US3432291A - Low alloy steel particularly suitable for cold forging - Google Patents

Description

United States Patent 3,432,291 LOW ALLOY STEEL PARTICULARLY SUITABLE FOR COLD FORGING George Mayer, Cobham, and Roger Kearsley Greenwood, Stourbridge, England, assignors to The International Nickel Company, Inc., a corporation of Delaware N0 Drawing. Filed Dec. 10, 1965, Ser. No. 513,080 Claims priority, application Great Britain, Dec. 18, 1964,

51,60 US. Cl. 75-124 6 Claims Int. Cl. C22c 39/36 ABSTRACT OF THE DISCLOSURE The present invention relates to ferrous-base alloys and more particularly to special low alloy steels characterized by a desired combination of metallurgical properties which render the steels particularly suitable for use as cold forging steels.

As is generally known by those skilled in the art of ferrous metallurgy, the fortunes of cold forging, a term which includes die stamping and cold extrusion, have been somewhat on the rise in recent years. For example, cold extrusion has gained ground for the mass production of certain types of small components, particularly in the automotive field, e.g., disc brake pistons, bushes, steering knuckles, and hydraulic tubes, since the high degree of precision obtainable results in reduced machining time and in increased recovery of metal (less scrap).

In one sense it could be advanced that the advantages of cold forgeability are somewhat limited since to be economically advantageous the forging must be applied to relatively soft alloys, i.e, usually less than about 200 D.P.N. (diamond pyramid number). For example, where extrusion techniques are used, harder alloys would necessitate higher extrusion pressures, a factor which would concomitantly bring about, other considerations being equal, excessive tool wear. However, it would be more accurate to say that relative softness is not the limiting factor; rather, it is but the natural result flowing from the fact that many applications require alloys of high tensile strength, i.e., above about 50 long tons per square inch (t.s.i.) and above, e.g., 60 to 70 t.s.i., and soft steels present difliculties during the further processing (subsequent to cold forging) necessary to attain the desired higher strength level, assuming the steel is capable of manifesting high strength in any event. Perhaps this is but another way of saying that if a steel is soft enough to be readily cold forged, it may not have the hardness and strength required in the final article in question. Accordingly, the steel used should be hardenable.

Steels from which articles have hitherto been made 3,432,291 Patented Mar. 11, 1969 by cold forging fall into two principal groups, namely, the mild steels and the low carbon, low alloy steels, such as the so-called carburizing s'teels. In the mild steels the necessary strength is achieved by work hardening. However, variations in working and the intermediate annealing heat treatments necessary during the production of articles of varying section by successive cold forging steps, both lead to lack of uniformity of hardness. Much of the benefit gained by work hardening in the process of forming one part of the article is lost on account of the intermediate annealing treatment necessary before another part of the article is formed. Further, mild steels in the work hardened condition are generally useful in only producing tensile strengths up to about 40 to 45 t.s.i. and then if the strength is to be uniform throughout the component, only in simple shapes.

When low carbon, low alloy steels are employed, the steel is first cold forged. Some of these steels are used in the cold forged (and thus work hardened) state without further treatment. As such, they suffer from the same disadvantages attendant the mild steels particularly with respect to low tensile strengths. To develop the higher strength levels required by sundry commercial applications, these steels are further treated after cold forging. Often this subsequent treatment includes the application of a quench, with or without tempering. (If these steels are carburized, the carburization is effected before quenching.) Now, it is well documented, as will be readily appreciated by those skilled in the art, that quenching not only imposes undesired limitations on the sec tion size of the forging produced but also brings about other problems. By way of illustration, to be soft enough for cold forging purposes these steels must have a low alloy content and, therefore, usually have low hardenability. Thus, they are more sensitive than steels of higher alloy content to differential rates of cooling throughout the section during quenching. Moreover, during the quenching and tempering, particularly the former, distortion takes place and there is also some scaling. Quenching as opposed to, say, air cooling is a rather drastic treatment which induces internal stresses, dimensional change, etc. Tempering can relieve the stresses but it requires special equipment and/or further processing to correct or obviate dimensional change. These effects make it difficult to produce forgings of precise dimensions.

It has now been discovered that certain precipitation hardenable alloy steels of special composition are particularly adaptable to cold forging since the steels (1) are sufiiciently soft to be cold forged without necessitating unduly high extrusion pressures where extrusion techniques are employed, (2) manifest high hardenability such that tensile strengths of, say, above about 50 t.s.i. are readily obtained and (3) do notrequire recourse to quenching to provide the high strengths. Further, the steels are amenable to ease of machining with the added attribute that less material is machined off than otherwise might be required.

