Patent Application
20170349986
Efficient railroad transportation systems require that railroad rails withstand the demands of high-axle loads, acceleration and deceleration friction and stress, and high usage. As schematically illustrated in the cross-sectional view of
Fracture toughness, or toughness, is a term used in the art to describe steel's resistance to crack propagation. Hardness is a term used in the art to describe steel's resistance to deformation such as surface indentation or abrasion. Yield strength is another term used in the art to describe a material's resistance to deformation. A steel having a high hardness is less prone to wear and abrasion. A steel having a high yield strength is more resistant to deformation under the applied forces of wheels in service. Ideal steel for rail would be one that has a high toughness, a high yield strength and a high hardness.
In its simplest form, steel is composed of a mixture of iron (Fe) and carbon (C) containing less than about 2 wt % carbon. During the production process, the mixture may be cooled from about 1000° C. to below about 723° C. Upon slow cooling, a mixture of iron and carbon with eutectoid composition of approximately 0.8 wt % carbon transforms at or below approximately 723° C. into a structure of alternating lamellae of ferritic iron, and very hard iron carbide, known as cementite (Fe3C). The resulting microstructural morphology of alternating lamellae of ferritic iron and cementite is called pearlite, which is the most common steel microstructure for use in rail. Eutectoid steel with a pearlitic microstructure is characterized as having a high tensile strength and hardness.
An iron and carbon mixture having less than about 0.8 wt % of C results in pearlitic steel that is hypo-eutectoid. That is, when the iron and carbon mixture is cooled from approximately 1000° C. to the equilibrium eutectoid transition temperature at approximately 723° C., some of the mixture transforms into ferrite. At and below approximately 723° C., the remaining iron and carbon can transform into pearlite. Therefore, hypo-eutectoid steel can comprise pearlite and pro-eutectoid ferrite.
Iron and carbon mixtures having more than about 0.8 wt % of C are referred to as hyper-eutectoid. That is, when the iron and carbon mixture is cooled from approximately 1000° C. to approximately 723° C., some of the mixture can transform into cementite. At and below approximately 723° C., the remaining iron and carbon can transform into pearlite. Therefore, hyper-eutectoid steel can be comprised of pearlite and pro-eutectoid cementite. Steel compositions having an increasing amount of wt % of C above about 0.8 wt % will produce steel having an increasing amount of cementite in the pro-eutectoid form as well as more cementite in the pearlite morphology. This tends to produce steel of increasing hardness and decreasing toughness. Hyper-eutectoid pearlitic steel is characterized as being very hard and therefore wear resistant, but less tough and ductile compared to eutectoid and hypo-eutectoid steels.
Since it is desirable for railroad rails to be substantially pearlitic, proeutectoid ferrite and proeutectoid cementite are not desired. Consequently, rail quenching techniques, such as forced air, can be applied to accelerate the cooling rate through the temperature ranges where proeutectoid ferrite or cementite form, thus reducing the amount of proeutectoid ferrite or cementite.
The presence of additional alloying elements in steel can have an effect on the eutectoid equilibrium conditions. For example, the presence of additional alloying elements can affect the eutectoid carbon content and/or the eutectoid transformation temperature. In addition, alloying elements can affect the rate of formation and the structure of pearlite, pro-eutectoid ferrite and pro-eutectoid cementite.
Railroad rail would benefit from being made from steel having both high toughness and high hardness. Increasing amounts of carbon along with alloying agents and adjustment of manufacturing processing parameters have been used in an attempt to retain the toughness of a hypo-eutectoid steel yet increase the hardness. Steels alloyed with manganese, silicon and optionally chromium (e.g., 0.74 to 0.86 wt % C, 0.75 to 1.25 wt % Mn, 0.1 to 0.6 wt % Si and 0.30 wt % max Cr) are used for standard grade rail steel. Rail steel compositions with higher amounts of carbon have been described in U.S. Pat. No. 7,288,159 (Cordova) and U.S. Pat. No. 8,361,246 (Ueda). In addition, rail steels with titanium additions have been described in U.S. Pat. No. 7,217,329 (Cordova). The cooling rate at which the steel is cooled from a high roll-forming temperature through the eutectoid temperature and finally to ambient temperature has a dramatic effect on the formation of the pearlitic structure, in which higher cooling rates produce a finer lamellar structure compared to low cooling rates. A finer structure pearlite is expected to increase yield strength and hardness.
