US2145989A - Method of treating metals - Google Patents

Description

- invention that by hot-working there is produced Patented Feb. 7,'1939 d UNITED STATES 2,145,989 I amn on or TREATING amas William J. Marten, Pittsburgh, Pa.', assignor to Bocjl Corporation, tion of Delaware Pittsburgh, Pa., a corpora- No Drawing. Application .lune 1a, 1937,

Serial No. 148,955

10 Claims.

This invention relates to' a method of treating steel to produce articles, especially springs and the.like, possessing particularly desirable mechanical properties.

It is among the objects of the invention to provide a method of treating steel which confers a combination of properties of a particularly desirable character, especially for the production of helical and elliptical springs, which eliminates the necessity for cold-working while providing satisfactory hardness, strength and other mechanical properties, is cheap and easily practiced, and provides a grain structure in the finished article that is particularly suited to the requirements of springs.

A particular object is to provide .steel articles possessing a fibrous structure comprising elongated grains disposed parallel to the main. axis of the article, and which in the hardened condition embody highly desirable mechanical properties.

7 The invention is predicated upon my discovery that its stated objects are attained by hot-working a piece of steel to form stock which is oversized relative to the final product, then sizing the stock to bring it to the dimensions of the finished article by working it hot but at a temperature below the recrystallization range, to thereby produce a hot-sized shape having a fibrous structure composed of elongate parallel grains disposed in alignment with the longitudinal axis of the shape. The sized article thus produced by hot-working is then subjected to a hardening operationwithout destruction of the fibrous structure which characterizes the product of the invention. This is accomplished by heating the article rapidly to a temperature within the austenite range, followed by quenching, both steps being so, conducted as to effect the desired hardening while preserving the fibrous structure substantially without change.

. Following this the heat-treated article may be tempered if desired.

Thus it will be seen that it is of the gist of this an article which is sized and possesses a fibrous .structure in which the fibers lie in the direction of working, and such structure is preserved in subsequent heat treatment for hardening.

The manner of conducting the hardening step constitutes an important feature oi. the invention -because it must be so conducted as to eflect hardening without material destruction of the fibrous structure. To this end the article is brought rapidly into the austenite temperature range, and the quenching is carried out quickly to preserve the grain size and fibrous structure of the article.

Stated otherwise, in the practice of the invention 1 the'conversion of the fibrous grains to equi-axed grains is avoided as far as possible so that the hardening step is efi'ected in such manner as substantially to avoid recrystallization and thus withsubstantial'preservation of the fibrous structure.

If the heating step of the hardening operation is carried out properly, the recrystallization will proceed only incipiently, and provided it does not proceed so far as to substantially alter the fibrous structure, such incipient recrystallization is desirable because apparently it produces an interlocking of the fibers with consequent increase in the cohesive forces between them, which increases the strength of the product.

When the article has been heated sufficiently, i. e., when the heat has penetrated the article, it is quenched with sumcient rapidity to avoid recrystallization to any substantial extent. Provided it be done rapidly, the article may be formed between the heating and quenching stages of the heat treatment. After quenching, the article may be tempered, or drawn, as need be.

Although not restricted thereto, the invention is especially adapted to the manufacture of springs or stock for making springs, for which reason it may be described with particular reference thereto, elliptical springs being chosen by way of example. In the practice of such an embodiment of the invention a billet or ingot of steel of suitable composition ishot-rolled in ac; cordance with standard practice except that whereas it is customary in the manufacture of elliptical springs :to hot-roll (i. e., at temperatures productive of recrystallization) down to the finished size of the spring leaves, in accordance with the present invention there is produced by hot-working stock, such as strip or rod, which is oversized, i. e., whose cross-sectional area is greater than that of the finished leaf. This oversized stock is then subjected to hot-finishing at a temperature slightly below the recrystallization range of the steel,'i. e., at a temperature below that used in true hot-working. Ordinarily it is possible to so correlate the temperature of the ingot or billet and the hot-rolling to the oversized shape that the shape will then be at a suitable temperature for the finishing pass, but if necessary or desirable for .any reason the oversized stock may be reheated until it is at a temperature appropriate for the hot-finishing pass (below the recrystallization temperature).

