UNS S32550, W.Nr. 1.4507, F61
UNS S32550 is a highly alloyed, super duplex (ferrite-austenite) solid solution trengthened alloy, which provides high strength and wear resistance while offering superior corrosion resistance compared to 316 stainless steel. Also, the material offers good ductility with a high fatigue strength, twice the yield strength of alloy 20, and stronger than duplex 2205. It is important to note that carbides, nitrides and sigma phase can formed if UNS S32550 stainless steel is allowed to soak between the temperatures of 1000°F and 1800°F.
| Material | Cr | P | S | Ni | Si | C | Mn | Mo |
|---|---|---|---|---|---|---|---|---|
| S32550 | 24.0-27.0 | 0.04 max | 0.03 max | 4.50-6.50 | 1.00 max | 0.04 max | 1.50 max | 2.90-3.90 |
| Material | Tensile strength | Yield strength 0,2 | Elongation | Hardness HB30 |
|---|---|---|---|---|
| S32550 | 750 N/mm² | 550 N/mm² | 25% | 290HB |
In most media, UNS S32550 display excellent corrosion resistance, including pitting and crevice corrosion resistance. Its also has high resistance to intergranular corrosion and high strength it performs well in abrasion/corrosion conditions. In addtion, UNS S32550 stainless steel performs well in sulphuric, phosphoric, and nitric and shows excellent resistance to organic acids such as acetic and formic acid.
UNS S32550 super duplex stainless steel cannot be hardened by heat treatment, but they can however be work hardened. Solution treatment or annealing can be done by rapid cooling after heating to around 1100°C.
Heating and pickling are common processes used for UNS S32550 duplex stainless steel to remove surface scale, oxide layers, and other contaminants. First, UNS S32550 is heated at a certain temperature range to eliminate organic materials and facilitate the pickling process. Then, it undergoes pickling by immersing it in an acid solution, typically a mixture of nitric acid and hydrofluoric acid , to dissolve and remove scale and oxides. After pickling, thorough rinsing with water is performed to remove residual acid. Passivation, an optional step, may follow to enhance corrosion resistance.
UNS S32550 super duplex stainless steel can be both hot and cold formed, although there are some considerations to keep in mind for each process. The hot forming of this material typically occurs above 1000°C (1832°F) to ensure the material is in the austenitic phase, and techniques such as hot rolling, hot forging, or hot extrusion can be used. However, cold forming is performed at room temperature and involves processes like bending, drawing, or stamping. Cold forming causes the material to work harden, increasing its strength and reducing its ductility.
Although machinable, the high strengths of UNS S32550 stainless steel makes machining difficult. Compared to 304, machining of UNS S32550 is around 20% slower. During machining, cutting edges must be kept sharp, dull edges cause excess work hardening. Cuts should be light but deep enough to prevent work hardening by riding on the surface of the material. Chip breakers should be employed to assist in ensuring swarf remains clear of the work. Also, low thermal conductivity of UNS S32550 stianless steel results in heat concentrating at the cutting edges, this means coolants and lubricants are necessary and must be used in large quantities.
UNS S32550 duplex stainless steel has good weldability and all standard welding processes can be used. They are not quite as easily welded as the austenitic grades but low thermal expansion in duplex grades reduces distortion and residual stresses after welding.
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