Patent Application
20150028165
This invention relates to a wrought steel for producing parts for railway crossings and switches and to a method for producing said parts.
Switches and crossings (S&C) are components of the rail system which are subjected to significant loads in use in a railway line.
Several techniques are being used to produce these S&C. A significant proportion of S&C parts are currently manufactured from cast austenitic-manganese steel (AMS). AMS has traditionally been used owing to its high work hardening capacity on impact, excellent toughness following solution treatment and water quenching, and very good resistance to wear in the work hardened condition. The nominal chemical analysis of AMS is 1.2% C, 13% Mn and 0.5% Si, which produces a bulk hardness in the region of 200 to 250 HB. Following the passage of a certain amount of traffic, the hardness of the S&C can reach levels of 500 to 550 HB.
AMS also has a number of drawbacks, such as the low 0.2% proof stress. Railway points and crossings commonly experience severe impact loading conditions during service resulting in plastic deformation and work hardening which raise the material strength to levels more resistant to further plastic flow. However, the associated dimensional changes that are unavoidable in the original unhardened condition are undesirable. In track, the differential loading leads to uneven hardening and localised plastic deformation, with the resulting poor ride quality eventually necessitating rebuilding of the deformed profile with weld deposits. As a consequence, AMS in heavy axle load applications requires frequent grinding to remove lipping and weld repairs to restore deformation height loss.
AMS is a difficult material to cast or machine into the complex shapes needed for S&C. Furthermore, any change to the footprint of the railway crossing requires a new casting mould, making the production of uncommon crossing profiles very expensive. The narrow freezing range of AMS results in many cavity-type defects which may be the starting point of cracking seen in service. Commonly, porosity in AMS crossings occurs at depths of around 10 to 15 mm from the new surface and once this depth is approached weld restoration to rebuild the crossing becomes impractical owing to the risk of cracks initiating from any residual porosity. For an AMS component, 10 years is commonly regarded as a normal life span as beyond this period the extent of the defects has become so severe that it is uneconomical to continue remedial weld repair and the components have to be replaced. A further problem with AMS is the thermal instability of the austenitic microstructure, which renders the material difficult to weld. This difficulty in welding is a problem not only when weld repairing the components in situ, but also during component manufacture, as rails have to be welded to the component prior to its installation in a railway line.
As a result of these issues, several alternatives have been developed to the conventional AMS crossing type. Some S&C are made up of materials in the form of a composite metal sandwich, where the part contacting the wheels of passing trains is made of a hard, wear resistant plate steel of different composition, microstructure and properties. The lower part of the item is manufactured from a basic steel composition. These solutions usually offer a cheaper alternative with better weld repairability, but the properties of the crossing nose are then dependent on the level of sophistication of the steel composition making up the crossing nose. In many cases, such compositions do not match the wear, and in particular the rolling contact fatigue, resistance offered by AMS crossings. Moreover, these solutions are more difficult to produce as a result of the composite sandwich of different steels.
Another alternative to AMS is to provide the high manganese parts with a work-hardened layer prior to installation of the parts in the line. Surface pre-hardening techniques may include shot blasting, rolling or explosive hardening. Of these techniques, explosive hardening is generally the preferred choice as it provides a hardened layer which is thick enough to meet the service requirements of the S&C.
However, it is difficult to control the thickness of the hardened layer with these surface hardening techniques. Moreover, surface hardening does not address the weld repairability issues and casting defects associated with cast high manganese parts.
The inventors therefore set out to devise a solution to these problems.
It is an object of this invention to provide a new wrought steel for S&C as an alternative to AMS and in particular to cast AMS.
It is also an object to provide a new wrought steel for S&C as an alternative to AMS which is readily weldable for in-situ repairing by a top-of-head weld repair procedure.
It is also an object to provide a new wrought steel which will be more resistant to nose batter.
It is also an object to provide a new wrought steel for S&C which is weldable to conventional pearlitic rails.
It is also an object to provide a new wrought steel for S&C which is flash-butt weldable to conventional pearlitic rails.
One or more of the objects of the invention is reached by providing a wrought steel for producing parts for railway, railway crossings or railway switches comprising (in weight percent):
The current invention allows to produce a single type of feedstock blank that can subsequently be machined to any crossing design required to satisfy the local conditions. Computer controlled machining results in closer tolerances at reduced costs. As railways have many different angled crossings to cater for the local needs, a variety of casting moulds would be needed to produce these from AMS castings and this is reflected in their relatively high cost. The current invention therefore offers a significant cost reduction.
