In a recent study published in the journal Borders in materials, Swedish researchers investigated the sensitivity of super duplex stainless steel (SDSS) products with the UNS (unified numbering system) code SDSS UNS S32750 under test conditions identical to natural environments and without simulated thermal aging.
To study: Fatigue behavior of super duplex stainless steel exposed in natural seawater under cathodic protection. Image Credit: Maxx-Studio/Shutterstock.com
Super duplex stainless steels
Super duplex stainless steels (SDSS) are used in offshore functions and have significant toughness, strength and corrosion resistance. The microstructure of SDSS consists of a mixture of ferrite and austenite in equal amounts. For offshore applications, SDSS are frequently mixed with carbon steel, through which they are cathodic protected (CP). CP can also be used to protect the SDSS when the operating temperature is above the maximum allowable temperature for alloy use in seawater.
Microstructure of UNS 32750 SDSS alloys (A) thin tube SDSS 1, (B) thin plate SDSS 2, and (VS) thick plate SDSS 3. The ferrite phase appears in dark gray and the austenite phase in light gray. Image credit: Vucko, F et al., Frontiers in Materials
A slight reduction in the corrosion potential of SDSS is beneficial because it reduces the chances of degradation of the stainless steel passivity and the formation of anodic sites, such as crevices. However, several failures in subsea applications have been reported due to cathodic protection, which ultimately results in hydrogen-induced stress cracking (HISC).
The present study aimed to investigate the fatigue behavior of SDSS UNS S32750 in seawater with and without CP. They also determined the impact of grain size, surface defects, load ratio, temperature and frequency on steel fatigue strength and material behavior. Additionally, the researchers also looked at the changes in the microstructure of the material when it is made by orbital welding.
Methodology
Three SDDS samples with different austenite spacings were selected for the study. The sample selection process was based on a linear intercept method derived from standard metallurgical practice. The first sample consisted of SDSS 1 and SDSS 4 tubes with very low austenitic spacing. The second was SDSS 2 thin plates with intermediate austenite spacing, and the third was SDSS 3 thick plates with larger austenite spacing.
In the study, four-point bending was used to perform the in-situ fatigue tests, and the experiments were performed with and without CP. Additional tests were carried out at a temperature of 80±1°C, and a fatigue curve was plotted using 12 samples.
The researchers developed a titanium tank to perform fatigue experiments under seawater. The counter electrode of the tanks was made of titanium mixed metal oxide (TiMMO) and was placed outside the tank to prevent chlorination . Natural seawater from the harbor of Brest was used to fill the tank, and seawater circulation was carried out to renew the total volume of the tanks. In addition, the biological activity of natural seawater was also maintained during the experiments.
The researchers tested the specimens without additional surface preparation, but machining treatment was performed to control the shape of the notches. They performed thermal desorption analysis (TDA) for hydrogen quantification, and a scanning electron microscope was used to examine fracture surfaces and cross sections after fatigue testing. TDA was used to measure hydrogen quantification after pre-charging SDSS samples for 1 to 30 days at temperatures of 20°C and 80°C.
As the mechanical strength of alloys decreases with an increase in temperature, the magnitude of the strength was normalized in the calculations to obtain accurate results.
Equipment to perform the 4-point bending fatigue test in natural seawater with CP. Image credit: Vucko, F et al., Frontiers in Materials
Results
The results indicate that the hydrogen content increases with charging time but stabilizes at certain stages of the experiment.
For fatigue tests with and without CP, the results demonstrated that seawater immersion did not affect fatigue performance. Moreover, these results were similar to those of previous studies conducted on fatigue performance.
Experiments conducted to identify the influence of test temperature indicated that fatigue life was reduced at 80°C compared to results at 20°C. SDSS 3 with greater austenite content had the least fatigue strength, while SDSS 1 and 2 with greater austenite spacing had similar results.
The fatigue performance of specimens tested under CP was not affected by stress concentration. However, the fatigue performance under CP was greater than the open circuit potential (OCP) at an equivalent notch depth, indicating that the addition of hydrogen had a significant impact on the fatigue performance of SDSS.
The fatigue performance of the welded specimens was lower than that of the specimens tested without the orbital weld. The fatigue life of the welded specimens was higher under CP than under OCP. Additionally, failure occurred consistently at the toe of the weld in all tested specimens due to microstructural changes and stress concentration.
Comparison of fatigue life of SDSS 1 (A) in the air and in natural seawater at OCP and (B) at OCP and under CP at -1,100 mV/SCE. Overruns (>1M cycles) are indicated by arrows. Image credit: Vucko, F et al., Frontiers in Materials
conclusion
The study evaluated the fatigue performance of UNS S32750 stainless steel in air and natural seawater with and without CP. The results indicate that SDSS is resistant to hydrogen embrittlement, but critical engineering review is needed for reliable offshore applications.
Source
Vucko, F., Ringot, G., Thierry, D., and Larché, N. Fatigue behavior of super duplex stainless steel exposed to natural seawater under cathodic protection. Boundaries in materials26. https://www.frontiersin.org/articles/10.3389/fmats.2022.826189/full
