Duplex stainless steel has a two-phase microstructure in which the volume fraction of ferrite and austenite is about 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 austenite 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 time, and operate in more extreme environmental conditions.
High-strength materials allow you to reduce part thickness and weight.For example, super duplex stainless steel can provide three to four times higher yield strength and higher resistance to pitting corrosion than 316 stainless steel.
Duplex stainless steels are classified into three grades based on gravimetric chromium (Cr) content and pitting resistance equivalent number (PREN):
One of the key aspects of welding DSS, SDSS, HDSS and special alloy stainless steels is the control of welding parameters.
The petrochemical industry welding process requirements determine the minimum PREN value required for filler metals.For example, DSS requires a PREN of 35 and SDSS requires a PREN of 40.Figure 1 shows DSS and its matching filler metal for GMAW and GTAW.Typically, the Cr content of the filler metal will match that of the base metal.One practice to consider when using GTAW for roots and hot channels is the use of superalloy filler metals.If the weld metal is not uniform due to poor technique, over-alloyed filler metal can provide the desired PREN and other values ​​for the welded sample.
As an example to demonstrate this, some manufacturers recommend using SDSS filler wire (25% Cr) for DSS-based alloys (22% Cr), and HDSS filler wire (27% Cr) in SDSS (25% Cr) based alloys.For HDSS based alloys, you can also use HDSS filler wire.This austenitic-ferritic duplex stainless steel has approximately 65% ​​ferrite, 27% chromium, 6.5% nickel, 5% molybdenum and is considered to be less than 0.015% low carbon.
Compared to SDSS, HDSS packing has higher yield strength and better resistance to pitting and crevice corrosion.It also has higher resistance to hydrogen-induced stress cracking and higher resistance to strongly acidic environments than SDSS.Its high strength means lower maintenance rates during pipe production, since finite element analysis is not required for weld metal of matching strength, and acceptance criteria can be less conservative.
Given the wide range of base materials, mechanical requirements and service conditions, please consider consulting with a DSS application and filler metal specialist before embarking on your next project.
WELDER, formerly Practical Welding Today, showcases the real people who make the products we use and work with every day.This magazine has served the welding community in North America for over 20 years.
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