Low temperature embrittlement of duplex stainless steels : A study of alloying elements’ effect

University essay from KTH/Materialvetenskap

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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