US3294595A

US3294595A - Precarburization and other prediffusion treatment of spiral rolled or differentially plastically formed and standard type drill steel

Info

Publication number: US3294595A
Application number: US505018A
Authority: US
Inventors: Tadeusz W Wlodek
Original assignee: Individual
Current assignee: Individual
Priority date: 1965-10-24
Filing date: 1965-10-24
Publication date: 1966-12-27
Anticipated expiration: 1983-12-27

Description

United States Patent PRECARBURIZATION AND OTHER PREDIFFU- SION TREATMENT OF SPIRAL ROLLED 0R DIF- FERENTIALLY PLASTICALLY FURMED AND STANDARD TYPE DRILL STEEL Tadeusz W. Wlodek, 297 5th Ave., Ottawa, Ontario, Canada No Drawing. Filed Oct. 24, 1965, Ser. No. 505,018 8 Claims. (Cl. 14812.1)

This application is a continuation-in-part of application Serial No. 148,854, filed October 31, 1961, now

abandoned, which is a continuation-in-part of application Serial No. 835,409, filed August 24, 1959, now abandoned, which is a continuation-in-part of application Serial No. 551,455, now Patent No. 3,017,697, filed December 6, 1955, and which, in turn, is a continuation-in-part of Serial No. 373,369, filed August 10, 1953, now abandoned.

This invention relates to a novel process for the manufacture of drill steel, hollow drill steel and other steel and metal structural members. More particularly, the invention concerns a method of improving the fatigue and corrosion fatigue resistance of drill steel by a carburizing treatment, followed by subjecting the steel to differential plastic deformation at ordinary or at high temperatures, with or without subsequent over-rolling.

This invention introduces also an additional process of precarburization of drill steel and other steel and metal structural members; and/ or precoating by thermal diffusion, preimpregnation and preplating and other coating treatments with other than carbon elements, all these treatments are called as preimpregnation treatments and all applied before final rolling and forming operations.

In general, it is the practice to form drill steel, and particularly hollow drill steel, from billets into rods having a cross section which may be round, hexagonal, or octagonal, the latter being commonly in the form known as quarter-octagon, by a rolling procedure; The resulting rods may be formed solid or with an internal longitudinal hole. Such drill steel rods and similar structural metal members are subject to a variety of stresses during use, which tends to hasten fatigue and failure of the steel.

In my Patent No. 3,017,697, there is disclosed a novel method for improving the fatigue resistance of drill rods and similar structural members which comprises subjecting the surface of the rods or metal members by cold working to pressures beyond the elastic limit of the surface portions. The pressures are applied in such a way as to result in differential plastic deformation of the surface at spaced intervals sufiicient to provide plastically deformed depressions of small dimensions, for example between one-thousandth of an inch and one-sixteenth of an inch in depth. The intervals are interspaced with substantially undeformed portions to provide locked-in residual stresses which resist fatigue.

Preferably, the distance between the depressions is kept between about one-eighth of an inch and about onesixty-fourth of an inch, the distance between the depressions being such that it is greater than the depth of the depressions. In this manner, plastic deformation is impressed on the metal surface at spaced intervals thereon, the intervals being so spaced as to leave between adjacent pairs of depressions a portion of the metal which has not been subjected to deformation, the interval portions 'remaining in the original elastic form. In general, juxtaposition of regions plastically deformed with regions of lesser plastic deformation, both regions being arranged in a regular or irregular pattern, is the topic of this treatment. The transition between these two regions may be gradual or abrupt, and the built-in residual stresses are ice substantially perpendicular to the direction of plastically deformed regions, i.e., direction of pattern.

It should be also emphasized that impression of grooves and depressions provide, through so-called mutual interlocking phenomena, a controllable mechanism of plastic deformation of treated surfaces, resulting in greatly improved physical qualities of material treated by this invention. These depressions may take the shape of helixes or of parallel grooves, for example. The grooves or depressions may be impressed by rolling, spiral rolling, ex truding, pressing, stamping, hammering, squeezing, drawing or twisting, or the like, depending upon the character of the rod or metal structural member. In the case of hollow or tubular members, such as pressure vessels or cylinders, the grooves or depressions may be applied to the inner or outer surfaces or to both.

The grooves or depressions made in the surface of the metal being plastically deformed should be at least 0.001 inch (0.025 mm.) in depth. Where extremely hard metal is employed, the depressions formed by pressure applied at spaced intervals are very shallow. The preferred range of depth is from about inch (0.12 mm.) to about inch (0.4 mm). It will be apparent that the spacing of the grooves or depressions and the depth used will be dependent upon the form of groove employed. The preferred distance between the grooves is from about A; inch (3.2 mm.) to about & inch (0.4 mm.), the distance generally preferred being about 1 inch (1.08 mm.).

