US3438802A - Siliconized alpha-delta ferrous alloy - Google Patents

Description

United States Patent Oiice 3,438,802 Patented Apr. 15, 1969 3,438,802 SILICONIZED ALPHA-DELTA FERROUS ALLOY Jerome J. Kanter, Palos Park, Ill., assigner to Crane C0., Chicago, Ill., a corporation of Illinois Filed May 25, 1962, Ser. No. 197,702 Int. Cl. B44d 1/09; C23c 13/02 U.S. Cl. 117-106 6 Claims This invention relates to the impregnation of metals with silicon, and especially to the formation of an adherent coating, or case, containing silicon upon ferrous metal articles.

It is well-known that silicon-iron alloys yare not only resistant to mechanical wear and abrasion, but are also corrosion resistant which makes then highly desirable for certain industrial uses. But such alloys are also brittle, of relatively low strength and toughness, and difficult to machine. For this reason, although their exceptionally desirable properties might have resulted in extensive industrial use, their concomitant undesirable properties has limited their use.

Attempts have been made by workers in the eld to produce articles having surface coatings, or cases, of high silicon-iron alloys adhering to strong tough cores of iron or steel. The desired articles are first fabricated in part or substantially to their desired final form from iron or steel of the desired strength, toughness and other mechanical properties. Then the surfaces of the articles, or some selected portions thereof, are siliconized to cause impregnation, penetration, or diffusion of silicon into the metal of a sufficient depth to provide a protective skin, layer, or case. This process is illustrated by the following United States patents: Ihrig Patent Nos. 2,163,753; 2,142,941; 2,157,902; and 2,109,485; Henderson Patent No. 2,501,051; Eckman Patent No. 2,897,093.

In spite of the -attractive potentials of the siliconizing processes illustrated by these patents, commercial utilization has been limited. Aside from other defects, the siliconized products have been characterized generally by poor adherence between the case containing silicon and the underlying core. As a result, siliconized articles have a pronounced tendency for the case to spall off, or for cleavage between the two layers. Spalling, or cleavage, is especially apparent upon subjecting the articles to wide changes in temperature, mechanical shock, and the wide variety of usages encountered in actual service conditions.

Commercial ulitization has been limited also to a restricted class of ferrous metals. Workers in the eld have heretofore believed that only carbon steels were commercially satisfactory, and generally regarded alloy steels unsatisfactory. The reason for this attitude has been due apparently to the fact that some processes in which free chlorine was present, the chlorine reacted with chromium in the steel alloys to form an undesirable refractory coating of chromium chloride.

A general object of the invention, therefore, is to avoid the difficulties and objections characterizing prior siliconizing processes, and to produce articles containing a silicon case superior to those heretofore obtainable. A more specific object of the invention, and one of importance, is to produce ferrous metal yarticles having a firmly adherent case, or coating, containing silicon, which is less susceptible to spalling or cleavage between the case and the underlying core. A further specific object of the invention is to produce ferrous metal articles having a high-silicon case on bases, or cores, of iron or steel `alloys of compositions generally regarded as impractical. It is a still further specific object of the invention to reduce the scrap loss in siliconizing processes, It is a yet further object of the invention to produce a case containing silicon of high density. It is a still yet further object of the invention to produce siliconized products characterized by fewer microiissures. It is a still yet 'further object of the invention to provide siliconized -articles having a higher corrosion resistance than obtainable herebefore. Another object of the invention is to provide a process for siliconizing steel alloys. Further objects and advantages of the invention will be apparent. from a study of the description and appended claims.

Briefly stated, the present invention is directed to siliconizing ferrous base articles which have a stable body centered cubic crystal structure, for example, alpha-delta alloys. More particularly the invention is directed to siliconizing ferrous base articles that are free from phase transformations up through the siliconizing temperatures, usually in the range from 1300 F. to about 2000 F.

The accompanying drawing illustrates diagrammatically in cross-section a product prepared in accordance with the present invention in which a ferrous article having a body centered cubic crystal structure for example, alpha-delta alloy, is siliconized to form an adherent case or coating.

The siliconizing process employed may be one, or more, of the processes set forth in the Ihrig, Henderson, or Eckrnan patents set forth hereinabove, `although the Eckman process is preferred. These processes are carried out by heating the article to be treated to elevated temperatures, usually in the range from 1300" F, to about 2000 F., more particularly from about 1500 F., to `about 1900 F., generally above 1650 F., and, in most instances, at around 1850 F. At elevated temperatures the articles are contacted with siliconizing reagents. The siliconizing reagents may be solids, gases, or combinations of the foregoing, including the reagents given in the above patents.

