Material properties
Materials used for duplex steel pipe fittings  refer to stainless steel with ferrite and austenite each accounting for about 50%. Generally, the content of less phases needs to be at least 30%. In the case of low C content, the Cr content is between 18% and 28%, and the Ni content is between 3% and 10%. Some steel also contain alloying elements such as Mo, Cu, Nb, Ti and N. Super duplex stainless steel types contain much molybdenum and nitrogen, and the standard grade is UNS S32750 (25Cr-7Ni-3.7Mo-0.3N). Some also contain tungsten and copper with a PREN value greater than 40, which are suitable for harsh medium conditions, have good corrosion resistance and comprehensive mechanical properties and are comparable to super austenitic stainless steel. The representative grade of duplex steel elbow is UNS S31803 (22Cr-5Ni-3Mo-0.15N), with a PREN value of from 32 to 33, and its corrosion resistance is between AISI 316L and 6%Mo+N austenitic stainless steel. After the normal solution treatment of duplex stainless steel (heating at temperatures of between 1020 ℃ and 1100 ℃ and water cooling), the steel contains about 50% to 60% austenite and 50% to 40% ferrite.

With the increase of heating temperature, the two phase ratio changes are not obvious. The continuous use temperature range of duplex stainless steel is between -50℃ and 250℃, and the lower limit depends on the brittle transition temperature of the steel. The upper limit is limited by the brittleness of 475℃, and the upper limit temperature cannot exceed 300℃. Duplex stainless steel requires rapid cooling after solid solution treatment. Slow cooling will cause the precipitation of brittle phases, which will lead to a decrease in the toughness of the steel, especially the partial corrosion resistance. The lower limit temperature of hot working and hot forming of duplex stainless steel containing much chromium molybdenum cannot be lower than 950℃, and super duplex stainless steel cannot be lower than 980℃; duplex stainless steel containing low chromium molybdenum cannot be lower than 900℃ so as to avoid the precipitation of brittle phase in the process, resulting in surface cracks. The relieving stress treatment with temperatures of between 650°C and 800°C should not be used for austenitic stainless steel, and solution annealing treatment is generally adopted.
 
After surfacing duplex stainless steel on the surface of low-alloy steel, when the overall  relieving stress treatment at temperatures of between 600°C and 650°C is required, the toughness and corrosion resistance caused by the precipitation of the brittle phase must be considered, especially the decline in partial corrosion resistance. Shorten the heating time in this temperature range as much as possible. The heat treatment of low-alloy steel and duplex stainless steel clad plates should also be considered in the same way. Materials used for duplex steel elbows are as follows:

Materials
ASTM/ASME SA789/790 UNS S31803, S32205, S32750 and S32760
ASTM A182 F51, F53 and F55
ASTM A815 UNS S31803, S32205, S32750 and S32760

Size ranges
Outer diameters: between 21.3mm and 1219mm

Wall thickness
SCH 10S to SCH XXS