Railroad rail steels resistant to rolling contact fatigue

Abstract

Railroad rail steels having a pearlitic structure and containing 0.720 to 0.860 wt % carbon; 1.000 to 1.280 wt % manganese; 0.450 to 1.000 wt % silicon; 0.010 to 0.100 wt % copper; 0.150 to 0.280 wt % chromium; 0.0010 to 0.0500 wt % aluminum; 0.050 to 0.120 wt % nickel; 0.100 to 0.260 wt % molybdenum; 0.100 to 0.210 wt % vanadium; 0.0010 to 0.0065 wt % nitrogen; 0.0010 to 0.0080 wt % phosphorus; 0.0010 to 0.0040 wt % sulfur; and 0.0100 to 0.0350 wt % niobium with the remainder of said steel being iron, can be used to make railway rails that are particularly resistant to rolling contact fatigue and, hence, shelling.

Description

BACKGROUND OF THE INVENTION[0001]1. Field of the Invention[0002]The present invention generally relates to railroad rail steels. More particularly, it is concerned with those railroad rail steels that are specifically alloyed to resist fatigue effects including rolling contact fatigue (RCF) and shelling in the head regions of such rails. The term “shelling” generally refers to loss of steel material as a result of deterioration arising from mechanical stresses. In the context of this invention, the term shelling is often contrasted with the term “spalling.” Spalling generally refers to loss of steel material as a result of metallurgical damage created by excessive heat that arises from the sliding of railroad wheels over railroad rails during extreme train braking operations. Since shelling and spalling often occur in conjunction they are often collectively referred to as “thermo-mechanical deterioration.”[0003]Various problems arise from each form of rail head material loss. For ex...

AI Technical Summary

Benefits of technology

[0054]Once applicants' liquid steel has been degassed and cast into blooms (e.g., bars of either round or square cross-section and of varying lengths), said blooms are then hot rolled into final rail products. This general process is often referred to as Thermo-Mechanical-Processing (TMP). During applicants' TMP process, the blooms are subjected to a number of rolling stages that vary in number from about 12 to about 16. Each rolling stage further reduces the cross-sectional area of the bloom and forces the original bloom towards the final shape of the rail product. Each rolling stage is done at a specific temperature and reduction rate. The representative TMP schematic depicted in FIG. 2 has been simplified to reflect two (2) reductions, each of which has the aim of simulating a number of combined rolling stages that a bloom experiences in the applicants' rail rolling processes. The range of rolling parameters for the applicants' rails are as follows:Reheating Temperature [° C.]:1120-12401st Reduction Temperature [° C.]:1070-11601st Reduction [%]:40-601st Cooling Rate in ° C. per second [° C. / s]:2.0-6.02nd Reduction Temperature [° C.]:840-9302nd Reduction [%]:40-702nd Cooling Rate [° C. / s]:4.0-6.0Coil Temperature [° C.]:500-650Coil Hold Time [minutes]: 5-20Air Cooling Target:Room Temperature

Problems solved by technology

Railroad rails eventually wear out as a result of normal usage.

Such rails are however often prematurely retired from service as a result of various forms of thermo-mechanical deterioration.

For example, a great deal of thermo-mechanical deterioration is associated with metallurgical transformations of the rail steel from the original, relatively tough, pearlitic microstructure to more brittle microstructures such as bainite and / or martensite—with associated loss of the austenite / bainite / martensite steel material through spalling.

Again, thermo-mechanical deterioration is caused by the heat generated by friction when the train's wheels skid on railroad rails during extreme braking operation.

In any case, the resulting brittle martensite steel then tends to crack and spall away from the rail head surface.

Again, RCF produces the undesired form of steel material loss known as “shelling” wherein the rolling action of a steel railroad wheel over a steel rail produces mechanical stresses in the rail that—in their own right—contribute to a rail's deterioration.

That is to say that rolling contact fatigue can occur even if the rail does not experience metallographic changes attributed to temperature effects.

Rolling contact fatigue is also associated with diminished shear fatigue strength of a rail's head surface.

In any case, rolling contact fatigue is related to both the strength of the rail surface and to the load applied to it.

Modern railroad rails are being called upon to carry out increasingly severe duties.

The relatively high loads carried by the rails lead directly to higher levels of rolling contact fatigue.

The use of hard steels notwithstanding, the incidence of shelling type defects in railroad rails is increasing as a result of the greater loads they are currently called upon to carry.

And as previously discussed, if a rail is heated to high enough temperatures, the stresses produced therein can exceed the yield strength of that rail steel.

For example, elevated temperatures in a steel rail serve to reduce its ability to resist mechanical loading owing to the steel's diminished mechanical strength above certain temperatures.

Moreover, the longer a steel rail experiences elevated temperatures, the greater the degree of shelling that will result from this time related circumstance.

Unfortunately, to varying degrees, these properties range from being metallurgically antagonistic to being metallurgically incompatible.

Conversely, when a steel is alloyed to be more resistant to thermo-generated deterioration, this usually implies that the steel will be less hard, and hence, inherently less wear resistant.

Such steels are not, however, particularly resistant to thermo-mechanical deterioration.

Conversely, it is also well known that medium carbon steels having carbon contents ranging from approximately 0.45 to 0.55 weight percent are more resistant to thermo-mechanical deterioration than harder steels, but they are generally less wear resistant.

However, it also should be appreciated that some of these custom based statements can lead to certain misunderstandings.

This all goes to say that the wear resistance versus thermo-mechanical resistance problem has a persistent dilemmatic quality that continues to thwart the railroad industry's attempts to extend the useful life of railway rails.

Conversely, heat-producing railroad wheel skids over such rails are relatively unpredictable.

Worse yet, thermo-generated deterioration tends to produce damage that is much more immediate and much more severe in nature.

Thus far, alloying practices have been of somewhat limited value in dealing with the wear resistance vs. thermo-mechanical deterioration dilemma.

For example, even though the constitution of three component steels can theoretically be deduced from ternary phase diagrams, they are often rather difficult to interpret.

Their practical value is also limited by the fact that they only describe equilibrium cooling conditions.

Therefore, since most modern railroad rail steels are both heat treated during their manufacture and contain more than three alloying components, much more complex graphing methods (e.g., Temperature Time Transformation diagrams) must be employed and interpreted—thus far with varying degrees of success as far as railroad rail steels are concerned.

Further complexities arise from various heat treatment processes to which steels are usually exposed.

In closing their comments concerning the prior art concerning steel railroad rails, applicants would say that even though a great deal has been learned about rolling contact fatigue in the rail head, the fact remains that such damage mechanism contributes in a significant way to the accelerated wear of rails.

Indeed, rolling contact fatigue problems are becoming more and more pronounced as rails are utilized to carry heavier and heavier loads as well as more tonnage of traffic.

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