Duplex stainless steel is among various metals used in corrosion-resistant applications.
Duplex steels have a mixed microstructure with about equal proportions of ferrite and austenite. Duplex stainless steels are a family of grades, which range in corrosion performance depending on their alloy content.
Duplex steels have improved strength over austenitic stainless steels and also improved resistance to localised corrosion, particularly pitting, crevice corrosion and stress corrosion cracking. They are characterised by high chromium (19–28%) and molybdenum (up to 5%) and lower nickel contents than austenitic stainless steels. The chemical composition based on high contents of Cr and Mo, improves intergranular and pitting corrosion resistance, respectively. Additions of nitrogen can promote structural hardening by interstitial solid solution mechanism, which raises the yield strength and ultimate strength values without impairing toughness.
When welding duplex alloys, the heat input and interpass temperature must be controlled to maintain the ferrite-austenite balance. Otherwise, the advantages of using the duplex material will be lost. The time spent at 1,300 °F to 1,800 °F must be controlled. The heat-affected zone (HAZ) time-at-temperature is critical in controlling the intermetallic phases.
Duplex 2205
Duplex 2205 (22% Chromium, 5% Nickel) is widely used in the Oil and Gas Industry. Grade S31803 has similar physical properties to that of S32205. The S31803 grade stainless steel has undergone a number of modifications resulting in UNS S32205. Duplex 2205 has excellent corrosion resistance, much higher than that of grade 316. It resists localized corrosion types like intergranular, crevice and pitting.
Super Duplex 2207
Super Duplex 2507 (25% Chromium, 4% Molybdenum and 7% Nickel) has a higher corrosion resistance. This high molybdenum, chromium and nitrogen content gives Duplex 2507 a high resistance to chloride pitting and crevice corrosion attack and exceptional resistance to chloride stress corrosion cracking. S32750 and S32760 super duplex steels are commonly used in the oil and gas industry.
Typical specification for Duplex piping
Following is a typical specification for class 150 Duplex stainless steel piping 275 psig @ 130° F
Pipes
- 1½" & below------Duplex SS, Schedule 40s, seamless, per ASTM A790 UNS S31803.
- 2" thru 10"------Duplex SS, Schedule 10s, seamless, per ASTM A790 UNS S31803.
- 12" thru 24"------Duplex SS, Schedule 10s, seamless or welded with 100% radiography, per ASTM A790 UNS S31803.
Fittings
- 10" & below------Butt weld fittings to ASTM A815 UNS S31803 (Class WP-S). Wall thickness same as pipe.
- 12" thru 24"------Butt weld fittings to ASTM A815 UNS S31803 (Class WP-W) with 100% radiography. Wall thickness same as pipe.
Flanges
- 24" and below-------Weld neck RF Flange to ASTM A182 Grade F51.