US2708159A

US2708159A - Heat treated, hardened alloy steel elements

Info

Publication number: US2708159A
Application number: US412942A
Authority: US
Inventors: Francis B Foley; Charles C Clark
Original assignee: International Nickel Co Inc
Current assignee: Huntington Alloys Corp
Priority date: 1954-02-26
Filing date: 1954-02-26
Publication date: 1955-05-10
Anticipated expiration: 1972-05-10

Description

States atent 2,70s,1ss

HEAT TREATED, HARDENED ALLOY STEEL ELEMENTS Francis E. Foley, Philadelphia, Pa., and Charles (3. Clark, Little Falls, N. 3., assignors to The International Nickel Company, Inc, New York, N. Y., a corporation of Delaware No Drawing. Application February 26, 1954, Serial No. 412,942

6 Claims. (Cl. '75124) The present invention relates to hard machine elements, such as gears, dies, and the like, which are required to have accurate dimensions in the hardened condition and to a material, such as bar stock, for the production of such articles.

Attempts have been made to provide the trade with hard machine elements made from a bar stock of tough, high strength steel capable of being finished directly to final and accurate dimensions and of being heat treated to relatively high hardness without substantially distorting or otherwise warping out of shape. The problem of distortion is particularly important in the manufacture of gears, especially automotive gears. In order for gears to run quietly in service, it is essential that they pass through the heat treating operations with the minimum amount of change in size and shape. It is frequently found after heat treatment that gears are not circular, that they have warped or that they have increased or decreased in diameter. These shape irregularities and variations are generally attributed to the heating and quenching operations employed in the hardening heat treatment. One of the remedies suggested to overcome this disadvantage has been to simplify the heat treatment of the gears. While this suggestion helped to a certain extent in alleviating the condition, gears still had to be produced with a sutficient oversize so as to allow for dimensional variation due to distortion which had to be corrected by grinding and machining. Although attempts were made to overcome the foregoing difficulties and other disadvantages, none, as far as we are aware, was entirely successful when carried into production on a commercial and industrial scale.

In searching for a solution to the above-described problem, it occurred to us that a nickel-aluminum dispersionor age-hardening steel might be employed to overcome the foregoing difliculties encountered in producing gears. However, it was found that while this steel could be employed in producing articles of finished dimensions with minimum distortion, the steel did not usually have the desired hardness and, furthermore, did not have the" required toughness and/ or strength for gears. Generally, it is desired that the gear steel be capable of hardening to at least 42 Rockwell C. It was found that in most instances, the nickel-aluminum steel was generally brittle and could not be employed under conditions involving sudden applications of stress such as occur in tractor gears and other automotive gears. For example, it was found that While a nickel-aluminum steel containing about nickel and about 2% aluminum could be heat treated and aged without substantially distorting, the steel did not have adequate hardness and also did not have the required toughness. Generally speaking, the steel was found to be too brittle and to have inferior resistance to impact stresses such as encountered in gears.

It has now been discovered that a hardened machine element, such as a gear, a die, etc., having high strength combined with relatively high hardness and toughness can be produced directly from bar stock of a special dispersion-hardening steel to its finished dimensions without substantially distorting or warping after hardening by heat treatment.

It is an object of the present invention to provide a machine element hardened to at least about 42 Rockwell C without substantially distorting or warping out of shape.

Another object of the invention is to provide a special machine element bar stock made of a dispersion hardenable steel capable of being heat treated without exces-. sive distortion to a hardness of at least about 42 Rockwell C combined with improved toughness.

The invention also contemplates providing improved machine elements, such as gears, dies, etc., which in use are subjected to wear and heavy static and impact stresses.

