US2798805A - Wrought railroad wheels made of alloy steels - Google Patents

Description

. steel at the tread surface may occur.

WRfiUGl-IT RAHLROAD WHEELS MADE OF ALLGY STEELS John M. Hodge and .lnseph M. Wandrisco, Pittsburgh,

Pa., assignors to UnitedStates Steel Corporation, a corporation of New Jersey No Drawing. Application September 3, 1954, Serial No. 454,206

2 Claims. (Cl. 75-123) The present invention relates to alloy steel wrought railroad-car Wheels.

The wrought-steel wheels used currently in railroad service are manufactured by rolling and/or by forging plain carbon steel. Because railroad wheels serve as brake drums as well as means of locomotion, the tread surface of the wheel is heated by friction when the brakes are applied. Therefore, large thermal gradients develop in the wheel during braking, so that the steel at the tread surface may be heated above its transformation temperature. On cooling, the steel that was heated above this temperature usually undergoes a transformation to a microstructure of higher hardness and lower ductility than the hardness and the ductility of the original micro structure.

This transformation results in an expansion which opposes the normal contraction of the cooling metal and tends to set up high stresses in the heat-affected portion of the rim. These stresses may be augmented by a further contraction which occurs when the wheel is reheated by repetition of the braking, and the hard microstructure is tempered. The sharp thermal gradient created by the brake heating tends to intensify these stresses resulting from transformation and tempering. These stresses may be high enough to cause cracking in the heat affected portion of the rim of the wheel, and such cracking, which is of common occurrence in plain carbon wheels, is known as thermal checking. Since the magnitude of the expansion and contraction from transformation and tempering is a function of carbon content, the susceptibility to thermal checking is, likewise, dependent upon the carbon content. This effect of carbon content on susceptibility to thermal checking has been long recognized in the trade and it is common practice to use lower carbon wheels for applications involving severe braking. In

plain carbon wheels, however, it is impractical to use carbon contents below about 0.50%, as the wheels then become too soft to satisfactorily resist wear and shelling from the rolling loads.

Because the thermal expansion of the heated steel at the tread surface is restrained by the unheated steel in the other parts of the wheel, plastic deformation of the On cooling, circumferential tensile stresses develop in the steel that has been plastically deformed and these stresses increase in magnitude with repeated heating and cooling cycles. These stresses may be of such magnitude as to result in deep cracks in the rim, known as thermal cracks, and on further repeated braking these thermal cracks may propagate into, or in some instances through, the plate res Patent fiice 2,798,805 Patented July 9, 1957 of the wheel, resulting in the serious type of wheel failure, known as explosive failure. Since the magnitude of this tensile stress depends upon the extent of the plastic deformation of the heated portion of the rim, the susceptibility to this type of failure can be decreased by using steels of higher elevated temperature strength which will resist this deformation at the elevated temperature.

The third requirement for a wheel which will perform satisfactorily in service is a high enough strength or hardness in the tread and rim to resist wear, or fatigue failure of the tread from stresses imposed by heavy rolling loads or flow which leads to the type of fatigue failure of the treads known as shelling. This need is generally recognized and it is common practice with plain carbon wheels to use higher carbon, harder wheels for service involving heavy loading and consequently a greater tend ency to shelling.

Still another requirement for a wheel which will perform satisfactorily in service is a sufficiently high strength level in the plate portion of the wheel to resist fatigue failure in this portion of the wheel.

It is accordingly an object of this invention to provide wrought steel railway wheels which have much lower carbon content than is possible in plain carbon steel wheels without sacrifice of resistance to wear or shelling and with an attendant gain of a much lower susceptibility to thermal checking or cracking.

Another object of this invention, is the provision of a restricted range of carbon content, within which the occurrence of thermal checking is insignificant even under the most severe braking conditions.

A further object of this invention is to provide wrought steel railway wheels having enhanced elevated temperature strength.

Still another object of our invention is: the use of a steel of sufiicient hardenability to permit the attainment of the requisite hardness and strength in the rim portion to minimize shelling and wear to permit the attainment of the desired hardness throughout the balance of the wheel.