In some aspects, the present disclosure provides steel railroad rails including carbon, manganese, silicon and copper. In other aspects, methods for manufacturing such steel rails are provided. The amount of copper in rail steel is generally limited. However, copper is commonly present in steel scrap used to produce rail steel. The ability to produce useful steel rails including higher than standard amounts of copper can provide a cost advantage by requiring less dilution of lower grade scrap with higher grade scrap, pig iron and/or direct reduced iron. Inclusion of greater than 0.45 wt % to 1%, 1.5% or 2% copper can increase the hardness and yield strength of the steel as compared to a steel including a lesser amount of copper.
In embodiments, the steel portion of the steel rail comprises from 0.65 to 1.1 wt % of C, 0.8 to 1.2 wt % of Mn, 0.26 to 0.80 wt % of Si, from greater than 0.45 to 2.0 wt % of Cu and less than or equal to 0.35 wt % of Cr. In further embodiments, the steel portion of the steel rail comprises from 0.7 to 0.95 wt % of C, 0.8 to 1.2 wt % of Mn, 0.26 to 0.80 wt % of Si, from greater than 0.45 to 1.0 wt % of Cu and less than or equal to 0.35 wt % of Cr. In further embodiments, the steel portion of the steel rail comprises from 0.9 to 1.1 wt % of C, 0.8 to 1.2 wt % of Mn, 0.26 to 0.80 wt % of Si, from greater than 0.45 to 1.0 wt % of Cu and less than or equal to 0.35 wt % of Cr. In additional embodiments, the steel rail comprises less than or equal to 0.25 wt % Ni, less than or equal to 0.050 wt % Mo, from 0.005 to 0.105 wt % Ti, less than or equal to 0.025 wt % S and/or less than or equal to 0.01 wt % Al. In some embodiments, when the carbon content is 0.7 to 0.95 wt % of C, the vanadium content is 0.0 to 0.02 wt %. In some embodiments, when the carbon content is 0.9 to 1.1 wt % of C, the vanadium content is 0.0 to 0.2 wt %. In further embodiments, the steel comprises from 0.6 to 1.0 wt % Cu, from 0.7 to 1.0 wt % Cu, from 0.8 to 1.0 wt % Cu, or from 0.85 to 1.0 wt % Cu. In embodiments, the balance of the steel composition comprises iron and less than 1 wt % additional alloying elements and/or impurities. In some embodiments, at least one additional alloying element present in the composition is present in an amount from 0.01 to 0.25 wt % or 0.01 to 0.20 wt % and is selected from the group consisting of Ni, Mo, Ti or N. In further embodiments, the total amount of impurities or other trace elements is less than 0.2 wt % or less than 0.1 wt %. In some embodiments, elements present in trace amounts include, but are not limited to, phosphorus, sulfur and combinations thereof. In additional embodiments, hydrogen is present in amounts of 2 ppm or less. It is noted that mill scale (e.g., a mixture of FeO and Fe3O4) is present at the surface of the rail in some embodiments; the above composition ranges do not encompass such surface oxides. It is also noted that near surface compositions may differ from the bulk composition due to carbon depletion from the billet, bloom, or ingot form of the steel prior to roll forming from furnace atmospheric chemical reactions with the steel depleting the near surface carbon content.
In embodiments, the steel portion of the rail is characterized by a substantially pearlitic microstructure. In examples, the microstructure of the steel is fully pearlitic or comprises less than 10% pro-eutectoid ferrite or pro-eutectic cementite. In additional examples, ferrite regions of the pearlite microstructure include precipitates (e.g., copper containing precipitates). Inclusions (e.g., sulfides and oxides) that result from the steelmaking process may also be present in the steel rail. In some examples, the interlamellar spacing is less than 500 nm as measured at a depth of 6 mm from the running surface or less than 300 nm as measured at a depth of 6 mm from the running surface.
In some embodiments, the ultimate tensile strength of the steel rails is from 1170 MPa to 1725 MPa or 1375 MPa to 1450 MPa. In additional embodiments, the yield strength of the steel rail is from 850 MPa to 1600 MPa or 925 MPa to 1000 MPa. In an embodiment the yield strength is an 0.2% offset yield strength. In further embodiments, the hardness of the steel can decrease with distance from the rail surface. In some embodiments, the hardness (as measured from a polished cross-section) 2 mm below the top surface of the rail surface (e.g., 14 in
In some embodiments, the present disclosure provides methods for manufacturing a steel rail, the methods comprising the steps of:
preparing a steel comprising the elements in a range from 0.65 to 1.1 wt % of C, from 0.8 to 1.2 wt % of Mn, from 0.26 to 0.80 wt % of Si, from greater than 0.45 to 1.0 wt % of Cu and from less than or equal to 0.35 wt % of Cr, wherein the balance of the steel is composed of Fe and less than 1 wt % additional alloying elements and impurities;
hot rolling the steel to have a rolling finishing temperature in a range from 850° C. to 1000° C. and thereby forming a rail; and
cooling the rail at a selected cooling rate in a range from 0.1° C./sec to 20° C. /sec beginning substantially at said rolling finishing temperature and continuing at least until 600° C.
