Stainless Steel World Magazine September 2024 | Page 28

[ Column ]

[ Column ]

Hyper duplex stainless steel

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 .
Q : Can hyper duplex stainless steel welding filler material with PREN above 45 be used for cladding ? Are there any challenges for possible intermetallic formation and 475 ° C- embrittlement versus cladding with super duplex stainless steel ? A : Duplex stainless steels ( DSS ) are a sustainable stainless steel family , providing high corrosion resistance and mechanical properties , giving high strength low maintenance solutions for a wide range of high-duty applications . They have ferritic – austenitic microstructure with normally fine grains . The typical microstructure of weld metal is shown in Figure 1 with ferrite as dark matrix and austenite as light grains . In my previous column 1 , I mentioned that hyper duplex stainless steel ( HDSS ) is one of the newest members of the DSS family with Pitting Resistant Equivalent ( PREN ) above 45 , providing superior mechanical properties and corrosion resistance compared to other alloys in this family thanks to their high chromium and nitrogen contents .
High and low temperature phase transformations It is known that higher chromium contents can increase the risk of intermetallic formation such as sigma , chi , etc . In addition , it can also promote low-temperature phase separations , causing 475 ° C-embrittlement ( which is a type of aging process that causes loss of plasticity in duplex stainless steel when it is heated in the range of 250 to 550 ° C ). Both intermetallic formation and 475 ° C-embrittlement occur in ferrite . One of the reasons is that the diffusion of elements is much faster in ferrite than in austenite , promoting intermetallic formation . For 475 ° C-embrittlement , the miscibility gap in ferrite is responsible for the separation of Cr- and Fe-rich regions , resulting in embrittlement . Therefore , it is the composition and thermomechanical history of ferrite , as well as the ferrite /
Table 1 . Nominal composition of some alloying elements
Grade Group Cr Ni Mo N PREN
ESAB Exaton 25.10.4 . L
ESAB Exaton 27.7.5 . L
SDSS welding wire
HDSS welding wire
Figure 1 : Microstructure of a hyper duplex stainless-steel weld metal , where ferrite and austenite appear dark and light , respectively . austenite interface , that mainly govern the phase transformation kinetics in DSS .
Hyper duplex stainless steel filler metal Table 1 shows the nominal composition of the main alloying elements for super DSS ( SDSS ) and HDSS welding wires . Although high chromium is expected to significantly increase the kinetics of sigma phase formation in HDSS clads compared to SDSS clads , the experimental data did not show this , which will be discussed later . Despite the higher content of chromium in HDSS compared to SDSS filler metal , the content of nickel is lower than that of SDSS . Such an alloy design , in addition to finetuning the other alloying
YS ( MPa )
UTS ( MPa )
25 9.5 4 0.25 42 700 880
27 6.5 5 0.4 49 750 900
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