US2804412A - Carburization of metal articles - Google Patents

Description

F. R. ANDERSON Fil-ed nec. '22, 1955' CARBURIZATION OF METAL ARTICLES Aug. 27, 1957 CARBURIZATION F METAL ARTICLES Floyd R. Anderson, Denver, Colo., assigner to Gardner- Denver Company, Denver, Colo., a corporation of Delaware Application December 22, 1955, Serial No. 554,767

6 Claims. (Cl. 148-19) This invention relates to a process for case hardening hollow metal members, and more particularly, to an improved process for case hardening both the interior and exterior of such members which may have diicultly accessible interior surfaces.

Although the instant invention may have application in a number of elds, particularly those involving the case hardening of both the interior and exterior surfaces of elongated hollow articles having relatively thin body portions separating the interior and exterior surfaces, the instant invention is particularly adapted to the formation of a novel drill rod structure. The so-called rock drill rods have generally accepted standard sizes ranging from about 2 to about 20 feet in length and from about 0.75 to 2 inches in diameter. Actually, hexagonally crosssectioned rods are usually employed with maximum crosssection dimensions of 7m inch up to 17/8 inches (but such' dimensions are considered to be diameters of a substantially round cross-section for the purposes of the instant description). The rods are formed from steel stock by hot or cold working usually over a suitable mandrel, which results in the formation of an approximately 1A inch axial bore in the rods. Although the particular manner in which the bore is formed in the rods is not so important, this bore does extend the entire length of the rod and the ratio of bore length to bore diameter is very substantial. v

Heretofore, drill rods have been made of high carbon steels which are difficult to work; but it has only recently been discovered that drill rods giving distinctly superior performances may be made from steels having a lower carbon content which are case hardened on both the outside surface and the inside surface defining the axial bore. The case on the interior surface is very important here, and it should be about equal in thickness to the exterior case. A number of difiiculties, however, have been encountered in attempting to fabricate such a case hardened drill rod.

cementing or carburizing step and then a'quenching step wherein the hot carburized article is immersed in a quenching liquid such as water or oil so as to quickly cool the :article from substantially the carburizing temperature to substantially room temperature (or at least to a temperature of about 300 F. so that further metallurgical `changes do not occur on cooling to room temperature). The carbon content of the carburized surface portions as well `as the rate :at which such surface portions are quenched will determine the hardness and other desirable properties in the case hardened layer. Differences in case thicknesses particularly in the difiicultly accessible interior surfaces may cause defects. Also, different quenching or cooling rates for different surface portions may make such a significant difference in the resulting material that flaws or defects may be created, if such surface portions are adjacent. In addition, excessive warpage, over and above that which may be obtained using ideal carburizing and htates Patent i APatented Aug. 27, 1957 ice quenching techniques, may result from lack of uniform quenching.

A peculiarity of drill rods and similar hollow articles having diicultly accessible interior surfaces is uniform carburization of the interior surfaces is difficult and that uniform quenching of such surfaces by contact with the quenching liquid cannot be obtained.

For example, the carburization operation per se presents a number of problems. Uniform carburization with gas has been found to be practically impossible because of the difficulty in having a uniform concentration of gas throughout the bore. Packing the rodsV in solid carburizing material also presents difficulties not only in the tendency for slightly slower carburization (which is actually a gas-solid reaction) within the bore but also in the tendency to bend the rods so packed because of shifting of the carburizing material during carburization. Liquid carburizing baths (KCN) present safety hazards.

As another problem, the initial quenching liquid entering the bore is only a very small amount of liquid which engages :a rather substantial amount of heated surface and this results immediately in vaporization of the quenching liquid which, in the case of such drill rods, has been found to violently force vapor and liquid out of the Opposite end of the bore. Such a fast flowing stream of heated steam or oil which is very hazardous and difficult tohandle is undesirable. In addition, the surfaces of the bore are not uniformly contacted with liquid, because of the presence of vapor in the bore, so that certain of the difficulties hereinbefore mentioned are encountered.

