MANUFACTURE OF CORROSION RESISTANT STEEL COMPONENTS

Abstract: A steel component is provided with a black wear and corrosion resistant finish by the steps of forming an epsilon iron nitride or carbonitride layer, bringing the component to gas oxidation temperature and oxidising by a gaseous medium to form a dense black coating, and carrying out a surface finish treatment.

Description: The invention relates to the manufacture of corrosion resistant steel components, and in particular to such components which have an aesthetically pleasing uniform dense black finish. It is known from, e.g. EP-A-0077627 to form a corrosion resistant epsilon iron nitride or carbonitride layer on an alloy steel component in a treatment vessel. According to GB-A-2180264 the treated layer is given a mechanical surface finish, followed by a gaseous oxidation to provide an oxide-rich surface layer. The invention is based on the realisation that if a component having a selected surface layer is subjected to a predetermined gaseous oxidation followed by a predetermined surface treatment the component is provided with both corrosion resistance and an aesthetically pleasing uniform dense black appearance. Further, such a component may be used without the need for a further coating, e.g. a wax sealant or paint or a film of oil. Accordingly, in one aspect, the invention provides a method of manufacturing a corrosion resistant steel component comprising forming an epsilon iron nitride or carbonitride surface layer on the component by nitriding or carbonitriding in a treatment vessel, and then applying a surface finish characterised by the sequential steps of forming the surface layer, contacting the component having the surface layer with a gaseous oxidation medium at a gas oxidation temperature to form by oxidation a uniform dense black coating which comprises Fe₃0₄, and then carrying out a mechanical surface finishing treatment. The invention also provides a method as defined characterised by the sequential steps of forming the surface layer, bringing the component to a gas oxidation temperature in the same treatment vessel as was used for forming the surface layer, introducing a gaseous oxidation medium into the same treatment vessel to form by oxidation a uniform dense black coating which comprises Fe₃O₄, removing the component from the vessel and then carrying out a mechanical the surface finishing treatment. It is an advantageous feature of this invention that the heat treatment stages of the method, i.e. the nitriding or nitrocarburising and gaseous oxidation can be performed in immediate succession in the same treatment vessel, preferably a hot wall vacuum furnace. In one preferred aspect of the invention the nitriding or carbonitriding is carried out in a treatment vessel therefor at a nitriding or carbonitriding temperature, and on completion of this stage, the temperature is lowered to a gaseous oxidation temperature and gaseous oxidation is then carried out in the same vessel. Preferably after the nitriding or carbonitriding the vessel is purged of the nitriding or carbonitriding atmosphere, filled with an inert gas during the cooling and then filled with the gaseous oxidation medium for the oxidation. One advantage of carrying out the gaseous oxidation in the same treatment vessel as that used to form the surface layer is that the conditions of gaseous oxidation can be predetermined, i.e. closely controlled, so that the oxide formed is substantially exclusively Fe₃O₄ as a result of which the layer has a uniform dense deep black colour. The gaseous oxidation may be carried out at any convenient temperature, preferably about 400 o to about 650 o C, more preferably at about 500 o C. The gaseous oxidation medium may comprise oxygen, exothermic gas, steam, CO₂, or a mixture of any of these or any one or more of these with nitrogen; preferably the gaseous medium is lean exothermic gas. Preferably the oxidation treatment is carried for a period of about one hour to form a layer consisting exclusively of Fe₃O₄ so that the component has a uniform dense black colour. At the end of the gaseous oxidation, the component is cooled and then released from the treatment vessel. The nitride or carbonitride surface layer may be formed in a fluidised bed furnace or by a plasma discharge method. The depth of oxide layer is preferably sufficient to resist the later application of a mechanical surface preparation treatment, e.g. polishing, lapping or the like. Preferably, the oxide layer is at least 0.2 µm deep and does not exceed 1.0 µm in depth. The component may be any steel, including carbon steels, non-alloy and alloy steels, and the like. In a specific preferred aspect the invention provides a method of manufacturing a corrosion resistant steel component of uniformly dense black appearance, the method comprising forming an epsilon iron nitride or a carbonitride surface layer on the component, and then applying a surface finish characterised in that the component is placed in a hot wall vacuum furnace, an inert gas is introduced, the temperature raised to a nitriding or nitrocarburising temperature, the nitriding or nitrocarburising atmosphere is introduced and the component is exposed thereto for a period to form the nitride or carbonitride surface layer, the component is cooled to a gaseous oxidation temperature while the vessel is purged of the nitriding or nitrocarburising atmosphere, a gaseous oxidation medium, preferably an exothermic gas, is introduced and the component exposed to the gaseous oxidation medium for a period to form a surface layer which is substantially formed of Fe₃O₄ only, the component is cooled to ambient temperature in an inert gas atmosphere, e.g. nitrogen, removed from the furnace, and then given a mechanical surface finishing treatment. The cooling may be carried out quickly by a conventional quenching method, or slowly in an oxidising or inert atmosphere. These may be performed within or outside the furnace. The invention includes a corrosion resistant black component manufactured by the method, including one subjected to the later quenching step. A component of the invention has high corrosion resistance and is of a deep black colour, and can be used directly, e.g. without a sealant such as wax or a film of oil. In order that the invention may be well understood it will now be described by way of illustration only with reference to the following example. EXAMPLE A damper rod of 080A37 material according to BS 970 was carbonitrided in an ammonia based carbonitriding atmosphere at 610 o C in a hot wall vacuum furnace for 90 minutes. The component in the furnace was cooled to 500 o C during which the carbonitriding atmosphere was purged using nitrogen. After the temperature was stable at 500 o C the nitrogen atmosphere was quickly replaced by a lean exothermic gas using a pump down and back fill procedure and this gas was held there for about 1 hour to oxidise the surface layer to form Fe₃O₄. A layer was formed extending to a depth of 0.5 µm and the colour was a desirable uniform dense black. The furnace and components were cooled to ambient temperature, a nitrogen atmosphere being introduced during the cool down period. The black damper rod was then removed from the furnace and polished to give a surface finish of 0.4 µm Ra maximum. No paint or wax sealant was applied. The polished, black, corrosion resistant damper rod was subjected to a neutral salt spray test according to ASTM B 117 and no corrosion attack took place after 200 hours of exposure.

