Duplex stainless steels have a two-phase microstructure containing ferrite and austenite in volumetric fractions approximately equal to 50%. Due to their two-phase microstructure, these steels combine the best properties of ferritic and austenitic stainless steels. In general, the ferrite phase (body-centered cubic lattice) provides high mechanical strength, good toughness, and good corrosion resistance while the austenitic phase (face-centered cubic lattice) provides good ductility.

The combination of these properties is why duplex stainless steels are widely used in the petrochemical, pulp and paper, marine, and power generation industries. They can withstand corrosive media, extend service times, and operate in more extreme environmental conditions.

High-strength materials allow you to reduce part thickness and weight. For example, compared to 316 stainless steel, super-duplex stainless steel can provide three to four times higher yield strength and a higher resistance to pitting corrosion.

Duplex stainless steel is categorized into three grades based on the chromium (Cr) content by weight and the pitting resistance equivalent number (PREN):

For comparison’s sake, 316 stainless steel has a 16% Cr content and a PREN of 22.6 to 27.9.

One of the critical aspects of welding DSS, SDSS, HDSS, and specially alloyed stainless steel is controlling weld parameters.

Petrochemical industry weld procedure requirements identify minimum PREN values necessary for filler metals. For example, DSS requires a PREN of 35, and SDSS requires a PREN of 40. Figure 1 shows DSSs and their matching filler metals for GMAW and GTAW. Generally, the filler metal’s Cr content will match that of the base metal. When using GTAW for the root and hot passes, a practice to consider is using an overalloyed filler metal. In the event that the weld metal is not homogenous due to poor technique, an overalloyed filler metal may provide the weld sample with the required PREN and other values.

As an example to lend credence to this, for a root and hot pass on pipe with a wall thickness of ½ in. or less, some fabricators recommend using SDSS filler wires (25% Cr) for DSS base alloys (22% Cr), and HDSS filler wires (27% Cr) for SDSS (25% Cr) base alloys. You can also use an HDSS filler wire for HDSS base alloys. This austenitic-ferritic duplex has approximately 65% ferrite, 27% Cr, 6.5% nickel, 5% molybdenum, and is considered to be low carbon with less than 0.015%.

HDSS filler offers improved yield strength and better pitting and crevice corrosion resistance compared to SDSS. It also has higher hydrogen-induced stress cracking resistance compared to SDSS and higher resistance to aggressively sour environments. Its high strength means lower repair rates during pipeline production because finite element analysis is not required for weld metal of matching strength and the acceptance criteria can be less conservative.

Given the wide range of base materials, mechanical requirements, and service conditions, consider consulting with a DSS application and filler metal expert before embarking on the next project.

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