Stainless Steel World Magazine June 2024 | Page 44

Q : Our team is considering hyper duplex for an upcoming chemical industry heat exchanger project , what do we need to consider when selecting welding filler material ? A : Duplex stainless steels ( DSSs ), with ferriticaustenitic microstructure , have an excellent combination of corrosion and mechanical properties . Since their development , they have been pushed towards higher contents of alloying elements , particularly chromium , nickel , and nitrogen , aiming for higherperformance applications . Typical composition and mechanical properties of different DSS are summarized in Table 1 . To rank duplex stainless steels , the Pitting Resistance Equivalent number ( PRE ) is used , where the higher the number , the higher the pitting resistance . There are different formulas among them , PRE based on nitrogen content ( PRE N ) is normally used to rank DSSs : PRE N = % Cr + ( 3.3 .% Mo ) + ( 16 % N ). Standard , Super , and Hyper DSSs have a PREN of 35-40 , 40-45 , and ≥ 45 , respectively . This makes hyper duplex stainless steels ( HDSSs ) the highest corrosion resistance alloy among all DSS groups . From the view of mechanical properties , HDSSs have higher yield and ultimate

tensile strength compared to the other grades . The unique alloy design provides high toughness values comparable with other DSS grades . Coupled with their excellent corrosion resistance , HDSS is the highest-performing grades among all DSSs .

Alloy design The thermodynamic calculation , Figure 1a , shows that an equal fraction of ferrite and austenite may be achieved at temperatures around 1100 ° C . Heat treatment or reheating below that temperature may result in the formation of detrimental phases such as sigma phase and nitride .

Meet the columnist

Dr Vahid A Hosseini

Dr Vahid A Hosseini joined ESAB in 2021 and is the manager of the Research Team at R & D Filler Metal , ESAB , Gothenburg . He received his doctoral degree related to Welding Metallurgy of Stainless Steels in 2018 . Before joining ESAB , he worked as an assistant professor at University West , Sweden . His background is in welding metallurgy , working on processing-microstructureproperties relationships in welding and additive manufacturing of ferrous and non-ferrous alloys . a ) b ) 1.0

52

Liquid

Ferrite 0.8

50

Mass fraction of all phases

0.6

Figure 1 : Thermodynamic calculations , a ) equilibrium phase fraction of SAF 2707 HDSS b ) PREN values of ferrite and austenite in SAF 2707 HDSS and 2507 SDSS

Table 1 . main alloying elements in weight percent and properties of some known DSS grades Grade Group Cr Ni Mo N Co PRE N

YS ( MPa )

UTS ( MPa )

2205 Standard DSS 22 5 3 0.2 – 35 485 680-880

2507 Super DSS 25 7 4 0.3 – 43 550 800-1000 SAF 2707 HD HDSS 27 7 5 0.4 1 48 700 920-1100 SAF 3207 HD HDSS 27 7 3.5 0.5 – 50 ≥700 ≥850

ESAB Exaton 27.7.5 . L

HDSS welding wire

27 7 5 0.4 1 49 750 900

* Alloys contain Si and Mn as well as traces of other elements . Max carbon content is 0.03 wt .%.

PRE N

48

Austenite HDSS Ferrite HDSS

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