The instant invention affords a unique solution to the problems peculiar to this art. First of all, the invention contemplates 'filling the bore of the rod with a normally solid carburizing material and sealing such material in the bore so that contact with the bore thereby at carburizing temperatures will result in a uniform carburization at a predetermined rate. The rod itself is heated to carburizing temperatures in 4a carburizing fluid which will, of course, be exposed only to the exterior surfaces of the rod :and which also carries out lcarburization of such exterior surfaces uniformly and at a predetermined rate. Preferably, the rod is suspended from one end in the carburizing fluid to minimize bending or warpage. In addition, the concentration of the carburizing material in the fluid can be controlled so that the carburizing rate is controlled and the resulting case thicknesses on the inside and outside surfaces may have the desired ratio. Forexample, diffusion techniques may be employed to correlate the two carburizing rates in a situation wherein the solid carburizing material in the bore is somewhat slower. In such situation, carburization in a gas atmosphere can be carried out to the extent desired on the exterior surface of the rod and the rod can then be maintained at carburization temperatures in a substantially inert atmosphere (or an atmosphere which will not cause decarburization) for an additional period of time, during which diffusion takes place along the exterior surface and additional carburization may take place in the bore of the rod.

Although quenching by -actual surface contact between the quenching liquid and the surface to be case hardened has heretofore been considered a necessary aspect of the procedure, the instant invention additionally provides a process which avoids this procedural step with respect to the difficultly accessible interior surfaces of the article, while still obtaining case hardening thereof. In other words, the instant invention contemplates quenching the rod by contacting the outside surface only thereof with the quenching liquid, while preventing contact between the quenching liquid and the inside surface. This is accomplished by sealing the open ends of the bore before carburization and retaining the seal during quenching;

and thev instant invention is based in part on the discovery that the somewhat less' rapid cooling ofthe'interio'r sur? faces by heat conduction through the thinshell or body of the rod results in not only an extremely uniform but also'an effective case hardening'of theinteio'rsu'r'face. The uniformity of the interior surface-case hardeninghas'A been found 'to be partcula'rl'y'important fromA a A'strength and performance point" of 'viewlr y `It is',` therefore, an important" object" of the instant invention' to provide an improved case hardeng operation for hollow drill rods and the like'artic'les.'

It is a further object of the'in'stantinvention to provide an improvedmethod of producing a steel 'drill rod having" an axial bore that"comprises 'filling the 4bo're with particulate carburizingrnaterial, heating the rod to carburizing temperaturev in the presence of a carburizing fluid, lsealing the ends of thebore, and quenching the rod by contacting the'outs'ide only of the rod with quenching liquid, whereby cooling of the inside surface is accomplished by heat conduction throughthe body of the rod.

Other and further objects, features and advantages of the present invention willV become apparent to those skilled in the art from`the following detailed disclosure thereof and the drawings attached hereto and made a part hereof.

On the drawings:

Figure 1 is a sectional elevational View of a case hardened drill rod made by the process embodying' the instant invention, and shown with sealing means closing the open ends of the bore;

Figure 2 is an enlarged cross-sectional view of the drill rod of Figure 1 taken substantially along'the line II-II of Figure 1; and v Figure 3 is a cross-sectional view taken substantially along the line III-III of Figure 2.

As shown on the drawings:

In Figure l, the reference numeral 10 indicates generally a drill rod embodying the instant invention at the completion of the quenching step. The drill rod 10 has a case hardened layer 11 defining its outer peripheral surface, case hardened layers 12 and 13 defining, respectively, the top and bottom end surfaces of the rod 10 and a case hardened surface portion or layer 14 defining an axial bore B extending the full length ofthe rod 10. Actually, the rod 10 comprises a first case hardened sleeve 14 defining the axial bore B, a second vcase hardened sleeve 11 dening the' outer surface for the rod 10 and a steel-body 15 interposed between the rst and sec-` ond case hardened sleeves 11 and 14. This structure ha's been found to be uniquely satisfactory for drill rods.

As shown in Figure l, the overall length "l of standard drill rods may range from 2. to 20 feet, whereasv the maximum' cross-sectional dimension or diameter D may be from 3A to2 inches and the diameter d'of the bore B is customarily about 1A inch. Although'it isy standard commercial practice to employ a bore diameter d of approximately 1A inch, it will be appreciated that this dimension d may range from as little as about s inch to as much as about 1/2 ich for the purposes of the instant invention. Because of the manner in which the bore B is usually formed during working of the steel, it is ordinarily not perfectly circular but only approximately so and the dimension d representsV generally the average diameter (or corresponding average cross-sectional dimension).

