic stainless steel is prone to stress corrosion cracking in neutral chloride solutions above 60 ° C. Heat exchangers, evaporators, and other equipment made with this type of stainless steel in trace chloride and hydrogen sulfide industrial media tend to experience stress corrosion cracking, while duplex stainless steel has good resistance.
(2) Molybdenum containing duplex stainless steel has good resistance to pitting corrosion. When having the same equivalent value of pitting resistance (PRE=Cr%+3.3Mo%+16N%), the critical pitting potential of duplex stainless steel is similar to that of austenitic stainless steel. The pitting corrosion resistance of duplex stainless steel and austenitic stainless steel is equivalent to that of AISI 316L. The resistance to pitting and crevice corrosion of high chromium duplex stainless steel containing 25% Cr, especially nitrogen, exceeds that of AISI 316L.
(3) Has good resistance to corrosion fatigue and wear corrosion. Suitable for manufacturing power equipment such as pumps and valves under certain corrosive media conditions.
(4) Good comprehensive mechanical performance. It has high strength and fatigue strength, with a yield strength twice that of 18-8 austenitic stainless steel. The elongation rate of the solid solution state reaches 25%, and the toughness value AK (V-shaped groove) is above 100J.
3. Structure and Type
Dual phase stainless steel has the characteristics of both austenitic stainless steel and ferritic stainless steel due to its dual phase structure of austenite and ferrite, and the content of the two phases is basically the same. The yield strength can reach 400Mpa~550MPa, which is twice that of ordinary austenitic stainless steel. Compared with ferritic stainless steel, duplex stainless steel has higher toughness, lower brittle transition temperature, significantly improved intergranular corrosion resistance and welding performance; At the same time, it retains some characteristics of ferritic stainless steel, such as 475 brittleness, high thermal conductivity, low coefficient of linear expansion, superplasticity, and magnetism. Compared with austenitic stainless steel, duplex stainless steel has higher strength, especially significantly improved yield strength, and significant improvements in properties such as pitting corrosion resistance, stress corrosion resistance, and corrosion fatigue resistance.
Dual phase stainless steel can be classified into four types based on its chemical composition: Cr18 type, Cr23 (excluding Mo) type, Cr22 type, and Cr25 type. For Cr25 duplex stainless steel, it can be divided into ordinary type and super duplex stainless steel, among which Cr22 type and Cr25 type are more commonly used. The duplex stainless steel used in China is mostly produced in Sweden, with specific grades including 3RE60 (Cr18 type), SAF2304 (Cr23 type), SAF2205 (Cr22 type), and SAF2507 (Cr25 type).
4. Classification
The first type belongs to the low-alloy type, represented by the grade UNS S32304 (23Cr-4Ni-0.1N), which does not contain molybdenum in the steel and has a PREN value of 24-25. It can replace AISI304 or 316 in terms of stress corrosion resistance.
The second type belongs to the medium alloy type, represented by the grade UNS S31803 (22Cr-5Ni-3Mo-0.15N), with a PREN value of 32-33. Its corrosion resistance is between AISI 316L and 6% Mo+N austenitic stainless steel.
The third type belongs to the high alloy type, generally containing 25% Cr, as well as molybdenum and nitrogen, and some also containing copper and tungsten. The standard grade is UNSS32550 (25Cr-6Ni-3Mo-2Cu-0.2N), with a PREN value of 38-39. The corrosion resistance of this type of steel is higher than that of duplex stainless steel with 22% Cr.
The fourth type belongs to the super duplex stainless steel type, containing high molybdenum and nitrogen. The standard grade is UNS S32750 (25Cr-7Ni-3.7Mo-0.3N), and some also contain tungsten and copper. The PREN value is greater than 40, which can be used in harsh medium conditions and has good corrosion resistance and mechanical comprehensive performance, comparable to super austenitic stainless steel.
5. Welding characteristics
Dual phase stainless steel has good welding performance. Compared with ferritic stainless steel and austenitic stainless steel, it does not have the welding heat affected zone of ferritic stainless steel, which greatly reduces plasticity and toughness due to severe grain coarsening, nor is it sensitive to welding hot cracks like austenitic stainless steel.
Due to its unique advantages, duplex stainless steel is widely used in industrial fields such as petrochemical equipment, seawater and wastewater treatment equipment, oil and gas pipelines, paper machinery, etc. In recent years, it has also been studied for use in bridge load-bearing structures and has great development prospects.