In further embodiments, the steel composition is as described for the steel rail above. For example, the steel comprises from 0.7 to 0.95 wt % of C, 0.8 to 1.2 wt % of Mn, 0.26 to 0.80 wt % of Si, from greater than 0.45 to 1.0 wt % of Cu and less than or equal to 0.35 wt % of Cr. In further a further example, the steel portion of the steel rail comprises from 0.9 to 1.1 wt % of C, 0.8 to 1.2 wt % of Mn, 0.26 to 0.80 wt % of Si, from greater than 0.45 to 1.0 wt % of Cu and less than or equal to 0.35 wt % of Cr.
In further embodiments, the steel composition is as described for the steel rail above. For example, the steel comprises from 0.7 to 0.95 wt % of C, 0.8 to 1.2 wt % of Mn, 0.26 to 0.80 wt % of Si, from 0.5 to 1.0 wt % of Cu and less than or equal to 0.35 wt % of Cr. In a further example, the steel portion of the steel rail comprises from 0.9 to 1.1 wt % of C, 0.8 to 1.2 wt % of Mn, 0.26 to 0.80 wt % of Si, 0.5 to 1.0 wt % of Cu and less than or equal to 0.35 wt % of Cr.
In some embodiments, the steel is in the form of a bloom or billet prior to hot rolling. In some embodiments, the bloom or billet is heated before being passed to the rolling mill(s). In further embodiments, the hot rolling process involves multiple passes of hot reduction. In embodiments, the total rolling reduction is from 85 to 95%. In embodiments, the cross-sectional area of the bloom or billet prior to rolling exceeds value of 8 times the cross-sectional area of the finished rail. The hot rolling process involves a finishing stage; in embodiments the temperature during this finishing stage, known as the rolling finishing temperature, is from 800° C. to 1100° C. After hot rolling, the steel rail is cooled to develop a pearlitic or substantially pearlitic microstructure. In embodiments, the endpoint of the controlled cooling process is a temperature above ambient or room temperature at which the transformation of the microstructure to pearlite is complete or nearly complete. This temperature may be termed the pearlite transformation completion temperature. In embodiments, the rail is cooled from the rolling finishing temperature to below 600° C. In embodiments, the cooling rate is 0.1° C./sec to 20° C./sec or 1° C./sec to 10° C./sec.
In the accompanying drawings, like reference numerals designate like parts.
In some embodiments, the amount of a given non-pearlitic phase is specified to be less than a certain amount. In an embodiment, the amount of a given phase may be determined from a polished cross-section of the sample. As an example, the amount of the phase is determined from area percentage of the phase in the polished cross-section.
In some embodiments, the range of chemical components for steel is as follows:
In embodiments, the amount of carbon (C) is from 0.65 to 1.1 wt %, from greater than 0.65 to 1.1 wt %, from 0.7 to 1.1 wt %, from 0.7 to 0.95 wt %, from 0.9 to 1.1 wt % or from greater than 0.9 to 1.1 wt % of C. Carbon, as explained above, contributes to the hardness of the steel. The amount of carbon directly determines if the steel will have hypo-eutectoid properties (i.e., pearlite with ferrite), eutectoid properties (i.e., pearlite only), or hyper-eutectoid properties (i.e., pearlite with cementite). The higher the amount of carbon, the steel is generally of higher hardness, but undesirable pro-eutectoid cementite, which reduces ductility and toughness, is more difficult to control.
In some embodiments, the amount of manganese (Mn) is from 0.8 to 1.2 wt %. Manganese, like silicon, is used to deoxidize the steel matrix. Further, manganese improves the steel's hardness. As the amount of manganese is increased, the manganese can segregate from the matrix, which is detrimental to the resulting steel's toughness.
In some embodiments, the amount of silicon (Si) is from 0.26 to 0.8 wt %. Silicon is used to deoxidize the steel matrix and improves the strength of the resulting steel. An amount of silicon approaching 1.0 wt %, in conjunction with the other specified alloying levels, is predicted to result in non-pearlitic microstructures.
