strathSEEA

Expanded Austenite Phase in Nickel

 

 

Nickel which encourages the formation of an austenitic phase is very important to the formation of an expanded austenite layer. The addition of nickel makes the austenite phase more stable at lower temperatures, as well as delaying the formation of chromium carbides precipitates which would lessen the corrosion resistant properties of the surface.

 

Two of the examined samples (Inconel 617 and Nikrothal) were nickel based chromium alloys and not iron based. The excellent corrosion resistant properties of nickel based alloys makes them ideal for use in the nuclear and aerospace industry. And as the nickel is of an FCC structure, it is possible for an expanded austenite later to form. High temperature treatments, typical to those undertaken on stainless steel, have been successful in hardening the surface, but came at the expensive of the alloys corrosion resistance. However most s-phase formation has been carried out using a nitriding process, with little information available on carburising processes on nickel based alloys.

 

S-phases formed via plasma nitriding processes in Ni-Cr alloys have shown some different results. The formation of an s-phase in Inconel 690 by ‘Czerwiec’ was found to be a lot slower compared to Fe-Cr-Ni alloys. It was also found to produce a dual layer structure which was further confirmed by a second study carried out by ‘Williamson.’ This was thought to be due to the “uphill diffusion of carbon in the bulk material.”

 

Therefore the analysis carried out in this investigation regarding carburised nickel based alloys will be relatively pioneering.