Super Duplex Stainless Steel: 2507 (UNS S32750/F53/DIN W.Nr. 1.4410)
What is 2507 (UNS S32750/F53/DIN W.Nr. 1.4410)?
UNS S32750 or DIN W.Nr. 1.4410, Super Duplex 2507 is a super duplex stainless steel containing 25% chromium, 4% molybdenum and 7% nickel, designed for demanding applications requiring superior strength and corrosion resistance, such as chemical processing, petrochemical and seawater equipment. Alloy 2507 (S32750) has a nominal composition of 25Cr-7Ni-4Mo-N. Compared to alloy 2205 (S32205), alloy 2507 has a higher chromium content, which improves corrosion resistance. Since chromium promotes the body-centered cubic structure of iron as a ferrite former, more nickel is needed to promote the formation of austenite to balance the effect of higher chromium. Higher chromium levels also promote the formation of deleterious intermetallic phases, which are sensitive to 475°C embrittlement due to precipitation of the chromium-rich α' phase, and to σ, chi, and other phase embrittlement at higher temperatures. Alloy 2507 (S32750) also has a higher nitrogen content, which not only promotes the formation of austenite and improves strength, but also retards the formation of intermetallic phases enough to allow machining and fabrication of duplex grades.
Fabrication, Heat Treatment, and Weldability
Forming: Duplex 2507 Pipe material can be fabricated by hot or cold forming methods. Hot forming temperatures should be between 1875°F and 2250°F.
Heat Treatment: This should be followed by a solution anneal at a minimum of 1925°F with a rapid air or water quench. Cold forming includes bending, drawing, extrusion and pressing. After cold forming, solution annealing and quenching heat treatments are required.
Welding: In addition, 2507 super duplex stainless steel has good weldability. 2507 has good weldability and can be joined to itself or other materials by stick arc welding (SMAW), gas tungsten arc welding (GTAW), plasma arc welding (PAW), flux cored wire (FCW) or submerged arc welding (SAW). ). It is recommended to use 100/P2507 filler metal when welding 2507 as it will produce a proper duplex weld structure.
There is no need to preheat 2507 except to prevent condensation on cold metal. Interpass welding temperature should not exceed 300°F or weld integrity may be adversely affected. The root should be shielded with argon or 90%N2/10%H2 purge gas for maximum corrosion resistance. The latter provides better corrosion resistance.
GTAW is recommended for the root pass if welding is to be done on only one surface and post-weld cleaning is not possible. GTAW or PAW should not be done without filler metal unless post-weld cleaning is possible. Heat input is 5-38 kJ/in. for SMAW or GTAW. Heat input is about 50kJ/in. for SAW.
Corrosion resistance
Pitting and crevice corrosion: S32750 super duplex stainless steel has better resistance to pitting and crevice corrosion than other duplex stainless steels after 24 hours of testing in different chloride solutions.
Stress corrosion in chloride solutions containing 8% ppm oxygen in an autoclave, duplex steels such as 32750 duplex steel do not change in yield strength under external stress equivalent to 232°.
Intergranular corrosion: The carbon content of S32750 duplex steel is usually less than 0.02%, generally controlled at 0.015%. No carbide precipitation was found in the organization. Therefore, the low carbon content and the presence of ferrite phase make the sensitization of the steel negligible.
Uniform corrosion: S32750 steel has good uniform corrosion resistance in some organic and inorganic acids, for example, it is comparable to high-alloy austenitic stainless steel in formic acid and acetic acid.
Wear corrosion: Due to the high hardness and wear resistance of S32750 duplex steel, the steel has good wear resistance in flowing media containing abrasive particles or media with high flow rate, such as used for conveying sand-containing seawater or manufacturing pumps and other mechanical parts used in corrosive media.
Physical properties
Welding performance: S32750 super duplex stainless steel can be welded by GTAW, GMAW, SMAW, sawing and other welding methods. The welding materials used are shown in the table below. 25.104l is used for GTAW welding, 25.104lr and 25.10.4lb are used for manual welding, and 25.10.4lb basic welding rods are mostly used for weldments with high toughness requirements.
Note: sanicro 60 welding wire is Cr22Mo9Nb3.5Ni alloy.
The recommended linear energy is 0.2-1.5kj/mm. For rapid cooling, the upper limit is generally used for thick plate welding and the lower limit is used for thin plate welding. The interlayer temperature shall not exceed 150℃.
During GTAW welding, pure argon or Ar + 2% N2 is used for protection.
Cold and hot processing performance: The hot forming temperature range is limited. When the temperature is too low, the ferrite with poor plasticity will crack severely in the deformation zone, so temperature control is very important. The hot forming temperature range of S32750 duplex steel is 1200-1025℃, and the workpiece after hot processing needs solid solution water quenching. Duplex stainless steel is very soft at the solid solution temperature. If the workpiece is not sufficiently supported, it will be twisted and deformed. This is very important for large-diameter thin-walled pipes, and it must be paid attention to. Twisted products are more difficult to form or straighten than austenitic stainless steel.
After cold forming such as cold bending, S32750 super duplex steel is heated at 1050-1120℃ and then air-cooled or water-treated.
