US3527624A - Controlling carbide surfaces - Google Patents

Description

United States Patent O 3,527,624 CONTROLLING CARBIDE SURFACES Vitti Julian Biretta, Elmer George Lemon, Jr., and Eugene John Vater, Cincinnati, Ohio, assignors to Jervis B. Webb Company, a corporation of Michigan No Drawing. Filed Sept. 8, 1967, Ser. No. 666,485 Int. Cl. C21d 1/06, 1/74 US. Cl. 148-14 8 Claims ABSTRACT OF THE DISCLOSURE A continuous carbide network within the carburized case of a ferrous alloy structure may be converted, by reducing it to a discontinuous carbide array or by entirely eliminating it, by coating the structure with a material which forms an effective mechanical barrier to the diffusion of carbon, such as copper, heating to a temperature within or above the transformation range of the ferrous alloy for a time sufficient to accomplish the extent of carbide network conversion desired, cooling the alloy, and then removing the coating.

BACKGROUND OF THE INVENTION Field of the invention The present invention relates to a method of converting i.e., reducing or eliminating a continuous carbide network from within the carburized case of a ferrous alloy structure.

Description of the prior art In modern practice, the process of carburizing is applied to ferrous metals and alloys thereof with the object of increasing the carbon content of the surface, so that when subjected to a suitable heat treatment, the surface portion thus carburized will be substantially harder than the underlying metal. The combined process of carburizing and hardening have long been known as case hardenmg.

The term car-burizing as used herein, relates to a process in which carbon is introduced into a solid ferrous alloy by heating the alloy in contact with a carbonaceous material-solid, liquid or gasto a temperature above the transformation range of the alloy and holding it at that temperature for a predetermined amount of time. Carburizing results in a ferrous alloy having a surface layer, known as a case, that has been made substantially harder following appropriate heat treatment than the interior portion-known as the core.

This combination of hard surface and softer interior, made possible by case hardening is of inestimable value in modern engineering practice. By the use of alloy steels, great strength and toughness in the core can be combined with extreme surface hardness, resulting in a composite structure capable of withstanding high stress. For less exacting requirements, there are many applications where low or moderate core properties, together with a high degree of surface hardness, can be obtained with cheaply fabricated, low priced carbon steels.

In the production of ferrous alloy structures, such as gears, pinions, push rods, valve rocker arms, piston pins, camshafts and the like, a case is formed by diffusing carbon into the ferrous alloy at high temperatures in such a manner that upon cooling, iron carbides in the grain boundaries are not formed. A failure to control carburizing process conditions has been known to cause an excess of carbon to be diffused into the ferrous alloy. Upon cooling, the excess carbon reacts with the iron to form a network structure, i.e., a structure in which the grains or crystals of ferrous alloy are partly or entirely enveloped by iron carbide. Where the envelopment is com- 3,527,624 Patented Sept. 8., 1970 SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to provide a method by which continuous carbide networks can be converted, i.e., reduced or eliminated.

It is another object of this invention to provide a method by which ferrous alloy structures, such as gears and the like, which have been rendered useless by the formation of continuous carbide networks within their carburized case, can be salvaged and the embrittlement in their surfaces eliminated.

Other objects and advantages will become apparent from the following description and appended claims.

Briefly, in accordance with the objects of this invention, a process, for converting a continuous carbide network within the carburized case of a ferrous alloy structure by reducing it to a discontinuous carbide array or by completely eliminating the carbide network, is provided comprising the steps of coating the ferrous alloy structure with a material which provides an effective mechanical barrier to the diffusion of carbon and heating the coated structure to a temperature within or preferably above the transformation range of the ferrous alloy for a time sufiicient to accomplish the extent of carbide network conversion desired. After the heating the ferrous alloy may be cooled and the coating stripped from the structure. It has been found that a coating consisting of plated copper is particularly effective as the barrier material of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The carburization process, has as its purpose to increase the surface hardness and wearability of the ferrous alloy structure. It has been found that the continuous carbide network resulting from the failure to properly control the carburization process can be reduced to a discontinuous carbide array or completely eliminated from the carburized case, whichever is more desirable under the circumstances. This is accomplished, generally by diffusion of the carbon from within the continuous iron carbide net Work into the core or by its redistribution, if possible, Within the case. The extent to which the diffusion is efiected determines whether the carbide boundary is merely broken up or is entirely eliminated. The effectiveness of the reduction or elimination can be ascertained by micro scopic inspection.