As will be noted from Figure 2, the overall cross-section of the rod 10 is hexagonal and the maximum cross-sectional dimension D is the dimension hereinbefore indicated and, for the purposes of the instant invention, the dimension D may be considered to be a diameterof'a substantially round cross-section (or substantially` the' average cross-sectional dimension).

As will be seen, the ratio of the bore length-l to the' bore diameter (on the basis of the standard'bore diameter of 1A inch) ranges from a minimum of about'lOOl to' a maximum of ab'otl000:l (although the'maximum is best chilling or quenching effect on the case hardened layer 14 by the instant quenching process. Wall thicknesses substantially greater than this dimension'will serve to slow down the cooling through the body of the rod to too substantial an extent to obtain the best results.

The first step in the practice of the instant invention is the carburizing step wherein both the inside surface layer 14 of the rod and the outside surface layer 11 of the rod are carburized. Carburization per se (sometimes referred to as cementation) is a well known process,

which comprises exposing the surface to be carburized to a carburizing material at carburizing temperatures. The carburizing material may be solid or fluid. For example, charcoal and/or coke in combination with chemical energizes in particulate compositions such as are described in Rodman U. S. Patents Nos. 949,448 and Y 1,432,416 may be employed. Also, a bath of a salt containing carbon, such as a potassium cyanide salt bath may be employed as a liquid carburizing material; or a hydrocarbon such as natural gas may be employed asa gas carburizing material. which may be used in the practice of the instant invention the carbon begins to penetrate the steel surfaces-at about 1300 F., but temperatures up to as much as 2000o F. may be used to accelerate the process. Preferably, a

y carburizing temperature of 1650 to 1700 M. is employed.

In the practice of the instant invention, the rst process step .is that offfilling the bore B with particulate carburizing material M. Such particulate carburizing material may be in the form of potassium cyanide salt particles (with or without other carburizing particulate materials suchas thosementioned by Rodman) but if such particles of a salt containing carbon are to be used as an essential ingredient in the particulate carburizing material which fills the bore B, it is necessary to pack suchparticles in the bore rather tightly in order to assure contact between the carburizing material and the bore walls 14 at the carburizing temperatures. In other words, the saltsof this type melt at the carburizing temperatures and, if packed loosely in the bore B, might only ll the lower portion of the bore B when melted. Preferably, the particular carburizing material used is a refractory material or a material which remains solid at carburizing temperatures. By the use of such refractory materials it is possible to maintain close contact between the carburizing material and the bore walls 14 in a uniform manner throughout the bore B. Preferably, the particulate carburizing material is a coked carbonaceous material such as coked charcoal, coal, etc., which contains a small amount of energizer (i. e. l to 20% by weight) such as an alkaline earth carbonate or an alkali metal carbonate, which has been ground down to a ne particle size (e. g. suitable to pass an 8 mesh screen) so that it can be conveniently fed into the bore B.

Referring to Figure l, it is generally preferable to' maintain the rod 10 in an upright position, and a bottom steel plug 17 is force-lit into the bottom of the bore B, and the particulate material is then poured into the bore B so'as to completely fill the same.y Then, the top steel plug 16 is force-fit'into the top of the bore B. At this point in the process, it will be appreciated that the carburized layers 11, 12, 13 and 14 are not yet on the rod 10.

In the next step, the rod 10 is suspended by its upper end in the furnace so as to hang freely.` This is of particular advantage since it 'tends to eliminate a substantial In such carburizing processes amount of warpage in the rod 10 during carburation; and this is one of the reasons for employing a liuid carburizing material to carburize the outer layer 11 of the rod 10. In the furnace the rod 10 may be suspended by its upper end in a fluid carburizing material such as a bath of a molten salt containing carbon (such as a potassium cyanide bath). One disadvantage of the use of potassium cyanide is the safety factor in view of the poisonous character of the fumes; but better control of carburization can also be obtained using a gas carburizing material as the fluid carburizing material herein. An important feature of the instant invention resides in the fact that the inner case thickness or depth c and the outer case thickness or depth C may be controlled adequately during the instant operation, so the use of gas for a more easily controlled carburization of the outer layer 11 is of particular advantage. Gases which may be used as carburizing fluids contain, as the carbon bearing component, such materials as natural gas, volatilized higher hydrocarbons, etc.

