SURFACE COATING OF MATERIALS FOR HYDROTURBINE APPLICATIONS

Abstract

The silt erosion resistance of non-nitrided and nitrided 16-5 martensitic stainless steel AISI 431 was evaluated according to ASTM G32 standard. Nitriding was carried out in BHEL Haridwar with a mixture of ammonia gas for the duration of 25hrs. The microstructure of the AISI 431 steel was characterized by optical and scanning electron microscopy, along with by X-ray diffraction. Expanded martensite and iron nitrides were produced at the surface of the 16-5 martensitic stainless steel.XRD analysis along with curves of weight loss, comparison between two steels and roughness parameters of both steels were also plotted as a function of exposure time of 16 Hrs. The 28.6+5 μm thick nitride layer has been formed and it showed two distinct regions: a first 5 μm thick of white layer and just underneath the surface comprising precipitated ɛ Fe3N and γ Fe4N nitrides and expanded α martensite and then rest is simplymartensite. Iron nitride precipitation significantly reduced the incubation period, letting detachment of entire grains due to the impact of shock-waves over the surface. Regardless of this, after elimination of the first 5 μm thick layer, the silt erosion resistance enhanced considerably. The correlation between weight loss and time-variation curves, wear mechanisms and surface roughness along with micro hardness were also conferred