It is an object of the present invention to provide new, cold forgeable low alloy steels having the combination of characteristics above-described.

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

Generally speaking, the present invention contemplates providing cold forgeable, precipitation hardening steels containing (percent by weight) from about 2.5% to 7% nickel, not more than 0.1% carbon, from 0.5 to 1.25% aluminum, at least one metal selected from the group consisting of chromium and copper in a total amount of from 0.5% to 2%, the chromium being present in an amount up to 2% and the copper being present in an amount up to 1.25 the sum of the aluminum, chromium and copper contents being at least 1.4%, e.g., 1.5%, and not more than 3%, the balance of the steels, except for the usual incidental elements and impurities present in nickel steels, being iron. Of these usual incidental elements and impurities, silicon, manganese, phosphorus and sulfur are all preferably kept as low as is convenient. In any event, the silicon content should not exceed about 0.25% and manganese should not be present in an amount above about 0.5%.

At least 2.5% nickel should be present in the subject steels to insure, upon aging, adequate precipitation and strength. With nickel contents much below 2.5%, extremely long aging times are required. However, the higher the nickel content, the greater is the hardness of the steel in the solution treated condition (the condition in which the steels are preferably cold forged) as a result of solid solution hardening of the ferrite matrix. Accordingly, it is preferred that the nickel content be from 4% to 6%.

Aluminum must be present to produce the precipitable phase. While up to 1.25 can be present, the preferred maximum content of this element is 1%. Too much aluminum causes embrittlement of the steels in the aged condition and also increases the hardness in the solution treated condition.

With respect to chromium and copper, it is advantageous that the steels contain chromium in an amount of at least 0.25% and be copper-free or have low copper contents. A chromium range of 0.5% to 1.5% is preferred and highly satisfactory in copper-free steels. In accordance herewith, chromium has the function of reducing the solubility of aluminum in the matrix and, therefore, improves age hardenability. Copper performs a similar function but, in addition, this element also acts as a supplementary hardener, that is to say, a precipitate of copper will occur in addition to the nickel-aluminum precipitate. While, broadly speaking, chromium need not be present when sufficient copper is present, the latter renders it much more difficult to obtain a desired hardness level in the annealed condition. Further, although copper confers a higher hardness in the aged condition, it also impairs toughness to a greater extent than chromium. Therefore, chromium is a more desirable hardening addition, although copper may be preferred where toughness is of less importance and maximum strength and hardness in the aged condition are required. If more than 1.25% of copper or 2% of chromium or 2% of both is present, the hardness in the solution treated condition is too great, thus making cold forging difiicult. Mention should be made that it is preferable when the combined content of aluminum, chromium and copper approaches 3% to keep the nickel content at the lower end of its range specified herein.

While the carbon content can be as high as 0.1%, the preferred maximum is 0.05 If the carbon content is increased, the work hardening rate is also increased, as is the hardness of the steel in the solution treated condition. Therefore, carbon contents above 0.05% are not desirable and, in general, the carbon content should be as low as feasible, e.g., not more than 0.03%.

A most advantageous alloy range as contemplated herein is as follows: about 4% to 6% nickel, about 0.65% to 1% aluminum, about 0.25% to 1.5% chromium, up to 0.3% or 0.35% copper, the sum of the aluminum plus chromium plus any copper being about 1.4% to 2.5%, carbon up to 0.05 the balance being essentially iron.

In carrying the invention into practice, the steels should be solution treated before cold forging but a distinct virtue of the steels is found in the fact that it is not necessary to quench them from the solution treating temperature to retain the solution or to attain the high strengths, e.g., to t.s.i., characteristic thereof. Rather, they can be cooled in air. While it is not absolutely mandatory that the steels be solution treated before being cold forged (the steels can be cold forged in the hot worked condition), it is considered that better and more consistent re sults are achieved. The steels can thereafter be precipitation hardened at low temperature without a phase change. The basic hardening mechanism of the steels of the invention is the precipitation during the aging treatment of a nickel-aluminum precipitate (Ni Al or gamma phase). The age hardening heat treatment can be carried out at a temperature between about 400 C. and 550 C. for about 1 to 300 hours, and preferably between 450 C. and 500 C. for about 3 to 16 hours. The duration of the aging treatment depends upon the temperature, longer periods being used with the lowest temperatures.

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

Several steels of various composition were prepared for test by standard procedures which comprised air melting the same in a high frequency induction furnace having a basic lining. The ingots (10 lbs.) produced were forged at about 1,150" C. to inch diameter rod and test specimens were subsequently machined from the rods in both the solution treated and aged conditions. The steels are given in Table I, Alloys A through F being within the invention, and Alloys 1 through 10 being outside the scope thereof.

