Interface - The Journal of Wheel/Rail Interaction

RAIL MATERIALS

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

Mechanically-Induced Flaws
The simplest mechanical form of railroad-induced flaw damage is an indention surface scar made by a misdirected blow of a track-man’s spike maul hitting the base of a rail while driving the spike into a wood tie. The depressed concave volume in a maul scar can have sharp edges. These edges will act as stress-intensifying notches. Track foremen prudently plug-replace any rail that has even been slightly maul-damaged.

A second mechanical flaw-generation mechanism in rail can occur when the wheel of a moving car experiences a rim failure. If the damaged wheel somehow continues to roll along the rail without derailing, the sharp-edged protrusions on the jagged rim surface of the rolling wheel will indent and scar the top surface of the rail.

Heat-Induced Service Cracks
A normal single wheel rolling along a rail with a significant traction force on the rail can generate a great amount of near surface plastic deformation work in the rail surface layer. This surface friction work from a single passing wheel will be converted to heat in the deformed rail’s surface layer. The temperature rise in the deformed surface layer will be a function of the rail steel’s thermal diffusivity and the energy input from the passing wheel. This friction heat spike generates significant temperature pulses in the extreme rail surface layer. The mass quenching effects of the subsurface rail layer materials then quenches the surface layer. Finite element thermal analysis of a freight car wheel sliding at 30 mph on top of a rail with a friction coefficient of 0.3 has shown that there is sufficient friction energy to heat the first 0.0001 inch of rail surface material to 1800 degrees F, and that the ultra-hot surface layer will be mass quenched to less than 400 degrees F in less than a second. This reaction is equivalent to the movement of a high-energy laser beam across the surface of the steel.

A thin ultra-hard and brittle layer of untempered martensite can be formed on the rail surface by the passage of a single wheel exerting sufficient traction force. Passage of a second wheel over an as-quenched hardened martensite surface layer will generate multiple micro-cracks. Each micro-crack has the potential to grow into a major spall defect in the rail (7).

Alternate Rail Materials
There has been extensive laboratory testing of many potentially improved rail materials. Testing has indicated there are a host of alternate materials with varying combinations of higher tensile yield strength, improved fracture toughness, better impact strength, and higher hardness than current pearlitic rail steels. There are numerous bainitic structure steels, with some promise for demonstrating better track-service durability than pearlitic rails.

Only one bainitic steel alloy has been roll-formed into a full-size rail section. That single bainitic rail is being tested in the AAR’s heavy-haul test loop at Pueblo, Colo. The bainitic steel's general abrasive wear resistance seems to be comparable to pearlitic rail’s wear, but its surface crack resistance has been slightly less than that of pearlitic rail. Bainitic rail steel's marginal track performance, together with its much higher alloy cost, has not generated any rush to manufacture any other bainitic rail steel.

Resistance of a material to cracking under extreme compression shear conditions is difficult to measure with conventional laboratory test equipment. It is difficult to measure and compare or rate any "new" or potentially improved rail material’s resistance to extreme compression shear cracking. It is possible that better measurement of this specific property could ultimately be the best laboratory sorting tool for developing a better rail material.

Non-metallic Inclusions
There are areas with potential for improving the durability of pearlitic rail materials. The inclusion content in pearlitic rail steels has been dramatically reduced with introduction of improved steel-making equipment. Steel mills can now make "clean" rail steel on a regular basis with a basic refractory melting practice, post-melting ladle treatment, vacuum degassing of melts, and continuous casting. The resultant rail materials, with smaller and fewer inclusions, have shown improved in-track durability.

There is considerable question as to how much value might be achieved by further reduction in rails’ inclusion content. Some rail mills have expressed caution that any across-the-board reduction in the maximum sulfur content in rail could trigger hydrogen fish-eye failures in finished rail. Other mills, with apparently superior vacuum degassing (and hydrogen removal) of their molten steel, have found hydrogen cracking has not been a problem with their finished rail.

Ultimate determination of any “best” amount of maximum inclusion material in rail will be the result of some sort of economic balance between railroads’ desire to procure long-lasting rail at a minimum price and rail mills’ ability to produce and deliver it profitably.

Effects of Micro-Alloying
The effect of micro-alloying on the durability of pearlitic rail steels is not a completely developed science. Small, but as yet undetermined, differences in alloy content can affect the performance of a rail in track. It is fairly well understood that very small amounts of vanadium, titanium and columbium in rail can improve its durability. The effectiveness of these additions is often a matter of how they are introduced into the rail during production, and not a simple function of their alloy content.

An example of what the potential for undiscovered effects of micro-alloying might be can be illustrated with an anecdote of a simple railroad experiment. A major railroad purchased premium rail materials from two different mills. The two rails’ resistances to curve wear was compared by making composite cwr test strings with the two rails, and then laying several composite cwr strings, with the joint between the two different rail materials at the center of each test curve. After a period of traffic over these two-rail composite rail curves, it was noted that the high rail head surfaces supplied by one mill were surface check cracked, while the high rail surfaces from the second mill were not cracked. Samples from the two different rails were sent to a laboratory for an explanation of the difference in performance.

Both rails’ compositions, as determined by conventional spectrometry, were virtually identical. The rails’ primary section hardnesses were identical. Their pearlitic grain structures were identical. Their inclusion contents were essentially alike. The only discernible difference was the rails' work-hardening capabilities. The extreme surface micro-indention hardness of the rail that cracked in service increased (normally) 100 points. The surface layer micro-hardness of the crack-free rail increased 150 points. Something in the better performing rail generated a higher hardness (and more resistant to continued deformation) material than the normal hardening steel.

The potential value point of this single example is that work hardening of rails is not a completely universal or even consistently understandable property for all rail materials. A lot of past emphasis has been made in developing new rail steels with higher yield strengths. Anything that will improve a pearlitic rail’s work-hardening capability will also generate deformed rail head surface hardnesses that have greater resistance to continued compression-shear overload — and in so doing, will minimize the formation of surface cracks.

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