US2333573A - Process of making steel - Google Patents

Description

NOV- 2, 1943. P. R. KALlscHER PROCESS OF MAKING STEEL Filed Feb. 12 1942' 2 Sheets-Sheet 1 #n m p Pear/#e fron Carb/deV /Cr/Ve l'l//f/VESSES.' Z 4 4.

'ffl-.ffanad Nov. 2, 1943 UNITED STATES PATENT OFFICEI Westinghouse Electric & Manufacturing Com-4 pany,v East Pittsburgh, Pa., a corporation of Pennsylvania Application February 12, 1942, Serial No. 430,658 4 Claims. (Cl. 'I5-22) This invention relates to powder metallurgical processes of making steel.

In powder metallurgical processes it is common practice to use combusted gas atmospheres containing appreciable quantities of carbon monoxide, methane, butane, or other hydrocarbons where one constituent of the powder compress is graphite. material in the gas atmosphere functions to prevent loss of graphite during sintering. graphite in such compacts functions as a lubricant where the iinished piece is subjected to friction and acts as a powder lubricant during pressing operations on the powders. In addition, the graphite also serves, where the part being formed is of ferrous material, to harden and strengthen the finished part by partial solution in the iron.

It is also known that where articles are to be formed from mixtures of pure iron powder and high carbon iron alloy powder that a combusted In such processes, the carbonaceous atmosphere should be employed as a protective V blanket to prevent decarburization. In such processes, the function of the iron-carbon alloy is to. furnish carbon to increase the hardness and strength of the final article. However, because the diiiusion of iron-carbide into iron is very slow, satisfactory results are not obtained by this process.

Whether the combusted atmospheres are em ployed for preventing loss of graphite or as a protective blanket, it is found that the resulting product sintered in such atmospheres has superor properties to an article formed by sintering pure iron powders, but that such properties will never approach the hardness and strength of carbon steel made by the more usual melting process and of the same carbon content. The lack of strength in the sintered article formed from the compacted powder has heretofore p revented the general use of articles formed by the known powder metallurgical processes, as `it has been impossible to produce such articles which will have the physical properties of parts made by machining from bar or plate stock in the more orthodox fashion.

it is an object of this invention to provide a process of making steel having a substantially uniform carbon content and high strength from metallic powders.

Another object of this invention is to provide for developing a hardened case on steel formed from metallic powders.

A more specic object of this invention is to provide for making a steel having a predetermined carbon' content from metallic powders and to provide a hardened case on the steel.

Other objects of this invention will becomeap` parent from the following description when taken in conjunction with the accompanying drawings, in which:

Figure 1 is a photomicrograph of a plain carbon steel produced in accordance with this invention;

Fig. y2 is a photomicrographof another plainy carbon steel produced in accordance with this invention;

Fig. 3 is a photomicrograph of a, plain carbon l steel having a uniform carbon content and -proporosity land decrease the time required for.

homogenization of the steel article which is to be produced. The alloying components are .preferably carbon-free, and when prepared as powders are mixed in any suitable manner in the required alloying proportions depending upon the composition of the steel which is to be produced.

In making steel, the steel must contain some element which will give the steel suiiiciently slow critical cooling rate as to permit hardening of the steel by some type of quenching treatment. Manganese is usually employed for this purpose in the making of steels by the usual melting practice. However, in making steel from metal powders in accordance with this invention, manganese is not satisfactory because of the special requirements on the sintering atmosphere fory preventing oxidation of ther'nanganese.y

Instead of manganese, asubstantially carbonfree ferro-manganese containing up to of manganese is employed 'as an essential alloying component of the steel made in accordance with this invention. With Asuch ferro-manganese, powdered to the particle size of the other, alloying components, it is found possible to avoid the t formation of manganese oxides'in the'presence of the sintering atmosphere. Likewise, other substantially carbon-free ferro alloys, suchv as I ferro-chromium, ferro-vanadium, ferro-tungsten,

and ferro-molybdenum, may be powdered and, employed as alloying components of A the steelmixed powders to compact them intothe required shape and size of the article which is to be produced. kThe forming pressure employed in making the powder compact depends somewhat on the die employed, the porosity desired in the nal piece, the thickness of the oxide nlms on assac'rs mined amount` of carbon in the steel. Thus, a steel having any predetermined carboncontent can be produced, the only requisite being that the `carbon component of the sintering atmosthe metal particles, and the hardness of the particles. For example, the allowable stress in the die must be considered and a pressure employed which will not damage thedie. if a high density, low porosity article is required, then a high forming pressure is employed. If

the metal powders have little or nooxidekfllm n on them, then relatively low forming pressures will kgive adequate surface contact between the metal powders so that solid diiusion can` readily take place, whereas if a thicker tenacious oxide fllmis present, then higher pressures are necessary in order to effect therupture of the oxide I'llm and the cold welding of the clean metalsurfaces. Onthe other hand, if the powders are relativelyk soft, kthe lower forming pressure may be employed in obtaining a given density than with harder powders.