According to the present understanding of the chemical mechanisms involved, each of the processes forms silicon tetrachloride which reacts with the surface of the metal to form silicon and luy-product chlorine gas. The elemental silicon thus formed inoculates the outer layer, or portion, of the treated ferrous article.

According to one procedure, the articles receiving the siliconizing treatment are placed in a horizontal drum type retort. The retort is connected to a source of substantially dry inert gas, such as nitrogen, neon, or argon, which is introduced to displace the air. After the preliminary flushing the ow of nitrogen, for instance, is reduced so as to be sufficient to make up for losses that may occur due to leakage. Then the retort is rotated and heat is applied, for example by electrical heating elements on the sides, to raise the temperature to proper siliconizing levels. The temperatures are not critical, but generally best results are obtained at approximately 1850 F.

After the siliconizing temperature is attained, the flow of the inert gas is shut off and silicon tetrachloride gas is introduced into the retort. This gas is generated by applying heat to a receptacle containing liquid silicon tetrachloride. Alternatively, the silicon tetrachloride may be introduced by a carrier gas which is bubbled through the silicon tetrachloride liquid and then conducted into the retort. After the retort has filled with the siliconizing reagent, with or without the presence of the carrier, the flow of the reagent is reduced to a slow, continuous in- -gress for the siliconizing process.

In the preferred process, as more particularly described in the United States patent to Eckman given above, silicon carbide is added to the retort, so that the reaction between the siliconized article and the silicon tetrachloride takes place in the presence of silicon carbide. The silicon carbide controls, or reduces, the reaction with the iron. In an alternative process, as more particularly described in the United States patent to Henderson given above, hydrogen is introduced into the retort. The foregoing and other agents, or controls, may be employed as deemed expedient.

Ordinarily, the exposure time of the treated articles to the siliconizing atmosphere is from about 0.5 to about ve hours. Usually satisfactory results are obtained within one to three hours.

By controlling the time of exposure to the siliconizing atmosphere, and the temperature, the depth of silicon impregnation and concentration of silicon in the case may be controlled according to need.

After receiving the siliconizing treatment, the articles may be allowed to cool in the retort to room temperature, or may be removed to separate containers to lower temperatures for handling.

The present invention is based on the discovery that ferrous base articles having a stable body centered cubic lattice can be siliconized to form a silicon-containing coating, or case, with many unique advantages, including those set forth hereinabove. Preferably, the ferrous articles must have a stable body centered cubic microstructure which is not `transformable into allotropes. Most desirably, the ferrous articles will not have allotropic transformations in the temperature range from ambient temperatures to siliconizing temperatures, the latter Of which may be as high as about 2000" F.

The stable alpha-delta iron alloys, which maintain a body centered cubic lattice structure throughout the solid phase, are well known, and only exemplary alloys are set forth below to illustrate ferrous base alloys which may be siliconized to produce the unique advantages of the invention. In general, the alloys may be utilized which contain constituents of the type, and in the amount, which close the gamma loop of ferrous articles, for example, silicon, tungsten, aluminum, vanadium, molybdenum, chromium, titanium, and the like.

Only small concentrations of silicon, for example, at least about two percent by weight are needed to produce a stable alpha-delta alloy possessing a body centered cubic microstructure throughout the solid phase. A siliconized case formed on this alloy reduces the tendency to spall.

Titanium exerts a strong influence in closing the gamma loop, and merely at least about 0.75% by weight is required to produce a stable alpha-delta alloy.

Tungsten in amounts of at least about six percent by weight is needed to close the ygamma loop. A siliconized case formed on this alloy has improved adherence of the case to the core.

Only a small concentration of aluminum of at least about one percent by weight is required to stabilize and form the alpha-delta alloy. A siliconized coating, or case, formed on an iron alloy of this type reduce the tendency to spall, and is characterized by exceedingly high corrosion resistance.

Only at least about 1.5% by weight of Vanadium is required to close the gamma loop. This alpha-delta alloy, when siliconized, has an improved adherence between the case and the underlying core.

Only at least about three percent by weight of molybdenum is required to close the gamma loop of iron. Molybdenum steel alloys reduce the cleavage between the siliconized case and the core.

Only at least about two percent by weight of niobium is needed to form a stable alpha-delta alloy.

Chromium closes the gamma loop of iron with at least about thirteen percent by weight concentration. Steel alloys containing chromium above these amounts produce improved siliconized cases.

