Low temperature embrittlement of duplex stainless steels : A study of alloying elements’ effect
Abstract: Duplex stainless steels (DSSs), consisting of an equal amount of ferrite and austenite phases, have wide applications in e.g. vehicles, chemical engineering as well as nuclear plant because of the combination of excellent mechanical properties and corrosion resistance. However, low temperature embrittlement has existed for a few decades restricting the application of DSSs over about 250 ºC. When the service temperature ranges from around 250 to 500 °C, DSSs would become brittle because of the phase separation in the ferrite phase. The phase separation is the main reason for the low temperature embrittlement, and the kinetic of this phase separation achieves the fastest rate at about 475°C, so it is also termed as ‘475°C embrittlement’. Plenty of research has been performed in this field, but the problem remains. The mechanism of phase separation is due to the existence of a miscibility gap in the iron chromium binary system, and previous research has reported some alloying elements can have the potency to delay the phase separation and the goal of my thesis is to investigate the influence of different alloying elements and select one which could be a plausible one to retard phase separation, and subsequently try to mitigate the low temperature embrittlement problem of DSSs. This work includes the literature survey of different alloying elements which could influence the microstructure and mechanical properties of DSSs in general. Subsequently the thermodynamic calculation was performed to identify the effect of the selected alloying elements addition on phase formation during heat treatment. Vanadium was selected to be a potentially suitable alloying element to be added into DSSs and experimental investigations were performed on the heat treatment process and the effect of V alloying. The main conclusions of the proposed thesis can be drawn as follow: The elemental addition of Al, Si, V, Nb and Ti are calculated by Thermo-Calc, they are all ferrite stabilisers and V addition seems most likely to be effective due its combination with the interstitial elements C and N. In the experimental part, vanadium additions combined with intermediate temperature solution treatment could be effective to retard the age hardening effect and the impact toughness test has a consistent tendency. Also, according to Thermo-Calc calculations and experimental results, the more interstitial elements that combine with V and precipitate from the ferrite phase the better was the performance of the duplex stainless steels.
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