In some embodiments, the amount of copper (Cu) is from greater than 0.45 to 1.0 wt %, from 0.6 to 1.0 wt %, from 0.7 to 1.0 wt %, from 0.8 to 1.0 wt %, from 0.85 to 1.0 wt %, from 0.7 to 0.9 wt %, from greater than 0.45 to 1.5 wt %, from 0.6 to 1.5 wt %, from 0.8 to 1.5 wt %, from greater than 0.45 to 2 wt %, from 0.6 to 2 wt %, or from 0.8 to 2 wt %.
In some embodiments, the amount of copper (Cu) is from 0.5 to 1.0 wt %, from 0.6 to 1.0 wt %, from 0.7 to 1.0 wt %, from 0.8 to 1.0 wt %, from 0.85 to 1.0 wt %, from 0.7 to 0.9 wt %, from 0.5 to 1.5 wt %, from 0.6 to 1.5 wt %, from 0.8 to 1.5 wt %, from 0.5 to 2 wt %, from 0.6 to 2 wt %, or from 0.8 to 2 wt %.
In some embodiments, the amount of chromium (Cr) has an upper limit. In further embodiments, the amount of chromium is less than or equal to about 0.35 wt %. Chromium can improve the strength of the resulting steel by making the lamellae of the pearlite thinner.
In some embodiments, the amount of nickel (Ni) is less than or equal to 0.25 wt %.
In some embodiments, the amount of molybdenum (Mo) is less than about 0.050 wt %. Molybdenum in a quantity up to 0.050 wt % is utilized for the hardenability characteristics of the resulting alloy.
In some embodiments, the amount of titanium (Ti) is from 0.005 to 0.105 wt %. Titanium is used to control austenitic grain growth in the hot rolling process, providing a finer grain size in the final product.
In some embodiments, the amount of vanadium (V) is up to 0.02 wt % or up to 0.20 wt %. As examples, when the carbon content is 0.7 to 0.95 wt %, the vanadium content is 0.0 to 0.02 wt % or when the carbon content is 0.9 to 1.1 wt % of C, the vanadium content is 0.0 to 0.2 wt %. Vanadium improves the hardness and strength of the resulting steel.
In some embodiments, the amount of aluminum (Al) is less than or equal to 0.01 wt %.
In some embodiments, the amount of sulfur (S) is less than or equal to 0.025 wt %. Sulfur is an inevitable impurity that is detrimental to the toughness of the resulting steel. It has been determined that as much as 0.025 wt % of S is acceptable for the steel of this invention, and therefore, is used as an upper limit.
In some embodiments, the amount of phosphorus (P) is less than about 0.025 wt %. Phosphorus is an inevitable impurity that is detrimental to the toughness of the resulting steel. It has been determined that as much as 0.025 wt % of P is acceptable for the steel of this invention, and therefore, is used as an upper limit.
In some embodiments, the amount of hydrogen (H) is less than or equal to 3 ppm, less than or equal to 2 ppm, less than or equal to 1.5 ppm or less than or equal to 1 ppm.
There are four predominant production methods used in the art to cool rail. They are air cooling, air/water cooling, oil submersion, and aqueous polymer submersion. Any method may be used in the present invention as long as the prescribed controlled rate of cooling is obtained.
The air/water cooling technique presents a mixture of air and water to the rail, cooling the rail in a dual process of heat of vaporization of the water and forced convection of the air. The cooling rates can be adjusted by controlling the air and water temperatures and flowrates, the form of the water such as droplets, mist or fog, and the timing and duration of the exposure to the quench media.
The oil submersion technique is where the rail is submerged into a tank of oil. The cooling rates can be adjusted by controlling the oil temperature, oil formulation, fluid flow and agitation, and duration of the immersion step or steps.
The aqueous polymer submersion technique is where the rail is submerged into a tank of aqueous polymer. The aqueous polymer has a high vaporization temperature effectively preventing boiling at the rail surface and producing a more uniform cooling environment. The cooling rates can be adjusted by controlling the polymer formulation and concentration, controlling fluid flow and agitation, and duration of the immersion step or steps.
The air cooling technique uses pressurized gas, traditionally atmospheric air but not limited to air, to cool the rail thru conduction of the rail heat to the gas. The cooling rates can be adjusted by controlling the air temperature, flow rates, and duration of the exposure to the cooling media.
