Patent Application
20110189047
1. Field of the Invention
The present invention generally relates to railroad rail steels. More particularly, it is concerned with those railroad rail steels that are specifically alloyed to resist fatigue effects including rolling contact fatigue (RCF) and shelling in the head regions of such rails. The term “shelling” generally refers to loss of steel material as a result of deterioration arising from mechanical stresses. In the context of this invention, the term shelling is often contrasted with the term “spalling.” Spalling generally refers to loss of steel material as a result of metallurgical damage created by excessive heat that arises from the sliding of railroad wheels over railroad rails during extreme train braking operations. Since shelling and spalling often occur in conjunction they are often collectively referred to as “thermo-mechanical deterioration.”
Various problems arise from each form of rail head material loss. For example, when railway rails experience thermo-engendered deterioration, surface cracks tend to propagate from such deteriorated areas and cause potentially dangerous defects in contiguous regions of the rail. Similar surface cracks are also created as a result of mechanically generated deterioration. Aside from their catastrophic accident causing potentials, such rail defects are also known to cause increased rail/wheel dynamic forces that, in turn, tend to produce consequential damage such as accelerated track deterioration. The railroad industry is therefore constantly looking for ways to minimize both aspects of thermo-mechanical deterioration of rails while still preserving, as far as possible, their wear resistance qualities.
2. Description of the Prior Art
Railroad rails eventually wear out as a result of normal usage. Such rails are however often prematurely retired from service as a result of various forms of thermo-mechanical deterioration. For example, a great deal of thermo-mechanical deterioration is associated with metallurgical transformations of the rail steel from the original, relatively tough, pearlitic microstructure to more brittle microstructures such as bainite and/or martensite—with associated loss of the austenite/bainite/martensite steel material through spalling. Again, thermo-mechanical deterioration is caused by the heat generated by friction when the train's wheels skid on railroad rails during extreme braking operation. That is to say that the above noted brittle steel materials are produced in rails when such frictional heat is sufficient to raise the temperature of the head region to the austenite transformation level. Upon rapid cooling, this austenite phase is then transformed to bainite and, in many cases, to martensite (with different level of retained austenite). The localized rail heating can occur in time periods as short as one second; indeed it can occur in time periods of less than one thousandth of a second. Thereafter, because the rest of the rail serves as a heat sink, such very high local temperatures are then quickly lowered. A process referred to as quenching. Thus, in skid producing braking situations, local areas of the rail head top surface are rapidly transformed to austenite, as the steel material rapidly heats—and then to bainite and/or martensite as the steel rapidly cools.
Those skilled in railroad rail manufacturing arts will appreciate that a martensite transformation progresses only while the steel is cooling rapidly (that is to say that more and more discrete volumes of a parent austenite solid solution transform as the steel cools). Martensite transformations can be prevented if the cooling process is interrupted at a temperature above the so-called “Start Martensite” temperature. Moreover, the amount of martensite formed per degree of decrease in temperature is not a constant (i.e., the number of martensite crystalline units produced at first is small, but increases exponentially as the temperature continues to decrease). In any case, the resulting brittle martensite steel then tends to crack and spall away from the rail head surface.
The mechanical aspects of thermal-mechanical deterioration are often referred to as “rolling contact fatigue” (or RCF). The present invention is particularly concerned with minimizing RCF in railway rails. Again, RCF produces the undesired form of steel material loss known as “shelling” wherein the rolling action of a steel railroad wheel over a steel rail produces mechanical stresses in the rail that—in their own right—contribute to a rail's deterioration. That is to say that rolling contact fatigue can occur even if the rail does not experience metallographic changes attributed to temperature effects. Rolling contact fatigue is also associated with diminished shear fatigue strength of a rail's head surface. This form of damage is usually considered in conjunction with the level of subsurface shear stress being applied to the rail head, especially in the region just below the rail head's wheel contact surface. In any case, rolling contact fatigue is related to both the strength of the rail surface and to the load applied to it. And, as previously noted, the strength of the rail head surface steel is also related to its hardness.
