Early Stages of Deleterious Phases in Super and Hyper Duplex Stainless Steel and Their Effect on Toughness
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Duplex stainless steels combine excellent mechanical properties with good corrosion resistance. Consequently, they are in frequent use in various highly demanding applications, like Oil & Gas, chemical processing, pulp and paper, and desalination plants. Due to their high alloying contents, super and hyper duplex are susceptible to the formation of detrimental secondary phases such as chromium nitrides, chi, and sigma phase.
The presented investigation deals with the precipitation of those phases in super and hyper duplex by employing rapid isothermal heat treatments on a Gleeble 3800 thermo-mechanical simulator. The Charpy-V impact toughness was measured as a function of the sensitization time and temperature in order to characterise their effect on low temperature toughness. Both alloys suffer from severe embrittlement at the early stages of sensitization. In super duplex at 850 °C for 20 s, chromium nitrides are precipitated, a chi-phase is formed after 200 s, and small amounts of sigma after 500 s. In hyper duplex, the precipitation sequence is the same but significantly faster. In this alloy, the sigma-phase is encountered after merely 50 s and ferrite is completely consumed after 500 s.
KeywordsSuper duplex stainless steel Hyper duplex stainless steel Isothermal heat treatment Sigma-phase Chi-phase Chromium Nitride Precipitation Embrittlement Impact toughness
Frühphasen schädlicher Phasen in Super- und Hyperduplex-Edelstahl und ihr Einfluss auf die Zähigkeit
Duplex-Edelstähle verbinden hervorragende mechanische Eigenschaften mit guter Korrosionsbeständigkeit. Daher werden sie häufig in verschiedenen, sehr anspruchsvollen Anwendungen eingesetzt, wie z. B. in der Öl- und Gasindustrie, der chemischen Industrie, in der Zellstoff- und Papierindustrie sowie in Entsalzungsanlagen. Super- und Hyperduplex sind aufgrund ihres hohen Legierungsanteils anfällig für die Bildung schädlicher Sekundärphasen wie Chromnitride, Chi- und Sigma-Phase.
Die vorgestellte Untersuchung beschäftigt sich mit der Ausfällung dieser Phasen im Super- und Hyperduplex durch den Einsatz schneller isothermer Wärmebehandlungen auf einem thermomechanischen Gleeble 3800 Simulator. Die Charpy-V Kerbschlagzähigkeit wurde als Funktion der Sensibilisierungszeit und -temperatur gemessen, um ihren Einfluss auf die Tieftemperaturzähigkeit zu charakterisieren. Beide Legierungen weisen in den frühen Phasen der Sensibilisierung eine starke Versprödung auf. Im Superduplex bei 850 °C für 20 s werden Chromnitride ausgefällt, nach 200 s bildet sich eine Chi-Phase und nach 500 s kleine Mengen Sigma. Im Hyperduplex ist die Fällungssequenz gleich, aber deutlich schneller. Bei dieser Legierung tritt die Sigma-Phase bereits nach 50 s ein und Ferrit ist nach 500 s vollständig verbraucht.
SchlüsselwörterSuperduplex-Edelstahl Hyperduplex-Edelstahl Isotherme Wärmebehandlung Sigma-Phase Chi-Phase Chromnitrid Fällung Versprödung Schlagfestigkeit
Duplex stainless steels consist of balanced amounts of ferrite and austenite, resulting in high corrosion resistance and excellent mechanical properties . By increasing the alloying content, super and hyper duplex were developed . Both duplex steels are stronger than equally resistant austenitic stainless steels and considerably cheaper than nickel base alloys. Hence, they represent a cost effective alternative when mechanical strength and corrosion resistance are required at the same time. However, the high alloying content in super and hyper duplex may result in the precipitation of unwanted detrimental phases. The most prominent in that regard are chromium nitrides, chi- and sigma-phase. They typically form between 700–1000 °C, due to inadequate cooling or excessive reheating during fabrication, processing, or welding. These secondary phases result in severe embrittlement and reduce the corrosion resistance due to segregating alloying elements.
Proper information about the precipitation kinetics of intermetallic phases in super duplex is available in the literature [3, 4, 5, 6], but it is limited for hyper duplex [7, 8]. However, most of the reported investigations consisted of quenching from the solution annealing temperature with subsequently reheating to a designated sensitization temperature. In many cases, this was done with a slow heat transfer from air to steel. Hence, short tempering durations include a substantial uncertainty in respect to the actual material temperature. This blurs transformation temperatures and kinetics substantially.