The role of carbon in this steel is to obtain sufficient hardness of the steel mainly by solid solution strengthening. On the other hand, a high carbon content leads to an increase in the amount of retained austenite, leading to a reduction in hardness. An increase in carbon content will significantly enhance the risk of grain boundary embrittlement in these steels due to the formation of carbide networks, both in the as-manufactured condition and also following welding. Therefore, to maintain the delicate balance between hardness and the risk of embrittlement, the carbon content needs to be between 0.01 and 0.15% for these steels (all compositions are given in weight percent, unless otherwise indicated). More preferably the carbon content is between 0.01-0.12%. As a consequence of their lower carbon content, most of these alloys are readily weldable. To further improve the weldability the carbon content preferably is at most 0.10, more preferably at most 0.08.
To achieve the desired microstructure the carbon content is at least 0.01% and preferably at least 0.02%. A suitable minimum carbon content from a steelmaking point of view is 0.04%.
Manganese is an austenite promoting element. It stabilises austenite i.e. increases the temperature range in which austenite exists.
Varying the manganese content in the steels according to the invention revealed that a maximum in hardness is obtained at a manganese content of at least 10%. At very high manganese levels of e.g. 15% the hardness decreases to an inadequate level. The hardness has a strong correlation with wear resistance and the resistance to wear is a determining factor for the life span of most railway parts, including S&C. A low wear rate means that repair of the part is needed less frequently. The significant difference in wear resistance between steels having a manganese content below 10% and those above 10% is attributed to the differences in microstructure. Manganese levels below 10% resulted in fully martensitic microstructures whereas levels above 10% displayed mixed microstructures of retained austenite, ε-martensite (hexagonal close-packed, or hcp martensite) and martensite. Preferably the manganese level is at least 11%. The wear resistance of steels having fully martensitic microstructures has been found to be poorer than those of mixed microstructures containing martensite and retained austenite. However, increasing the manganese content also results in an increase in retained austenite. At manganese contents above 15% the levels of retained austenite become sufficiently high that the increasing hardness of the martensite phase is more than offset by the increasing proportion of the softer austenite, with the result that the overall hardness of the steel falls, along with the wear resistance. Resistance to crack propagation is high and is associated with very sluggish progressive failures. Because of this, there is an increased opportunity for any fatigue cracks that develop to be detected, and the affected part or parts removed from service or repaired before complete failure occurs. Based on the above reasoning the manganese content is preferably at least 11 and at most 15%. As manganese is also a costly alloying element, a suitable maximum manganese content was found to be 14% or even 13%. A suitable minimum content of manganese was found to be 11.5%. The maximum value of hardness and wear resistance was achieved when the manganese content was between 12 and 13% Mn. At these levels the amount of retained austenite+ε-martensite on the one hand and hard martensite on the other is about 50:50, thereby providing a satisfactory combination of impact toughness and hardness.
Molybdenum is effective in increasing the impact toughness. In addition, due to the scavenging effect of molybdenum for phosphorus, temper embrittlement phenomena are prevented. At a level of 0.6% Mo, the increase in impact toughness is already notable, but a further increase is obtained at values above 0.6%. The increase in impact toughness levels off at values of 1.5%. Consequently, the molybdenum addition in this steel needs to be between 0.6% and 3.95%, and preferably the molybdenum content is at most 2.95% and/or least 1.25%. A molybdenum content of 1.5% was found to be a suitable minimum value for stable impact toughness values. A molybdenum content of 1.90% was found to be a suitable maximum value from a combined cost and technical perspective as the additions of values above 1.90% result in only a modest further improvement.
Silicon was found to have little effect on the impact toughness and wear resistance of these steels, although it does provide an increase in tensile strength and hardness via solid solution strengthening. It also serves as a killing agent during steel production. On this basis, a maximum value of 0.5% Si is recommended. A suitable minimum content was found to be 0.10 or even 0.15%, and/or a suitable maximum was found to be 0.40 or even 0.35%.
Nickel (Ni), cobalt (Co) and copper (Cu) have a similar effect as manganese by way of their being austenite promoting elements. To a certain extent these elements can be added instead of, or in addition to, manganese. Ni, Co and Cu may be added to a maximum of 1.0% per element, totaling not more than 3%. Preferably the maximum of Ni, Co and/or Cu is 0.5%.
The alloys according to the invention have proven to be readily machinable. One or more additions of sulphur, calcium, tellurium, or selenium or any other known machinability enhancing elements may be made to further these alloys if necessary.
The phosphorus content is generally maintained below 0.08%, preferably below 0.05% and preferably below 0.02% to minimize the tendency for hot cracking. Phosphorus is a residual element in these steels. If no sulphur is to be added to enhance machinability, then the sulphur content is generally maintained below the impurity level of 0.02%. If sulphur is to be added, then a suitable maximum amount is 0.08%, preferably 0.05%. If the following elements are added as alloying elements, then preferable ranges are as follows: between 0.02 and 0.20% V, between 0.02 and 0.10% Nb, between 0.02 and 0.20% Ti, between 0.02 and 0.20% Zr, from 5 to 50 ppm B and from 10 to 250 ppm N. Suitable maximum contents are 0.10% V, 0.075% Nb, 0.10% Zr and/or 0.10% Ti. B, V, Nb, Zr and Ti contribute to a grain refinement of the steel.
