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

[0053]It might also be noted here that the use of the herein described amounts of aluminum, phosphorous and sulfur is specifically aimed at reducing the amount of non-metallic inclusions in the final rail product. These non-metallic inclusions have been linked to degradation in mechanical properties (e.g., ductility)

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

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