Duplex Stainless Steel Pipe Dimensions

ARE THERE ANY ISSUES WITH 22%CR DUPLEX STAINLESS STEEL?

Whilst the combination of good strength, corrosion resistance and reasonable weldability make 22%Cr duplex stainless steel the ideal choice for a lot of applications, it is important to understand some of the issues associated with its manufacture and fabrication.

The metallurgy of 22%Cr Duplex Stainless Steel Pipe Dimensions is much more complex than Type 316/316L stainless steel.  In addition to ferrite and austenite, 22%Cr duplex stainless steel can also contain undesirable third phase (intermetallic) precipitates if the steel is not properly processed.  The most critical operations are heat treatment and welding.

The presence of intermetallic precipitates can be extremely damaging as they can dramatically reduce corrosion resistance and toughness of the material. Sigma phase is probably the most likely intermetallic precipitate to occur.  It is produced during manufacture or welding, if the cooling rate is too slow.

To remove any intermetallic precipitates that may occur during manufacturing, 22%Cr duplex stainless steel should always be supplied in the solution heat-treated condition.  As its name suggests, solution heat treatment is a process by which any intermetallic precipitates are dissolved back into solution such that the microstructure is comprised solely of ferrite and austenite.  

Heat treatment should not be required after welding.  In modern 22%Cr duplex stainless steels, the chemical composition is balanced in such a manner that the reformation of austenite in the heat-affected zone of the weld takes place quickly. This results in a microstructure that gives corrosion properties and toughness roughly equal to that of the parent metal. Welded joints in 22%Cr duplex stainless steel should easily pass the intergranular corrosion test according to ASTM A262 Practice E (Strauss' test) (1).

There are limitations on both the maximum and minimum service temperatures at which 22%Cr duplex stainless steel can be used.

At elevated temperatures, 475 degree embrittlement can occur in 22%Cr duplex stainless steel due to the formation of a phase called α′ (alpha prime).  Although the worst temperature is 475°C, it can still form at temperatures as low as 300°C (7).  For this reason, 22%Cr duplex stainless steel is normally limited to a maximum service temperature of 250°C.

At low temperatures, unlike Type 316/316L stainless steel 22%Cr duplex stainless steel exhibits ductile-brittle transitional behaviour as indicated in Figure 5 (1, 7).  Whilst, in theory, 22%Cr duplex stainless steel can be used at temperatures below minus 50°C, this is its normal temperature limit in service.