As is characteristic of products made in accordance with this invention, the hot-sized shape is provided with a fibrous grain structure in which the grains are elongated in the direction of working so that they are parallel and are aligned with the longitudinal axis of the shape. Such a structure is highly desirable for many purposes, and through it there is attained, after appropriate hardening, a highly desirable combination of mechanical and physical properties.

In the practice of the invention as applied tov the production or elliptical springs, the hot-sized shape is cut to lengths appropriate for the manufacture of spring leaves. rapidly, as described above, in a suitable medium, such as a fused salt bath, a mixture of equal parts of sodium and potassium chlorides affording a bath suitable for many purposes. The bath is heated to a temperature such that the article will be brought rapidly to the desired temperature, the mass of the bath being such that its temperature will not be materially decreased in consequence of the introduction of'the piece.

When the heat has penetrated the piece sufliciently it is removed, placed in the forming dies, pressed quickly to shape, and immediately quenched, suitably while still held in'the dies, precautions being taken to insure that the quenching is sufficiently rapid to produce maximum hardness and also retain the fibrous structure. For many purposes the quenching medium may be a sodium chloride brine, and if desired lime may be suspended in the brine.

Fused salt baths are desirable because when the article is removed it is coated with a film of the salt which protects it against scaling and decarburization during the forming operation. It also may act as a heat insulator, to minimize cooling prior to quenching. The addition of lime to the quenching bath is desirable because it minimizes the formation of vapor pockets and thus insures rapidity and uniformity of quenching. 1 After the article has been quenched it is removed from the dies, cleaned, and tempered to a hardness appropriate for itsintended service.

As exemplifying the benefits which may be derived from thepractice of the invention, referencemay be made to tests in which there was used a silica-manganese spring steel containing 0,64 per cent of carbon, 0.89 per cent of manganese, 0.021 per cent of phosphorus, 2.2 per cent of silicon, and 0.023 per 'cent of sulfur. The steel was'hot-rolled to produce round rod, such as is used in the manufacture of helical springs, having a diameter somewhat greater than 3/;

' inch, the intended final diameter. The rod was then given a finishing pass at about 1200 F. to

bring it to inch diameter, thus producing the fibrous structure referred to hereinabove. The hot-sized rod was then out to lengths which were immersed in a salt bath and heated to 1550 F. in two and one-half minutes, which time in-' cluded' the interval necessary to bring the rod to temperature and effect complete penetration of the heat. At the end of the interval stated the rod was removed and quenched. It was then tempered at 800 F. for, one hour. The rod treated in this manner showed mechanical properties as follows, these being determined from test bars of standard shape and size:

Strength, lbs/sq. in.

Elongation, Reduction B u percent in of area, g Yield Ultimate two inches percent 635 point tensile The combination of properties represented by the foregoing tests is unusual and is highly desirable for spring steel purposes. The properties of the hot-sized material are presumably actually better than those given because in the preparation of the standard test bars used the outer portion ofthe bar, which is the most highly fibered, is removed. It follows that aspring made from These are then heated the hot-sized bar, without machining, would have still better properties.

As further exemplifying the practice of the invention, steels of similar composition were rolled to produce strip such as is used in making elliptical springs. These steels contained from 0.58 to 0.62 per cent of carbon, from 0.78 to 0.86 per cent of manganese, from 0.022 to 0.026 per cent of phosphorus, from 1.86 to 1.98 per cent of silicon, and from'0.025 to 0.029 per cent of sulfur. The ingots were hot-rolled to produce a fiat whose sectional area was somewhat greater than that of the finished spring, and they were then brought to finished size by a hot-finishing pass at about.

