Metal treatment – Stock – Ferrous
Reexamination Certificate
2002-04-17
2003-10-14
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S333000
Reexamination Certificate
active
06632297
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to steel railway wheels, and especially those formulated to resist spalling caused by martensite transformations in the steel that constitutes the tread and/or flange regions of such wheels. Spalling in these wheel regions causes several problems. For example, spalling of the wheel tread will cause the wheel itself to have flat spots and the quality of “out-of-roundness”. Moreover, when railway wheels experience spalling, surface cracks tend to propagate from spalled areas and cause pieces of the martensite steel to detach from the wheel, especially as the spalled area suffers rolling contact fatigue. These wheel defects also increase wheel/rail dynamic forces that produce consequential damage such as broken rails and accelerated track deterioration.
2. Description of the Prior Art
Steel railway wheels wear out as a result of normal usage. They are also prematurely removed from service as a result of spalling. Spalling occurs in railway wheel tread and/or flange regions as a result of metallurgical transformations caused by the heat generated when a train's wheels skid during brake application. In effect, these skids produce local heating to temperatures above 1300° F. (704.4° C.). These high temperatures produce metallurgical transformations in small spots of the steel in the tread and/or flange regions of such wheels. These spots transform to martensite when they cool. The resulting brittle material then cracks and falls away. Again, spalling takes place in addition to the “normal” wear experienced by railway wheels.
The railroad industry has dealt with normal wear/spalling of its wheels in three general ways: (1) machining of tread and flange surfaces, (2) scrapping the wheel and (3) imparting improved metallurgical properties to those steels from which railway wheels are made. As far as scheduled and unscheduled machining of railway wheels are concerned, it should be noted that, since normal wear/spalling of railway wheels has certain safety implications, these matters are the subject of governmental regulation. In the United States for example, the Federal Railroad Administration (“FRA”) has promulgated various regulations concerning the dimensions of various parts of a railway wheel's profile. Many of these regulations express themselves in terms of the height and width of a railway wheel's flange.
For example, these regulations call for new (or newly machined) wheel flanges to have a height of {fraction (16/16)}'s inches (i.e., 1 inch) and a width of {fraction (21/16)}'s inches (i.e., 1{fraction (5/16)} inches). A railway wheel is considered to be in violation of FRA regulations if the height of its flange—as measured from the crown of the tread surface of the wheel—reaches {fraction (24/16)}'s inches (i.e., 1½ inches), or if the width of the wheel flange reaches {fraction (15/16)}'s inches. If a wheel reaches either of these states of wear, it should be machined to the required dimensions or scrapped. Those skilled in the railway wheel maintenance arts will appreciate that in order to achieve these dimensions in a worn wheel, a great deal of the wheel metal is machined away—and hence, “wasted”. This waste has a very direct bearing on a wheel's useful life. Hence, many machining procedures have been employed to minimize such waste. For example, U.S. Pat. Nos. 4,134,314 and 4,711,146 teach several wheel reprofiling machining techniques that serve to bring railway wheels back into compliance with regulations with minimum waste of wheel tread and flange material.
Ideally, the steel from which railway wheels are made would have high levels of at least two general properties. They would be highly wear resistant; and they also would be highly heat-crack resistant. Unfortunately, these two properties have certain contrary metallurgical aspects, especially in the context of railway wheel exposure to the heat generated by heavy braking situations. The first metallurgical problem arises because, in order to enhance its wear resistance, the hardness of the steel must be raised. Unfortunately, increased hardness in a steel usually implies decreased spall resistance. On the other hand, making a steel more spall resistant usually implies that the steel will be less hard, and hence less wear resistant. Moreover, both of these properties (wear resistance and spall resistance) must be achieved without greatly sacrificing the pearlitic structure that imparts the quality of wear resistance to a steel.
Generally speaking, increased hardness can be brought about through addition of certain alloying elements (in certain concentrations) to a steel formulation. For example, when wear resistance is the more desired property, high carbon steels having carbon contents ranging from about 0.65 to about 1.0 weight percent are employed. Such steels are especially hard and, hence, especially wear resistant. Such steels are not, however, particularly spall resistant.
Their loss in spall resistance generally follows at least in part from the fact that martensitic crystalline structures (or bainitic crystalline structures) are more likely to be produced in those railway wheel steels alloyed to gain greater hardness. These martensite crystalline structures are produced when frictional heat is imparted to railway wheel tread/flange areas in braking situations where wheel slide takes place. Such heat is often sufficient to raise temperatures of the tread/flange steel to austinite-producing levels in those local regions known as “hot spots”. Thereafter, because the rest of the railway wheel serves as a heat sink, hot spot temperatures are quickly lowered to martensite-forming levels. Thus, in a braking situation, local areas of the tread and/or flange are transformed from pearlite to austenite to martensite as their steel rapidly heats—and rapidly cools.
Viewing the overall hardness versus heat-cracking resistance problem from the spalling resistance point of view, one finds that other alloying materials (and/or other concentrations of certain commonly employed alloying materials such as carbon) have been added to (or, in the case of carbon, reduced) certain steel formulations for the specific purpose of imparting spall resistant qualities to railway wheels. For example, medium carbon steels having carbon contents ranging from about 0.45 to about 0.55 weight percent have proved to be more spall resistant than the previously noted harder steels having 0.65 to 0.85 carbon concentrations. It also has been found that many of the other alloying materials (and/or different concentrations of identical alloying materials, e.g., the different carbon concentrations noted above) tend to have unacceptably low wear resistance. Thus, this wear resistance versus spall resistance problem has a certain dilemmatic quality that has for many years thwarted the industry's attempts to extend the useful life of railway wheels.
Those skilled in this art also will appreciate that spalling has proven to be the more intractable aspect of the wear resistance versus heat crack resistance dilemma. This generally follows from the fact that normal wear is somewhat predictable, and gradual, in nature. Heat producing wheel skids on the other hand are relatively unpredictable. Worse yet, spalling tends to produce damage that is much more immediate and much more severe in nature. Nonetheless, most prior art railway wheel steel compositions tend toward satisfying railroad industry requirements for greater wear resistance, while “silently” conceding that spalling due to heat cracking caused by wheel skids will be dealt with by: (1) physically machining railway wheel tread/flange regions on a scheduled basis to meet the wheel flange dimension requirements previously noted, or (2) by machining heavily spalled wheels on an “as needed” basis, or (3) by simply scrapping the wheel.
To some extent, the patent literature reflects the railway industry's attempts to deal with the wear resistance vs. heat c
Sawley Kevin James
Stone Daniel Hunter
Dorr, Carson , Sloan & Birney, P.C.
Transportation Technology Center Inc.
Yee Deborah
LandOfFree
Railway wheels resistant to martensite transformation does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Railway wheels resistant to martensite transformation, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Railway wheels resistant to martensite transformation will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3171610