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

Generally speaking, the present invention contemplates a hardened machine element made of a special dispersionhardenable steel hardened by heat treatment to a hardness of at least about 42 Rockwell C without substantially distorting or warping out of shape. The present invention is based on the discovery that machine elements made of nickel-aluminum steels have an improved combination of physical properties, particularly improved hardness in combination with high strength properties, improved toughness and improved resistance to impact, when the steel contains in combination with controlled amounts of nickel and aluminum critical and controlled amounts of the elements chromium and molybdenum. The steel contemplated by the invention is unique in that it can be hardened by utilizing an austenitizing treatment, a tempering treatment and a low temperature aging treatment to produce the required hardness in combina- 'tion with improved toughness and impact resistance without encountering excessive distortion or warping.

In general, the steel provided by the invention for use in machine elements contains as critical and essential elements about 3.5% to 6% nickel, about 1.5% to 2.5% aluminum, about 0.4% to 1.25% chromium, about 0.2% to 0.3% molybdenum and about 0.18% to 0.3% carbon the balance of the composition being essentially iron. It is preferred for optimum physical properties that the steel contain, together with the essential elements chromium and molybdenum, vanadium in amounts not exceeding 0.15%, the vanadium content being preferably at least about 0.05%. In producing the steel within the foregoing composition range, it is essential that the carbon content be correlated to the nickel and aluminum contents expressed stoichiometrically by the formula NiAl. Thus, when the NiAl content of the alloy falls within the range of about 4.5% to 5.5%, the carbon content falls Within the range of about 0.25% to 0.30%. Likewise, when the NiAl content of the steel is within the range of about 5.5% to 7.5%, the carbon content is within the range of about 0.18% to 0.25%.

The machine elements provided by the present invention, such as gears, dies, etc., and made of the steel described herein are characterized by the following improved properties and characteristics: adequate strength and hardness to resist wear and abrasion together with suflicient resistance to failures resulting from stress con centrations as evidenced by relatively high notched impact strength. The machine elements contemplated by the invention may be employed at elevated temperatures up to about 1100 F. Although the temperature in service may occasionally exceed 1100 F. for short periods. of

time, such service should not be too prolonged as itcauses decrease in hardness.

As has been indicated hereinbefore, it is preferred foroptimum physical properties that all three of the elements a chromium, molybdenum and vanadium be present in amounts ranging from about 0.4% to 1.25% chromium, about 0.2% to 0.3% molybdenum and about 0.05% to 0.15% vanadium. It is also preferred that the steel contain silicon and manganese, each in amounts up to about 1%. A preferred embodiment of a steel especially suitable as machine element bar stock contains about 5% nickel, about 2% aluminum, about 0.22% carbon, about 0.5% chromium, about 0.25% molybdenum, about 0.1% vanadium, about 0.25% silicon, about 0.35% manganese, and the balance essentially. iron. The expression the balance or the balance essentially as applied herein to iron does not exclude the presence of other elements in amounts which do not adversely affect the properties of the alloy. Thus, as mentioned hereinbefore, silicon and manganese may be present in the steel provided by the invention, each in amounts not exceeding 1% of the total composition. Of course, other elements may be present in the steel as a result of the use of steel scrap in making up a charge, as a result of the use of master addition alloys,

from the use of deoxidizers, degasifiers, purifiers and the like during processing, etc. Examples of other elements which may be present without substantially adversely affecting the properties of the steel employed in producing bar stock for the machine elements provided by the inven- 2 tion include up to 0.1% copper, up to 0.02% magnesium, up to 0.03% titanium, up to 0.01% calcium, up to 0.05% of phosphorus and sulfur, etc., the total amounts of such elements, including silicon and manganese, usually not exceeding about 2.5% of the steel composition. The iron content of the steel may range from about 87% iron up to about 94%.