It is to be understood that the alpha-delta alloys are not equally desirable, or useful. The alloying constituents themselves are more, or less, desirable for various intended objects and uses. By way of example, alloying constituents may be classified into two -groups as to their reactivity with chloride; (A) those which are more reactive with chlorine than iron, for instance, cerium, yttrium, magnesium, aluminum, manganese, titanium, and chromium; and (B) those which are less reactive with chlorine than iron, for instance, carbon, tungsten, molybdenum, nickel, cobalt, and copper. The alloying constituents of group (A) above tend to form spongy porous cases as their concentration increases n the process of the above-identified Eckman patent. Chromium, for instance, in concentrations above 5% by weight results in a thick spongy case which increases in thickness and sponginess in relation to the chromium content. The alloying constituents of group (B) above tend to form denser cases than those of group (A) in high concentrations, in the process of the above-identified Eckman patent.

In some instances it may be desirable to employ several alloying constituents to close the gamma loop. For instance, at least about thirteen percent by weight of chromium is needed to form an alpha-delta alloy. At these concentrations the case tends to be spongy, even though the tendency to spall off is reduced. Nevertheless, if a chromium containing alpha-delta alloy is desired, the chromium may be maintained in concentrations below about five percent by weight to avoid a spongy case, and other alloying elements may be used to close the gamma loop, such as titanium, aluminum, silicon, or the like. Various combinations of alloying elements which will attain a stable alpha-delta alloy, with other desirable properties for the intended uses, will be apparent to those skilled in the field.

While not intended to be limited thereby, the theoretical considerations which are believed to produce the improved siliconized case, are as follows. The siliconized case, or coating, contains Fe3Si, which has a crystal structure of ferrite, body centered cubic with a double lattice parameter. Siliconizing has ordinarily been produced upon articles that are austenitic at siliconizing temperatures. Despite the transformation of austenite to ferrite in the base materials, with the accompanying volume increase on the order of 5%, the siliconized case containing Fe3Si remains adherent in many alloys. This is probably due to plastic flow or creep, which diminishes the shear force and maintains the adherency between the siliconized case and the core.

In the event, however, iron or steel is alloyed to the extent that suppressed transformation occurs at a black heat, below the creep range of steel, the shearing modulus and the shearing stress is substantially increased and probably accounts for spalling in many instances when the article is subjected to wide changes in temperature, or mechanical shock.

In the present invention, there are no phase transformations, and thus, no Volume changes in the underlying core that produce shear forces between the case and the core, which tend to spall the case. Furthermore, the underlying core has the same type of microstructure, a body centered cubic lattice, as Fe3Si produced in the case.

Other modes of applying the principle of the invention may be employed, change being made as regards the details described, provided the features stated in any of the following claims, or the equivalent of such be employed.

I claim:

1. As a new article of manufacture, a ferrous base metal alpha-delta alloy having a stable body centered cubic crystal structure in sufficient amounts in its solid solution phase to suppress allotropic transformations in the temperature range up to about 2,000" F., said ferrous base metal alpha-delta alloy provided with an adherent siliconized case.

5 6 2. The new article of claim 1 in which said alpha-delta References Cited alloy contains at least about 2% by weight silicon.

3. The new article of claim 1 in which said alpha-delta UNITED STATES PATENTS alloy contains at least about 0.75% by weight of titanium. 2,501,051 3/1950 Henderson et al 4. The new article of claim 1 in which said alpha-delta 5 FOREIGN PATENTS alloy contains at least about 1% by weight aluminum.

5. The new article of claim 1 in which said alpha-delta 543495 7/1957 Canada' alloy contains at least about 3% by weight basis of molyb- RALPH S KENDALL Pmary Examiner denum.

6. The new article of claim 1 in which said alpha-delta lo U-S- C1- XR.

alloy contains at least about 3% by weight niobium. 117-1351 U.S. DEPARTMENT 0F COMMERCE PATENT OFFICE Washington, D.C. 20231 UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTIQN Patent No. 3,438,802 April l5 196 Jerome J. Kanter It is certified that error appears in the above dentfed patent and that said Letters Patent are hereby corrected as shown below:

Column 4, line 5, "chloride" should read chlorine Signed and sealed this 7th day of April 1970.

(SEAL) Attest:

Edward M. Fletcher, Jr.

Attesting Officer Commissioner of Patents WILLIAM E. SCHUYLER, JR.

Claims (1)

1. AS A NEW ARTICLE OF MANUFACTURE, A FERROUS BASE METAL ALPHA-DELTA ALLOY HAVING A STABLE BODY CENTERED CUBIC CRYSTAL SSTRUCTURE IN SUFFICIENT AMOUNTS IN ITS SOLID SOLUTION PHASE TO SUPPRESS ALLOTROPIC TRANSFORMATIONS IN THE TEMPERATURE RANGE UP TO ABOUT 2,000*F., SAID FERROUS BASE METAL ALPHA-DELTA ALLOY PROVIDED WITH AN ADHERENT SILICONIZED CASE.

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