US2291904A - Wrought steel wheel - Google Patents

Description

Patented Aug. 4, 1942 A "WROUGHTSTEELWHEEL Reid L. Kenyon, Middletownpohio and Harry Tobin, Butler, Pa., assignors to The American Rolling Mill Company, Middletown, Ohio, a corporation of Ohio N Drawing.

ApplicationFebruary 28,1940; Serial No.321,276

icia ms. (01.29am)- This application is a continuation in partof our copending application Serial'No.126,734, filed February 19, 1937.

A railroad car wheel presents one of the most important problems in the mechanical departments of the railroads, and in thedevelopment of the railway industry various types of wheels have been tried in an effort to find One which has the best combination of safety,utility and economy. The principal varieties of railroad wheels have been cast iron, cast steel and wrought steel. The cast ironwheels, while at one time used on all types of railway equipment, have for a number of years been restricted to freight cars on account of their frequent breakage due to the inherent low strength and brittleness of the cast iron from which they are made. Cast steel wheels have been tried in an effort to overcome these disadvantages of the cast iron wheels and to obtain advantages. from control of variable compositions during casting and uniformity of dimensions by use of the casting process, butthese Wheels have also given trouble due to breakage which probably results from the dendritic cast structure and its accompanyinglow ductility which are. inherent from the process of manufacture. In an attempt to overcome this disadvantage of the cast steel The superior physical properties of the wrought steel wheel have m'adeof importance a shortcoming of steel compositions which is ahazard. to safety and which has been the cause of large expenditures by. the railroads forreplacement and repairs. This is a condition known as thermal cracking. -Therma1 cracking has been described-in the Wheel and Axle vManualof the Association of American Railroads, but briefly it consists in' the development of. transverse cracks in the tread .or fiange-surfacesiof the wheel, and results from. the alternate heating-by the brake shoes and cooling by contact with the rail vand difiusionof the heat into. other parts of the wheels. In some cases .where hub faces of wheels rub against journal boxes without adequate lubrication thermal cracks may develop on the faces of the hub. The cracksare practii and flanges have substantially a radial direction wheel, it has been customary to subject it to an additional step in the process of manufacture consisting of a special annealing treatment which troduction over thirty years ago,havehad a very satisfactory record of performance as far as toughness, strength and freedom from breakage are concerned. They'are not only being used today underthe most severeconditions of service where cast iron and cast steel wheels, failed to meet the requirements, but they are replacing these types of wheels in other less severe classes of service. During this periodthe composition cally always normal to. the surface which'is subject to heating'and cooling andthose in the rim fromqthe tread surface toward the wheel center and are also normal to the direction of tensional stress in the rim as describedhereinafter.

These thermal cracks tend to grow larger in service andthis is particularly, hazardous in the 1 ly a rigid service-inspection must be maintained. -When wheels are removed from service on.ac-

count of thermal cracks jtheymay be machined in .a lathe until allof the cracks areturned out providing the rim is of suflicientremaining thickness to meet the present Association of American Railroads regulations that passengercar wheels must have at least;oneinch rim thickness. and freight car.,wheels three fourths inch thickness. The depth-of the crackscannot be determined by inspection and. frequently they are so-deep thatthey cannot be machined out withbut to this-must, beadded, the vcostof removalof the wrought steel Wheels has been held closeto the present Association of American Railroads specifications which call for 0.65 to 0.85% carbon and 0.60 to 0.85% manganese.

outreducing the rim thickness ;below the condemning limit; thus cit is seenqthatothermal cracking not only .necessitatesthe machining away of considerable amounts of "service metal for service.

The general object of our invention may be v stated as the provision of a wrought steel car wheel which has all of the advantages of said wrought steel wheels aforementioned and in addition is characterized by the elimination or very great lessening of the tendency toward thermal cracking.

Another object of our invention is to provide a wrought steel wheel which not only. has a very greatly lessened tendency toward thermal cracking but which has sufficient hardness and toughness to meet service requirements.

