ESAB Technical Paper Offers Guidance For Welding Duplex Stainless Steels
November 30, 2020
Based on requests from Saudi Aramco and other leading petrochemical companies in the region, a team of engineers from ESAB Welding & Cutting Products’ Process Centre in Dubai, United Arab Emirates, demonstrated that the pitting resistance equivalent number (PREN) value of duplex stainless steel alloys may lead to a misleading prediction of pitting corrosion resistance in the as-welded material. ESAB research results, published a technical paper in the April 2020 issue of Saudi Journal of Engineering & Technology, conclude that in the event of an imbalance of d (delta) ferrite and austenite volume fraction, the chemical composition of the phases may be very different from that of the alloy.
PREN values are a predictive measurement of a stainless steel's resistance to localized pitting corrosion based on its chemical composition. PREN is frequently specified when stainless steels will be exposed to seawater or other solutions with a high chloride concentration.
“We presented our customers with the science necessary to explain critical aspects of welding duplex, super duplex, hyper duplex and specially-alloyed stainless steel materials,” says Roberto Luiz de Souza, Segment Manager, ESAB, one of the paper’s four co-authors. “The research helps them understand the importance of controlling weld parameters and the potential impact on welded joint properties if they do not follow recommended welding procedures. We would like to thank our customers and the publishers of the Saudi Journal for supporting our research, which has been made publicly available on ESAB’s website.”
A Leader in Stainless
The paper is titled Study of the Distribution of Cr, Mo, Ni and N in d Ferrite and Austenite in Duplex Stainless Steels. ESAB researchers observed that the d ferrite phase presents higher chromium and molybdenum contents than the austenite phase; on the other hand, the austenite presents nickel contents higher than the d ferrite (these elements, along with nitrogen, control formation of ferrite or austenite).
“For all duplex stainless steel studied, the PREN of one of the phases, d ferrite or austenite, was below the PREN of the alloy,” noted de Souza, who holds a B.S. in Mechanical Engineering.
Duplex stainless steels (DSS) are steels than contain ferrite and austenite in volumetric fractions approximately equal to 50 percent. Due to their biphasic microstructure, these steels present a favorable combination of the properties of ferritic and austenitic stainless steels, including high mechanical strength, good toughness and ductility and good corrosion resistance in various media. This favorable combination of properties makes this class of stainless steels widely used in the petrochemical, pulp and paper and power generation industries.
The paper’s co-authors are André de Albuquerque Vicente, Process Centre Manager, ESAB (PhD in Chemical Engineering, MSc Metallurgical & Materials Engineering and Welding Specialist), Italo Leonardo dos Santos, Segment Manager, ESAB (B.S. in Mechanical Engineering, post-degree in Welding Automation and Inspection) and Renato Rodrigues de Aguiar, Filler Metals Director, ESAB (B.S., Electrical Engineering, emphasis on Power Electronics, and Master’s in Welding Engineering).
ESAB is a leading world-wide provider of stainless steel filler metals and associated application engineering expertise. Sold under its ESAB and Exaton™ brands, the company offers a broad portfolio of alloys for welding duplex stainless steel with the GTAW, GMAW, FCAW, SMAW and SAW processes. Figure 1 provides guidance on filler metal options, including an innovative hyper duplex filler metal, EXATON 27.7.5.L, with a PREN >50.
“Companies such as Brunei Shell Petroleum Company recommend hyper duplex stainless steel filler metals, such Exaton 27.7.5.L, for the root and hot pass welding of super duplex stainless steels,” adds Martin Denault, Product Manager – Specialty Alloys, ESAB. “Exaton 27.7.5.L offers pitting corrosion temperature (CPT) up to 78 oC, a yield strength of 690 MPa, a tensile strength of 890 MPa and an impact strength of 190J at 20 oC.”
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