Introduction:
Called Duplex Pipe because of its 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. 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. Moreover, the two-phase microstructure guarantees higher resistance to pitting and stress corrosion cracking in comparison with conventional stainless steels.
Duplex is a stainless steel made from a mixture of austenite and ferrite phases. Like most austenitic stainless steels, duplex has a strong resistance to corrosion, similar to those of a Type 304 and 316.
Unlike similar steels, duplex also displays an improved resistance to localized corrosion, particularly pitting, crevice corrosion and stress corrosion cracking. And because duplex has a lower nickel and molybdenum content than other austenitic stainless steels, it can prove a more cost effective option due to a lower alloying content.
Due to its Ferritic qualities, duplex steel also shows very good resistance to stress corrosion cracking when compared to standard austenitic.In some cases, the strength of duplex steel can be up to double that of the most commonly used grades of stainless steel.
Duplex becomes brittle at extreme temperatures so its use is normally restricted to a maximum temperature of 300 degrees. Duplex also shows signs of embrittlement at -50 degrees.
Benefits:
Stronger than 300 series stainless steel which also brings weight advantages.
Cheaper than some stainless steels.
High resistance to pitting, crevice corrosion and stress corrosion cracking.
Higher heat conductivity and lower thermal expansion than austenitic steels.
Uses:
Pipes for production and transportation of oil and gas
Structural and mechanical components
Heat exchangers
Cooling pipes
Cargo vessels and containers
High strength wiring
Standard of ASTMA789
1. Scope*
1.1 This specification2 covers grades of nominal wall thickness, stainless steel tubing for services requiring general corrosion resistance, with particular emphasis on resistance to stress corrosion cracking. These steels are susceptible to embrittlement if used for prolonged periods at elevated temperatures.
1.2 The values stated in either inch-pound units or SI units are to be regarded separately as standard. Within the text, the SI units are shown in brackets. The values stated in each system are not exact equivalents; therefore, each system must be used independently of the other. Combining values from the two systems may result in nonconformance with the specification. The inch-pound units shall apply unless the M designation of this specification is specified in the order.
2. Referenced Documents
2.1 ASTM Standards: 3
A 480/A 480M Specification for General Requirements for Flat-Rolled Stainless and Heat-Resisting Steel Plate, Sheet, and Strip
A 1016/A 1016M Specification for General Requirements for Ferritic Alloy Steel, Austenitic Alloy Steel, and Stainless Steel Tubes
E 527 Practice for Numbering Metals and Alloys (UNS)
2.2 SAE Standard:4
SAE J 1086 Practice for Numbering Metals and Alloys(UNS)