While this aspect of the invention is applicable to such structural members as steel, aluminum and magnesium alloy plates, and the like, it is especially applicable to drill steel rods.

In accordance with the first aspect of the present invention, it has been found that drill steel and other structural metal members, which are subjected tothe differential plastic deformation treatment disclosed in the abovementioned copending applications, are further improved with respect to their fatigue resistance and other physical properties if they are first subjected to carburization, or to carburization together with diffusion treatment, or also to impregnation treatments prior to plastic deformation in the manner described.

It is the present industrial practice to carburize drill steel and other structural members only after all rolling and forming operations are completed. Thus, a typical process starts with a length of drill steel 10 to 20 feet long and from 1 to 1.5 inches in diameter. This is placed in a long canburization furnace packed with carburization mixture, and heated for 8 to 12. hours at about 1700"- 1800 F., then quenched and cleaned. This results in a case hardened steel which can be subjected to plastic deformation by impressing shallow grooves or depressions on its surface which serve to increase the life of the steel.

In accordance with the second aspect of the invention, it has been found that by applying plastic deformation in the form of grooves or depressions to the surface of a drill steel which has been precarburized and/or preimpregnate-d with other metals before it is rolled into a drill rod, the same effect or even a heightened effect in improvement of fatigue resistance and life of the steel can be obtained with shallower grooves. This is believed due to the fact that precarburization prior to rolling enables the rolling operation to bring about a better and more even distribution of the carbon and other alloying elements along the surface of the drill steel, a greater depth of penetration of the case hardening carbon, and a gen eral diffusion of the carbon within the steel coupled with refinement of its grain structure, resulting-in increased strength.

Accordingly, the manufacture of drill steel, hollow drill steel, and other solid and hollow structural members, in accordance with the invention, is based on precarburiza- 3 tion and/or preimpregnation with other metals and other prediifusion treatments of the exterior, or of both the interior and exterior, of such members, before the application of final rolling and forming operations. If desired, further diffusion of the precarburization carbon and prediffused alloys can be carried out prior to the final rolling or other operations, in accordance with our procedures, or they may also be applied after rolling and forming.

In order to simplify the descriptions, the precarburization, preimpregn-ation, and preditfusion treatments in this application identify treatments applied, as recommended in this invention, i.e., before final forming and rolling operations. The same treatments applied after forming operations are identified as carburization, impregnation and diffusion treatment-s with omission of pre.

Preimp-regnation processes, in general, involve the introduction of other than carbon hardening elements or alloying substances into a ferrous or nonferrous alloys by heating the metal in contact with hardening materials which can be solid, liquid or gas, to a temperature above the characteristic transformation range of treated metal, for a few hours, usually from 4 to 8 hours, for a lower temperature range about 100 F. above the characteristic transformation temperature, and from 30 min. to 4 hours for a very high temperature range about 100 F. to 500 F. above the characteristic transformation temperature, depending on the type of metal treated and applied and required thickness of the hardened case, usually from 30 to inch deep. Preimp-regnation treatments, with other than carbon elements i.e., with such metals as Cr, Mo, Si, Ni, Va, Co, U, Ti, columbium, boron and tungsten and their alloys, recommended for the application in the proposed method of production, e.-g., before final rolling and forming of hollow and solid drill steel and other structural metal members, include transfer techniques of selected elements into the surface of the prefabricated billet. These selected techniques are: chromizing, siliconizing, tungstenizing, in general coating with metals specified above.

Cladding with layer of a selected metal or alloy specified in the above list, is accomplished by heating in a granular packing of the metal to be applied in a protective atmosphere, or by heating in a gas from which the metal is deposited, e.-g., from a chloride of the metal. Moreover, cladding for the proposed preimpregnation treatment, with specified above metals, is accomplished by thick electroplating technique, by metallizing with blowpipe, hot-dipping and electrochemical depositions followed if required by a prediffusion treatment. A typical example of a preimpregnation treatment is given below:

Example I p The preimpregnation with chromium, molybdenum or the like, in accordance with the invention is applied both to solid or hollow drill steel billets or other structural metal members by thermal diffusion of selected metals into the surface of treated billets in order to convert a Zone at the surface of steel into a corrosionand fatigue-resisting alloy. This transformation is effected by the impregnation of the surface with chromium, molybdenum or other metals listed above by chemical and physical reactions taking place at a high temperature. In one particular example of preim-pregnation with chromium or molybdenum, a treated billet is heated to between 1900 F. and 2000 F. in chromium or molybdenum ferroalloy dust and the surface of billet is enriched with the respective metal. The depth of impregnation depends on time and temperature of treatment within the range of 4 to 12 hours at 2000 F., the required depth of the alloyed case inch being reached.

