By James R. Hornaday, Jr.
Contrary to many railroaders’ and rail mills’ opinions, there are no innocuous inclusions in rail. Any simple oxide particle, any complex oxide particle, any sulfide particle, or any complex oxide-sulfide particle in a rail will initiate a crack in the rail if it happens to be located in a critically stressed section of rail.
Dr. M. Nabil Bassim at the University of Manitoba demonstrated this fact in several research studies (1, 2, 3), indicating that the mechanism for crack initiation for each type of inclusion in rail is different.
• Aluminum oxide particles generate a tensile residual stress field around each alumina particle. It takes less applied stress on the rail to initiate a crack in the pearlite surrounding the alumina particle.
• Conventional oxides and complex oxides are inherently brittle. They crack easily under minimal loading. The resultant crack will extend into the surrounding pearlite matrix.
• Sulfide particles have absolutely no bond with their surrounding pearlite matrix material. They contribute no “strength” to the pearlite when it is stressed. The action is analogous to cracking open a pea pod — the peas do not hold anything together. The shear crack resistance of any sulfide particle is equivalent to a void of the same size and shape.
Controlled laboratory test work on specially prepared rail steels by Drs. J. Kalousek, D.M. Fegredo, E.E. Laufer, and M.T. Shehata generated the conclusion that the total volume fraction of sulfide particulate in the steel was a detrimental factor for avoiding or minimizing rail head wear (4, 5, 6).
Small inclusions in a railhead’s near-surface layer will definitely be overload-compression shear-stressed. At least one major heavy-haul railroad considers the extent of surface corrugation damage (or spalling) in premium rails to be one of the most serious causes for rail damage and replacement. Since near-surface railhead material is continuously wheel-loaded, it is subject to unavoidable near-infinity shear strain. It is not hard to extrapolate laboratory test results to recognize that any non-metallic particle in that high-strain field will severely limit the pearlitic structure’s ability to continue plastic deformation without initiating a surface spall crack.
Current inclusion-related quality specifications for rail intended for heavy-axle-load service merely prohibit the presence of single, large ultrasonically detectable inclusions in the rail, and only partially limit the size and aggregation of inclusions that can be seen in macro-etched transverse section test surfaces.
Rail breaking dynamically under stress reacts consistently with predictions derived from modern fracture mechanics, so long as the perceived flaw is a single, isolated flaw. The analysis becomes murky whenever two or more inclusion flaws are in close proximity. There is no effective procedure for assessing the real enhanced flaw potential of a cluster or an aligned string of non-metallic inclusions in a rail.
All so-called undersize particles, regardless of their clustering or aggregation, are commonly considered to be benign at the time of rail acceptance. The fact that initiation of rail failure may be traced to a clustered inclusion site is of little help to a railroad arguing with a supplier mill that the inclusion cluster in the supplied rail initiated failure.
Four conclusions can be made about non-metallic inclusions in rail:
• There is no such thing as a truly benign inclusion in rail.
• The only reason any non-metallic inclusion will not initiate rail failure during its track service life is because it didn’t experience sufficient service stress pulsing to initiate a crack.
• Rail mills may have little effective control over the size and distribution of inclusions in their finished rail, but they can control and minimize the total volume fraction of inclusions in their rail.
• Complete elimination of non-metallic inclusions in rail will never completely prevent rail failure or surface corrugation damage in track service. However, further minimization of inclusions will slow the incidence of sudden fracture and corrugation damage in current production rail.
Sulfur and Hydrogen Content in Rail
Traditionally, excess hydrogen in finished rail generated a high probability that the rail would spontaneously form serious hydrogen “fish-eye” flaws in the rails’ head sections, causing sudden failure in track. Over the years, the problem of excess hydrogen was greatly reduced by slow cooling of finished rail in insulated cooling boxes. The hydrogen problem has since been virtually eliminated by vacuum degassing of every molten heat of rail steel.
