DIFFUSION COATINGS OF STEELS: FORMATION MECHANISM AND MICROSTRUCTURE OF ALUMINIZED HEAT-RESISTANT STAINLESS STEELS.

Abstract

The effect of base-alloy composition on the microstructure and mechanical and thermal stabilities of aluminum diffusion coatings has been studied for 316, 310, and I800H stainless steels by optical, microprobe, transmission/scanning transmission electron microscopy, and microhardness testing. In all the diffusion aluminized alloys, two distinct coating layers form: an outer aluminide layer and an inner, interdiffusion layer. The substrate austenite stability is the single most important parameter affecting the thickness, phase distribution, and microchemistry of these two layers. TEM/STEM analyses showed that the interdiffusion layer is a 'natural composite' made up of a uniform dispersion of the hard nickel aluminide phase (B2) in a soft ferrite matrix. Formation of this layer involves 'ferritization' of the substrate, a process akin to pearlitic transformation in carbon steels. The interdiffusion layer demonstrated high hardness with good mechanical integrity.