In one embodiment in accordance with the method of manufacturing the rail, controlled-rate in-line forced-air cooling is performed. In-line cooling consists of cooling the rail immediately after it is rolled. This is in contrast to re-heating previously cooled rail to the desirable temperature at a different location off of the rolling line and cooling it using the desired cooling rate. In-line cooling is preferable in terms of manufacturing efficiency and achieving mechanical properties in the head of the rail.
In some embodiments, steel having the composition as described above is roll-formed at a temperature range of approximately 800-1200° C. to a net shape of the finished rail, in accordance with known roll-forming techniques. The roll-formed rail enters a heat treating unit which controls the cooling rate of the rail, and is otherwise known as a head hardening unit or line-slack quench (LSQ) unit. The rail is cooled at a controlled rate in a range from 0.1° C./sec to 20° C./sec using a forced air quench operation. The rail is cooled at this rate until the rail reaches a desired temperature. In embodiments, rail is cooled at this rate until the rail reaches a temperature below 600° C.
A head hardening unit apparatus suitable for use in the manufacture of rail in accordance with the present invention comprises a conveyor and an air-handling system. Rail is placed into the air cooling position with the conveyor and is then heat-treated (cooled) with air. The air-handling system comprises a series of nozzles strategically placed around the rail from which air is discharged under pressure. The air handling apparatus controls the cooling rate of the rail by controlling the air pressure. After heat-treatment, the rail is moved out of position in the head hardening unit to the next processing step.
Every formulation or combination of components described or exemplified can be used to practice the invention, unless otherwise stated. Specific names of compounds are intended to be exemplary, as it is known that one of ordinary skill in the art can name the same compounds differently. When a compound is described herein such that a particular isomer or enantiomer of the compound is not specified, for example, in a formula or in a chemical name, that description is intended to include each isomers and enantiomer of the compound described individual or in any combination. One of ordinary skill in the art will appreciate that methods, device elements, starting materials and synthetic methods other than those specifically exemplified can be employed in the practice of the invention without resort to undue experimentation. All art-known functional equivalents, of any such methods, device elements, starting materials and synthetic methods are intended to be included in this invention. Whenever a range is given in the specification, for example, a temperature range, a time range, or a composition range, all intermediate ranges and subranges, as well as all individual values included in the ranges given are intended to be included in the disclosure.
As used herein, “comprising” is synonymous with “including,” “containing,” or “characterized by,” and is inclusive or open-ended and does not exclude additional, unrecited elements or method steps. As used herein, “consisting of” excludes any element, step, or ingredient not specified in the claim element. As used herein, “consisting essentially of” does not exclude materials or steps that do not materially affect the basic and novel characteristics of the claim. Any recitation herein of the term “comprising”, particularly in a description of components of a composition or in a description of elements of a device, is understood to encompass those compositions and methods consisting essentially of and consisting of the recited components or elements. The invention illustratively described herein suitably may be practiced in the absence of any element or elements, limitation or limitations which is not specifically disclosed herein.
The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments and optional features, modification and variation of the concepts herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention as defined by the appended claims.
In general the terms and phrases used herein have their art-recognized meaning, which can be found by reference to standard texts, journal references and contexts known to those skilled in the art. The following definitions are provided to clarify their specific use in the context of the invention.
All references cited herein are hereby incorporated by reference to the extent not inconsistent with the disclosure herewith. All patents and publications mentioned in the specification are indicative of the levels of skill of those skilled in the art to which the invention pertains.
Although the description herein contains many specificities, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of the invention. For example, thus the scope of the invention should be determined by the appended claims and their equivalents, rather than by the examples given.
The invention may be further understood by the following non-limiting examples.
Six alloys with copper levels varying from 0.07 percent by weight or (wt %) to 0.85 wt % were produced to the same rail section following the same processing procedures used for all rail production at Evraz Pueblo. Processing consisted of heating the round cast blooms with dimensions of 311 mm (12.25 in) diameter by 5.89 m (19.33 ft) length in a natural gas walking beam re-heat furnace, a total rolling reduction of 88.7% of hot reduction to the final nominal dimensions shown in
Tables 1A and 1B show the chemical composition of selected copper containing alloys in weight percentage except as noted.
Full head specimens of 22.2 mm (0.875 in) thickness were sectioned from the 7 Cu, 38 Cu, and 85 Cu alloys according to the schematic in
Tensile properties were measured on samples machined from 254 mm (10 in) lengths of full rail cross-section. Longitudinal tensile specimens with a gage diameter of 8.75 mm (0.350 in) were sectioned from the gauge corner of the head (
Five tensile specimens per alloy were prepared and tested.