1200 F. The hot-sized strip was then cut into lengths for spring manufacture which were heattreated in the same manner as in the foregoing example. Two of the bars, which were 1% inches wide and inch thick, showed ultimate tensile strengths of 229,000 psi and 284,480 psi. Other results were as follows:

Elonga- Size of Yield Ultimate tion, ,13% Brinell flat lbpyint, lbtensile, pergent a hard- S. sq. in. S. sq. 111. 111 W0 HESS inches percent min-4 198,900 206,189 s o 11.7 '415 211%: 175,760 219,059 8 5 16.9 41s zylxua 174,719 195,720 9 o 9.1 ass The foregoing'tests exemplify the manner in which the hardening operation may suitably be conducted. It will be observed that in the spe imens of the dimensions given the article w s heated to 1500 F. in two and one-half minutes. It will be understood, however, that exact times of heating can not be given for all purposes because the time of heating will depend upon the composition of the steel, the section of the piece,

and the treating temperature. As to composition, it is known to those skilled in the art that some steels are more resistant to the effect of heat treatment than others. vFor instance, the chromium steels generally tend to recrystallize somewhat sluggishly and where steels exhibit this property they may be exposed longer than others without danger of impairing the fibrous structure. Also, the section or mass of the piece will play its part, it being generally true that the greater the thickness the greater the time thatwill be required to bring the article to temperature. For the reasons stated it is not practicalor possible to give precise heating times which will produce optimum results under all conditions and for the production ofall combinations of properties, but generally speaking, as the time of heating is increased there is a tendency to destroy the fibrous structure whichresults from hot-working and hot-sizing, and due regard must be given to this factor. The criterion is that the heating period must be short enough and quenching carried out sufficiently rapidly that the fibrous structure is substantially retained. It may be said, however, that-in the manufacture of springs a heating time of a few minutes, say I two to three minutes, sufiices.

Similarly, the temperature of heat treatmnet will vary according to the composition of the steel as is known to those skilled in the art.

Reference has been made to the use of fused salt baths for rapid heating of the article in the hardening step. The advantages of such heating media have been explained. It is possible, however, to heatiin other manners, as in electric furble. Thus, although the invention has been exemplified by reference to silica-manganese spring steel, it is applicable generally to steels whether of the plain carbon or of the alloy variety. In both cases the carbon content, and in the latter instance the s'pecific alloying elements and their amounts, will be selected according to service requirements. In any event, the application of the invention provides properties which exceed those of the same steel fabricated according to prior art procedures. I

Also, in the foregoing tests reference has been made to hot-rolling but it will be understood that other types of hot-working are applicable, such as die-forging, hammering, and the like.

The precise degree of oversize of the stock prior to hot-sizing will depend in part upon the size of the final shape and the required properties. In sizing the fibrous structure is most pronounced at the surface if the stock is not reduced much .in sectional area in hot-sizing it. The greater the longitudinal direction of the bar or rod. For instance, in a leaf spring the maximum stresses are tension and compression largely concentrated adjacent the upper and lower surfaces of the leaf. The fibrous structure of products made in accordance with this invention is particularly desirable for spring purposes because the grains are thus disposed in the direction of the major stresses, and through increased resistance to tension and true compression, coupled with reduction of shear and torsion stresses, improved life is assured. m

The fibrous structure is further desirable in the case of helical springs because when the spring is put under tension or compression in use the coiled rod is subjected to torsional stresses normal to the direction of the fibers. metal undergoes torsional strain, a component of stress longitudinally of the bar is introduced as a result of the disposition of the fibers, which permits the helical spring to resist the compressive and torsional stresses to which it is subjected in use better than such springs made in accordance with prior practice in the art.

As noted hereinabove, the heat treatment is conducted to harden the article. This involves, generally, a change from one type of space lattice to another, and the reaction is more rapid than that involved in complete recrystallization, al-

though recrystallization can not be wholly avoid-' ed. I believe that the incipient recrystallization is valuable, however, because the fibers are bonded thereby, improving strength. While I do not not confine myself to this theory, I believe also that such incipient equi-axing of the grains sets up internal stresses which are beneficial in that the service stresses, ,for instance in the case of Asthe' springs, must overcome this internal stress before their effect upon the spring in the direction of the service stress is encountered. In other words, the internal strain opposes the service stress which increases the safe imposed stress value of the part, or it withstands a greater elastic loading than a part not having the structure provided by the invention.

The same properties adapt the articles made in accordance with the invention to other uses also, as will be understood in the art.

