US2987805A - Process for surface protection of parts subject to high thermal stress - Google Patents

Description

United States Patent 2,987,805 PROCESS FOR SURFACE PROTECTION OF PARTS SUBJECT TO HIGH THERMAL STRESS Gerhard Kubera and Gisela Prasse, Blnmberg, Baden,

Germany, assignors to Alfred Teves Maschinen- Armaturenfahrik KG., Frankfurt am Main, Germany No Drawing. Filed May 24, 1957, Ser. No. 661,276 Claims priority, application Germany May 26, 1956 1 Claim. (Cl. 29-156.7)

The present invention relates to the surface protection of parts subject to high thermal stress and corrosion, such as valve cones of intake and exhaust valves of combustion engines, turbine blades of jet units, and similar parts exposed to high temperatures and the incidence of corrosion.

It has been found that aluminum coatings are an effective protection against corrosion and scale formation. Thus, it became widely known to coat the exposed surfaces of intake and exhaust valve cones with aluminum in order to extend their useful life.

Aluminum coating is effected by dipping the parts into an aluminum melt or by spraying them with aluminum. Subsequent heat treatment in salt baths results in a diffusion of the aluminum to the marginal zone of the base material. An iron-aluminum alloy is formed thereby at the surface which has high corrosion resistance.

However, the known methods have certain disadvantages. Thus, during the diffusion-heating the accurately finished parts are somewhat distorted, and since in most cases an after-machining is not possible, a large amount of Waste products will result. The applicability of this method is restricted thereby to few cases only.

It is the object of the present invention to overcome the above mentioned shortcomings of known coating methods and to provide a process for surface treatment of parts subject to high thermal stress which is simple, dependable and permits to obtain the desired coated parts with a minimum quantity of waste to be discarded.

According to the invention this is accomplished by the following operational steps: Parts subject to high thermal stress, e.g. intake and exhaust valves of combustion engines, combustion chambers. turbine blades, and the like. are first preformed to a part somewhat thicker than the final shape and coated with aluminum in a manner known per se. Subsequently the parts are heated in an oxygen atmosphere to forging temperature or higher temperature so that a diffusion of aluminum will occur into the base metal as well as oxidation of aluminum at the surface, according to the equation 4Al+3O =2Al,O,. Thus, ceramic material is formed on the surface which, as generally known, is resistant to highest temperatures. After this procedure the parts are forged or pressed to final shape. By that operation, the highly heat-resistant oxides are forged into the base metal consisting of aluminumiron so that they cannot become detached later.

Patented June 13, 1961 Articles treated in this manner will have a much higher heat and corrosion resistance than those produced by conventional methods due to the contents in ceramic components forming part of the surface.

In the following, the method according to the invention will be described by way of example with reference to a valve cone, but it should be understood, that this is only given by way of illustration and not of limitation and that many changes in the details can be made without departing from the spirit of the invention.

Example A valve cone of an exhaust valve of a combustion engine made of known valve steel f.i. silcrom 1, is first extruded. Then the aluminum is sprayed on the head of the valve cone in a known method.

In the next operation the valve cone is heated to a temperature of 960 C. in an atmosphere of oxygen. This will cause a diffusion of the aluminum into the base metal to take place, while simultaneously oxidation at the surface will set in. This treatment is carried out over a period of 3 hours and results in the formation of A1 0 i.e. a highly heat resistant ceramic material. In a final step the articles are forged into final shape. This operation results in the secure union of the ceramic material with the alloy metal and in the formation of an ex cellent protective coating.

What we claim is:

A process for eifecting surface protection of valve cones made of ferrous metals and subject to high thermal stress, by applying to the surface of said valve cones a ceramic material, resistant to high temperatures, which comprises coating said cones with molten metallic aluminum, heating the cones thus coated at approximately 960 C. in an atmosphere of oxygen for approximately 3 hours to allow diffusion of the aluminum into the ferrous metal to occur, while oxidation takes place at the surface of the aluminum-coated cones with formation of highly heat-resistant aluminum oxide, then heating said cones to forging temperature and forging them into the final shape.

References Cited in the file of this patent UNITED STATES PATENTS 1,335,024 Peschko Mar. 30, 1920 1,409,017 Ortiz Mar. 7, 1922 1,823,869 Baur Sept. 15, 1931 1,866,145 Wilson July 5, 1932 2,167,701 Whitfield Aug. 1, 1939 2,303,869 Quinlan et al. Dec. 1, 1942 2,573,229 Stern Oct. 30, 1951 2,845,365 Harris July 29, 1958 FOREIGN PATENTS 731,281 Great Britain June 8, 1955