Modern railroad rails are being called upon to carry out increasingly severe duties. For example freight car wheels frequently subject the rails over which they travel to local contact stresses in excess of 160,000 p.s.i. The relatively high loads carried by the rails lead directly to higher levels of rolling contact fatigue. It should also be borne in mind that modern railroad rails must be made from relatively hard steels in order to impart acceptable wear life characteristics. The use of hard steels notwithstanding, the incidence of shelling type defects in railroad rails is increasing as a result of the greater loads they are currently called upon to carry.
And as previously discussed, if a rail is heated to high enough temperatures, the stresses produced therein can exceed the yield strength of that rail steel. Moreover, when such rails cool down, residual tensile stresses may remain within the rail and subsequently serve to further open any surface cracks that may be present. Those skilled in the rail manufacturing arts will also appreciate that the phenomenon of shelling due to rolling contact fatigue is much more pronounced in rails residing on long and steep grades, e.g., in mountainous regions, where a train's brakes are much more heavily employed.
Again, those skilled in the rail manufacturing arts will appreciate that RCF in rails often occurs in intimate conjunction with the thermo aspects of thermo-mechanical deterioration. For example, elevated temperatures in a steel rail serve to reduce its ability to resist mechanical loading owing to the steel's diminished mechanical strength above certain temperatures. Moreover, the longer a steel rail experiences elevated temperatures, the greater the degree of shelling that will result from this time related circumstance. Thus, in formulating rail steels resistant to wear, thermo deterioration and/or mechanical deterioration, one must always appreciate that these phenomena are often intimately related.
Ideally, steels from which railway rails are made will, simultaneously, have high levels of the three general properties previously described. That is to say that such steels would be highly wear resistant, highly resistant to thermo-generated deterioration and highly resistant to mechanical deterioration resulting from rolling contact fatigue. Unfortunately, to varying degrees, these properties range from being metallurgically antagonistic to being metallurgically incompatible. For example, increased hardness in a steel usually implies decreased resistance to thermo-generated deterioration. Conversely, when a steel is alloyed to be more resistant to thermo-generated deterioration, this usually implies that the steel will be less hard, and hence, inherently less wear resistant.
The ability of a given alloying element to create and/or stabilize certain metallographic phases is of great importance. Indeed, many steel alloying elements are categorized around this concept. For example, nickel and manganese are often referred to as austenite-forming elements. Chromium, silicon, molybdenum, tungsten and aluminum are frequently referred to as ferrite-forming elements. Another group of elements known as carbide-forming elements includes chromium, tungsten, molybdenum, vanadium, titanium, niobium, tantalum and zirconium. In most cases however, any given desired resistance to thermo-mechanical deterioration through the use of alloys must be considered in the context of the degree of sacrifice of a steel's pearlitic structure that will be the result of the specific alloying elements employed. This remains a very important consideration because a pearlitic microstructure serves to impart the quality of wear resistance to steel railroad rails.
It has been long known that liquid steels, including those used to make railroad rails, can be further purified by exposing them to subatmospheric pressure (commonly referred to as a “vacuum”). In effect the presence of such vacuum conditions serves to remove dissolved gasses formed during chemical reactions of various elements in molten steels. Applicants' degassing processes are specifically directed at diffusing and removing various non-metallic inclusions such as manganese sulfide (MnS) and aluminum oxide (Al2O3) from the molten steel—and thereby precluding their presence in the solidified rail steel. A liquid steel under vacuum conditions forces the gas density flux to flow down the concentration gradient towards the vacuum. Ultimately, this serves to reduce the porosity of Applicants' solidified rails.