By maintaining the nickel-aluminum steel within the composition limits indicated hereinbefore, optimum hardness combined with toughness and high strength can be assured for machine elements produced from bar stock thereof by employing a controlled heat treatment. This heat treatment comprises first subjecting the steel stock to a preliminary treatment consisting of hardening by quenching or rapidly cooling the steel from above the critical temperature (i. e., an austenitizing temperature) followed by tempering at a temperature just below the critical temperature and rapidly cooling. The thus-treated material is then fabricated by hobbing, milling, grinding and the like into a machine element, such as a gear, hav- 7 ing the desired final dimensions. The machine element is thereafter aged to the required hardness by heating at a temperature below the tempering temperature. The purpose of heating the steel stock above the critical temperature and rapidly cooling it is to harden it completely by a forming martensite. The purpose of the second heating or tempering treatment is to dissolve the age-hardening constituent associated with the nickel and aluminum contents and to fully temper the martensite and thereby put the steel stock or blank in a relaively soft condition so that it can be finish machined to the desired final dimensions of the machine element, for example, a gear, being produced. The article is then aged or dispersion hardened by heating at the lower temperature. The use of this lower temperature enables the production of hardened gears and intricate dies of close tolerance without excessive distortion and with an improved combination of properties, including high strength combined with high toughness and impact resistance.

The high temperature or austenitizing treatment mentioned hereinbefore may be conducted over the range of 1600 F. to 1750 F. for about 1 hour to 2 hours followed by'rapid cooling, such as, for example, by oil quenching. The steel stock'or blank may then be tempered by heating over the range of about 1200 F. to 1275" F., but never above the Aci temperature of the steel, for about 1 hour to about 4 hours and thereafter rapidly cooled. The aging treatment may be conducted at a temperature from about 950 F. to 1100 F. for about 1 hour to 24 hours. Generally, when aging the steel to a hardness of at least 42 Rockwell C over the temperature range of 950 F. to 1100 F., the time of heating at the aging temperature is higher the lower the temperature and vice versa. Thus, at 950 F. the aging time would preferably range from about 2 hours to 24 hours. At 1000 F., the desired hardness would preferably be achieved by using times ranging from about 1 hour to 16 hours. At 1050 1 the desired hardness can be achieved by aging from about 1 hour to 12 hours. At i100 F., only a short time would be required to achieve the desired age hardness, e. g., about one-half or one hour, etc., as longer times, e. g., 2 hours or longer, tend to produce overaging or softening to below 42 Rockwell C.

In evaluating the suitability of a material for a machine element, the physical properties generally relied upon include tensile strength, toughness or resistance to impact determined by the Charpy keyhole impact test, hardness in terms of the Rockwell C numbers, etc. When, in accordance with the invention, the nickel-aluminum steel employed in the production of a machine clement contains the two essential elements chromium and molybdenum and is hardened in the manner described hereinbefore, unexpectedly improved properties are obtained which are superior to those obtained in a similar steel which does not contain or which is deficient in these elements. Generally, the steel provided by the invention in the hardened state will have a high tensile strength of at least about 190,000 p. s. i. (pounds per square inch) and a hardness of at least 42 Rockwell C combined with a consistently higher level of Charpy keyhole impact values which usually are at least about 5 foot pounds.

For the purpose of giving those skilled in the art a better understanding of the invention and a better appreciation of the advantages of the invention, the following illustrative examples are given:

Example I A steel forged bar embodying the present invention was produced which contained about 4.97% nickel, 2.03% aluminum, 0.22% carbon, 0.49% chromium, 0.26% molybdenum, 0.095% vanadium, 0.18% silicon, 0.31% manganese and the balance essentially iron (hteel No. 1). The nickel and aluminum contents in the steel were sufficient to provide a stoichiometric equivalent of about 6.45% NiAl. The bar stock of the steel was austenitized at a temperature of about 1650" F. for about one hour followed by oil quenching and then solution treated at about 1275 F. for about one hour followed by quenching in water. Finished machined test pieces were produced from the bar and aged variously from about onehalf hour to eight hours at temperatures ranging from about 950 F. to 1100 F. The foilowing properties were obtained after aging It will be noted that the steel bar when aged over the temperature range of 950 F. to 1100 1 exhibited a hardness of at least 43 Rockwell C, ranging to as high as 46 Rockwell C. The tensile strength averaged over 200,000 p. s. i., while the Charpy impact values were generally in excess of 5 foot pounds and averaged about 9 foot pounds. It was found that when the agin temperature was not lower than about 1000 F. or more, preferably not lower than about 1050 F., the level of properties was particularly high, especially with regard to impact resistance. The steel described in this example is very satisfactory for use as bar stock to produce a machine element, for example, a gear, in accordance with the invention.