According to the provisions of the patent statutes, I have explained theprincipl and manner of practicing my invention, and have described what I'now consider to be its best embodiment. However, I desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

I claim: a

1. That method of treating steel which comprises hot-working a piece of steel to form oversized stock, then hot-sizing said stock at a temperature close to but below the recrystallization range of the steel, thereby producing a sized shape having a fibrous structure extending inwardly a substantial depth from the surface, and heating such shape in the austenite range and quenching to harden it while substantially preserving said structure.

2. That method of treating steel which comprises hot-working a piece of steel to formoversized stock, then hot-sizing said stock at a temperatureclose to but below the recrystallization range of the steel, thereby producing a sized shape having a structureof fiberselongated in the direction of working and extending inwardly a substantial depth from the surface, and heating such shape in the austenite range and quenching it with substantial avoidance of recrystallization. 3. That method of treating steel which comprises hot-working a piece of steel to form oversized stock, thenhot-sizing said stock at a temperature close to but below the recrystallization range of the steel, to produce thereby a sized shape having a fibrous structure of parallel elongate grains aligned with the longitudinal axis of the shape and extending inwardly a substantial depth from the surface, heating such shape rapidly to a temperature within the austenite range for a period of time such that said structure is not substantially destroyed, and quenching the thus heated steel rapidly to preserve such structure.

4. That .method of treating steel which comprises hot-working a piece of steel to form oversized stock, hot-sizing said stock at a temperature close to but below the recrystallization range to bring it to finished size, thereby producing a sized shape having a fibrous structure and extending inwardly a substantial depthirom the surface, heating the sized shape rapidly to a temperature within the austenite range, and quenching the heated piece before said fibrous structure has been substantially'changed.

5. That method of treating steel which comprises hot-working a piece of steel to form a shape close to but somewhat oversize, then bringing the shape to finished size by a hot-finishing pass at a temperature close to but below the recrystallization range, to produce thereby a sized shape having a fibrous structure of elongate parallel grains extending inwardly a substantial depth from the surface, heating the sized shape rapidly to a" temperature within the austenite sized shape having a fibrous structure extending I inwardly a substantial depth from the surface and composed of elongate grains parallel to the longitudinal axis of the shape, heating the sized shape rapidly to a temperature of about 1550 F., and quenching the heated piece before said 11- brous structure has been substantially changed.

'7. That method of making springs which comprises hot-working a piece of steel to form oversized bar or strip stock, then hot-sizing said stock at a temperature close to but below the recrystallization range of the steel, thereby producing a sized shape having a fibrous structure of elongate grains and extending inwardly a substantial depth from the surface, cutting said hot-sized stock to spring length, heating such length rapidly to a temperature within the austenite range for a period of time such that said structure is not substantially altered, shaping the spring while so heated, and quenching the thus heated steel rapidly to preserve said fibrous structure.

8. That method of treating steel which comprises hot working a piece of steel-to form oversized stock, hot sizing-said stock at a temperature close to but below the recrystallization range of the steel to bring it to finished size, thereby producing a sized shape having a fibrous structureand extending inwardly a substantial depth from the surface, heating the sized shape to a temperature to convert the steel to the austenitic condition, and quenching when such condition has" been produced but before equi-axing of the fibrous grains has occurred.

9. That method of treating steel which com priseshot working a. piece of steel to form oversized stock, hot sizing said stock at a temperature close to but below the recrystallization range of the steel to bring it 'to finished size, thereby producing a sized shape having a fibrous structure and extending inwardly a substantial depth from the surface, heating the sized shape to a temperature to convert the steel to the austenitic condition, quenching when such condition has been produced but before equi-axing of the fibrous grains has occurred, and tempering after quenching to produce a troostitic-sorbitic structure in the steel.

10. As a new article of manufacture, a hotworked and heat-treated steel article made by hot working a piece of steel to form over-sized stock, hot sizing said stock at a temperature close to but below the recrystallization rangeto bring it to finished size thereby producing a sized shape having a fibrous" structure composed of elongated substantially parallel grains substantially stable, on reheating, against grain growth due to residual Working strain, and heating the sized shape