A ferrous alloy structure, whether it be a gear, pinion or any other carburized structure, which is found by microscopic inspection to have a continuous carbide network Within its carburized case, may be treated by the method of the present invention. The structure is initially coated with a material which forms an effective mechanical barrier to the diffusion of carbon. The coating may be placed directly upon the case and serves to preclude the transfer of carbon between the atmosphere and the alloy. Depending upon the configuration of the structure and the extent and location of the carburized case, it may be desirable to expose only selected portions of the structure to the method described herein. This invention contemplates selective as well as total treatment of the ferrous alloy structure. The coating material must be nonreactive with the atmosphere, which generally includes a low carbon potential, and should be capable of being easily removed from the structure surface at the conclusion of the heat treatment. It is particularly important that the coating form a substantially nonporous barrier to the diffusion of carbon. Generally, any material which is commonly used as a mask to protect specific surface areas during known selective carburization processes will be effective in the process of the present invention. The smaller the porosity of the barrier coat, the more desirable is its use. In known carburization process, the mask or mechanical barrier coating precludes carburization of selected surface areas. The barrier coating as employed herein serves to preclude the decarburization of the case.

Copper is particularly effective as a mechanical barrier coating material. The copper prevents the already carburized structure from becoming decarburized, while at the same time permitting the heat treatment to effectively reduce or eliminate the continuous network. It may be applied to the ferrous alloy by any well known coating method. We have found that electrolytic methods of copper plating are the most efficient. However, copper in the form of tubing, bars, cups, sleeves, disks and in any other appropriate form depending upon the configuration of the structure, can be employed. Commercially pure copper has been found to be of sufiicient purity for use herein. No requirement exists for reagent grade copper. The copper coat is preferably applied in a thickness of from about /21 /2 mils (mil= inch). Porosity consideration dictate the smallest thickness which can be effectively employed. A thickness of /2 mil is relatively easy to apply and seems to provide, in most cases, the necessary mechanical barrier. Thicknesses in excess of 1 /2 mils serve no real purpose and merely waste copper. Other effective mechanical barriers include materials employed as masks for selective carburization and may include such materials as carbonaceous cements, solutions or pastes, cements made of fire clay and sodium silicate and the like.

After the mechanical barrier is applied to the structure the coated ferrous alloy is heated to a temperature within or preferably above the transformation range of the ferrous alloy (i.e. above the lower transformation temperature). The transformation range of an alloy is generally defined as the temperature interval within which austenite forms while ferrous alloys are being heated. It is also the temperature interval within which austenite disappears while ferrous alloys are being cooled. The limiting temperatures of the transformation range depend primarily upon the composition of the alloy. Above the upper transformation temperature, the alloy is totally austenitic. We have found that heating the structure to within or above the transformation range increases the afiinity of gamma iron for the carbon which we are attempting to diffuse by the process of the present invention. Although success has been noted where temperatures below the upper transformation temperature have been employed, progress is significantly slower and it is preferred that the temperature employed be above the upper transformation temperature, i.e., above the transformation range of the alloy wherein the alloy is fully austenitic.

uvork conversion. Another factor which influences the heating time is the amount of carbide present in the case. It requires a greater amount of thermal energy to diffuse large amounts of carbide than to diffuse lesser amounts. This increased thermal energy may be supplied by increasing the time at which the structure is exposed to the heat treatment. For example, it has been found that, depending upon the extent of conversion desired, the method of the present invention is effective when the heating is continued for as little as one hour or less and up to less than 20 hours at about 16001700 F. for A.I.'S.I. 9310 steel. Twenty hours or more results in excessive case depth and too soft a case whereas tests at 1, 5, 10 and 15 hours were acceptable. Therefore, in determining the appropriate heat treatment time in any particular instance, it should be recognized that too short a heat treat might not accomplish the desired conversion whereas too long a heat treatment may cause extensive diffusion and an attendant undesirable increase in the case depth.

Thermal energy may be applied to the ferrous alloy structure by any of the well known processes of radiation and conduction. Heat may be applied directly by the passage of electrical current through the structure, or indirectly through the use of furnaces heated by oil, gas, coke or coal; furnaces heated by electrical resistors of both metallic and non-metallic types; and any other well known means not inconsistent with the requirements of this method and the intended use of the ferrous alloy involved. The rate of heating should be determined by a consideration of the ultimate metallurgical properties which are desired in the ferrous alloy. Rate of heating is not critical to the reduction or elimination of the carbide network.

In a preferred form of the present invention it has been found desirable to heat the ferrous alloy structure in an atmosphere which has a controlled carbon potential. The carbon potential is provided as a precaution in the event that the applied coating is not totally effective in presenting an impermeable barrier to the diffusion of carbon. This is not an uncommon occurrence since the coating may have minute imperfections regardless of the type of coating which is employed. If in fact there is a defect in the mechanical barrier, and if there were no carbon potential in the atmosphere, there might be a tendency for the case to decarburize at the temperature to which it is heated. The controlled carbon potential in the atmosphere insures that there is no gradient favoring decarburization. In theory, if it were certain that the mechanical barrier was totally effective, there would be no need for a carbon potential in the atmosphere. In lieu of the carbon potential atmosphere, dry inert gases such as argon, helium and nitrogen, dry reducing gases such as hydrogen, or a vacuum would provide satisfactory analogous protection.

