Ferrite percentage prediction of duplex stainless steels as a function of cooling rates

Liao, Longlong
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Highly alloyed CD3MWCuN was chosen to investigate the influence of cooling rates on the ferrite percentage of duplex stainless steels. It is known that cooling rate can influence the overall ferrite / austenite ratio, and achieving the proper balance of phases is crucial in producing a casting with suitable mechanical and corrosion properties. In this thesis cooling was initially investigated as a function of the solidification rate from the molten state and the rate of cooling from the homogenization temperature. It was quickly realized that given a suitable homogenization temperature and time the solidification cooling rate was of minimal importance. However, homogenization temperature and the cooling rate from that temperature has a large effect on ferrite percentage. Homogenization cooling rates ranging from 10à ° to 1000à °C/hr and temperatures of 1150à °C, 1205à °C, and 1250à °C were selected to investigate how the cooling rate and temperature will influence the ferrite content. In addition to the base alloy six different composition steels based on CD3MWCuN (containing à ±10% relative amounts of Mo, Cr, and Ni) were produced to investigate how changing the chromium / nickel equivalent value ratio (Creq / Nieq) will influence the ferrite content. It was found that ferrite content increases with an increase of homogenization cooling rate and temperature. In addition, the ferrite content increases with the increase of Mo and Cr content and decreases with Ni content.

Diagrams for ferrite content in CD3MWCuN as a function of cooling rates and homogeneous temperature are proposed. One diagram proposed allows the ferrite content of CD3MWCuN homogenized between 1150à °C to 1250à °C and cooled at rates between 500à ° to 1000à °C/hr to be predicted. Another diagram allows ferrite content to be predicted as a function of Creq / Nieq ratio, enabling the performance of other alloys besides CD3MWCuN to be estimated. This proposed diagram covers the Creq / Nieq ratio between 1.34 to 1.75 and rapid cooling rates between 500à ° to 1000à °C/hr.

BSE-SEM, Cooling rate, Duplex stainless steels, Ferrrite content, Point-count