Abstract: A process for forming an outer diffusion surface layer on a metal substrate or member includes in a first activation stage of an inert particulate refractory material and a metal based material including metals and metal halides. An inert gas and hydrogen halide gas is introduced into the inert particulate refractory material and the metal based material to activate an outer surface of the metal based material. The metal substrate is pretreated to form a diffusion zone extending inwardly from the outer surface of the metal substrate having nitrogen forming an inner diffusion zone and an outer compound or white layer of an iron nitride, an iron carbide or an iron carbonitride compound without an oxide layer. A subsequent diffusion stage treats the metal substrate in an inert gas, in the absence of hydrogen halide gas to form the diffusion surface layer on the metal substrate.

Abstract: The specification discloses a method and apparatus enabling the formation of a diffusion surface layer on a surface of a metal substrate, typically a ferrous based metal substrate, wherein in a first stage in a first fluidized bed furnace, a diffusion zone is formed extending inwardly from the surface of the metal substrate in which nitrogen has been diffused to form a nitride or carbo nitride inner zone and an outer white layer that is substantially free of porosity, treating the substrate formed in the first stage to prevent formation of or remove any surface oxide on the surface of the substrate, and in a second stage separate from the first stage, holding the thus treated substrate in a fluidized bed furnace operated under an inert atmosphere and fluidized by a flow of inert gas or gases, the substrate in the fluidized bed furnace being treated in the presence of a halide gas and a particulate metal or metal alloy.