Column: Hyper duplex stainless steel

Figure 1: Microstructure of a hyper duplex stainless-steel weld metal, where ferrite and austenite appear dark and light, respectively.

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 column¹, 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/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 fine-tuning the other alloying elements, resulted in quite a similar ferrite composition in SDSS and HDSS weld metal. Acuna² showed that at the salvus temperature of sigma phases, there is no significant difference between the ferrite composition of HDSS and SDSS weld metal. More interestingly, the PREN of ferrite and austenite is about 48 for HDSS. However, in SDSS, PREN is 48 for ferrite and 38 for austenite at the solvus temperature of the sigma phase. This means that we expect quite similar kinetics for sigma phase formation in both alloys as well as a higher corrosion resistance for HDSS, if no unwanted secondary phases are present. Experimentally, Acuna et al.³ verified that the kinetics of sigma phase formation in HDSS is quite similar to SDSS clads particularly up to 1% sigma phase. After 1%, however, HDSS showed slightly faster kinetics. However, it is already known that 1% sigma phase is above the limit that can degrade the mechanical properties and corrosion resistance of the welds.

Hosseini et al.⁴ also showed that the kinetics of 475°C-embrittlement in HDSS weld is slower than that of SDSS welds in the solution-annealed condition at 1150°C, where it was observed that ferrite in both HDSS and SDSS has quite similar chromium content, but higher nickel in SDSS. Therefore, the faster embrittlement of SDSS weld can be attributed to the higher nickel in SDSS, known for promoting the 475°C-embrittlement.

Cladding using HDSS

A systematic study for cladding of low-carbon steel by HDSS filler metal was performed at Ohio State University2. It was seen that HDSS was not prone to the sigma phase formation during cladding using mechanized gas tungsten arc welding (GTAW). The reported critical pitting resistance temperature of the HDSS overlays was in the range of 60-75°C using different welding parameters. The cladded samples showed yield and ultimate tensile strengths around 700 MPa and 1100 MPa, respectively.

Final remark

HDSS welding wires may be used for cladding as a higher-performance alternative for SDSS where high corrosion resistance is required. HDSS welds/clads showed similar kinetics of sigma phase formation and 475°C- embrittlement compared to SDSS welds/clads but higher PREN, making them more resistant to local corrosion.

References

1. Hosseini VA. Hyper duplex stainless steel: a sustainable choice for high-performance applications. Stainless Steel World. 2024;36:44-45.
2. Fischdick Acuna AF. An ICME Approach for Sigma Phase Formation Kinetics on Highly Alloyed Duplex Stainless Steels: The Ohio State University; 2023.
3. Acuna A, Riffel KC, Ramirez A. Sigma phase kinetics in DSS filler metals: A comparison of sigma phase formation in the as-welded microstructure of super duplex stainless steel and hyper duplex stainless steel. Materials Characterization. 2024;207:113433.
4. Hosseini VA, Thuvander M, Lindgren K, et al. Fe and Cr phase separation in super and hyper duplex stainless steel plates and welds after very short aging times. Materials & Design. 2021;210:110055.

About this Tech Article

Appearing in the September 2024 issue of Stainless Steel World Magazine, this technical article is just one of many insightful articles we publish. Subscribe today to receive 10 issues a year, available monthly in print and digital formats. – SUBSCRIPTIONS TO OUR DIGITAL VERSION ARE NOW FREE.

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