Abstract
Super Duplex Stainless Steels (SDSS) have excellent corrosion resistance due to their high concentration of alloying elements like Cr, Mo, and N. There is still, however, disagreement on the role of tungsten in the corrosion resistance of stainless steels. In this regard, the influence of tungsten on tertiary phase precipitation kinetics remains a chief source of controversy. In this study, three different SDSS with different tungsten contents have been investigated, namely, UNS S32750 (W-free), S32760 (0.6 wt% W), and S39274 (2.1 wt% W). Different isothermal aging conditions were studied, followed by microstructure characterization using scanning electron microscopy, energy dispersive X-ray spectroscopy, electron backscatter diffraction, and transmission electron microscopy to quantify the type and volume fraction of tertiary phases and intermetallic compounds. Time-Temperature-Transformation-Corrosion maps were constructed by quantifying the changes in pitting corrosion resistance caused by the precipitation of incremental amounts of deleterious phases. Results showed that 2.1 wt% W additions retarded the precipitation kinetics of all tertiary phases—including σ-phase—favoring the formation of χ-phase. Both χ- and σ-phase affected corrosion resistance, reducing the critical pitting temperature by 10 °C–20 °C at concentrations well below 1 vol%.
Highlights
The Role of Tungsten on the Phase Transformation Kinetics and its Correlation with the Localized Corrosion Resistance of 25Cr Super Duplex Stainless Steels
An analysis of the results presented by Nilsson et al and Hertzman et al.[36,41] shows that the deleterious phases nucleated faster in the W-free filler as the isothermal aging time increased in the 800 °C to 950 °C temperature range, surpassing the volume fraction of the W-rich filler at longer times
critical pitting temperature (CPT) values dropped according to the concentration of deleterious phases within the same Isothermal heat treatments (IHT) temperature, indicating a strong correlation between corrosion resistance and tertiary phase precipitation kinetics
Summary
The Role of Tungsten on the Phase Transformation Kinetics and its Correlation with the Localized Corrosion Resistance of 25Cr Super Duplex Stainless Steels. To cite this article: Cristian Torres et al 2020 J. Soc. 167 081510 View the article online for updates and enhancements. It is well established that the localized corrosion resistance of stainless steels can be improved by alloying with Cr, Mo, and N.1–4. Lorenz and Medawar[5] and later Truman[6] proposed an empirically derived parameter referred to as Pitting Resistance Equivalent (PRE). To correlate the observed localized corrosion resistance of stainless steels with composition (Eq 1). In Eq 1, the sub-index N indicates that the original PRE formula proposed by Lorenz and Medawar was modified to include N.7. PRE and similar expressions only consider that alloying elements are in solid solution and ignore the effects of deleterious phases such as chromium carbides and σ-phase, which can precipitate during welding or improper thermomechanical processing.[7,8,9]
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