Surface treatment of metal objects

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.

1 . A method of forming a diffusion surface layer extending inwardly of an outer surface of a metal substrate, said method including:
 (i) in an activation stage, providing an activation treatment furnace containing an inert particulate refractory material and a metal based material for forming said diffusion surface layer, said activation treatment furnace having a flow of an inert gas introduced into the inert particulate refractory material and the metal based material in the activation treatment furnace for a first period of time to treat an outer surface region of said metal based material in the presence of a hydrogen halide gas to form an activated metal based material with an activated surface region; and   (ii) in a diffusion stage, providing a diffusion treatment furnace and introducing said metal substrate into said diffusion treatment furnace, said metal substrate having been pretreated to form a diffusion zone extending inwardly from the outer surface of the metal substrate in which nitrogen has been diffused to form an inner diffusion zone and an outer compound or white layer formed, at least in part, by an iron nitride, an iron carbide or an iron carbonitride compound without an oxide layer on said outer surface of the metal substrate, treating the metal substrate in the diffusion treatment furnace, sealed against the ingress of atmospheric air and under an inert gas atmosphere, in the absence of hydrogen halide gas for at least a second period of time, in the presence of said activated metal based material, to form said diffusion surface layer on said metal substrate.   
     
     
         2 . A method according to  claim 1  wherein the activation treatment furnace is the same as the diffusion treatment furnace. 
     
     
         3 . A method according to  claim 1  wherein the diffusion treatment furnace is different to the activation treatment furnace. 
     
     
         4 . A method according to  claim 1  further including:
 (i) in a pretreatment stage, forming said diffusion zone extending inwardly from the surface of the metal substrate in which nitrogen has been diffused to form said inner diffusion zone and said outer compound or white layer of an iron nitride, iron carbide or carbonitride compound; and 
 (ii) treating the metal substrate formed in said pretreatment stage to either prevent formation of a surface oxide on said surface or to remove any said surface oxide formed on said surface prior to said metal diffusion stage. 
 
     
     
         5 . A method according to  claim 1  wherein the inert gas flow in said activation stage is nitrogen and/or argon. 
     
     
         6 . A method according to  claim 1  wherein the inert particulate refractory material is aluminium oxide or silicon carbide. 
     
     
         7 . A method according to  claim 1  wherein the diffusion treatment furnace contains an inert particulate refractory material that is fluidized by a flow of an inert gas during said metal diffusion stage. 
     
     
         8 . A method according to  claim 1  wherein the diffusion treatment furnace contains an inert particulate refractory material that is, at least partly fluidized by vibration means during said metal diffusion stage. 
     
     
         9 . A method according to  claim 1  wherein ammonia is not supplied to the diffusion treatment furnace during said metal diffusion stage. 
     
     
         10 . A method according to  claim 1  wherein said second period of time is greater than said first period of time. 
     
     
         11 . A method according to  claim 1  wherein during said second period of time a hydrogen halide gas flow is pulsed to said diffusion treatment furnace for periods of no hydrogen halide gas flow and at least one period of hydrogen halide gas flow. 
     
     
         12 . A method according to  claim 1  wherein said diffusion treatment furnace contains an inert particulate refractory material and an inert gas flow is provided to said diffusion treatment furnace during said second period of time, said inert gas flow being variable between a zero flow rate and a flow rate at or above a minimum fluidization velocity for said diffusion treatment furnace. 
     
     
         13 . A method according to  claim 1  wherein a temperature of between 500 and 750° C. is maintained in said activation treatment furnace for said first predetermined period of time. 
     
     
         14 . A method according to  claim 1  wherein a temperature of between 500 and 750° C. is maintained in said diffusion treatment furnace for said second predetermined period of time. 
     
     
         15 . A method according to  claim 1  wherein said hydrogen halide gas is supplied to said activation treatment furnace continuously during said first predetermined period of time. 
     
     
         16 . A method according to  claim 1  wherein said hydrogen halide gas is supplied to said activation treatment furnace during said first predetermined period of time in a pulsed manner for periods of supply separated by periods of non supply. 
     
     
         17 . A method according to  claim 1  wherein said hydrogen halide gas is selected from hydrogen chloride gas, hydrogen bromide gas, hydrogen fluoride gas, and hydrogen iodide gas. 
     
     
         18 . A method according to  claim 1  wherein the hydrogen halide gas is mixed with a said inert gas prior to entry into the activation treatment furnace. 
     
     
         19 . A method according to  claim 1  wherein the hydrogen halide gas is mixed with a said inert gas prior to entry into the diffusion treatment furnace. 
     
     
         20 . A method according to  claim 1  wherein ammonium chloride is supplied to said activation treatment furnace during said activation stage, said ammonium chloride being heated while being introduced to disassociate into nitrogen gas and hydrogen chloride gas for activation of said metal based material. 
     
     
         21 . A method according to  claim 18 , wherein the hydrogen halide gas and the inert gas mixture enters the activation or diffusion treatment furnace at a lower region thereof. 
     
     
         22 . A method according to  claim 1  wherein the metal based material for forming the diffusion surface layer is chosen from at least one of:
 (i) a solid metal or metal alloy; 
 (ii) a metal or metal alloy coated on a substrate carrier; 
 (iii) a particulate or powder metal or metal alloy; 
 (iv) a metal or metal alloy coated on an inert particulate refractory material; 
 (v) a metal halide particle or powder (anhydrous or hydrated); and 
 (vi) a metal halide material (anhydrous or hydrated) coated on an inert refractory particulate material or a substrate carrier. 
 
     
     
         23 . A method according to  claim 1  wherein the metal based material for forming the diffusion surface layer is selected from chromium, titanium, vanadium, niobium, tantalum, tungsten, molybdenum, manganese, and alloys thereof including ferrous based alloys, or metal halides comprised of a metallic element of the aforesaid metals and a halide selected from chlorine, bromine, iodine and fluorine. 
     
     
         24 . A method according to  claim 1  wherein the metal substrate is a ferrous based metal or a ferrous based metal alloy. 
     
     
         25 . A method according to  claim 1  wherein said inert gas introduced into said diffusion treatment furnace during said second predetermined period of time is nitrogen.