Since the formation of deleterious phases in super and hyper duplex occurs rapidly, a precise time-temperature control is necessary. To ensure that in the presented investigation, a Gleeble 3800 thermo-mechanical simulator with conductive heating was used to apply a proper heat treatment.
Chemical composition and PREN(W) of SDSS and HDSS, in wt. %
After applying the heat treatment, a notch was machined at the connecting sites of the thermo-couple to ensure a constant temperature in the tested volume.
The impact toughness was determined according to , at a temperature of −46 °C. Electrochemical etching with 4% NaOH was applied to reveal the microstructure in the tested area after impact testing. A SEM Zeiss Ultra 55 was used for further microstructural characterisation; all samples were grinded and subsequently polished with OPS.
During the early stages of sensitization, SDSS exhibits a low toughness below 850 °C. The lowest impact values after an extended temperature exposure is obtained at 850 °C, which corresponds to the maximum rate of embrittlement (the nose temperature).
Regardless of the actual alloying composition, small amounts of precipitates cause severe embrittlement in super and hyper duplex stainless steels. At −46 °C even precipitate free hyper duplex hardly reaches 30 J of impact toughness and consequently falls behind super duplex, which maintains 300 J in a precipitate free condition. This difference may be caused by dissimilar ductile-to-brittle transition temperatures. Yet, due to limited information regarding the transition temperature in hyper duplex, this is up for debate. Regardless, hyper duplex exhibits a lower toughness in every tested combination of time and temperature.
Both alloys initially form chromium nitrides during the early stages of sensitization at their respective nose temperatures. Nitrides are most frequently present at the ferrite/ferrite grain boundaries. Further sensitization leads to the precipitation of the chi-phase, mainly decorating ferrite/ferrite grain boundaries and, in smaller quantities, ferrite/austenite interfaces. A longer temperature exposure results in the formation of a sigma-phase at both interfaces. This precipitation sequence is particularly well documented for super duplex, as can be seen in Fig. 6a–d. In this case, the sigma-phase is preceded by chi and chromium nitrides and is eventually formed after 500 s of sensitization. In hyper duplex, the precipitation at its nose temperature is considerably faster, and noticeable amounts of a sigma-phase are already present after 50 s.
Nevertheless, in both alloys, the sigma-phase is not solely responsible for a low impact toughness, especially during the early stages of sensitization. This becomes particularly obvious by comparing the high and low sensitization temperatures in the super duplex. At 900 °C and after 50 s, the microstructure is free of nitrides, and the toughness remains at 200 J. Yet, after sensitizing at 750 °C for 50 s, nitrides form, and the toughness drops to roughly 100 J. The subsequent formation of chi further reduces the toughness substantially, even before sigma precipitates. This detrimental effect of nitrides and chi is known in literature, though sigma is mostly seen as the root cause for low impact values, which is clearly not the case in the presented investigation.
These unforeseen results most likely arise from diverging heat treatment parameters between this investigation and those in literature. Firstly, most of the available literature concerns quenched and tempered material. By quenching to ambient temperature and subsequently reheating, diffusion processes may influence the precipitation sequence. Secondly, nitrides and the chi-phase form rapidly, especially at the nose temperature. Uncontrolled reheating conditions can distort early precipitation states and hide the formation of chromium nitrides and the chi-phase.
Therefore, inaccurate sensitization conditions are of particular concern for hyper duplex, with its rapid precipitation kinetics. Sigma formation occurs in under one minute of tempering, and insufficient reheating conditions would hide the detected precursor phases.
Hyper duplex forms detrimental phases multiple times faster than super duplex, which leads to a rapid embrittlement, especially at low service temperatures.
Minute quantities of detrimental phases result in a substantial reduction of impact toughness. No discrimination between the contributions of the individual phases was possible.
The following precipitation sequence was observed in both alloys: chromium nitride over to chi-phase over to sigma-phase. This was shown for the peak transformation temperatures of 850 °C and 900 °C in super and hyper duplex, respectively.
Chromium nitrides form especially fast at temperatures below the nose temperature and mainly cover the ferrite/ferrite interfaces.
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