Example I] Another illustrative steel within the scope of the invention which was found suitable for use as bar stock in the production of machine elements is one containing about 4.98% nickel, 2.0% aluminum, 0.21% carbon, 0.44% chromium, 0.25% molybdenum, 0.28% silicon, 0.38% manganese, and the balance essentially iron (Steel No. 2). The equivalent NiAl content of Steel No. 2 was about 6.35. This steel was tested as described for Example I except that it was austenitized at 1700 F. for about one hour and quenched in oil and then solution treated for about one hour at 1275 F. followed by quenching in water. The finish machined specimen pre pared from the thus-treated bar aged to at least about 42 Rockwell C hardness and exhibited high tensile strength combined with adequate toughness in the aged condition. There is little, if any, distortion after age hardening and bar stock made of this steel is suitable for use in machine elements.

For the purpose of better illustrating the advantages of the invention, a series of comparative tests were conducted with a group of nickel-aluminum dispersion, hardening steels having compositions outside the invention. The compositions of these steels are illustrated in the following schedule:

be noted that the steels within the invention are characterized by high tensile strength in combination with a high reduction of area of over 40% and a Keyhole Charpy impact of over foot pounds and at least 8 foot pounds and as high as 10 foot pounds. On the other hand, the steels outside the scope of the invention (A to E, inclusive) exhibited a much lower order of reduction of area ranging from 1 to about 26% and a lower order of Keyhole Charpy impact not exceeding 4.5 foot pounds. The comparative data show clearly that molybdenum in an amount of about 0.25% and vanadium in an amount of about 0.20%, each whether alone or together (Steels A to D), were insufiicient to produce adequate toughness with a hardness of at least 42 Rockwell C. Of the steels within the scope of the invention, Steel 1, which contained all three of the elements chromium, molybdenum and vanadium, exhibited optimum properties as compared to the steel in which only chromium and molybdenum were employed in accordance with the invention.

Results have indicated that the tendency toward brittleness exhibited by the nickel-aluminum steel outside the scope of the invention appears to be associated with the retention of a ferritic phase after the austenitizing treatment. Tests have indicated that the amount of the retained ferritic phase appears to be a function of the aluminum content of the steel. For example, in a steel containing about 4.5% nickel, it has been found that the amount of ferrite increases from about 1% with an aluminum content of about 1.16% to a ferrite content of about 85% with an aluminum content of about 4.15%.

This is illustrated by the following data for a steel con- Per- I Per- Per- Per- Per- Per- Per- Per- Per- Per- Steel cent cent cent cent cent cent cent 7 cent cent cent Ni Al N 1A1 0 Cr Mo Si Mn Fe 5. 43 2. 20 6. 98 0.21 0. 26 0.16 0.34 Bal. 4.93 2.06 6. 56 0.22 0.25 0.28 0.37 Bal. 5. 32 2.13 6. 78 0. 21 0.26 0.20 0 11 0. 23 Bal. 5. 41 2. 38 7. 56 0.20 0.20 0. 12 0.31 Bal. 5. 77 2. 23 7.10 0.22 0. 24 0. Bal.

Comparative data illustrating the advantages of the invention are given in the following schedule which compares the steels of Examples I and 11 provided by the invention with the aforementioned steels outside the invention.