The most commonly used molten steel degassing systems generally fall into three categories—recirculating degassers, tank degassers and stream degassers. Recirculating degassers insert two snorkels into a ladle of liquid steel. The steel in the ladle is drawn into a vacuum chamber wherein argon is injected to promote turbulence. The molten steel is then exposed to vacuum conditions in order to remove undesired gases. The degasified molten steel is then recirculated back into the ladle via the second snorkel. Representative recirculation degassers are disclosed in U.S. Pat. Nos. 2,893,860 and 3,099,699. Tank degassers are vessels into which the ladle is sent and stirred by argon injection. The chamber is then depressurized to remove the undesired gas. Thereafter, the ladle is removed from the vessel. Representative tank degassers are disclosed in U.S. Pat. Nos. 1,131,488 and 2,993,780.
The technical and patent literature reveals that many alloying materials have been added to (or, in the case of carbon, taken from) a host of railroad rail steel formulations for the purposes of striking a balance between imparting hardness (and hence wear resistance) to a given steel while imparting, as far as possible, resistance to thermo-mechanical deterioration. By way of general example only, it is well known that in situations where wear resistance is the more desirable property in a rail, high carbon steels having carbon contents ranging from about 0.73 to about 1.0 weight percent are preferred. Such steels are especially hard and, hence, relatively wear resistant. Such steels are not, however, particularly resistant to thermo-mechanical deterioration. Conversely, it is also well known that medium carbon steels having carbon contents ranging from approximately 0.45 to 0.55 weight percent are more resistant to thermo-mechanical deterioration than harder steels, but they are generally less wear resistant. It is also common knowledge that virtually all other steel alloying elements (other than carbon) tend to produce decreased wear resistance in railroad rails as the concentrations of such elements are increased.
The literature also shows that it has been a long standing custom to consider steel alloying elements in terms of the properties they confer upon a steel (e.g., chromium makes a steel hard, nickel and manganese make it tough, and so on). However, it also should be appreciated that some of these custom based statements can lead to certain misunderstandings. For example, when a statement to the effect “chromium makes a steel hard, and hence, wear-resistant,” is encountered, one should realize that author of such a statement probably has in mind a steel having a relatively high (e.g., 1.2%) carbon concentration and a relatively high (e.g., 2.0%) chromium concentration. If, however, a steel contained the same 2.0% chromium concentration—but only a 0.10% carbon concentration—the hardness of that steel would be considerably lower than that of the 1.2% carbon, 2.0% chromium steel. Similarly, if a statement to the effect that “manganese makes a steel tough” is encountered, one should realize that the author of such a statement probably has a steel with a high (e.g., 13%) manganese concentration in mind because, in fact, steels containing lower manganese concentrations (e.g., 1.0% to 5.0% manganese), especially in conjunction with other alloys, can have relatively higher levels of toughness.
This all goes to say that the wear resistance versus thermo-mechanical resistance problem has a persistent dilemmatic quality that continues to thwart the railroad industry's attempts to extend the useful life of railway rails. It also should be noted that railroad rail designers have long accepted that thermo-generated deterioration is the more intractable aspect of the wear resistance versus thermo-mechanical deterioration resistance dilemma. Aside from economic considerations, this acceptance generally follows from the fact that normal rail wear is somewhat predictable, and gradual, in nature. Conversely, heat-producing railroad wheel skids over such rails are relatively unpredictable. Worse yet, thermo-generated deterioration tends to produce damage that is much more immediate and much more severe in nature. Nonetheless, most railway rail steel compositions are still designed toward trying to (for economic reasons) satisfy railroad industry requirements for greater wear resistance, while “silently” conceding that thermo deterioration due to railroad wheel skids, and/or mechanical deterioration in its own right, will be dealt with by: (1) physically machining the rail head region on a scheduled basis, or (2) by machining heavily spalled rails on an “as needed” basis, or (3) by simply scrapping heavily damaged rails.
Thus far, alloying practices have been of somewhat limited value in dealing with the wear resistance vs. thermo-mechanical deterioration dilemma. For example, even though the constitution of three component steels can theoretically be deduced from ternary phase diagrams, they are often rather difficult to interpret. Their practical value is also limited by the fact that they only describe equilibrium cooling conditions. Therefore, since most modern railroad rail steels are both heat treated during their manufacture and contain more than three alloying components, much more complex graphing methods (e.g., Temperature Time Transformation diagrams) must be employed and interpreted—thus far with varying degrees of success as far as railroad rail steels are concerned.
