Interface - The Journal of Wheel/Rail Interaction

RAIL MATERIALS

What Kind of Rail Materials Will Survive in Heavy-Haul Service? (continued)

The stress vectors of wheels transmitted across a rail-wheel interfacial area can be quite different; a wheel exhibiting pure frictionless rolling will generate a simple compression overload stress on the rail. The compression deformation strain will flatten the rail surface in the interface, and will ultimately lead to observable change in rail profile. The maximum stress induced anywhere in the rail will be beneath the rail surface, as shown in Figure 2.

If the maximum subsurface resolved shear stress material contains a local material flaw or weakness, a subsurface crack will form. That crack, with passing wheels, will initiate a longitudinally oriented shell defect beneath the rail head surface.

Any tangential force exerted across the wheel/rail interface by a rolling wheel will generate quite a different response in the rail. This situation is schematically shown in Figure 3.

The simple formula for shear strain shown in Figure 3 does not readily indicate how much plastic deformation strain will be generated in a primary rail steel micro-cube element as a function of rotation angle theta. The relationship is graphically shown in Figure 4.

Proper quality rail steel, meeting current AREMA specification requirements for tensile ductility requirements, can be tensile strained 11% without breaking. The shear strain angle, equivalent to an 11% uniaxial tensile strain, (as shown by arrow in Figure 4) is 6 degrees. The relationship shown in Figure 4 is further illuminating in that as the plastic deformation strain produced by shear deformation approaches infinity as the theta angle approaches 90 degrees. In other words, the rail steel head surface structure that has demonstrated significant shear angle distortion has been strained beyond its nominal uniaxial tensile strain capabilities, and it can’t contain a stress-induced crack, or the rail will have initiated a spalling corrugation crack.

Metallurgical examination of as-rolled rail, as shown previously in Figure 1, indicates several pertinent observations:
• Pearlitic colonies’ crystal orientations are randomly distributed.
• The iron carbide platelets within each colony are generally continuous and are not broken into small discrete aligned particles.

High magnification section examination of the near running head surface layer material of a worn rail indicates there are progressive changes in the rail’s pearlitic structure:
• The orientations of whole pearlite colonies are spontaneously and progressively changed. The iron carbide platelets become progressively aligned with the predominant direction of rail surface shear strain.
• The flat iron carbide platelets break up into small, aligned particles.

At 1000x magnification, the discernible rail material at the extreme running surface appears to be free ferrite and small carbide particles that are aligned in a direction approaching parallel to the extreme surface. In other words, the nominal shear deformation angle of all the running surface rail material is extremely close to 90 degrees. The combined plastic compression-shear deformation capability of pearlitic rail without initiating a surface crack is phenomenal.

This primary characteristic of pearlitic carbon low alloy rail steel that makes it superior to any current challenger rail material is that it can withstand tremendous amounts of combined compression-shear deformation overload before it initiates a surface spall crack. No other alternate rail material can do this without initiating spall cracks with lesser service exposure. On the other hand, there are reasons why pearlitic steel rail, with all its good characteristics, gives track workers fits and a desire for a better rail material

High carbon steels are all extremely notch sensitive. If there is any small flaw or weak spot in a pearlitic structure rail, it is subject to a sudden cleavage fracture with the application of, sometimes, minimal stress loading. Micro-cracks or flaws in rail will easily propagate into larger cracks with what would be considered entirely normal service stress loading. Once a crack or flaw is present in a rail, it will eventually grow in service to a size that makes it a candidate for sudden, brittle fracture. Flaws can be found anywhere in a rail section.

Rail Flaws Generated by Primary Mill Processing
Some flaws are original defects produced in the rail mill. A heavy-haul railroad’s procurement of rail with proper strength and produced by good "clean-steel" melting procedures is merely an assurance that the rail will not catastrophically fail within a short service period. Rail meeting heavy-haul service quality standards still contains small flaws that can grow to critical size in subsequent track service. These small non-specified flaws can generate sudden failure in a single rail after moderate use. They can aggravate corrugation wear of rail.

Variations in these small, but still specification-acceptable flaws between individual rails within a production heat, or variations between individual mills’ quality, will generate questions in Roadmasters’ minds, such as: Why did a single piece of rail in the middle of the curve go to hell? It may raise questions in System Track Engineers' minds: Why does mill X’s rail last longer than mill Y’s rail in the same kind of service?

It all comes down to the fact that "clean-steel" manufacturing processes used to produce the rail meeting current industry standards never generates a totally flawless material. There will always be "acceptable" variations in the amount, size and distribution of micro-flaws within the rail. Murphy’s Law applies in the prediction of rail service life. If there is a single flaw or an aggregation of flaws in a rail with the potential to initiate a crack, stresses generated by passing wheels will find it.

James R. Hornaday, Jr., is President of Alpha Gamma Transform, Inc.

This article is based on a presentation made at Advanced Rail Management’s Wheel/Rail Interaction Seminar, May 2009.

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