All of the steels, with the exception of Steel E, were austenitized for one hour at a temperature within the range of 1650 F. to 1750" F. followed by quenching in oil. These same steels were then given a solution heating for one hour at 1275 F. followed by quenching in water and thereafter aged at a temperature within the range of about 1025 F. to 1050 F. for about two to eight hours. Steel E, on the other hand, was austenitizcd for one hour at 1600 F. and then oil quenched followed by a solution heating for one hour at 1275 F. followed by quenching in water. This steel was then age hardened at 950 F. for about 24 hours. It will be noted from the comparative data that while all the steels had an age hardness of at least about 42 Rockwell C, the best and optimum combination of properties was obtained by Steels Nos. 1 and 2 within the scope of the invention. It will taining nickel, aluminum and carbon with the balance essentially iron.

Steel Percent Percent Percent Percent Percent Ni Al NiAl C Ferrite 5 It will be noted from the above tabulation that only a small amount of aluminum in the neighborhood of about 1% can be tolerated in avoiding the presence of harmful amounts of ferrite in a steel having an adequate amount of nickel. However, Steel F could not be hardened to 42 0 Rockwell C, but had a hardness of only 33 Rockwell C.

The presence of free ferrite in the other three steels greatly aifected the resistance of the steels to the Keyhole Charpy impact test. This was evident by the results obtained on Steel E mentioned hereinbefore which contained about 2.23% aluminum. After being subjected to the austenitizing treatment and tempered and aged to a hardness of 43 Rockwell C, Steel E exhibited a Charpy impact value of only 2 foot pounds and a reduction in area of only 1% for a low strength value of 187,000

70 p. s. i. The addition of the essential elements chromium 75 ness in combination with high hardness, high strength properties and high reduction in area.

As has been stated hereinbefore, it is essential in producing the special nickel-aluminum steel provided by the invention for use as a machine element that the nickel and aluminum contents expressed stoichiometrically by the formula NiAl range from about 4.5% to about 7.5% NiAl. When the NiAl content is reduced below about 5%, and especially below about 4.5%, the minimum Rockwell C hardness of 42 is hardly attained after aging, even when the composition is bolstered by sutlicient alloy additions to inhibit retention of ferrite during the harden- Tests have indicated it is essential that the carbon content be correlated to the NiAl content of the steel in. order to obtain the results of the invention. Thus, when the NiAl content ranges from about 4.5% to 5.5 the correlated carbon content should range from about 0.25% to 0.3%. Likewise, when the NiAl content of the steel ranges from about 5.5% to 7.5%, it is essential that the correlated carbon content range from about 0.18% to 0.25%.

In order to achieve the minimum age hardness of at least 42 Rockwell C contemplated by the invention, it has been found that the solution quench hardness prior to aging should range from about 25 to 32 Rockwell C. Generally, the higher the solution hardness the higher will be the aged hardness. The increment of hardness obtained by aging tends to be lower the higher the solution hardness before aging. This relationship is illustrated by the following Rockwell C hardness determinations on a nickel-aluminum steel containing about 6% to 7% NiAl and about 0.20% to 0.25% carbon:

The foregoing data confirm that when the solution quench hardness is about 25 Rockwell C, an age hardness of at least about 42 Rockwell C is obtained. able for the solution quench hardness prior to aging to exceed 32 Rockwell C as hardnesses in excess of 32 Rockwell C make it diiiicult to machine the steel to the final dimensions of the machine element prior to aging and also adversely affect the toughness of the steel in the finally aged condition. When the steel contains 6% to 7% and higher of NiAl together with moderate additions of the essential elements chromium and molybdenum, with or without vanadium, in accordance with the invention, a solution quench hardness of up to 32 Rockwell C is readily obtainable. When either of the essential elements chromium and molybdenum, or both, is deficient in the amount stated hereinbefore or absent, even though 6% to 7% NiAl be present, two undesirable effects are generally indicated in the final product: (1) lack of full hardness after aging and (2) noticeable brittleness after aging.

It has been found that in order to insure high toughness with high hardnesses in excess of 42 Rockwell C, the steel provided by the invention should have a solution quench hardness of the order of about 30 Rockwell C or greater and should be aged at a temperature of about 1050 F. When the steel contains about 2% aluminum or more and has a high NiAl content, c. g., above 6% NiAl, it is preferred that the austenitizing temperature employed in the preliminary heat treatment be of the order of about 1700 F. in order to insure optimum properties in the aged condition.