Indeed, it seems fair to say that even though modern steel metallurgy is a highly skilled science, it nonetheless has certain elements of empiricism in many circumstances wherein even relatively minor changes in the identity and/or relative concentrations of any given alloying element can potentially make very significant changes in the resulting properties of a given steel. Further complexities arise from various heat treatment processes to which steels are usually exposed. These competing considerations are very nicely summarized by Dr. Edgar C. Bain on page 4 of his now somewhat dated, but still very highly regarded, work on this subject: “Functions of the Alloying Elements in Steel.” There, he said:
This concession to empiricism in the steel making arts has not changed much over the years since Dr. Bain's seminal work was published. For example, in discussing the alloying of steels, the Encyclopedia Britannica Online makes a much more up-to-date concession to steel alloying empiricism with the statement:
The patent literature reflects the railway industry's continued attempts to deal with the wear resistance vs. rolling contact fatigue. For example, U.S. Pat. No. 4,575,397 describes a wear resistant steel railroad rail comprising 0.50 to 0.85 wt. % carbon, 0.10 to 1.00 wt. % silicon, 0.50 to 1.50 wt. % manganese, less than 0.035 wt, % phosphorus, less than 0.035 wt. % sulfur, and less than 0.050 wt. % aluminum.
U.S. Pat. No. 4,426,236 discloses a high strength steel rail containing 0.65 to 0.85% C, 0.50 to 1.20% Si, 0.50 to 1.50% Mn, 0.005 to 0.050% Al, 0.004 to 0.050% of one or both of Nb and Ti, with the balance being iron and unavoidable impurities. These rails are designed to have a surface layer (to a depth of 10 mm or more) that has a fine pearlite structure with a tensile strength of more than 120 kg/mm2. They also exhibit a reduction of area of more than 40% and a surface hardness at the surface layer of HV 350 or more. Moreover, this rail steel is particularly well suited to welding operations.
U.S. Pat. No. 5,759,299 describes a bainitic railroad rail steel containing 0.15 to 0.45 wt % of C, 0.05 to 1.0 wt % of Si, 0.1 to 2.5 wt % of Mn, 0.03 wt % or less of P, 0.03 wt % or less of S, 0.1 to 3.0 wt % of Cr, 0.005 to 2.05 wt % of Mo, with the balance being iron and incidental impurities. This steel is particularly characterized by its resistance rolling contact fatigue.
U.S. Pat. No. 5,676,772 describes a high-strength bainitic steel having 0.2 to 0.5 wt % C, 0.1 to 2.0 wt % Si, 0.3 to 4.0 wt % Mn, 0.035 wt % or less of P, 0.035 wt % or less S, and 0.3 to 4.0 wt % Cr, with the balance being Fe. This steel is particularly characterized by its damage resistance properties.
U.S. Pat. No. 5,711,914 discloses a rail steel consisting of 0.5 to 0.75% carbon, 0.10 to 0.50% silicon, greater than 0.90 and up to 1.70% manganese, less than 0.025% aluminum, not more than 0.05% phosphorus, a tellurium content of less than 0.004%, and a sulfur content such that the tellurium/sulfur ratio is from 0.1 to 0.6, with the balance being iron and incidental impurities.
U.S. Pat. No. 5,762,723 describes a pearlitic rail steel comprising 0.85 to 1.20 weight % carbon. This steel is further characterized by having a pearlite lamella spacing of not more than 100 mm, and a ratio of cementite thickness to ferrite thickness of at least 0.15.
U.S. Pat. No. 5,830,286 discloses a wear resistant steel containing 0.85 to 1.20% C, 0.10 to 1.00% S, 0.40 to 1.50% Mn, 0.0005 to 0.0040% B, with the balance being iron. The rail head region is hardened to a depth of at least 20 mm and exhibits a pearlitic structure having a hardness of at least Hv 370. The range in the hardness within this depth is not more than Hv 30.