In another example of chromizing, a steel billet of SAE 1080 drill steel is heated to 1800-2000 F. in a controlled atmosphere furnace in a granular mixture of ferrochromium. Maintaining a reducing atmosphere inside 'the furnace, chromium is brought to the steel surface in the form of a vaporized compound-metal chloride. Normal-1y the time required for this treatment is between 6 to 12 hours in order to obtain the desired -alloyed case Of %000 iIICh.

The precarburization step will be illustrated with reference to drill steels with low (0.15-0.2S%) or medium (0.30-0.45 carbon content, which also contain one or more other alloying elements such as Ni, Cr, Mo, Mn, Si, U, V, Co, Cb, Cu, W, and B, and also with reference to a No. 1080 SAE type plain high carbon steel.

A typical low carbon nickel-chromium-molybdenum drill steel may have, for example, either of the following two compositions:

(1) Carbon 0.15O.25%; chromium 0.400.80% nickel 1.5-2.50%; molybdenum 0.20-0.50%; silicon 0.25- 0.35%; manganese 0.400.60%; phosphorus and sulphur 0.02% each, maximum.

. (2) Carbon 0.20-0.30%; chromium ZOO-3.5%; nickel 0.20-0.40%; molybdenum 0.300.60%; silicon 0.30 0.60%; manganese 0.60l.00%; phosphorus and sulphur 0.02% each, maximum.

A typical medium carbon nickel-chromium-molybdenum drill steel suitable for use in connection with the invention is the following:

Carbon 0.300.45%; chromium 0.300.50%; nickel 2.53.5%; molybdenum 0.200-.30%; silicon (MS-0.35%; manganese 0.60l.00% phosphorus and sulphur 0.020% maximum each.

Two typical high carbon plain steels which are adapted to the process of the invention are:

(1) Plain carbon steel 0.60-0.80% carbon and small additions of other alloying elements like Mo, Va, Ca.

(2) Carbon-chromium steel 0.70 to 1.00% carbon and about 1.00% chromium with small additions of other elements.

Precarburization rand/or preimpregnation, in accordance with the third aspect of the invention, may be applied both to solid drill steel or to hollow drill steel billets. The precarburization may be effected by treatment with any conventional suitable carburizing medium, Whether gaseous, liquid or solid. Preferably a solid carburizing medium rich in carbon is used. Typical carburizing media are mixtures of hardwood charcoal and barium carbonate, such as 2 parts barium carbonate to 3 parts wood charcoal, or a molten cyanide salt bath containing 50% sodium cyanide and 50% sodium carbonate. A typical gaseous medium is carbon monoxide or a gas rich therein.

The rod, billet or member to be precarburized is packed in a carburizing oven with .a suitable amount of solid medium, depending upon the size of the member.

Where a gaseous .medium is used the billet can be precarburized in a gas carburizing furnace employing carbon monoxide, or a hydrocarbon gas such as methane, ethane, propane or butane, or mixtures of city gas or natural gas.

The precarburizing temperature employed will range from about 1600 to about 2300 F., depending upon the medium employed and the type of metal being treated. After the required case has been formed, the member may be heat treated in any desired manner, for example utilizing oil or water quenching. The precarburization time may be any desired period, preferably from about A to about 8 hours. The thickness of the case may be any desired thickness, but ordinarily will vary from about 0.02 to 0.2 inch.

The fourth aspect of the invention is the application of very high temperature precarburization, preimpregnation and prediffusion treatments. The carburization, impregnation and diffusion treatments, all high temperature treatments of previously finally formed structural elements, are treatments of which the upper limit of temperature is limited in order to avoid undesirable excessive grain growth, dimensional changes, intercrystalline se' regations and other changes which effect, very substantiah 5 l'y reduce the fatigue strength and durability of hollow and solid drill steel, e.g., the upper limit of carburization is limited to about 1750 F., and the temperature of other impregnation and diffusion treatment specified above is kept as low as possible above the required characteristic transformation temperatures of treated metals.

In the new and improved method of the present invention for the fabrication of precarburized and/or preimpre-gn-ated, and prediffused drill steel, the upper limit of preca-rburization can be increased from 1750 F. up to 2300 F., because final extensive rolling and other plastic deformations will eliminate the detrimental effects introduced by very high temperature of precarburization, preimpregnation and prediffusion treatments with other than carbon elements.