at a temperature of between 1000 and 1200 C.

for a period of time of up to 21/2 hours in a controlled sintering atmospherewhich will be described more fully hereinafter. Preferably, the powder compact is sintered at a temperature of about 1100 C. for a period of time of two hours in producing the steel in accordance with this invention.

In effecting the sintering oi the powder compact, a sintering atmosphere containing between and 25% of carbon monoxide and not more than 0.1% of each of carbon dioxide and oxygen with the balance hydrogen and nitrogen with small quantities of up to 0.5% of methane, is employed, it being found that the carbon pressure of such gas can be easily controlled. Preferably the atmosphere employed will have a dew point lower than -40 C. in order to prevent oxidation'and decarburization. The control of the carbon pressure is readily effected through the use of the apparatus and method disclosed and claimed in the copending application, Serial No. 295,058, led September 15, 1939 of John R. Gier,

Thus, depending upon the carbon content of the steel which is to be produced, the carbon pressure of the sintering atmosphere is controlled so that the sintering atmosphere is, in effect, a carburizing atmosphere and is sufciently rich in the carburizing component to be in carbon equilibrium at the sintering temperature with the predetermined carbon content of the steel. When a powder compact is thus treated for the period of time required for the sintering, namely, up to 21/ hours, it is found that the sintering atmosphere so effectively carburizes the powder compact as to effect a substantially uniform distribution of a predeter- Further,

phere be so closely Acontrolled that the carburizing component of the atmosphere will be in carbon kequilibrium at the sintering temperature with the predtermined carbon' content of the steel.

. Referring to Fig. 1 of the drawings, there is shown a photomicrograph ata magnification of 100 times of an annealed plain carbon steel having the analysis of 0.7% manganese, 0.2% silicon, 0.3% carbon, and the balance iron produced in accordance with this invention by sintering a powder compact at a temperature of 1100 C. for a period of time of two .hours in a carburizing atmosphere having an analysis of 0.1% carbon dioxide, 0.1% of oxygen, 18.5% of carbon monoxide, 39.4% of hydrogen, 0.1% of methane, and

41.8% of nitrogen. As shown, all of the carbon.

is in solution in the steel, the resulting steel kkbeing very ne grained. This particular steel had a hardness as quenched in waterof 54 Re.

Fig. 2 is a photomcrograph at a magnincation of 100k times of an annealed plain carbon steel having a carbon content of 1.5% produced from a powder compact under the same conditions with the exception that thecarburizing atmosphere had an analysisof 19.5% carbcnrnonoxide,

38.6% of hydrogen, 0.4% methane, and 41.5% of nitrogen, there being no carbon dioxide or oxygen present. In this case, although the steel has slightly abnormal structure, it is very nne grained and quite free `of non-metallic inclusions. The ne grain exhibited by this steel aswell as by the steel shown in Fig` 1 is characteristic of plain carbon and low alloy steels made from powders in accordance with this invention.

For many applications, it is necessary to provide the steel article, such as gears and cams,

with a case hardenedsurface. kIn accordance with this invention, a case may be provided on articles produced from metal powders by enriching the carburizing component of the sintering atmosphere after the steel is provided with its predetermined carbon content and while continuing the sintering of the steel. For example, by increasing the methane content of the carburizing and sintering atmosphere up to 21 or 3%, after the steel has been given its predetermined uniform carbon content, and continuing the sintering treatment for a period of time ranging up to about 5 minutes, a hardened case isprovided on the steel. The depth of the case will depend upon the length of time of the sintering treatment in the high carbon pressure gas, and the carbon content of the case will depend upon the carbon pressure of the gas. l

Referring to Fig. 3 of the drawings, there is illustrated a photomicrograph at a magnification of 100 times of a case carburized plain carbon steel produced in accordance with this invention. In this particular example, the powder compact was sintered at a temperature of 1100 C. for a period of time of two hours in a carburizing atmosphere suihciently rich in the carburizing component to give the steel a resulting substantially uniformly distributed carbon content of 0.3%. After the steel was treated in the carburizing atmosphere to impart to the steel the required uniformly distributed carbon content, the carburizing atmosphere was enriched by the addition of 1% of methane thereto and the sintering treatment was continued for a period of time of 3 minutes to impart to the article the case clearly shown in the photomicrograph. The case hardness of the steel illustrated was found to be 65 Re.