We have discovered that the foregoing objects can be accomplished by maintaining the alloying contents within certain restricted ranges as hereinafter described. The first essential feature of the steel of this invention is the establishment of a restricted low range of carbon content, below 0.50%, in order to insure adequate resistance to thermal checking. This range is 0.10 to 0.30% carbon with a preferred range of 0.12 to 0.24%.

The second essential feature is the use of alloying elements conducive to elevated temperature strength and we prefer to use molybdenum and vanadium for this purpose. The ranges of these elements which may be included in steels of this invention are 0.10 to 1.50% molybdenum and 0.01 to 0.50% vanadium and preferably .30 to .75 molybdenum and .01 to .15% vanadium. Other elements such as columbium, titanium, tungsten and cobalt could be used to partially at least replace the foregoing.

The third essential feature is the use of alloying elements conducive to hardenability. We prefer to use the alloying elements nickel, manganese, chromium and boron for this purpose. The ranges of these elements which may be required to obtain adequate hardenability precipitation of a finely divided alloy carbide duing cool ing from the austenitizing temperature and on tempering. This is particularly beneficial in obtaining an adequate strength level in the plate portion of the wheel to resist fatigue failure.

Thus, the composition range of the steel of this invention is as follows:

Range,

Preferred Percent Range Balance iron and other elements in residual amounts.

Testing machines, originally designed for the testing of brake shoes, are used to study the effect of simulated conditions of braking on the susceptibility of full-scale wheels to thermal checking, to thermal cracking, and to the sudden fracture that results from the stresses developed by repeated braking (see The Effect of Brake Shoe Action on Thermal Cracking and on Failure of Wrought Steel Railway Car Wheels, University of Illinois Engineering Experiment Station, Bulletin Series No. 387, June 1950).

The stop test, which simulates the braking conditions that exist when a rapidly moving train is stopped suddenly in an emergency, is used to determine the susceptibility of wheels to thermal checking and to thermal cracking. The drag test, which simulates the braking condition that exists when a train descending a long grade is slowed intermittently, is used to determine the susceptibility of wheels to the sudden, eXplosive-type failure that results from the propagation of a thermal crack. To simulate a thermal crack, a radial saw-cut is made in the rim of the Wheel before the drag test is conducted.

Braking tests, such as those described abovehave established that alloy steel wheels of the type of this invention have a resistance to thermal checking, thermal cracking, and explosive failure far superior to that of currentlyproduced plain carbon Wheels. The results of some of these tests, conducted with wheel composed of steels of the following analyses are hereinafter set forth to illustrate the performance of alloy steel wheels of this invention.

I. Stop tests All stop tests were conducted from aninitial speed of 115 M. P. H. and with a pressure of 20,000 pQllnds applied toeach of twobrake shoes.

Steel Heat Treat- Test Procedure Results No. ment 50 Full Stops-100% energy absorption. Partial Stops5% energy absorption. 5 Partlal Stops-% energy Wheel did not absorption. thermal 1 Rim Tough- 5 Partial Stops% energy check or ened. absorption. thermal 5 Partial Stops% energy (Hack absorption. 10 Partial Stops-% energy absorption. 5 Partial Stops% energy absorption. 50 Full Stops100% energy absorption. 5 Partial Stops-10% energy Light thermal 2 do absorption. checking- 5 Partial Stops-20% energy no thermal absorption. cracking. 5 Partial Stops-30% energy absorption. do.- 50 Full Stops100% energy Heavy thermal absorption. checkingno thermal cracking.

II. Drag tests Each drag test was of 30-m1nute duration with the brakes apphed for 50 seconds during each minute. The test wheel was rotated at a constant speed of 45 M. P. H.

Steel Heat Treat- Depth of Brake N o. of No. ment Saw Out; Shoe Drags Results Percent Pressure 1 'Rim 'Iough- None 4, 000 20} Wheel did not .ened. 4, 000 tail. 3 do 75 4, 000 100 Do.