Throughout this period, there has been some recognition of the presumed fact that sulfide particles in a finished rail can act as hydrogen sink traps, and sulfide particulate would tend to prevent the formation of fish-eye flaws. Many rail mills continue to use this rationale for arguing that a rail with less than 0.025% or possibly 0.035% sulfur is a rail with some potential for forming fish-eye flaws. This argument is not universally true. Rail mills with consistent vacuum treatment facilities can consistently produce rail with less than 1 p.p.m. hydrogen. The mills that produce low hydrogen content rails can safely limit their sulfur contents to the 0.005% – 0.015% range. Heavy-haul railroads using these ultra-low sulfur and consistently properly degassed rail steels have not experienced any hydrogen fish-eye-type failures.
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.
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.
Improved Fracture Toughness
High carbon pearlitic rail steel has very low fracture toughness compared to other types of steels. Practically all commercial rail steels have critical fracture toughnesses in the range of 40 to 45 ksi/(square root Inch). This is the one mechanical property of rail steel that quantifies the rail steel’s poor notch-crack sensitivity. Any future improvement in pearlitic rail steel’s fracture toughness will enhance rails resistance to crack initiation and resistance to crack propagation.
There are two specific ways in which current pearlitic rail materials could develop higher fracture toughness through reduction in their prior austenitic grain size. It is well known that the toughness of most materials is inversely proportional to their grain sizes. In the case of rail steel, the critical structure parameter is the austenitic grain size of the hot rail when it is being transformed to its ambient-temperature pearlitic structure.
Most commercial rails have common austenitic grain sizes and common fracture toughnesses. Most rails achieve this combination of properties because:
• Progressive hot rolling and reduction of billets into finished rail accomplishes all this plastic deformation process while the steel has an austenitic or face-center-cubic crystal structure.
• Each step in the deformation process breaks up the existing austenitic grains, and generates new, very fine austenitic grains. The new grains will grow with time. The rate of grain growth of the new fine grain austenite is inversely proportional to the rolling temperature.
• Each deformation rolling step is accomplished at progressively lower temperatures. The resistance of the steel for further deformation progressively increases as it cools.
• Most mills must finish their rail rolling in a common narrow temperature range. If they attempted to finish rolling rail at any lower temperature, the finishing rolls would elastically spread open and let the almost finished section rail go through without achieving the desired finish section dimensions.
• Standard rail is placed on walking beam cooling beds, with traversing speeds across the beds adjusted to permit the complete transformation of the rail’s austenitic grains into a pearlitic structure (body-center cubic crystal structure ferrite platelets + Fe3C carbide platelets) within a specified hardness range.
• On-line head hardened rail is quickly transferred into controlled cooling stations that transform the rail’s austenitic grains into a finer grain pearlitic structure within a higher specified hardness range.
Rails’ prior austenitic grain sizes (and resultant limited toughnesses) are essentially controlled by the rails’ finish rolling temperatures. Review of this “conventional” mill practice suggests a few processing options, each with some potential for improving rail’s fracture toughness:
• Off-line head hardening of rail has shown capabilities for producing rail with measurable higher fracture toughness and in-service wear resistance (8, 9).
• Any change in rolling procedure that would finish the rail section to size at a lower temperature would improve that rail’s fracture toughness.
Off-Line Head Hardening
Most rail mills have made large capital investments in their on-line head-hardening equipment. Their initial economic choice between using off-line and on-line processing was typically made based on the difference in unit processing costs. Off-line treatment of rail requires a significant energy input and additional cost to austenitize a cold rail to a temperature of 1500 degrees F prior to controlled cooling. On-line treatment uses the existing heat of just-finished rolled rail.
The improved fracture toughness of ordinary rail that can be achieved by off-line head hardening was demonstrated by work performed by Dr. J. Igwemezie (8, 9). These tests, characterizing one mill’s off-line head hardened rails, demonstrated fracture toughnesses of 52.9 ksi/(square root Inch.)
While it is recognized that most mills’ on-line head hardening processes will continue to be used, it is conceivable that any rail mill installing a new head-hardening process could enjoy a competitive technical advantage in the world rail market by using off-line processing equipment that was capable of producing pearlitic rail with enhanced fracture toughness.