Employing intensive and substantial plastic deformation will refine the grain size and the metallographic structure as a whole. Coarse crystal structure and concentration of grain boundary intercrystalline segregations of carbon and other alloying constituents built-up and developed during high temperature treatments will be 'worked through and better redistributed during final rolling deformation, an important step of the invention provided for that purpose. Applied after precarburization and/ or preimpregnation, and prediflusion treatments, very effective refining treatment of the coarse structure and large columnar grains, through extensive plastic deformation will cause also a better longitudinal alignment of microand macrofibrous structure into the direction of final rolling What will: also increase the fatigue strength of longitudinal fibrous of surface layers.

Such very high temperature treatments from 1800 F. up to 2300 F. for fast precarburization treatments, require only a fraction of time for completion of this treatment as compared to standard treatments, which is a substantial and very important saving in production time and equipment, and a significant improvement in the quality of final product.

Similarly by very high temperature fast preimpregnation and prediffusiontreatments from 100 to 500 F. above the required transformation temperature recommended, the time of treatment will be reduced and the effectiveness increased substantially.

After the completion of the precarburization treatment, the billet, without quenching, and at substantially within the carburization temperature range of about 1600 and 2300 F. is rolled down or formed to the required dimensions, and shapes, and then, if desired, subjected to differential plastic deformation, as described fully in my Patent No. 3,017,697.

Thus there is provided by the present invention, a method of producing hollow drill steel and other hollow steel members having a 'hard surface characterized by high fatigue and corrosion resistance which comprises the steps of heating a hollow billet to a temperature between about 1600 F. and about 2300 F. and above the transformation point of the steel in a carbon rich medium, maintaining the billet at that temperature in the carbon rich medium for a time sufficient to effect carburization of at least the exposed inside surfaces of the billet, and thereafter, without quenching, and at substantially the carburization temperature range, mechanically deforming the billet, to materially alter its cross-sectional area and to effect grain refinement and distribution of carbon and other alloying elements at the carburized surface.

There is also provided by the present invention a method of producing hollow drill steel and other hollow steel members having a hard surface characterized by high fatigue and corrosion resistance which comprises the steps of heating a billet at a temperature at least about 100 F. above its transformation point with an alloying element other than carbon, such as Cr, Mo, Si, Ni, V, Co, U, Ti, Cb, B or W, or their alloys, to effect diffusion of the element into the billet surface, then heating the billet to a temperature between about 1600" F. and 2300 F., maintaining the billet at this temperature for a time sufficient to effect diffusion into at least the exposed inside surfaces of the billet, and thereafter, without quenching, and at substantially the diffusion temperature range, mechanically deforming the billet, to materially alter its cross-sectional area and to effect grain refinement and distribution of carbon and other alloying elements.

The novel concept of high temperature carburization or diffusion followed by hot finishing at temperatures close to those of carburization or diffusion, permits very rapid carburization or diffusion as compared with prior art methods, drastically reducing carburization time, for example, from usual 8 to 12 hours, to as little as 30 minutes, because the very detrimental coarse grained structure built up during intensive carburization at high temperatures is subsequently refined, in accordance with the invention, by the rolling or other mechanical deformation step at substantially the carburization temperature range. This procedure is illustrated in Example 6.

Precarburization results in a surface layer which may have a carbon content up to about 1.3% C, thus provid ing sufficient carbon for further diffusion into the interior of the drill steel. Such diffusion, which may be achieved by further heat treatments so that the original relatively shallow high carbon case of 1.3% C and about 0.015 inch depth can be extended to a deeper case of about 0.9%- 1.0% carbon and about 0.040 inch depth, e.g., by heating at 1600 to 1900 F. for 2 to 6 hours to achieve diffusional redistribution of the carbon. Such precarburization, and diffusion treatments may be multiplied as desired, and can be interspaced with other deformation and rolling treatments. By creating a carbon gradient, precarburization and prediffusion provide reinforcement of the surface and allow deeper penetration of the locked-in stresses to prevent premature failure of the steel, by reducing substantially the amount of retained soft austenite.

A fifth aspect of the invention is double or multiple carburization, i.e., precarburization followed by diffusional redistribution of the first case and a secondary carburization cycle resulting in double-ply or multiple carburization or preimpregnation with other than carbon media, which will provide additional reinforcement of the surface of treated element.

Double-ply carburization involves, first a precarburization treatment; usually a high-temperature treatment applied in the upper limit of permissible temperature range 1750-2100 F., in some cases up to 2300 F., in order to save on time and increase the intensity and carbon gradients. This is followed by prediffusion treatment and afterwards 'by a substantial rolling and forming operation.

Manufactured in this way a bar, tube, or the like, after cutting and machining, is exposed to a final carburizing treatment (second carburization), applied in the lower temperature range 1650 to 1750 F., in order to limit the grain growth, during which treatment the second carbonply is built up.

