Investigation of diffusion annealing processes of steels with multi-layer Ni–Al coating

Abstract

Ni–Al intermetallics have high heat resistance and therefore they are used as coatings for steel parts running under high temperature conditions. A method for liquid-phase aluminizing of preliminary nickel-plated steel samples followed by diffusion annealing was offered to form such coatings. Liquid-phase aluminizing can form the second aluminum layer on the nickel-plated steel and diffusion annealing provides formation of a coating based on Ni–Al intermetallics. Diffusion annealing (t = 650÷850 °C, τ = 1, 2 and 5 hours) was done at samples coated with aluminum at 750 °С. It was found that the technology offered ensures forming surface intermetallic layers with a thickness depending on the temperature and time of annealing. Annealing at 650÷800 °С during 1÷5 hours provides forming mainly up to 50–140 μm thick Ni2Al3 layer, while NiAl and Ni3Al layers featuring the highest heat resistance are formed at 850 °С during 5 hours. The heat resistance of these coatings showed that due to refractory NiAl layer formation on the surface, the sample resists to failure at a testing temperature of 750 °С during 300 hours.