The welding performance issues that often arise with "economical duplex steel" are not a problem. However, welding standard duplex steel is not a problem, and regardless of the process used, there are welding materials suitable for these applications. From a metallographic perspective, welding 2101 (1.4162) is not a problem at all. In fact, it is even easier to weld than standard grade duplex steel because this material can actually be welded using acetylene welding process. For standard duplex steel materials, this process must always be avoided. The actual problem faced by welding 2101 is that the viscosity of the molten pool is different, so the wettability is slightly worse. This forces operators to use arc welding more during the welding process, which is precisely why The problem lies. Although it can be compensated for by selecting superalloys, we often want to choose matching welding materials.
6. Material and grade
Many duplex stainless steel grades have been added to the new standard GB/T 20878-2007 "Stainless Steel and Heat Resistant Steel Grades and Chemical Composition" in China. For example: 14Cr18Ni11Si4AlTi 022Cr19Ni5Mo3Si2N, 12Cr21Ni5Ti. More grades can be found in the standard.
In addition, the famous 2205 duplex steel is equivalent to China's 022Cr23Ni5Mo3N.
Finally, some web pages and even papers mistakenly refer to duplex stainless steel as duplex stainless steel. Biphase refers to the presence of two types of metallographic structures, rather than directional "orientation".
Chemical composition
Grade |
C≤ |
Mn≤ |
Si≤ |
S≤ |
P≤ |
Cr≤ |
Ni |
Mo |
Cu≤ |
N |
S32750(SAF2507) |
0.03 |
1.2 |
0.8 |
0.02 |
0.035 |
24.0/26.0 |
6.0/8.0 |
3.0/5.0 |
0.5 |
0.24/0.32 |
00Cr25Ni7Mo4N |
||||||||||
S31803(SAF2205) |
0.03 |
2 |
1 |
0.02 |
0.03 |
21.0/23.0 |
4.50/6.50 |
2.50/3.50 |
0.08/0.20 |
|
00Cr22Ni5Mo3N |
||||||||||
S31500(3RE60) |
0.03 |
1.2/2.00 |
1.4/2.00 |
0.03 |
0.03 |
18.0/19.0 |
4.25/5.25 |
2.50/3.00 |
0.05/0.10 |
|
00Cr18Ni5Mo3Siz |
7. Requirements for welding material selection
The welding material used for duplex stainless steel is characterized by a duplex structure dominated by austenite in the weld seam, with the main corrosion-resistant elements (chromium, molybdenum, etc.) having a content equivalent to that of the base material, thereby ensuring comparable corrosion resistance to the base material. To ensure the content of austenite in the weld, it is usually necessary to increase the nickel and nitrogen content, that is, to increase the nickel equivalent by about 2% to 4%. In duplex stainless steel base materials, there is generally a certain amount of nitrogen content, and it is also desirable to have a certain nitrogen content in welding materials, but it should not be too high, otherwise porosity will occur. This makes the high nickel content a major difference between the welding material and the base material.
Choose welding rods that match the chemical composition of the base metal according to different requirements for corrosion resistance and joint toughness. For example, when welding Cr22 duplex stainless steel, Cr22Ni9Mo3 welding rods such as E2209 welding rods can be used. When using acidic welding rods, slag removal is excellent and the weld seam is aesthetically pleasing, but the impact toughness is low. When high impact toughness is required for the weld metal and full position welding is required, alkaline welding rods should be used. When welding the bottom of the root, alkaline welding rods are usually used. When there are special requirements for the corrosion resistance of the weld metal, alkaline electrodes with super duplex steel composition should also be used.
For solid gas shielded welding wire, while ensuring that the weld metal has good corrosion resistance and mechanical properties, attention should also be paid to its welding process performance. For flux cored wire, when the weld formation is required to be beautiful, rutile or titanium calcium type flux cored wire can be used. When high impact toughness or welding under high restraint conditions is required, flux cored wire with high alkalinity should be used.
For submerged arc welding, it is advisable to use welding wires with smaller diameters to achieve multi-layer and multi pass welding under small and medium-sized welding specifications, in order to prevent the embrittlement of the welding heat affected zone and weld metal, and to use matching alkaline fluxes.