Double-ply carburized surfaces are recommended for elements exposed to severe working conditions such as ball and roller bearings, its races, drill steel, heavily stressed bolts, etc.

Double-ply or multi-ply precarburization and/or impregnation treatments, or straight double ca-rburizingdiffusion-impregnation-treatments, if required, followed by spiral-rolling will result in the most durable and fatigue resistant surfaces.

The double-ply or multiple carburization and impregnation-diffusion treatment described above amounts, in simple terms, to various combinations of proposed surface treatments such as, for example, one or two layers of carburized case combined, if required, with one or two layers of impregnated and diffused cases of elements other than carbon and their alloys, interspaced as would be desired, e.g., carbon case followed by impregnated metal of about 0.015 to 0.030".

7 case and so on, applied before, during or after the final forming processes.

In the manufacture of solid drill steel, the precarburi- Zation and/ or preimpregnation step is applied only to the outside surface of steel billets used to manufacture hexagonal, quarter-octagon, and round cross sections of drill steel of diameter from A" to 2 /2", or to drill pipes of outside diameter from 2" to 6" and wall thickness from M1" to 1" and length from 2 to 30 feet and is illustrated by the following example:

Example 2 A drill steel billet of a medium carbon nickelcbromium-molybdenum steel, 6" square and 40" long, was carburized by heating in a bed of powdered coke for about 6 hours at a temperature of about 1800 F., to form a hardened case of a depth between about 0.015 and 0.060". The billet was removed, and rolled to form a round 1.5" diameter drill rod, and then allowed to cool.

There were then applied to the outer surface of the rod by means of rolls, circumferential grooves or depressions of a hollow configuration, alternating with circumferential crest portions, the grooves having a depth of about & and the distance between grooves being about The resulting drill rod exhibited improved physical and mechanical properties over standard drill rod stock.

In the new and improved process as applied to the man ufacture of hollow drill steel, i.e., containing an inside longitudinal hole, the precarburization and/or other preimpregnation and predilfusion steps are applied to both the inside hole and the outer surface simultaneously or separately. Such hollow drill steel may be prepared, for example, by starting with a drill steel billet 4 to 10" square and 30" to 50" in length, drilling a large hole (1" to 3" in diameter) through the center with a mechanical drill to allow for free access of the carburizing medium. The billet is then precarburized and/or preimpregnated by any suitable method, a suitable mandrel or other means of maintaining the inside geometry of the hole is inserted, and the billet is rolled to a length of 20 to 30 feet, and a diameter (round, hexagonal, or quarteroctagon) of 1- 1.5". Alternatively, if required, the billet with central mandrel therein may be rolled to the desired shape and length and then carburized after the mandrel is removed.

The process for hollow drill steel is illustrated by the following examples:

Example 3 A drill steel billet of low carbon nickel-chromiummolybdenum steel having the analysis previously disclosed, 6 inches square and 40" long, and drilled to produce a central hole 2" in diameter, is precarburized with a charcoal-barium carbonate mixture at a temperature of 2000 F. for about 4 hours until the carburized case attained a depth between 0.040" and 0.080 on both the inside and outside surfaces. After completion of the precarburization, the billet is cooled and the inside and outside surfaces cleaned. An austenitic manganese steel rod 2" in diameter is inserted into the center hole of the billet in order to maintain the inside geometry of the hole. The billet is then heated to a rolling temperature of about 19 00 F. for about 2 hours, and rolled down to a 1" quarter-octagon section with inside hole of about diameter, and then air colled to a surface hardness of 50-60 Rockwell C units. The manganese steel rod is removed and the drill rod cut into the required lengths. In some instances the time of heating for rolling may be extended and the billet held for 4 to 6 hours at rolling temperatures to diffuse the carbon deeper into the interior ,of the billet and to widen the case from 0.060" to 0.120".

After the completion of this additional prediffusion during the extended heating cycle, and after the rolling operation, the precarburized, predilfused case will have a depth The drill rod lengths if required are then subjected to surface differential plastic deformation as described in Example 2.

The precarburization, predifiusion and other preim pregnation treatments of the invention are especially well suitable for treatment of inaccessible surfaces, openings, channels, and the like, before rolling is applied to members containing these, for instance conical or threaded drill rod attachments. Here the precarburized drill steel is heated at both ends and subjected to forging and machining operations. After completion of these operations, in order to restore the carbon case at both ends, a final gas carburization is applied at a lower temperature from about 1600 to about 1700 F. for 6 to 8 hours, followed by oil quenching and drawing at 450 F. for 1 hour.

