The consolidation and transformation of an ultra-ferritic stainless steel by hot isostatic pressing

[摘要] Ferritic stainless steels possess a number of properties which render themsuperior to austenitic stainless steels in certain applications. The ferritic stainlesssteels are highly resistant to stress-corrosion cracking and are generally of lowercost. Previous work had shown that the corrosion properties of the ferritic steelswere optimised at a chromium content of forty percent. Extensive research hadalready characterised the mechanical and corrosion behaviour of the Fe40Cralloy in the cast and wrought form. This investigation involves the developmentof the material by powder metallurgy processing. Particular emphasis is placedon a proprietary powder production technique as well as on powderconsolidation by Hot Isostatic Pressing. The effect of Ni, Mo, Ru, Nb, Al and Feadditions and well as various techniques of introducing these additions areexamined. The unique effect of powder metallurgy manufacture onmicrostructure and the kinetics of sigma phase formation is highlighted.Corrosion tests in various concentrations of sulphuric acid and an industrial fieldtrial were performed. It was found that combined Ni and Mo contents exceedingfour percent resulted in severe embrittlement due to widespread formation ofsigma phase during the HIP process. The use of powder metallurgy techniqueswas also found to enhance the kinetics of sigma phase formation in a particularalley when compared to the as-cast state. Direct additions of Fe powder weresuccessful in inhibiting embrittlement, possibly due to a mechanism of Fediffusion into Cr-rich regions. This lowered of the Cr content in these regions,thereby reducing the tendency for sigma precipitation. Corrosion tests indicatesatisfactory resistance for the Fc-40Cr-2Ni-2Mo alloys in 70 weight percentsulphuric acid at 50°C. Alloys with 0.2 percent Ru additions were found to beespecially resistant, due to the role of Ru as a cathodic modifier. The method ofmixing and diffusion bonding metallic powders of varying composition wasfound to be feasible and certain novel combinations of Ni and Fe-rich alloysexhibited satisfactory corrosion resistance. A mathematical diffusion model wasfound to provide an order of magnitude approximation of the time required forhornogentsation in a certain mixture of powders.

[发布机构] University of the Witwatersrand