Rolls used to finish hot-roll rail, with their massive diameters, give the appearance that they should be extraordinarily rigid and not capable of elastic deflection. In practice, rail going through its finishing mill pass is significantly colder than it was when it went through the first roll stand. The rail’s compressive strength, which must be overcome in each roll pass to effect any plastic deformation, has increased as the rail cools down. It is conceivable in most commercial rail rolling operations, that if the rail going into the final roll pass is below some definable empirical temperature, the last roll pass will spread open too much, and the finishing roll pass will not be able to squeeze the rail hard enough to achieve its acceptable finish section profile.
Deforming a rail into its final section shape at a lower temperature will generate a finer austenitic grain size prior to transformation into pearlite. This is a practical limitation for any existing finishing rolling stand. The rolls’ diameters and the linear distance between the rolls’ bearings determine the finishing roll stand’s squeeze capabilities. Roll diameters can’t be increased too much. However, a final roll stand with minimal linear distance between its bearings will be able to finish roll rail at a lower temperature and produce rail with enhanced fracture toughness.
This article is based on a presentation made at the Wheel/Rail Interaction Seminar, May 2009.
James R. Hornaday, Jr., is President of Alpha Gamma Transform.
1) “Evaluation of Fatigue Crack Initiation at Inclusions in Fully Pearlitic Steels,” C.D. Liu, M.N. Bassim, S. St. Lawrence, Department of Mechanical and Industrial Engineering, University of Manitoba, Winnipeg Manitoba Canada, Material Science and Engineering, A167, (1993), 107-113
2) “Detection of the Onset of Fatigue Crack Growth in Rail Steels Using Acoustic Emission,” M.N. Bassim, S. St. Lawrence and C.D. Liu, Department of Mechanical and Industrial Engineering, University of Manitoba, Winnipeg Manitoba Canada, Fracture Mechanics Vol. 47, No. 2, pp. 207-214.
3) “Dependence of the Fatigue Limit of Rail Steels on Stress Intensity Factor Near Inclusions,” C.D. Liu, M.N. Bassim, S. St. Lawrence, Department of Mechanical and Industrial Engineering, University of Manitoba, Winnipeg Manitoba Canada, Fracture Mechanics Vol. 50, No. 2, pp. 301-307, 1995
4) “The Effect of Spheroidization of the Dry Wear Rates of Standard C-Mn and Cr-Mo Alloy Rail Steels,” D.M. Fegredo, J. Kalousek, Wear of Materials, 1987, ASME, pp. 121-132.
5) “The Effect of Sulfide and Oxide Inclusions on the Wear Rates of Alloyed Rail Steels,” D.M. Fegredo, J. Kalousek, M.T. Shehata, A. Palmer, Proceedings 3rd International Heavy Haul Railway Conference, October 1986, pp. 152-164.
6) “The Effect of Inclusion Type and Control on the Wear of an Alloy Rail Steel,” D.M. Fegredo, M.T. Shehata, A. Palmer, J. Kalousek, in “Strength of Metals and Alloys,” Pergamon Press, 1985, pp. 1536-1568.
7) “Prolonging Rail Life through Rail Grinding,” A.W. Worth, J.R. Hornaday Jr., and P.R. Richards, Proceedings of 3rd International Heavy Haul Railway Conference, Vancouver, 1986, pages 106 to 116.
8) “Evaluation of Weld Rails that were Head Hardened using the Permatrack Off-Line Process,” J.O. Igwemezie, S.L. Kennedy, (Prepared for the Illinois Central Railroad, ARRT Inc. Report No. 044/296, April 1996)
9) “Residual Stress Measurements Volumes 1 & 2,” J.O. Igwemezie, L.L. Kennedy, T.E. Bryan, R.E. King, P.P. Odede, (Prepared for Transportation Development Center, Policy and Coordination, Transport Canada, TP Report No. 11946E, ARRT Inc. Report #4-194, and CIGGT [Queens University] Report No. 93-4 January 1994m 99 pp + appendix)