Differential plastic deformation, that is the impression of shall-ow depressions at spaced intervals on the surface of treated pieces, if required, can be basically applied in three ways:

(1) Simple differential plastic deformation, by grooving the surface, applied at room temperature, i.e., by cold working, or else applied at a temperature below the recrystallization temperature of the metal, which for ferrous metals is usually below 1000 F.

(2) Differential plastic deformation at high temperatures lying above the recrystallization temperature and above the full stress relief temperature of the metal. This is followed by overrolling with a roll having a smooth surface at room temperature or at a temperature below recrystallization and full stress relief temperature, viz, about 1000 F.

(3) Differential plastic deformation as described above at (1) combined with overrolling with a roll of smooth surface at room temperature or at a temperature below the recrystallization temperature and full stress relief temperature of the metal.

The machines and rollers used in differential plastic deformation are described in my copending application Serial No. 551,455, now Patent No. 3,017,697, and this description is incorporated herein by reference. The smooth rolls are used for smoothing or overrolling the differential plastic deformation patterns impressed during that stage of the process.

In accordance with a sixth aspect of the invention, therefore, precarburization and/or preimpregnation followed, if required, by diffusion treatments, or standard carburization precedes differential plastic deformation, and the latter step is followed by overrolling. This last sequence is illustrated by the following example:

Example 4 A A2" hexagonal 1080 SAE carbon drill steel rod in the as-rolled condition, without heat treatment, and having the composition: C 0.77, Mn 0.27, Si 0.19, P 0.018, S 0.025, was subjected to differential plastic deformation as described in Example 1 by impressing 24 grooves per inch on the drill steel having a groove depth between 0.004" and 0.006" with a roll at a pressure of 550 pounds, with radius at the rolling edge of the rolls. Overrolling was then applied with smooth rolls with /4" radius at the rolling edge and 550 pounds pressure. This resulted in a very substantial increase in fatigue strength. Thus, under a rotating bending moment of 5000 inchpounds and maximum surface stress about 61,500 pounds per sq. inch the standard untreaded steel resisted 90,000

' cycles to failure, as compared with 1,055,000 cycles for 9v deformation at high temperatures lying above the recrystallization temperature and the full stress relief temperature of the metal, generally above 1000 F., followed by over-rolling with smooth rolls at a temperature below the 1000 F. temperature. In this treatment the plastic deformation grooves are applied while the metal is hot, at a temperature above the recrystallization and stress relief temperature. Stress relief temperature may be defined as the temperature at which the effects of cold working and the locked-in residual stresses resulting therefrom,

begin to disappear. The grooved surface is then overrolled at room temperature as described previously. This over-rolling eliminates protruding crests which are partially flattened, elaving the adjacent valleys with lesser plastic deformation.

In accordance with still another aspect of the invention, there is contemplated the combination of plastic deformation either at ordinary temperature or at high temperatures (above 1000 F.), followed by over-rolling, the precarburization, preimpregnation or prediffusion step being omitted. The benefits arising from the comb-ination of over-rolling with either of these two types of plastic deformation have been pointed out. It will be understood, however, that the properties of the final product are nevertheless still further improved when precarburization and/ or preimpregnation is included in the process of the invention, for the reasons stated below.

The application of differential plastic deformation to the surface of drill steel, or hollow drill steel, increases its fatigue resistance and corrosion-fatigue resistance so effectively that on the average, a threefold increase in drilling life is achieved. 9

Differential plastic deformation of precarburized and/ or prediffused hollow drill steel is considerably more efiective in increasing fatigue resistance. This is because the presence of the carburized outer surface, supplemented by the strengthening effect of differential plastic deformation, provides an enchancing of the fatigue resistance beyond that of the additive effect of these two operations. The simultaneous precarburization of the inner hole portion of the hollow drill rod acts to prevent the initiation of fatigue cracks and radiation thereof to the outer portions of the hollow rod. Thus, tests data indicate in the case of a 4;" hexagonal low carbon Ni-Cr-Mo hollow drill steel rod, which has been carburized on inside and outside surfaces, under a rotating bending moment of 7000 inch-pounds and maximum stress of :99,000 pounds per sq. inch, the rod which was not plastically deformed after carburization withstood 97,000 cycles, Whereas the same rod after carburization followed by plastic deformation, withstood 950,000 cycles, a tenfold improvement. This clearly demonstrates the unexpected improvement conferred by the combination of precarburization and plastic deformation, in accordance with the present invention. The fatigue strength of the outside surface is supplemented by the reinforcing effect of the uniformly precarburized inside hole to provide a superior drill steel. Similarly, precarburized conical drill rod attachments with plastic deformed (spiral rolled) conical tapers are much stronger than when not precarburized.