U.S. Pat. No. 5,879,474 describes a method of producing a wear and rolling contact fatigue resistant bainitic rail steel. The method comprises of the hot rolling steps the steel composition by weight % includes 0.05 to 0.50% carbon, 1.00 to 3.00% silicon and/or aluminum, 0.50 to 2.50% manganese, and 0.25 to 2.50% chromium with the balance being iron. The steel can be continuously cooled from its rolling temperature in air, or by accelerated cooling techniques.
U.S. Pat. No. 6,254,696 describes a bainitic steel rail containing, by weight %, 0.15 to 0.45 percent carbon, 0.10 to 2.00 percent silicon, 0.20 to 3.00 percent manganese, and 0.20 to 3.00 percent chromium, with the remainder consisting of iron. This steel is particularly characterized as having excellent resistance to surface fatigue failures.
U.S. Pat. No. 6,406,569 describes a method for thermal treatment of the rail's head region that involves immersing a rail head (at an initial temperature of about 720.degree.C.) in a cooling agent that contains a synthetic cooling additive, and then withdrawing the rail head from the cooling agent upon obtaining a head surface temperature between 450 and 550.degree.C. without temperature equalization over the entire cross-section of the rail head.
U.S. Pat. No. RE40,263 describes a pearlitic rail steel having improved wear resistance and comprising of 0.85 to 1.20 wt. % C. This steel is further characterized by the fact that its structure is pearlitic and it has a lamella spacing that is not more than 100 nm. The ratio of cementite thickness to ferrite thickness in the pearlite is at least 0.15.
U.S. Pat. No. RE40,263 describes railroad rails with 0.85 to 1.20 wt. % C, 0.10 to 1.00 wt. % Si, 0.40 to 1.50 wt. % Mn and if necessary, at least one member selected from the group consisting of Cr, Mo, V, Nb, Co and B. The head portion of this rail is rapidly cooled at a rate ranging from 1 degree to 10 degree C./sec from an austenite boundary temperature to a cooling stop temperature of 700 degrees to 500 degrees C. The hardness of the head portion of such rails is at least Hv 320. This steel is especially characterized by its wear and damage resistance qualities.
In closing their comments concerning the prior art concerning steel railroad rails, applicants would say that even though a great deal has been learned about rolling contact fatigue in the rail head, the fact remains that such damage mechanism contributes in a significant way to the accelerated wear of rails. Indeed, rolling contact fatigue problems are becoming more and more pronounced as rails are utilized to carry heavier and heavier loads as well as more tonnage of traffic. It is therefore an important object of this invention to provide steels for railway rails that have increased resistance to rolling contact fatigue (and hence shelling) by virtue of their alloy formulations—without unduly sacrificing their wear resistant and thermo deterioration resistant qualities. Moreover, certain added advantages can be imparted to these particular railroad rail steels by various thermo-mechanical processes hereinafter more fully discussed.
Steel alloys characterized by their virtually fully-pearlitic, hypo-eutectoid microstructures having the alloying formulations given below can be used to make railway rails that are particularly resistant to rolling contact fatigue—and hence shelling. It also should be understood that various treatments of the molten forms of these steels (e.g., degassing) and their solid bloom forms (e.g., thermo-mechanical processes) may be employed during the manufacture of such rails in order to improve their metallurgical properties. The degassing treatments of the molten steels may include recirculation degassing, tank degassing and/or stream degassing in ways known to this art. The thermo-mechanical processes that may be applied to the bloom forms of these steels may include forging, quenching, hot working, cold working and the like. It also should be appreciated that these thermo-mechanical processes may be more specifically directed at the top surface (“head”) regions of Applicants' railroad rails.
