PSI - Issue 82

S.S.M. Tavares et al. / Procedia Structural Integrity 82 (2026) 192–198 Tavares et al. / Structural Integrity Procedia 00 (2026) 000–000

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1. Introduction Duplex (DSS) and superduplex (SDSS) are austenitic-ferritic stainless steels with high strength and corrosion resistance. These alloys are selected where mechanical strength and good toughness are required in harsh conditions. As a family of stainless steels DSS/SDSS undergoes an increase of production with expansion of applications. Old and new fabrication routes, as well as new commercial grades have been developed over the last few years (Francis and Byrne, 2021). The UNS S39274 is SDSS grade with ~ 25%Cr, ~ 7%Ni, ~ 3%Mo, ~ 0.28%N and 1,5 – 2.5%W (ASTM, 2022). Nowadays, this steel is frequently selected for pipes for the oil and gas industry. Specifically, in the production column below the wet Christmas tree, cold worked seamless pipes of SDSS UNS S39274 are employed in severe sour services (Gunn, 1997). As a relatively new SDSS grade, other applications requiring high pitting resistance may be considered. As mentioned in several previous works, the microstructure that optimize mechanical properties and corrosion resistance of DSSs and SDSSs is composed of ~ 50% of austenite ( g ), ~ 50% of ferrite ( d ), and no other phases (Alvarez Armas and Degallaix-Moreuil, 2009; Chan and Tjong, 2014; Gunn, 1997). Intermetallics such as chi ( c ) and sigma ( s ) are considered deleterious phases because they drastically decrease toughness and corrosion resistance (Alvarez Armas and Degallaix-Moreuil, 2009; Chan and Tjong, 2014; Gunn, 1997; Wang, 2021). The interval for s precipitation in austenitic-ferritic steels is about 600-1000oC with maximum kinetics at 750-850oC (Alvarez-Armas and Degallaix-Moreuil, 2009; Chan and Tjong, 2014; Gunn, 1997; Wang, 2021). The c phase is a Mo-rich cubic phase with chemical formula Fe 36 Cr 12 Mo 10 , which precipitates from the ferrite phase in the temperature range 700-900°C. In general, the intermetallic phases are formed from the ferrite phase, nucleating in g / d and d / d boundaries. Alloying elements such as Cr, Mo, W and Si accelerates the kinetics of precipitation of s and c , while Mo, W, and Si extend the interval of s phase precipitation to higher temperatures (Gunn, 1997). Although other phases, such as chromium nitrides (CrN and Cr 2 N) (Bettini et al., 2014; Gunn, 1997; Knyazeva and Pohl, 2013), G and a ’ phases (Chen et al., 2018; Gunn, 1997; Gutiérrez-Vargas et al., 2021) are also detrimental to corrosion resistance and toughness, the intermetallic s phase is considered the most deleterious, and, thus, is the most studied. Secondary austenite ( g 2 ), which can precipitate with s ( d®s + g 2 ) or independently ( d®g 2 ) is also considered prejudicial to corrosion resistance because of the low Cr and N contents of g 2 (Gunn, 1997). Despite the numerous research works about deleterious phases in austenitic-ferritic stainless steels, the precipitation of intermetallic phases in W-alloyed UNS S39274 grade has not been investigated. Biezna et al. (Biezma et al., 2021) presented a review article analyzing the performance of the four techniques used to detect σ phase: Ultrasounds, Eddy current, electrochemical potentiodynamic reactivation (EPR), and field metallography. R. Wang (Wang, 2021) concentrated his review work on the detection of s phase by the EPR method. Tavares et al. (Tavares et al., 2010) used the ferritoscope to detect the intermetallic phases precipitation in UNS S31803 (“2205”) aged for different periods of time at 800 o C. Since numerous failures of DSSs and SDSSs are attributed to intermetallic phases precipitation (Hitchcock et al., 2001; Smiderle et al., 2014; Tavares et al., 2018), there is a need to develop simple and sensible methods for the detection of these phases in DSSs and SDSSs. In this work, the ferritoscope and linear sweep voltammetry (LSV) methods were studied as methods for early detection of s and c phases in the UNS 39274 steel. 2. Experimental The materials studied was a cold worked seamless pipe of UNS S39274 superduplex stainless steel with chemical composition shown in Table 1. Specimens with 50 x 20 x 5 mm were cut and machined from the as received material. Six heat treatments described in Table 2 were performed to produce different microstructures, with different amounts of deleterious phases.

Table 1. Chemical composition of the UNS S39274 steel. C Cr Ni Mo W S

P

N

Fe

0.028

24.79

6.600

3.220

1.750

0.002

0.017

0.025

Bal.