Example 5 This example was performed for the illustration of the precarburization, prediffusion and double-ply carburization process on three one inch square and hollow samples of low carbon (0.20-0.30% C) nickel-chromium-molybdenum steel having the analysis as specified above.

All three samples were precarburized in a gas carburizing furnace employing hydro carbon mixtures at a temperature of 1725 F. for six hours then air cooled and reheated for the standardization of the structure to 1550 F. for 45 minutes and air cooled, the precarburized case was about inch thick and show surface hardness of 55-60 Rockwell C.

After completion of the precarburization the first sample was then heated to a rolling temperature of about 1900 F. for about one hour and rolled down to inch and then air cooled to a surface hardness of about 55 Rockwell C units. The thickness of the precarburized case after rolling operation was then about 10 to inch and, as was to be expected according to the invention, evenly distributed on the inside and outside surfaces and microstructure refined according to the invention. The second sample was precarburized in the same way as the first sample and additionally prediffused at a temperature of 1700" F. for 4 hours in protective atmosphere and air cooled to surface hardness of about 55 Rockwell C units. The precarburized case was then prediffused from original thickness of inch to inch, i.e., it increased by about 50% showing a smoother and better redistribution of carbon and a substantial decrease of soft retain austenite on the inside and outside surface of the sample. After completion of the precarburization and prediffusion treatments, the second sample was heated to a rolling temperature of about 1900 F. for about one hour and rolled down to 7 inch and then air cooled to a surface hardness of 50-55 Rockwell C units. The thickness of the precarburized and prediffused case after rolling was about A inch and the inside and outside case showed all improvements provided by the invention. The third sample was precarburized and prediffused in a similar way as the second sample, but before final rolling operation was precarburized for the second time at 1725 F. for two hours and air cooled in order to demonstrate the so called double-ply carburization, i.e., the reinforcement of the treated surface with two precarburized layers interspaced by a preditfusion treatment. Before the final rolling operation the second precarburized layer was inch thick and the first precarburized layer was effectively diffused into the surface by about inch thick. After final rolling the first carburized case was about inch thick and second case about 7 inch. These three tests proved, in all aspects listed above, the effectiveness of the precarburization, prediffusion and double-ply carburizati'on in the manufacture of durable drill steel. In all three tests according to the invention, it was proved that a very effective grain refining and better redistribution of carbon and other alloying elements takes place resulting in a very substantial fatigue and corrosion fatigue strength of structural members manufactured according to this invention.

Example 6 A drill steel billet of low carbon nickel-chromiummolybdenum steel having the analyses previously disclosed. 8 inches square 40" long, and drilled to produce a central hole 3 inches in diameter, is precarburized at a temperature of 2300 F. for about 30 minutes until the carburized case attains a depth between 0.040" and 0.080" on both the inside and outside surfaces.

After completion of the precarburization the inside and outside surface is cleaned and an austenitic manganese steel rod 3 in diameter is inserted into the center hole of the billet in order to maintain the inside geometry of the hole and afterwards without quenching and at substantially the hot rolling temperature range 1900 to 2300 F. is rolled down to 1 hexagonal section and cooled to a surface hardness of 45 to 55 Rockwell C units.

I claim:

1. Method for improving the fatigue resistance of a drill steel member which comprises the steps of carburizing the exposed surfaces of said member by heating in a medium rich in carbon at a temperature between 1600 and 2300 F., for a period of time sufiicient to effect carburization of said exposed surfaces, then subjecting the outer carburized surface to plastic deformation by cold working by applying to portions of said surface and at spaced intervals, pressure beyond the elastic limit of the steel, said pressure being sufficient to provide small plastically deformed depressions at intervals, said intervals being interspaced with substantially undeformed portions, the distance between said depressions being greater than the depth of said depressions, and further subjecting said undeformed portions to over-rolling with a smooth roll at a temperature between room temperature and about 1000 F.

2. Method for improving the fatigue resistance of a drill steel member which comprises the steps of carburizing the exposed surfaces of said member by heating in a medium rich in carbon at a temperature between about 1600" and 2300" F., and above the transformation point of the steel for a period of time sufficient to effect carburization of said exposed surfaces, then subjecting the outer carburized surface to plastic deformation at a temperature above 1000 F. by applying to portions of said surface and at spaced intervals, pressure beyond the elastic limit of the steel, said pressure being sufficient to provide small plastically deformed depressions at intervals, said intervals being interspaced with substantially undeformed portions, the distance between said depressions being greater than the depth of said depressions and then subjecting the surface to over-rolling with a smooth roll at a temperature between room temperature and about 1000 F.

