Mechanism of accelerating creep rupture under high stress level of pack cementation aluminide coatings deposited onto T92 steel tube

An aluminide coating was deposited on commercial T92 steel by pack cementation. The coating exhibits a typical bilayer structure composed of an outer FeAl layer and an inner interdiffusion layer. The creep rupture life of the coated and uncoated T92 was investigated. Results show that the rupture life of the coated T92 decreases significantly, i.e., decreases by 46 % at the creep stress of 110 MPa compared with the uncoated T92. By examining the microstructure and fractography of the coated specimen, the mechanism of aluminide coating deteriorating creep rupture life is clarified. The main fracture mode of T92 substrate with and without coating is microvoid accumulation fracture. The characteristic microstructure of the aluminide coating is responsible for the premature creep rupture. The formation of columnar grains and brittle nature of FeAl phase make it tend to crack during creep. The phase transformation and needle-like AlN precipitates in the inner interdiffusion layer contribute to the inward propagation of cracks and accumulation of creep damage.