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 ferritic 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.
Due to the combination of these properties, duplex stainless steels are widely used in the petrochemical, pulp and paper, marine and energy industries. They can withstand harsh environments, extend service life and operate in more extreme environmental conditions.
High-strength materials reduce the thickness and weight of the part. For example, super duplex stainless steel can provide three to four times the yield strength and pitting resistance of 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 requirements for the welding process in the petrochemical industry dictate the minimum PREN required for filler metals. For example, DSS requires a PREN value of 35, while SDSS requires a PREN value of 40. 1 shows DSS and its corresponding filler metal for GMAW and GTAW. As a rule, the Cr content in the filler metal corresponds to the Cr content in the base metal. One method to consider when using GTAW for roots and hot channels is the use of superalloy filler metals. If the weld metal is inhomogeneous due to poor technique, an over-alloyed filler metal can provide the desired PREN and other values for the weld specimen.
As an example, to demonstrate this, some manufacturers recommend using SDSS (25% Cr) filler wire for DSS (22% Cr) based alloys and HDSS (27% Cr) filler wire for SDSS (25% Cr) based alloys. HDSS filler wire can also be used for HDSS alloys. This austenitic-ferritic duplex contains 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 a higher yield strength and better resistance to pitting and crevice corrosion. It also has higher resistance to hydrogen stress cracking and higher resistance to strongly acidic environments than SDSS. Its high strength means lower maintenance costs in pipe production, as weld metal of adequate strength does not require finite element analysis and acceptance criteria can be less conservative.
Given the wide range of base materials, mechanical requirements and operating conditions, please consult with a DSS and filler metal application specialist before proceeding with your next project.
WELDER, formerly called Practical Welding Today, represents the real people who make the products we use and work with every day. This magazine has been serving the welding community in North America for over 20 years.
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