The Vtime at which the ferrous metal surface is exposed to the carburizing material at the carburizing temperature will, of course, determine the depth `of the carburized layer and the depth of the resultant case hardened layer. For the purposes of the instant invention the carburized or case depths c and C on the inside and outside surfaces, respectively (Figure 2) may range from about 0.010 to about 0.125 inch for the practice of the instant invention, although the preferred case depth c and/ or C for use in the practice of the instant invention in the fabrication of drill rods is 0.050 to 0.090 inch (preferably c and C are 0.05-0.07 D). The carbon content in these carburized portions is Oil-1.2%.

As a typical example, using a 11/2 inch hexagonal crosssectioned SAE 4320 steel, 20 foot drill rod having a 1A inch axial bore, the bore is completely filled with a solid carburizing material, in particulate form suitable to pass an 8 mesh screen, composed of coked charcoal and/or coal (100 parts by weight), barium carbonate (5 parts by weight) and sodium carbonate (5 parts by weight). The rod is then suspended freely from one end thereof in a furnace, heated (in an inert atmosphere) to 1700 F. then natural gas is fed into the furnace (at 200 cu. ft./hr.) to create a carburizing atmosphere; and the rod is maintained at 1700o F. for S hours which is followed by a 2 hour diffusion period at l700 F. in an inert atmosphere. This results in carburized layers 111 and i4 Von the outside and inside surfaces having carburized depths C and c each of 0080-0090.

Comparable results are obtained by maintaining the same rod immersed in a molten potassium cyanide bath for the same period of time and at the same temperature.

If either of the foregoing procedures is repeated except that a 7/8 inch hexagonal cross-sectioned SAE 8620 steel, 10 foot drill having a 1A inch axial bore is employed at a carburizing time of 41/2 hours and a diffusion time of 11/2 hours, the resulting carburized outside and inside depth C and c are each 0.05-0.06 inch.

The sealing of the opposite ends of the axial bore B, in each case, may be accomplished merely by forceiitting plugs 16 and 17, respectively (Figure 1) therein. Such plugs may be made of steel or other suitably refractory material, but are preferably made lof steel so as to stand up under the shock of the quenching operation. Quenching in the customary manner from the carburizing temperature (or from a temperature of at least about l500 F.) is carried out by plunging the carburized rods into quenching oil (or optionally Water). During the quenching, the rods are quickly cooled to a temperature which may be considered to room temperature, in that the temperature is low enough (perhaps 300 F. or less) that further metallurgical changes do not take place on cooling to room temperature.

After quenching of the carburized rods just described, the plugs are removed; and it is found that the tensile strength of the body is about 125,000 p. s. i. whereas 6 that of the case hardened layerslll and 14 is about 300,000 to 400,000 p. s. i; (andthe Rockwell C hardness is 55-63, as compared to 3.5-43 in the starting material).

The resulting rods are then straightened by (comparatively easy) cold working to eliminate the slight warpage, if any; and the rods may be given customary subsequent heat treatments to remove strains, etc.

Although the instant invention may be employed using any ferrous materials, certain materials are preferred for drill rods. As mentioned, heretofore, drill rods have been made of high carbon steels such as SAE 1080 (specification: O15-0.88% C, 0.60 0.90% Mn, 0.040% P maximum, 0.050% S maximum, remainder Fe). The steels preferred for use in the practice of the invention have relatively low carbon contents of 0.l0-0.30% C and may contain 0.40-l.00% Mn (plus 0.040% P max., 0.040% S max., and O20-0.35% Si); but these steels also contain small amounts of one or more elements such as Ni, Cr and Mo which retard the martensite (or which serve to retain at least 20-30% austenite in thecarburized rod inside surface upon quenching). The amounts of such elements are G40-3.75% Ni, 0.40- l.l0% Cr and/or 0.15-0.30% Mo. The preferred steel for use in the invention is SAE 4320 (or AISI 4320), specification: 0.17-0.22% C, OAS-0.65% Mn, 0.040% P max., 0.040% S max., O20-0.35% Si, l.65-2.00% Ni, 0.40-0.60% Cr max., O20-0.30% Mo, in the case of most sizes of drill rod stock; but for the smaller size stock of one inch diameter or less SAE 8620 (or AISI 8620) is preferred and this has a lower Ni content and slightly higher Mn content, specification: 0.18-0.23% C, U70-0.90% Mn, 0.040% P max., 0.040% S max., 0.200.35% Si, 0.40-0.70% Ni, G40-0.60% Cr and 0.l5-0.25% Mo. rThe additional Mn in SAE 8620 compensates for the reduced Ni in retarding martensite formation.

lt will be understood that modifications and variations may be effected without departing from the spirit and scope of the novel concepts of the present invention.