In producing the machine elements contemplated by the present invention, the special steel or bar stock made It is not desirthereof may be produced in the conventional manner well known to those skilled in the art. Thus, the steel may be produced in an electric furnace. Likewise, the charge may be composed of the various constituents either in elemental form or as special master alloys or ferro-alloys. Of course, scrap may also be used in the charge as is well known. Similarly, the molten metal may be treated in the conventional manner to produce sound and clean metal for poured ingots. The ingots can then be mechanicall worked in a conventional manner into bar stock, such as by forging and rolling.

As pointed out hereinbefore, the present invention is particularly applicable to the production of hardened machine elements, such as gears, dies, cams, hydraulic valve seats and valve stems and other hydraulic valve parts. etc. These machine elements have the common characteristic that they all require a high order of hardness to resist wear and distortion which heretofore could only be obtained by employing high temperature hardening heat treatments together with laborious and expensive tooling of the hardened steel to correct distortion or undesirable surface defects produced in the high temperature hardening operation. illustrative examples of gears which may be produced in accordance with the present invention include tractor gears, automobile gears, reduction gears, transmission gears of various kinds, etc. Illustrative examples of dies include die-casting dies for easting molten aluminum and zinc, which dies are designed not to heat in service to temperatures high enough, for

xampi over 1100 F., to cause overaging and softening.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understod that modifications and variations may be resorted to Without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. A hardened gear which in use is subjected to wear and heavy static and impact stresses, said element being comprised of a heat treated steel containing about 3.5% to 6% nickel, about l.5% to 2.5% aluminum, about 0.4% to 1.25% chromium, about 0.2% to 0.3% molybdenum, about 0.05 to 0.15% vanadium, up to about 1% silicon, up to 1% manganese, and about 0.18% to 0.3% carbo the carbon content being correlated to the nickel and aluminum contents in the alloy expressed stoichiometrically by the formula NiAl, such that when the NiAl content of the steel is within the range of about 4.5% to about 5.5%, the carbon content is within the range of 0.25% to 0.30% and when the NiAl is within the range of about 5.5% to 7.5%, the carbon content is within the range of about 0.18% to 0.25%, the balance of said steel being essentially iron.

2. A hardened machine element which in use is subjected to wear and heavy static and impact stresses, said element being comprised of a heat treated steel containing about 3.5 to 6% nickel, about 1.5 to 2.5% aluminum, about 0.4% to 1.25% chromium, about 0.2% to 0.3% molybdenum, about 0.05% to 0.15% vanadium, and about 0.18% to 0.3% carbon, the carbon content being correlated to the nickel and aluminum contents in the alloy expressed stoichiornetrically by the formula NiAl,

such that when the NiAl content of the steel is within the range of about 4.5% to 5.5%, the carbon content is within the r of about 0.25% to 0.30% and when the NiAl is within the range of about 5.5% to 7.5%, the carbon content is within the range of about 0.18% to 0.25%, the balance of said steel being essentially iron.

3. A hardened machine element which in use is subcted to wear and heavy static and impact stresses, said element being comprised of a heat treated steel containing about 3.5% to 6% nickel, about 1.5% to 2.5% aluminum, about 0.4% to 1.25% chromium, about 0.2%

to 0.3% molybdenum, up to about 0.15% vanadium, up to about 1% silicon, up to about 1% manganese, and about 0.18% to 0.3% carbon, the carbon content being correlated to the nickel and aluminum contents in the alloy expressed stoichiometrically by the formula NiAl, such that when the NiAl content of the steel is within the range of about 4.5 to 5.5 the carbon content is Within the range of about 0.25% to 0.30%, and when the NiAl is within the range of 5.5% to 7.5%, the carbon content is within the range of about 0.18% to 0.25 the balance of said steel being essentially iron.

