US3253942A - Chromium diffusion process and article - Google Patents

Description

United States Patent 3,253,942 CHROMIUM DIFFUSION PROCESS AND ARTICLE Karl Bungardt, Krefeld, and Gottfried Becker, Dusseldorf, Germany, assignors to Howe Sound Company, New York, N.Y., a corporation of Delaware No Drawing. Filed Dec. 15, 1961, Ser. No. 159,740 3 Claims. (Cl. 117-50) This invention relates to the provision of chromium diffused surfaces on various components. In particular it is the purpose of this invention to provide chromium diffused surfaces on articles which are to be subjected to .extreme conditions including exposure to high temperatures and to corrosive environments. Various alloys, ineluding steels and high temperature alloys, have found wide use in applications where stress is experienced under adverse environmental conditions. For example, various steels in view of their high strength are desirably employed in corrosive atmospheres. Similarly, certain iron, nickel and cobalt base alloys containing chromium and other elements are known to have suitable strength at extremely high temperatures.

There is a constant demand for materials which can withstand corrosion in more severe environments and which also can hold up under even higher temperature conditions. In the production of aircraft, rockets, missiles, space vehicles and the like, extreme corrosive atmospheres are realized in areas near the combustion chambers and ever increasing strength-temperature combinations are prevalent. It is also well known that various industrial facilities are subject to extreme corrosive conditions requiring metals which will not deteriorate and which will retain their physical properties over extended periods.

It has been proposed that steels and various other alloys could be surface treated in a manner such that their oxidation resistance to corrosion would be increased and, accordingly, the properties of the alloys would be relatively stable under various extreme conditions. Siliconizing, chromizing and aluminizing techniques have been attempted as surface treatments, but the results have not been entirely satisfactory. Surface layers produced by known techniques have not provided a sufficiently impervious barrier to the corrosive mediums whereby corrosion will eventually proceed as if there were no protective barrier. In addition, prior surface layers have not adequately withstood thermal shock, and these layers have exhibited a tendency to fall away when subjected to rapid and severe temperature differentials. I

It is an object of this invention to provide a'method for improving the oxidation resistance, the resistance to intergranular corrosion and to improve in general the elevated temperature characteristics of alloys including carbon and low alloy steels and the various iron, cobalt and nickel base alloys containing chromium and other alloying ingredients.

It is an additional object of this invention to provide an improved surface treatment for alloys of the above described types whereby the desired stability under adverse environmental conditions can be achieved.

It is a more specific object of this invention to provide a chromizing technique whereby various alloys can be provided with improved oxidation resistance, resistance to corrosion and improved elevated temperature characteristics.

These and other objects of this invention will appear hereinafter, and it will be understood that the specific embodiments hereinafter set forth are provided for purposes of illustration and not by way of limitation.

It has been previously known to enrich the surfaces of iron and steel parts with chromium by diffusion techniques in order to provide corrosion resistance. In the past such ice parts were heated to temperatures between 1000 C. and 1100 C. in the presence of a chromium compound and in the absence of oxidizing gases. Thus, chromium or ferrochromium pieces, along wtih aluminum oxide, porcelain sillimanite and a halogen salt such as ammonium chloride, have been placed in a retort along with parts to be treated in order to diifuse chromium into the surfaces of the parts.

Although the known methods have been practiced only to a limited extent and, in most cases, on alloy steels, it has recently become desirable to provide chromium on parts composed of higher alloy steels and nickel and co balt alloys. Thus, it has been found that various high temperature alloys are strongly attacked at temperatures as low as 700 C. when disposed in hot combustion atmospheres. Similarly, scaling of the components has been detected at temperatures as low as 1000 C. in the absence of strongly corrosive atmospheres. It will be obvious that the expensive high alloyed materials are severely damaged and become useless after a short period of use once they begin to scale or corrode. Conventional chromizing techniques have been employed on such alloys. However, these techniques have only delayed corrosive attack and have not substantially reduced scaling resistance.

It has been found, in accordance with this invention, that high alloyed steels and nickel and cobalt base alloys can be successfully provided with a chromized 'layer whereby the articles will effectively resist corrosion and scaling. It has additionally been found that the procedures of this invention can be profitably employed in place of conventional techniques, even in the case of carbon and low alloy steels. In connection with the procedures of this invention, it has been determined that the chromium diffusion treatment is only effective if the surfaces of the articles to be chromized are prepared in a distinct manner prior to the chromizing operation. Specifically, it has been found that the roughness of the surfaces of the articles to be chromized must be kept below 98 micro-inches and prefrably below 79 micro-inches.

After preparation of the surfaces of the articles, a specific chromizing technique must be employed in order to provide the results of this invention. It must be specifically noted that conventional processing, if applied to articles having the above noted prepared surfaces, will not provide the results of this invention.

In the'specific chromizing process employed, a chromium powder is added in an amount of about 20% to the chromizing compound. The chromium powder is not only used in this relatively small amount, but it is also characterized by a particle size of less than 5 microns.

In a further distinction of the process of this invention, the chromium powder is employed along with inactive oxides such as aluminum oxide or magnesium oxide. The particles of the inactive oxides are preferably retained between the range of 50 and 100 microns. It is important that the inactive oxides or oxide mixtures do not influence the course of the process. It has been suggested that porous ceramic pieces be employed in the chromizing mixture which will serve to retain chromium values and which will serve to retain a consistent concentration within the treating chambers. However, these ceramic pieces or other materials including porcelain and sillimanite are specifically avoided in the present process since these materials have been found to increase the surface roughness of the articles chromized.