Charpy impact specimens were tested at room temperature to assess the influence of copper on the dynamic fracture behavior of the steels. Eight specimens per alloy were prepared for testing at room temperature. Due to the inherently high strength and fully pearlitic microstructure of the alloys evaluated, a 2 mm U-notch was employed rather than the typical 2 mm V-notch or 5 mm U-notch as referenced in ASTM-E23. The Charpy blanks were sectioned out of the gauge corner and machined to 10 mm×10 mm×55 mm with the long dimension parallel to the rolling direction. A 1 mm radius U-notch, parallel to the transverse direction, on the surface closest to the running surface, was introduced by broaching. The specimens were tested on a 406.7 J (300.0 ft-lbf) capacity Charpy impact test frame at an ambient temperature of 22.7° C. (72.9° F.) and 80% relative humidity. Energy absorbed during the test was recorded and fracture surfaces were evaluated visually for percent shear.
Table 2 summarizes the observed average room temperature (22.7° C.) Charpy U-notch (2 mm) impact energies for the six experimental alloys along with the calculated standard deviations based on eight replicates for each condition. Measured impact toughness values ranged between 9.0 J and 10.8 J and the data do not exhibit a relationship with Cu content. The impact toughness values indicate that all specimens fractured in a brittle manner, a conclusion that is consistent with analysis of the fracture surfaces. All samples exhibited brittle fracture without the presence of discernable shear lips.
Table 3 shows average K1C Fracture Toughness for the alloys listed. The 7 Cu alloy had the highest average K1C at 38.6 MPa Im while the 38 Cu alloy had the lowest average K1C at 34.2 MPa √m. This difference is most likely the result of normal variation experienced when testing the same grade of steel. The data do not demonstrate that Cu alloying influences the fracture toughness of the material.
Fatigue crack growth rate was also assessed. The results indicated that the copper level had no discernable influence on fatigue crack growth rate.
Dry, twin-disc wear testing was also performed on 7 Cu, 38 Cu and 85 Cu alloys. A summary of the results is displayed in
Surface roughness measurements were also obtained on three wear samples of each of the three alloys. The study was performed to reveal any difference in the depth of gouging or the size of flaking that is contributing to the difference in wear rate. Samples from all three copper containing steels show flaking on the surface. The average surface roughness (Ra) was from 1 mm to 3 mm and the maximum peak-to-valley height (Rt) was from approximately 20 μm to 140 μm. The surface roughness of the samples was comparable between the three alloys.
The influence of Cu on the anticipated performance of the rail was simulated through rolling-sliding contact fatigue (RCF) testing on a twin-disc tribometer. The 0.07-Cu, 0.38-Cu, and 0.85-Cu materials were selected for the twin-disc testing.
Rail samples were machined from the head of the rail. The samples were then ground to a 1.77 in. (45 mm) diameter with a 0.394 in. (10 mm) wide running surface and a 0.984 in. (25 mm) crown radius. The rail samples were tested against a 1.819 in. (46.2 mm) diameter, 0.787 in. (20 mm) wide cylindrical wheel sample made from quench and tempered 4140 with a hardness of approximately 50 HRC.
An industrially available top-of-rail friction modifier was applied at the point of contact of the rail and wheel samples by a time controlled micro pump as a means of controlling the friction and preventing wear that would inhibit the formation of RCF cracks. The use of the friction modifier resulted in a coefficient of traction of approximately 0.1.
The RCF tests were conducted under constant load conditions, where the force between the rail and the proxy wheel samples resulted in a nominal maximum Hertzian contact pressure, P0, of between 406 and 493 ksi (2800 and 3400 MPa). The RCF tests were performed with a 20% slip ratio, which is the ratio of the difference between the surface velocities to the average of the surface velocities, where the wheel was driving and the rail was braking (wheel velocity>rail velocity). The tests were monitored by use of an eddy current sensor. The tests were terminated when the signal from an eddy current probe surpassed a threshold value, which was established with a machined reference flaw. If the sample achieved a 1,000,000 cycle count without the eddy current signal exceeding the signal threshold the test was arrested as a runout.
An example of the contact surface after testing is shown in
The conformal contact pressure versus cycles to failure RCF results are shown in
This application claims the benefit of U.S. provisional application 62/346,836, filed Jun. 7, 2017 which is incorporated by reference herein in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 62346836 | Jun 2016 | US |