4. A hardened machine element which in use is subjected to wear and heavy static and impact stresses, said element being comprised of a heat treated steel containing about 3.5% to 6% nickel, about 1.5% to 2.5% aluminum, about 0.4% to 1.25 chromium, about 0.2% to 0.3% molybdenum, up to about 1% silicon, up to about 1% manganese, and about 0.18% to 0.3% carbon, the carbon content being correlated to the nickel and aluminum contents in the alloy expressed stoichiometrically by the formula NiAl, such that when the NiAl content of the steel is within the range of about 4.5% to 5.5 the carbon content is within the range of about 0.25% to 0.30% and when the NiAl is within the range of about 5.5% to 7.5%, the carbon content is within the range of about 0.18% to 0.25%, the balance of said steel being essentially iron.

5. A hardened die-casting die which in use is subjected to wear and heavy static and impact stresses, said element being comprised of a heat treated steel containing about 3.5% to 6% nickel, about 1.5% to 2.5% aluminum, about 0.4% to 1.25% chromium, about 0.2% to 0.3%

molybdenum, about 0.05% to 0.15% vanadium, up to about 1% silicon, up to about 1% manganese, and about 0.18% to 0.3% carbon, the carbon content being correlated to the nickel and aluminum contents in the alloy expressed stoichiometrically by the formula NiAl, such that when the NiAl content of the steel is within the range of about 4.5% to 5.5 the carbon content is Within the range of about 0.25 to 0.30% and when the NiAl is within the range of 5.5 to 7.5 the carbon content is within the range of about 0.18% to 0.25 the balance of said steel being essentially iron.

6. A dispersion-hardenable steel suitable for the production of hardened machine elements such as gears, dies and other articles which in use are subjected to wear and heavy static and impact stresses which comprises about 3.5 to 6% nickel, about 1.5% to 2.5% aluminum, about 0.4% to 1.25% chromium, about 0.2% to 0.3% molybdenum, and about 0.18% to 0.3% carbon, the carbon content being correlated to the nickel and aluminum in the alloy expressed stoichiometrically by the formula NiAl, such that when the NiAl content of the steel is within the range of about 4.5% to 5 .5%, the carbon con tent is Within the range of about 0.25% to 0.30% and when the NiAl is Within the range of 5 .5 to 7.5%, the carbon content is Within the range of about 0.18% to 0.25%, the balance of the steel being essentially iron.

References Cited in the file of this patent UNITED STATES PATENTS

Claims

1. A HARDENED GEAR WHICH IN USE IS SUBJECTED TO WEAR AND HEAVY STATIC AND IMPACT STRESSES, SAID ELEMENT BEING COMPRISED OF A HEAT TREATED STEEL CONTAINING ABOUT 3.5% TO 6% NICKEL, ABOUT 1.5% TO 2,5% ALUMINUM, ABOUT 0.4% TO 1.25% CHROMIUM, VANADIUM, UP TO ABOUT 1% DENUM, ABOUT 0.05 TO 0.15% VANADIUM, UP TO ABOUT 1% SILICON, UP TO 1% MANGANESE, AND ABOUT 0.18% TO 0.3% CARBON, THE CARBON CONTENT BEING CORRELATED TO THE NICKEL AND ALUMINUM CONTENTS IN THE ALLOY EXPRESSED STOICHIOMETRICALLY BY THE FORMULA NIAL, SUCH THAT WHEN THE NIAL CONTENT OF THE STEEL IS WITHIN THE RANGE OF ABOUT 4.5% TO ABOUT 5.5%, THE CARBON CONTENT IS WITHIN THE RANGE OF 0.25% TO 0.30% AND WHEN THE NIAL IS WITHIN THE RANGE OF ABOUT 5.5% TO 7.5%, THE CARBON CONTENT IS WITHIN THE RANGE OF ABOUT 0.18% TO 0.25%, THE BALANCE OF SAID STEEL BEING ESSENTIALLY IRON.