3. Method for improving the fatigue resistance of a drill steel member which comprises the steps of subjecting the outer surface of said member to plastic deformation by cold working by applying to portions of said surface and at spaced intervals, pressure beyond the elastic limit of the steel, said pressure being sufficient to provide small, plastically deformed depressions at intervals, said intervals being interspaced with substantially undeforrned portions, the distance between saiddepressions being greater than the depth of said depressions, and then subjecting said member to over-rolling with a smooth roll.

4. Method for improving the fatigue resistance of a drill steelmember which comprises the steps of subjecting the outer surface of said member to plastic deformation at a temperature above about 1000 F. by applying to portions of said surface and at spaced intervals, pressure beyond the elastic limit of the steel, said pressure being suflicient to provide small, plastically deformed depressions at intervals, said intervals being interspaced with substantially undeformed portions, the distance between said depressions being greater than the depth of said depressions, and then subjecting said member at a temperature between room temperature and about 1000 F. to over-rolling with a smooth roll.

5. Method for producing hollow steel members comprising heating a short billet of large cross-sectional area and having a large bore to a temperature above the transformation point of the steel in a carbon rich medium and maintaining the billet at that temperature for a period of time sufficient to effect substantially uniform carburization and diffusion of hardened elements into both the inner and outer surfaces of said billet, then hot-rolling said billet to reduce the cross-sectional area and increase the length of the billet several fold and to effect grain refinement and longitudinal orientation, and then subjecting the outer surface 'of the rolled member at spaced intervals to the impression of shallow plastically deformed depressions about one sixty-fourth of an inch deep, said intervals being spaced with substantially undeformed portions, the dis tance between said depressions being about one twentyfourth of an inch.

6. The method of claim 5 in which the member, after the impression of shallow plastically deformed depressions, is further over-rolled at a temperature below the recrystallization temperature of the steel.

7. The method of producing hollow drill steel and other hollow steel members having a hard surface characterized by high fatigue and corrosion resistance which comprises the steps of heating a hollow billet to a temperature between about 1600 and 2300 F. and above the transformation point of the steel in a carbon rich medium, maintaining said billet at said temperature in the carbon rich medium for a time sufficient to effect carburization of at least the exposed inside surfaces of the billet, and thereafter, without quenching, and at substantially the said carburization temperature range, mechanically deforming the billet, to materially alter the cross-sectional area thereof and effect grain refinement and distribution of garbon and other alloying elements at the carburized surace.

8. The method of producing hollow drill steel and other hollow steel members having a hard surface characterized by high fatigue and corrosion resistance which comprises the steps of heating a billet at a temperature at least about F. above its transformation point with an alloying element other than carbon to effect diffusion of said element into the billet surface, then heating the billet to a temperature between about 1600" to 2300 F., maintaining said billet at said temperature for a time sufficient to effect diffusion of at least the exposed inside surfaces of the billet, and thereafter, without quenching, and at substantially the said diffusion temperature range, mechanically deforming the billet to materially alter the crosssectional area thereof and effect grain refinement and distribution of carbon and other alloying elements.

References Cited by the Examiner UNITED STATES PATENTS 4/1959 Thorn et a1 l48-l2.1 1/ 1962 Wlodek 72377

Claims

1. METHOD OF IMPROVING THE FATIGUE RESISTANCE OF A DRILL STEEL MEMBER WHICH COMPRISES THE STEPS OF CARBURIZING THE EXPOSED SURFACES OF SAID MEMBER BY HEATING IN A MEDIUM RICH IN CARBON AT A TEMPERATURE BETWEEN 1600* AND 2300*F., FOR A PERIOD OF TIME SUFFICIENT TO EFFECT CARBURIZATION OF SAID EXPOSED SURFACES, THEN SUBJECTING THE OUTER CARBURIZED SURFACE TO PLASTIC DEFORMATION BY COLD WORKING BY APPLYING TO PORTIONS OF SAID SURFACE AND AT SPACED INTERVALS, PRESSURE BEYOND THE ELASTIC LIMIT OF THE STEEL, SAID PRESSURE BEING SUFFICIENT TO PROVIDE SMALL PLASTICALLY DEFORMED DEPRESSIONS AT INTERVALS, SAID INTERVALS BEING INTERSPACED WIH SUBSTANTIALLY UNDEFORMED PORTIONS, THE DISTANCE BETWEEN SAID DEPRESSIONS BEING GREATER THAN THE DEPTH OF SAID DERPRESSIONS, AND FURTHER SUBJECTING SAID UNDEFORMED PORTIONS TO OVER-ROLLING WITH A SMOOTH ROLL AT A TEMPERATURE BETWEEN ROOM TEMPERATURE AND ABOUT 1000*F.