The following results of elevated temperature tensile tests on samples cut from wheels of this steel are illustrative of the superior elevated temperature strength .Which i as described above, an important factor in increasing the resistance to thermal cracking and explosive failure:

Tempera- Yield Reducture of Strength Tensile Elon tion Steel No. Test, (0.2% 011- Strength, gation, of

F. set), p. s. i. percent Area,

p. s. 1. percent 900 67, 800 84, 800 -20. c 61. 5 1 -1, 000 64, 800 79, 700 18. 0 58.0 1, 100 58, 800 69, 800 14. 0 33. 0 900 81, 000 101, 200 22. 0 65. 0 3 1, 000 77, 000 90, 500 20. 0 66. 0 1, 100 58, 300 77, 400 24. 0 54. 0 4 900 50, 800 80, 000 31. 0 73. 0 ;o7 carbon 1, 000 41, 000 60, 400 39.0 71. 0 Plain carbon. 1, 100 25, 200 40, 600 as. 0 74. 5

The following resultsof fatigue tests on samples cut from the platesof an alloy wheel of the type of this in- .vention and a similar plain carbon wheel will illustrate the superior resistance to plate fatigue failureof the alloy steel wheel.

Endurance Hardness Steel No. Heat Treatment Limit, BHN

1. Rim Toughened 61, 500 223/229 5- do 49, 500 223/229 The above examples are illustrative of the superior performance and properties of alloy steel wheels of the type of this invention. As a result of our discoveries, we have produced a superior wrought railway Wheel which is characterized by substantial freedom from thermal checking and cracking under severe braking conditions due to its containing less than 0.50% carbon and having an elevated temperature yield strength in the rim portion in excess of 60,000 p. s. i. at 1000 F. when heat treated to a hardness in excess of 250 BHN and having a room temperature yield strength in the plate portion in excess of 60,000 p. s. i. when heat treated to a hardness value in excess of 220 BHN.

While we have shown and described several specific embodiments of our invention, it will be understood that these embodiments are merely for the purpose of illustration and description and that various other forms may be devised within the scope of our invention, as defined in the appended claims.

We claim:

1. A wrought steel railway wheel having an elevated temperature yield strength in excess of 60,000 p. s. i. at 1000 F. in the rim portion when heat treated to a hardness in excess of 250 BHN and having a room temperature fatigue strength in the plate portion in excess of 60,000 p. s. i. when heat treated to a hardness value in excess of 220 BHN, said wheel being characterized by substantial freedom from thermal checking and cracking under severe braking conditions, the steel of said wheel containing with the balance iron and residual amounts of other elements.

2. A wrought steel railway wheel having an elevated temperature yield strength in excess of 60,000 p. s. i. at 1000 F. in the rim portion when heat treated to a hardness in excess of 250 BHN and having a room temperature fatigue strength in the plate portion in excess of 60,000 p. s. i. when-heat treated to a hardness value in excess of 220 BHN, said wheel being characterized by substantial freedom from thermal checking and cracking under severe braking conditions, the steel of said Wheel containing References Cited in the file of this patent UNITED STATES PATENTS 2,586,042 Hodge et a1. Feb. 19, 1952

Claims (1)

1. 2. A WROUGHT STEEL RAILWAY WHEEL HAVING AN ELEVATED TEMPERATURE YIELD STRENGTH IN EXCESS OF 60,000 P.S.I. AT 1000* F. IN THE RIM PORTION WHRN HEAT TREATED TO A HARDNESS IN EXCESS OF 250 BHN AND HAVING A ROOM TEMPERATURE FATIGUE STRENGTH IN THE PLATE PORTION IN EXCESS OF 60,000 P.S.I. WHEN HEAT TREATED TO A HARDNESS VALUE IN EXCESS OF 220 BHN, SAID WHEEL BEING CHARACTERIZED BY SUBSTANTIAL FREEDOM FROM THERMAL CHECKING AND CRACKING UNDER SEVERE BRAKING CONDITIONS, THE STEEL OF SAID WHEEL CONTAINING