The steel according to the invention is preferably silicon-killed. Provided the cleanness of the steel remains in accordance with the specifications in terms of maximum value of aluminium oxide inclusions, the steel may also be aluminium-killed or aluminium-silicon killed. When added as an alloying element, the maximum total aluminium content is 0.2%. Preferably the total aluminium content (when added as an alloying element) is between 0.02 and 0.15%. The metallic aluminium content (i.e. not present as an oxide) will be lower, dependent on the oxide content of the steel melt when adding the aluminium.
The steel according to the invention has a hydrogen content of is below 5 ppm, preferably of below 3.5 ppm and more preferably below 2.5 ppm. Although chromium is preferably kept below the impurity level of 0.15%, i.e. the chromium is not deliberately added, for some applications chromium may be added up to a level of 0.3%. A suitable maximum chromium content is 0.2%.
It should be noted that the steel composition according to the invention could also be used to produce castings, but as the low carbon composition is more expensive than the normal AMS Hadfield type steels with high carbon content whilst not producing better properties in its cast condition than AMS, it is economically unattractive to use the composition according to the invention to produce cast materials.
According to a second aspect the invention is also embodied in a method for producing a wrought steel part for use in a railway track such as in railway crossings or railway switches comprising the steps of:
The wrought steel can be used to produce a part like or for a crossing by machining it from a hot-rolled and cooled plate. The wrought steel can also be provided in the form of a rail having the desired geometrical profile, and these hot-rolled and cooled rails can be welded to the part or be used to be machined into a switch blade. The rails can also be used as such.
The method according to the invention allows the production of blanks of different lengths that can then be machined into crossings with a wide range of angles. Also hot-rolled plate can be slit into thinner lengths that are subsequently machined into switch blades. However, to form switch blades it may be preferable for the cast bloom to be hot-rolled into a rail of the desired geometrical profile and thereafter machining the rail to manufacture a switch blade.
According to a third aspect the invention is also embodied in the use of the wrought steel part produced according to the invention and/or having a composition according to the invention in a railway, railway crossing or railway switch, preferably wherein the steel part has been, at least partly, weld restored by an in-situ weld repair procedure.
Flash butt welding of the steel according to the invention is preferably performed by using a stainless steel insert to be welded to the crossing first before it is welded to a pearlitic rail with the stainless insert acting as a sandwich filler to improve compatibility for high integrity welds.
The steel according to the invention can be used to produce parts for railway crossings and switches such as a frog in a common crossing as shown in
A frog also forms part of a railroad switch, and is also used in a level junction (flat crossing). The frog is designed to ensure the wheel crosses the gap in the rail without “dropping” into the gap; the wheel and rail profile ensures that the wheel is always supported by at least one rail. To ensure that the wheels follow the appropriate flangeway, a check-rail or guard rail is installed inside the rail opposite the frog.
The invention is now described in more detail by means of the following, non-limiting examples.
A series of casts were produced and the chemical compositions are given in Table 1.
The mechanical properties of the steels are excellent as shown in Table 2.
An assessment of the rolling contact fatigue, RCF, resistance of the steels according to the invention is performed by means of twin disc wear tests at room temperature with a rolling contact stress of 900 MPa, a level of slip of 5% and a small amount of lubricant applied throughout the duration of the test. This test and the rig used in these tests is described in Fletcher & Beynon, “Development of a machine for closely controlled rolling contact fatigue and wear testing”, J. Test. Eval. 28 (2000), 267-275. The RCF resistance is defined as the number of cycles to crack initiation. The test data indicate the new material outperforms alternative rail steels currently in the market by a factor of two. In the case of premium grade heat treated rail, for example, the number of cycles prior to crack initiation is typically 110000 cycles. Furthermore, the new material displays RCF resistance comparable to that of AMS and Maraging Steels (AMS and MS respectively in
Comparison with other rail materials revealed that the steels according to the invention show good impact toughness values which are more than adequate for the purpose of producing parts for railway crossings and switches.
The material has a very high work hardening rate resulting in an increase in stress during low cycle fatigue testing of about 400 MPa after three cycles from +1 to −1% in the standard NF12 test specimen (see
An assessment of the rolling contact wear resistance for the steels according to the invention is also performed by means of twin disc wear tests at room temperature, but with a rolling contact stress of 750 MPa with the wear rate being measured after 2130 cycles (see
The weldability of the steel according to the invention is excellent due to its low carbon content and is much better that than of AMS, making the new steels a preferred option for applications where weldability is an issue, such as in the production and use of parts for railway crossings and switches.
| Number | Date | Country | Kind |
|---|---|---|---|
| 12152514.1 | Jan 2012 | EP | regional |
| 12153948.0 | Feb 2012 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2013/051517 | 1/25/2013 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 61595014 | Feb 2012 | US |