Although the elimination or lessening of the tendency toward thermal cracking has been indicated as an ultimate goal in the development of our new wrought steel wheel, there were many other requirements which had to be met. The ductility, yield strength, tensile strength, fatigue strength, impact value, hardness and wear resistance of the wrought steel wheel must be satisfactory and yet its fabrication cost and the cost of the material from which it is made must be low enough to strike an economic balance between the cost and the benefits which are derived from the combination of increased resistance to thermal cracking with the desirable properties mentioned above.

A further object of our invention is to provide a wrought steel wheel.having the above desirable combination of properties together with resistance to thermal cracking at a cost that is teachings hereinafter set forth.

After an extended investigation, we believe that we have discovered both the: cause and the cure for thermal cracking in steel, and will set forth hereinafter the theoretical considerations involved; it being understood, of course, that the theories propounded are not limitations on our invention as claimed.

tween the parts of the piece that'are above the Ar point and the parts of thepiece which are below it. Since the relief of these stresses occurs through plastic deformation, it will be clear that the portions of the piece which were above the A1 point, and were upset in the stress-relieving process will be dimensionally deficient upon cooling. This results in residual stresses after the piece has been brought to a uniform low temperature. If these stresses are sufllciently high, they will exceed the tensile strength or the endurance limit in fatigue, and will result in thermal cracking.

The amount of upsetting which occurs in the portions of the piece that are heated above the A1 point depends upon the yield strength, modulus of elasticity, and coefficient of thermal expanion at that temperature. The greater the extent to which the internal stresses are relieved at the elevated temperature, the greater will be the magnitude of the stresses after the entire piece has cooled to a uniform temperature. This action is the action which we believe occurs in car wheels due to the intense but very localized When a stress-free piece of steel is heated non- I uniformly, the difference in expansion in different parts of the piece produces certain in-- ternal stresses. The production of these stresses is, of course, dependent on the nonuniformity of heating, or cooling for that matter, in different parts of the piece. When all parts of apiece of steel are at the same temperature, there are no thermal stresses present, even though the temperature of the entire piece may be rising or falling. On the other hand, when different parts 'of the piece are atdifl'erent temperatures at the same instant, the differential expansion sets upstresses within the piece, as will be clear. If the highest temperature attained in the piece is below the A1 transformation point, it is likely that these stresses will not exceed the yield strength of the metal at theelevated temperature, and ,that consequently-when the piece is subsequently brought to a uniform lower temperature throughout, the parts which expanded upon heating will contract upon cooling, and

little or no residual stresses will remain in the steel, provided that no permanent deformation has taken place. If, on the other hand, certain parts-of the piece are heated to the A1 transformation-point or above, the stresses are likely to exceed the yield strength. The yield strength,

- of course, sinks to a low value at the Ar point,

heating which results from severe braking applications from high speeds.

We have found that the stresses resulting from differential heating and expansion when steels are heated through the thermal critical range are more completely relieved, at high temperature, in steels having a narrow transformation range than in those in which the transformation occurs over a wider range or is made to be sluggish due to the addition of suitable amounts of modifying agents. In the case of hypo-eutectoid steels, the transformation range is very much wider and very much less sharply marked than in eutectoid compositions, and thus the stresses are not so completely relieved, because only a portion of the metal is passing through the allotropic transformation at a given We have discovered that although lowering the carbon will reduce the tendency to thermal cracking, this results in a wheel which (when heat treated by methods which result in low internal stresses) is soft and subject to shelling and wear, whereas by simultaneously lowering the carbon and raising the manganese it is possible by means of standard heat treating methods to get the desirable combination of thermal cracking resistance, wear resistance and other properties already referred to.