Claims: A method of manufacturing a corrosion resistant steel component comprising forming an epsilon iron nitride or carbonitride surface layer on the component by nitriding or carbonitriding in a treatment vessel, and then applying a surface finish characterised by the sequential stops of forming the surface layer, contacting the component having the surface layer with a gaseous oxidation medium at a gas oxidation temperature to form by oxidation a uniform dense black coating which comprises Fe₃0₄, and then carrying out a mechanical surface finishing treatment. A method according to Claim 1, including the sequential steps of forming the surface layer, bringing the component to a gas oxidation temperature in tie same treatment vessel as was used to form the surface layer, introducing a gaseous oxidation medium into the same treatment vessel to form by oxidation a uniform dense black coating which comprises Fe₃O₄, removing the component from the vessel and then carrying out a mechanical surface finishing treatment. A method according to Claim 1 or 2, wherein the treatment vessel is a hot wall vacuum furnace. A method according to Claim 1, 2 or 3, wherein the component is brought to a gas oxidation temperature of from about 400 o to about 650 o C, preferably about 500 o C. A method according to any preceding Claim, wherein the nitriding or nitrocarburising is carried out at a nitriding or nitrocarbirising temperature and the temperature in the treatment vessel is then lowered to a gaseous oxidation temperature and gaseous oxidation is carried out. A method according to Claim 5, wherein after the nitriding or nitrocarburising, the vessel is purged of the nitriding or nitrocarburising atmosphere, filled with an inert gas during the cooling and then filled with the gaseous oxidation medium for the oxidation. A method according to any of Claims 2 to 6, wherein the component is cooled from the nitriding or nitrocarburising temperature to ambient temperature and later reheated to the gas oxidation temperature. A method according to any preceding Claim, wherein the gaseous oxidation is carried out in a gaseous medium comprising oxygen, exothermic gas, steam, CO₂ or any one or more of these and nitrogen. A method according to Claim 8, wherein the gaseous medium is lean exothermic gas. A method according to any preceding Claim, wherein the oxidation treatment is carried for a period of one hour to form an oxide layer about 0.2 to 1.0 µm thick and consisting essentially of Fe₃O₄ so that the component has a uniform dense black colour. A method according to any preceding Claim, wherein the mechanical surface finishing treatment is a polish to a maximum roughness of 0.4 µm Ra. A method according to any preceding Claim, wherein the epsilon iron nitride or carbonitride surface layer is formed by a gaseous technique, a fluidised bed technique or a plasma discharge technique. A method according to any preceding Claim, wherein the component having a uniform dense black colour is subsequently subject to quenching.

Inventors: SMITH, JOHN DAVID, VANES, STEPHAN ERIC
Applicants: LUCAS INDUSTRIES PUBLIC LIMITED COMPANY
Application year: 1988
Main classification: C23C8
IPC codes: C23C8