In Fig. 4 there is illustrated a, photomicrograph at a magnification of 100 times of another case carburized plain carbon steel produced from metal powders by sintering the powder compact in an atmosphere of cracked ammonium plus 0.5% of hydrocarbon gas to give the required uniform distribution of carbon after which the sintering atmosphere is so enriched as to have a hydrocarbon content of 2% of a gaseous hydrocarbon, such as butane, to produce a case thereon. 'Ihe steel of this example was sintered at 1100 C. for a period of two hours to give a required carbon content of 1.0% after which it was sintered for 5 minutes in the enriched atmosphere to provide the deep case illustrated. In this particular example, the hydrocarbon decomposes in the furnace and deposits carbon on the surface of the powder compact with the result that carburization takes. place in a manner similar to that eiected by pack carburization. As

illustrated, the steel resulting from such a process is not quite as uniform as that illustrated in Fig. 3.

By practicing this invention' it is possible to produce a plain or alloy carbon steel of any pre-A determined carbon content. The resulting steel has an exceedingly ne grain and a high strength. Thus, it is possible to produce articles of any predetermined shape and size directly from metal powders in steel having predetermined alloying components and having a uniformlydistributed predetermined carbon content. Further, by case carburizing the steel as a part of the sintering treatment, economies may be eected in the producing of special articles, such as gears and cams or the like, where it is necessary to provide the article with a hardened case.

Although this invention has been described with reference to a particular embodiment thereof, it is, of course, not to be limited thereto except insofar as is necessitated by the scope of the appended claims.

I claim as my invention: 4

1. The process of making steel containing a predetermined carbon content comprising the steps of, preparing substantially carbon free alloying components into powders of a predetermined particle size, mixing the component powders in predetermined proportions, compressing the mixed powders to a predetermined shape and size, and sintering the compressed powders in a carburizing atmosphere, the carburizing atmosphere being suiiiciently rich in a carburizing coniponent to be in carbon equilibrium at the sintering temperature with the predetermined carbon content of the steel to effect a. uniform distribution of a predetermined amount of carbon in the steel.

2. The process of making steel comprising the steps of, preparing substantially carbon freey alloying components into powders of a predetermined particle size mixing the component powders in predetermined proportions, compressing the mixed powders to a predetermined shape and size, sintering the compressed powders in car.- `burizing atmosphere, the carburizing atmosphere being sufficiently rich in a carburizing compo; nent to be in carbon equilibrium at the sintering temperature with the predetermined carbon content of the steel to effect a substantially uniform distribution of a predetermined amount of carbon in the steel, and enriching the carburizing component of the carburizing atmosphere after the steel is provided with its predetermined carbon content while continuing the sintering of the steel to provide a carburized case on the sintered steel.

' 3. The process of making steel containing a predetermined carbon content comprising the steps of, mixing metallic powders, having a particle size of not more than -325 mesh of substantially carbon free alloying components in prede-- termined alloying proportions, compressing the mixed powders under a pressure of from 30 to tons per square inch to a predetermined shape and size, sintering the compressed powders in a carburizing atmosphere having an analysis of l5 to 25% CO, from traces to 2% CH4 and the balance Hz and N2 and a dew point of not more than 4`0 C., and controlling the carburizing component of the sintering atmosphere to maintain the carburizing component in carbon equi-A librium at the sintering temperature with the predetermined carbon content of the steel to effect asubstantially uniform distribution of a predetermined amount of carbon in the steel.

4. The process of making steel containing a predetermined carbon content comprising the steps of, mixing metallic powders having a partie cle size of not more than '-325 mesh of sub` stantially carbon free alloying components inV predetermined alloying proportions, compressing the mixed powders under a pressure of from 30 to 100 tons per square inch to' a predetermined shape and size, sintering the compressed powders in a carburizing atmosphere having an analysis of 15 to 25% CO, from traces to 2% CI-I4 andthe balance H2 and N2 and a dew point of not more than 40 C., controlling the carburizing component of the sintering atmosphere to maintain the carburizing component in carbon equilibrium at the sintering temperature with the predetermined carbon content of the steel to effect a substantially uniform distribution of a predetermined amount of carbon in the steel, and then enriching the carburizing component of the carburizing atmosphere while continuing the sintering to provide a carburized case on the sintered steel.

' PHILIP R.

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