It will be seen that the principles developed above would indicate that the tendency toward thermal cracking is most marked in steel compo- .sitions whichare in the neighborhood of the eutectoid and that the tendency becomes markedly less the further one goes into the hypoeutectoid types of compositions in steels. Thus, it follows that in. hypo-eutectoid types of metal, thermal cracking can be avoided if one keeps far enough away from the eutectoid range.

While this principle which we have discovered for making non-thermal cracking steels is in itself quite simple,'in the manufacture of comni'ercially successful car wheelsmany other factors must be taken into consideration. These factors include, for example, sufficient hardness, tensile strength, toughness, and wear resistance to meet the service conditions economically and safely.-

The Association of American Railroads has worked out, over a period of years a specification for the analysis of wrought steel wheels, which has in the past satisfactorily met the above physical requirements, but has not been non-thermal cracking. These specifications call for an analysis of plain carbon-manganese steel which is of, or very nearly of, eutectoid composition. Nearly all of the wrought steel wheels now in railroad service are of this analysis, and under the present conditions of high speed service are very susceptible to thermal cracking.

In accordance with our invention, it might appear that the thermal cracking of car wheels might be avoided by merely lowering the carbon content to a sufficient degree. However, the problem is not as simple as merely carrying the analysis of the steel sufiiciently far into the hypoeutectoid range. For example, hypo-eutectoid teels will tend to be too soft and have insufiicient wear resistance. We have found that raising the manganese above its usual range-will accomplish the desired efiect.

In the commercial manufacture of wrought steel wheels we have found, in balancing the various requirements in non-thermal cracking tendency and mechanical properties, that a, carbon steel having, say, 50% carbon, was free from thermal cracking tendencies in service, and with the addition of 1.25% manganese would meet all of the service requirements of regular analysis car wheel steel. Carbon contents under .40% are likely to result in insufficient strength and hardening capacity, while with carbon above a 55% in the hypo-eutectoid type of car wheel there is a markedly increased tendency toward thermal cracking. .In hypo-eutectoid steels of the character referred to, manganese is available as a modifying agent in amounts ranging, say, from 1.00% to 1.50%. The silicon content of the material should preferably be in the neighborhood of .15 to 30%. This range is not limiting, but is based on good practice for obtainin killed steel of this analysis; or the steel may be killed by the addition of suitable amounts of any other deoxidizer. Compositions within the ranges set forth have notonly proved excellent upon laboratory tests, but have shown to be exceptional in their results on large scale actual service tests. In the hypo-eutectoid ranges, we prefer to employ compositions as follows: a

Balance substantially all iron excepting for the normal impurities.

the essentials in these compositions, the silicon being present only to secure a killed steel, and as. before mentioned, this result can be obtained by other deoxidizers.

Of course, in evaluating a thermal cracking tendency, it is necessary to adoptan arbitrary standard of severity. Inasmuch as there is a wide variation in the severity of service conditions, it is diflicult to draw absolute lines of demarcation between thermal cracking and nonthermal cracking compositions in service. There may be, therefore, some variation in these standards; but where we have described a wheel as non-thermal cracking w mean a wheel which can be used and worn away in service without thermal cracking.

While we have mentioned a specific composition as illustrating the applications of the general principles enunciated for the production of non-thermal cracking wheels, our invention is not to be limited to it. We claim as within the scope of our invention all wheels having compokill the steel, and the balance substantially all iron except forthepresence of a normal amount of impurities.

2. A wrought steel wheel for railway car service or the like, consisting of between .40 and .55 per cent carbon, between 1.00 and 1.50 per cent manganese, between .15' and .30 per cent silicon, and the balance substantially all iron except for the presence of a normal amount of impurities.-

3. A- wrought steel wheel for railway car service or the like, consisting of substantially .50 per I cent carbon, substantially 1.25 per cent manganese, a deoxidizer in suflicient quantity only to kill the steel, and the balance substantially all iron except for the presence of a normal amount of impurities.

REID L. KENYON.

HARRY TOBIN.

The proportions of carbon and manganese are