The steel rails of this patent disclosure are comprised of:
0.720 to 0.860 wt % carbon;
1.000 to 1.280 wt % manganese;
0.450 to 1.000 wt % silicon;
0.010 to 0.100 wt % copper;
0.150 to 0.280 wt % chromium;
0.0010 to 0.0500 wt % aluminum;
0.050 to 0.120 wt % nickel;
0.100 to 0.260 wt % molybdenum;
0.100 to 0.210 wt % vanadium;
0.0010 to 0.0065 wt % nitrogen;
0.0010 to 0.0080 wt % phosphorus;
0.0010 to 0.0040 wt % sulfur; and
0.0100 to 0.0350 wt % niobium
with the balance being iron.
It might also be noted here that the use of the herein described amounts of aluminum, phosphorous and sulfur is specifically aimed at reducing the amount of non-metallic inclusions in the final rail product. These non-metallic inclusions have been linked to degradation in mechanical properties (e.g., ductility) in the final rail products owing to the fact that they act essentially as pre-formed cracks in the metal matrix. Examples of some of the more common non-metallic inclusions that applicants seek to minimize are MnS, Al2O3 (commonly known as manganese sulfide and alumina, respectively), as well as other complex oxides.
Once applicants' liquid steel has been degassed and cast into blooms (e.g., bars of either round or square cross-section and of varying lengths), said blooms are then hot rolled into final rail products. This general process is often referred to as Thermo-Mechanical-Processing (TMP). During applicants' TMP process, the blooms are subjected to a number of rolling stages that vary in number from about 12 to about 16. Each rolling stage further reduces the cross-sectional area of the bloom and forces the original bloom towards the final shape of the rail product. Each rolling stage is done at a specific temperature and reduction rate. The representative TMP schematic depicted in
Finally, it should be understood that these thermo-mechanical operations (e.g., hot rolling) can also be specifically directed at the head regions of applicants' rails in order to improve their hardness and mechanical properties.
Applicants have found that the presence of the previously described alloying elements, in the concentrations given, are especially significant factors in imparting rolling contact fatigue occurrence in the railroad rails at the contact zone between the rolling wheel and the rail head surface. Another key point with respect to these steel formulations is that a pearlitic transformation of such steels takes place at relatively long coil hold times, see for example the continuous cooling transformation (CCT) diagram depicted in
Applicants have also found that for most practical considerations, 900° C. usually constitutes an optimum austenitic temperature. Applicants also favor holding the subject steels at that temperature for time periods indicated in
Applicants have also found that heat treatments conducted at 900° C. or higher for more than 1000 seconds of austenization, as well as the use of the cooling rates shown in
The selection of the Carbon content affects the formation of pro-eutectoid cementite (Fe3C) at the austenite grain boundaries during solidification. In current hypereutectoid rail steels the Fe3C tends to form at the grain boundaries prior to the formation of pearlite. This is a concern since this cementite is hard and brittle, exhibiting low ductility and low impact toughness. As a result in hypereutectoid steels the Fe3C phase becomes the preferred site for crack nucleation in rolling contact fatigue. The proposed steels suppress the Fe3C formation during rail processing. In particular, Silicon was selected as the solute element that suppresses Fe3C formation (as Si level increased, then Fe3C growth rate decreased). In the proposed steels Chromium and Silicon act synergistically to improve the mechanical properties by solid solution strengthening and by preventing the coarsening of the Fe3C phase. Manganese additions reduced the amount of pro-eutectoid ferrite, and lowered the temperature at which Fe3C begins to form.
Applicants' railroad rail steels will preferably have the following micro-structure and mechanical properties:
The mechanical properties of this steel are as follows:
An added plus for the applicants' steels is their ability to be made with substantially the same manufacturing processes used to make various prior art railroad rail steels. Moreover, the relative cost of the applicants' rail steels remains competitive—especially given their improved rolling contact fatigue resistance qualities.
Finally, those skilled in the steel railroad rail making arts will appreciate that, while this invention has been described in detail and with reference to certain specific embodiments thereof, various changes and modifications can be made therein without departing from the spirit and scope of this patent disclosure.
