Interface - The Journal of Wheel/Rail Interaction

RAIL MATERIALS

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

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.

Rolling Procedures
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.

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

This article is based on a presentation made at the Wheel/Rail Interaction Seminar, May 2009.

References
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)

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