TAB LE I Chemical composition 1 Contained 1.54% Mo and 1.95% 00. i! Contained 1.55% Mo and 1.90% 00. 3 Alloy N 0. 6 contained 0.72% Ti and Alloy N0. 8 contained 0.60% Ti.

After solution treating at 920 C. for about one hour, the hardness, D.P.N., of each of the steels was determined and the steels were then aged (various temperatures and times being used) as set forth in Table II, the aged, as well as the annealed, hardness being given therein. Certain of the steels were subjected to further testing to ascertain (a) ultimate tensile strength (U.T.S.), (b) percent elongation (EL, percent) and (c) percent reduction in area (R.A., percent). These results are also reported in Table II.

TABLE II Hardness, D.P.N.

Alloy U.T.S., EL, R.A.,

N 0. An- 3 hrs. 1 hr. 3 hrs. 1 hr. t.s.i. percent percent nealed at at at at 4 4K U.T.S., percent EL, and percent RA. determined after aging 3 hours at 500 C. for Alloys A, B, 9 and 10, and 16 hours at 500 C. for Alloys through F and 1.

The data of Tables I and II illustrate that steels with- 20 be illustrative. Such modifications and variations are conin the invention (Alloys A through F) exhibit a good level of hardness in the solution annealed condition, the steels being readily cold forgeable. Further, the data also show that these same steels manifested a satisfactory hardening response upon aging such that a high level of tensile strength was achieved. In contradistinction thereto and generally speaking, Alloys Nos. 2, 3, 6, 8, 9 and 10 were somewhat hard in the annealed condition to be readily cold forged. Hardness in this condition should not exceed about 225 D.P.N., e.g., 220 D.P.N., for acceptable cold forging characteristics and preferably should not exceed about 200 to 210 D.P.N. for the optimum with regard to ease of cold forging. While annealed hardness was not of significance regarding the remaining steels outside the invention, these steels did not age harden sufficiently to exhibit acceptable strength levels. It is worthy of note to mention that Alloys Nos. 2, 5, 6, 7 and 9 were devoid of aluminum. The absence thereof was not compensated for by additions of molybdenum and cobalt (Alloy No. 2), titanium (Alloy No. 6) or by a high copper content (Alloys Nos. 7 and 9).

The present invention affords the advantages over the mild steels and the carburizing steels in that the precipitation hardening effect produces the higher strengths and hardnesses and, moreover, any difficulty caused by nonuniform hardening, distortion or scaling is much reduced, since the precipitation hardening temperature is low and quenching is not required after cold forging in order to develop the high strength and hardness levels.

The composition of the steels according to the invention is such that they can be nitrided if high surface hardness is required and any such nitriding can be effected while they are being precipitation hardened.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood 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. For example, while the primary application of the steels is for cold forging, the steels can be employed (and are so intended) wherever the properties characteristic of the steels would be useful. Case hardened steels would sidered to be within the purview and scope of the invention and appended claims.

We claim:

1. A new and improved steel suitable for cold forging and which consists essentially of about 2.5% to 7% nickel, not more than 0.1% carbon, about 0.5 to 1.25% aluminum, from about 0.5% to not more than 2% of at least one metal from the group consisting of chromium and copper with the chromium not exceeding 2% and the copper not exceeding 1.25%, the aluminum plus chromium plus copper being correlated such that the sum thereof is at least about 1.4% and not greater than 3%, the balance being essentially iron.

2. The steel composition of claim 1 in which the carbon content is not greater than 0.05%.

3. The steel composition of claim 1 in which the metal selected from the group consisting of chromium and copper is chromium in an amount of from 0.5 up to 1.5%, the aluminum content is from about 0.5 to 1% and the nickel content is from 4% to 6%.

4. The steel composition of claim 1 and containing nickel from about 4% to about 6%, carbon in an amount not greater than 0.05%, aluminum from about 0.65% to about 1%, chromium from 0.25% to 1.5%, up to 0.35% copper, with the sum of the aluminum plus chromium plus copper being from 1.4% to 2.5

5. The steel composition of claim 1 in the cold forged condition.

6. The steel composition of claim 4 in the cold forged and aged condition.

References Cited UNITED STATES PATENTS 2,679,454 5/1954 Offenhauer l24 2,694,627 11/1954 Epstein 75l24 3,155,495 11/1964 Nakarnura 75-124 HYLAND BI ZOT, Primary Examiner.

U.S. Cl. X.R. 75125