The use of a halogen salt along with the chromizing compound is feasible in accordance with this invention. However, the amount of the salt preferred is unusually small and should-only be about 0.01%. The chromizing temperatures and treatment periods are conventional.

However, the highest practical treating temperature should be employed, since this will tend to increase the upper temperature limit of use.

The chromized articles produced in accordance with the described process are characterized by a final surface finish of less than-'98 micro-inches and, in the preferred situation, below 79 micro-inches. The articles produced have an extremely great resistance to corrosion at normal temperatures, and this resistance is retained at temperature limits beyond what heretofore has been considered 1 an upper limit. In addition, the chromized layers have proven exceedingly tough and, accordingly, there is no danger of the layers chipping off when the articles are subjected to thermal shock. The process of this invention has been found satisfactory in connection with extremely low alloyed iron materials. For example, soft iron with a carbon content of less than .05% exhibited a rough surface when chromized by conventional techniques. In accordance with this invention, the desired smooth diffusion surface can be obtained. The value of the disclosed concepts is even more apparent when considering the ability of high alloyed materials to withstand scaling, corrosion and thermal shock. Obviously, turbine vanes, combustion chamber parts and similar articles which are subjected to severe conditions can be expected to perform more satisfactorily when treated in accordance with this invention. The following examples will serve to further illustrate the value of the disclosed chromizing process:

Example I An alloy of the following composition was employed in a chromizing treatment:

Percent Ni 70 Cr 12 W Al 5 Mo 3.5 Ti, Nb, Ta 2.5 Fe r Balance Components prepared from the above composition were placed in a retort along with a chromizing mixture consisting of chromium and the remainder aluminum oxide. The chromium particles exhibited a particle size of less than 5 microns, while the aluminum oxide exhibited an average particle size of about 80 microns. The retort was heated to. 1100 C. whereby a chromized layer was produced in the components with the surface of the com- Fe ,1, =,,ga, Remainder Components composed of alloys of this composition, when treated in accordance with the procedure of Example I, have exhibited chromized layers having the characteristics of this invention.- Specifically, the chromized articles described in Examples I and II having a final surface finish of 98 micro-inches and they reveal an exceptional ability to resist scaling at elevated temperatures and to resist the effects of corrosive atmospheres up to extreme temperatures. The chromized layers, in addition, prove to be tenacious and exceedingly tough whereby thermal shock conditions did not result in deterioration of the component surfaces.

It will be understood that various modifications can be made in the chromium diffusion concepts disclosed which provide the characteristics of this invention without departing from the spirit thereof, particularly as defined in the following claims.

We claim:

1 1. A process for the production of metal articles adapted to withstand surface deterioration at elevated temperatures comprising the steps of processing said articles to achieve a surface roughness of less than 98 micro-inches and chromizing the articles by heating to a temperature above 1000 C. in a powder mixture of metallic chromium and inactive oxides containing about 20% by weight of chromium and in which the average particle size of the v chromium is less than 5 microns and in which the inactive oxides are selected from the group consisting of aluminum oxide and magnesium oxide and having an average particle size within the range of 50 and microns. '2. A process according to claim 1 wherein a halogen salt in an amount of about .01% is included in said mixture.

3. A process for the production of metal articles adapted to withstand surface deterioration at elevated temperatures comprising the steps of processing said articles to achieve a surface roughness of less than 79 micro-inches and chromizing the articles by heating the articles to a temperature above 1000 C. in a powder mixture containing about 20% chromium, about .0l% of a halogengt and inactive oxides and in which the metallic chromium has a particle size of less than 5 microns and wherein the inactive oxides are selected from the group consisting of aluminum oxide and magnesium oxide and have a particle size between 50 and 100 microns.

References Cited by the Examiner UNITED STATES PATENTS 1,853,369 4/1932 Marshall 117 22 2,141,640 12/1938 Cooper 117 22 2,811,466 10/1957 Samuel 11722 2,999,766 9/1961 Ashworth et al 117-50 X FOREIGN PATENTS 363,954 12/1931 Great Britain.

RICHARD D. NEVIUS, Primary Examiner. R. S. KENDALL, Assistant Examiner.

Claims (1)

1. A PROCESS FOR THE PRODUCTION OF METAL ARTICLES ADAPTED TO WITHSTAND SURFACE DETERIORATION AT ELEVATED TEMPERATURES COMPRISING THE STEPS OF PROCESSING SAID ARTICLES TO ACHIEVE A SURFACE ROUGHNESS OF LESS THAN 98 MICRO-INCHES AND CHROMIZING THE ARTICLES BY HEATING TO A TEMPERATURE ABOVE 1000*C. IN A POWDER MIXTURE OF METALLIC CHROMIUM AND INACTIVE OXIDES CONTAINING ABOUT 20% BY WEIGHT OF CHROMIUM AND IN WHICH THE AVERAGE PARTICLE SIZE OF THE CHROMIUM IS LESS THAN 5 MICRONS AND IN WHICH THE INACTIVE OXIDES ARE SELECTED FROM THE GROUP CONSISTING OF ALUMINUM OXIDE AND MAGNESIUM OXIDE AND HAVING AN AVERAGE PARTICLE SIZE WITHIN THE RANGE OF 50 AND 100 MICRONS.