I claim as my invention:

l. A method of carburizing a steel drill rod having an axial bore that comprises filling the bore with particulate carburizing material, closing off the ends of the bore to retain the particulate material therein, sealing the ends of the bore, suspending the rod from one of its ends, heating the rod to carburizing temperature in the presence of a carburizing fluid, and then quenching the rod by contacting the outside surface only with quenching liquid, while retaining the seal of the ends of the bore.

2. A method of carburizing a steel drill rod having an axial bore that comprises filling the bore with particulate carburizing material, sealing the ends of the bore, heating the rod to carburizing temperature in the presence of a carburizing fluid, quenching the rod while retaining the seal of the ends of the bore, and then removing the seal and the particulate carburizing material from the bore.

3. A method of carburizing a ferrous metal article having an elongated hole therein, which comprises filling the hole with particulate carburizing material, sealing the material therein,heating the article to carburizing temperature in the presence of a carburizing uid, whereby carburization of the surface defining the hole and the outside surface of the article is accomplished, and quenching the article by contacting the outside surface only with quenching liquid, while retaining the seal on the particulate material in the hole.

4. A method of carburizing a ferrous metal article having an elongated hole therein, which comprises filling the hole with particulate carburizing material that remains solid at carburizing temperature, sealing the material therein, heating the article to carburizing temperature in the presence of a carburizing fluid, whereby carburization of the surface defining the hole and the outside surface of the article is accomplished, and then quenching the article by contacting the outside surface only with quenching liquid, -whileretaining' the seal on the-particulate material inthe hole.A

5. A method of carburizing a steel drill rod having an axial bore that comprises filling the bore with particulate carburizing material, closing off the ends vof the bore to 5 `of` a carburizing uid,and.'quenching the rod by .conretain the particulate material therein, sealing the endsV tactingtheoutsideonly ofthe rod with yquenching liquid, of the bore, suspending the -rod` from onev of.' its ends, wherebycooling' of theinside surface is accomplished by heating the rod to carburizing temperature inthe presheatcon'duction lthrough'the body of the rod, said steelv ence of a carburizing fluid, and quenching the rod by rod havin'g'the followinglcompositiont' contacting the outside only of the rod with quenching 10 018;() 23% C' o 40 0 70% Ni liquid, whereby cooling ofthe inside surface is accom- 0'70 0`90% Mn 0'40 0`60% Cr plished by heat conduction through the body of the rod, 0'0407'0 -P *maxi 0'15 0'25% `M0 said steel rod-having the following composition: 0'040% AS Remail'lder. Fe` 0.l7-0.22% C 1.65-2.00% Ni 15 0.20-0.35%Si 0.45-0.65% Mn OAG-0.60% Cr 0.040% P max. O20-0.30% Mo References Cited in the'ile'of this patent 0.040% S max. Remainder Fe UNITED S'Ifyntsy PATENTS O20-0.35% Si 1,459,409 McQuald June 19, 1923 6. A method of carburizing a steel drill rod having-an `20 2,398,809 f Snyder Apr. 23, 1946 axial bore that comprises lling the bore with particulate carburizingmaterial,-'closing ot the ends ofsthe bore to retain thetparticulate material therein, sealing the ends of ,the' bore,-suspending the rod'from one of its ends,y

heating the rod to-carburizing temperature in the presence

Claims (1)

1. A METHOD OF CARBURIZING A STEEL DRILL ROD HAVING AN AXIAL BORE THAT COMPRISES FILLING THE BORE WITH PARTICULATE CARBURIZING MATERIAL, CLOSING OFF THE ENDS OF THE BORE TO RETAIN THE PARTICULATE MATERIAL THEREIN, SEALING THE ENDS OF THE BORE, SUSPENDING THE ROD FROM ONE OF ITS ENDS, HEATING THE ROD TO CARBURIZING TEMPERATURE IN THE PRESENCE OF A CARBURIZING FLUID, AND THEN QUENCHING THE ROD BY CONTACTING THE OUTSIDE SURFACE ONLY WITH QUENCHING LIQUID, WHILE RETAINING THE SEAL OF THE ENDS OF THE BORE.

Images