Such an atmosphere including carbon monoxide or a hydrocarbon will be effective to provide the needed carbon potential. The absolute carbon concentration in the atmosphere can vary over a relatively wide range. It is adequate if there is sufiicient carbon including gas in the atmosphere to provide the carbon potential necessary to counteract the decarburization gradient under the circumstances. In other words, at the high temperatures to which the carburized alloy is exposed, the tendency of the case to decarburize through an imperfection in the barrier coating must be overcome by the carbon potential in the atmosphere. We have found that a carbon-including atmosphere, for example, a carbon monoxide-including atmosphere controlled and measured at about 1-2% carbon dioxide as a measure of the carbon monoxide content, is satisfactory to prevent decarburization of the structures of ferrous alloys used as gear steels. The range represents an example of a workable embodiment rather than any limitation upon the present method and it should be understood that within the broad guidelines suggested, i.e., sufficient carbon potential to overcome the tendency to decarburize, the present invention contemplates any carbon concentration.

At the conclusion of the heat treatment, the coated ferrous alloy may be removed from the heating chamber and cooled by any well known method, as for example by permitting it to reach thermal equilibrium with the ambient, fan cooling it or any other means which will not be detrimental to the desired ultimate metallurgical properties in the converted structure.

After cooling the barrier coating may be removed by any effective method. If copper plate was employed it may be removed either electrolytically or chemically.

To demsonstarte the effectiveness of the present invention in salvaging carburized ferrous alloy structures having a continuous carbide network within the carburized case, more than 3400 gears of A.I.S.I. 9310 steel were treated by the method of the present invention. Due to the nature of the serivce to which gears are exposed, their surfaces must exhibit an adequate load carrying capacity as well as resistance to wear, scoring, pitting fatique, bending fatigue and chipping. Hight surface hardness, as for example conferred by carburizing, is required for withstanding this type of service. As a result, it is not uncommon that gears are found to have continuous carbide networks resulting from improper decarburization.

The gears were cleaned and copper plated over their entire surface to a one mil thickness and were placed in a furnace having a controlled carbon potential through a carbon monoxide atmosphere measured as 1.6% carbon dioxide. They were heated at 1650 F. for 5 hours, and then were subjected to normal heat treat procedures such as to harden and temper. The copper plate was stripped from the gears and the gears were microscopically examined to determine the existence of a carbide network within the case. It was found that the resultant metallurgical structure in the carburized case was free of a continuous carbide network. Additional testing has shown that heating at about 1650 F. for 1 hour is satisfatcory as well.

While the present invention has been fully and completely described with reference to particular embodiments thereof, it will be understood that numerous descriptions may be made by those skilled in the art without actually departing from the scope of the invention.

What we claim is new and desire to secure by Letters Patent of the United States is:

1. A method of converting a continuous carbide network within the carburized case of a ferrous alloy structure comprising the steps of:

(a) coating said carbun'zed case of a ferrous alloy structure with a material which forms an effective mechanical barrier to the diffusion of carbon; and

(b) heating said coated alloy to a temperature above the lower transformation temperature of said alloy for a time sufiicient to accomplish carbide network conversion.

2. A method, as claimed in claim 1, wherein sail alloy is heated to above the upper transformation temperature.

3. A method, as claimed in claim 1, wherein said heating is accomplished in an atmosphere which includes a carbon potential.

4. A method, as claimed in claim 1, wherein said coating is copper.

5. A method, as claimed in claim 4, wherein the copper coating is applied electrolytically.

6. A method, as claimed in calim 3, wherein said coating' comprises electrolytically applied copper, said atmosphere has a controlled carbon potential through presence of carbon monoxide measured at about 1-2% carbon dioxide and said coated alloy is heated to about 1600- 1700 F. for about 1 to less than 20 hours.

7. A method, as claimed in claim 1, wherein said coating is removed from said alloy after carbide network con version.

8. A method, as claimed in claim 7, wherein said coated alloy is cooled before said coating is removed.

References Cited UNITED STATES PATENTS 2,201,202 5/1940 Reed 148-19 2,335,295 11/1943 Millard 148-l9 2,406,539 8/1946 Greenslade et al. 14819 3,344,817 10/1967 Connard 14816.5

L. DEWAYNE RUTLEDGE, Primary Examiner W. W. STALLARD, Assistant Examiner US. Cl. X.R. 148-39 32 3 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 527, 624 Dated September 8, 1970 Inventor(s) Vitti J. Biretta et a1 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 5, delete the words Jervis B. Webb Company, a corporation of Michigan" and insert General Electric Company, a corporation of New York in place thereof.

SIGNED AND QEALE NW 2 4% M H- Fletcher, Ir.

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