Rails and Rail Steel
Rails and Rail Steel
The rail, as the most important component of the track superstructure, means the running surface, carrier and guiding element for the rolling wagons. It is the defining feature and most important component of the rail track. Rail steel is used to make rails for the rail track as well as the tracks for moving equipments like cranes, transfer cars, charging and pusher cars in coke oven battery, and material handling equipment (stacker, reclaimers, and bender reclaimers) etc.
The rail profile is the cross sectional shape of a rail, perpendicular to its length. The weight of a rail per unit length is an important factor in determining rail strength and hence axle loads and speeds. Weights are measured in pounds per yard or kilograms per metre. Cross-sections of IRS 52 kg/m and UIC 60 kg/m rails is given in Fig 1.
Fig 1 Cross-sections of IRS 52 kg/m and UIC 60 kg/m rails
Earlier wooden rails were used on horse drawn wagon ways. By 1760s strap iron rails, which consisted of thin strips of cast iron fixed onto wooden rails came into use. These were superseded by cast iron rails which were flanged (i.e. ‘L’ shaped) and with the wagon wheels flat. In 1789, the edge rails where the wheels were flanged were introduced and, over time it was realized that this combination worked better. The earliest of this type of rails used were the cast iron ‘fish-belly’ rails which were so called because of their shape. Rails made from cast iron were brittle and broke easily. They could only be made in short lengths which would soon become uneven. In 1820 as rolling techniques improved, wrought iron rails were introduced and replaced cast iron rails. These rails contributed significantly to the explosive growth of railroads in the period 1825-40. The cross-section of these rails varied widely from one line to another line. The parallel cross-section which developed in later years was referred to as Bullhead.
When Henry Bessemer discovered the process for producing steel on an industrial scale in the 1850’s it soon became economically feasible to use steel for railway construction. The first steel rails used anywhere in the world were laid in Derby station on the Midland Railway in 1857. Steel is a much stronger material, which steadily replaced iron for use in the production of rails and allowed much longer lengths of rails to be rolled. Ever since, in principal, all the rail track materials are made of steel. The hardness and wear resistance of rail steels have progressively increased over the past century as steelmaking methods have improved along with increase in the carbon level, selected alloy additions and advanced accelerated cooling processes.
The properties of rail steel are achieved through control of carbon and manganese contents. Carbon content of rail steel can go upto a maximum of 0.82 % and manganese content upto a maximum of 1.7 %. The normal rails are made of steel containing 0.7 % carbon and 1 % manganese, which are called as carbon-manganese rail steel
Nowadays, rails are mostly produced by continuous casting followed by immediate multi-stage process in the rolling mills. They are normally made of carbon-manganese steel composition with pearlitic or bainitic microstructure. Rails are to be free from internal cracks which are caused by hydrogen trapped in the liquid steel as it cools. Hydrogen needs removal either by vacuum degassing of the liquid steel before it is cast or by letting hot rails cool down very slowly. Rail steel is also to be resistant to fatigue or surface cracking so that it can have a long fatigue life. For this rail steel needs improved cleanliness.
Some of the basic features of refinement used for making of rail steel include (i) very accurate control of the chemistry of the rail steel with carbon and manganese at optimum level, (ii) control at low levels of the potential embrittling compounds such as sulphur, phosphorus, nitrogen and hydrogen etc., and (iii) steel is made cleaner with fewer numbers of stress-concentrating entrapped inclusions from the steelmaking and casting process.
Rails have asymmetric shape. The thinnest part of the rail section after its rolling in the rolling mill cools the fastest. Since the rail head contains a large mass it retains the heat to a higher degree. Hence, the contraction in the head is lesser than at the rail flange with the result that, during cooling after rolling, the rail sweep naturally or pull towards the its foot, causing a bow in the rail, which makes it extremely unwieldy for further processing. To overcome this condition, rails are given a pre-camber in opposite direction while they are hot so that in the process of cooling rails straightens themselves.
The primary function of the rail is to provide a smooth and continuous level surface for movement and to provide guidance in lateral direction for movement of the wheels. In the process, it transfers the load from the wheels to the track structure below. The rails are subjected to stresses. To be able to withstand these functions, the rails and rail steel is to meet the requirements of (i) high wear resistance, (ii) high resistance to deformation caused by compression, (iii) high fatigue strength, (iv) high yield strength, toughness / tensile strength, and hardness, (v) high resistance to brittle fracture, (vi) good weldability, (vii) high degree of steel purity and good texture, (vii) profile evenness and dimensions accuracy by inspection and acceptance, and (viii) low residual stresses after manufacturing and straightening. Some of these requirements are contradicting each other, which makes difficult the choice of rail profile and rail steel grade.
Steel is a versatile material. If the suitable alloying additions are made, or the correct heat treatment chosen, then the desired steel structures can be produced. Rail steel is required to resist plastic deformation, wear, rolling contact fatigue, bending stress and thermal stress during rail welding process and rails resurfacing.
The combinations of alloying and heat treatment are possible in rail steel and hence a range of grades can be produced. In the case of rail steel, tensile strength and toughness can be increased by heat treatment. The heat treatment can be applied to either the whole cross-section or to only the rail head providing a very large wear resistance. Head hardening rails are normally used in heavy loaded tracks, sharp curves and in turnout elements.
These days, the railway system is subject to intense use, with fast trains and large axle loads. There are many criteria which determine the suitability of steel for rail track applications. The primary requirement is structural integrity, which can be compromised by a variety of fatigue mechanisms, by a lack of resistance to brittle failure, by the localized plasticity, and by excessive wear. All of these depend on interactions between engineering parameters, material properties and the environment. The track material is to be capable of being manufactured into rails with a high standard of straightness and flatness in order to avoid surface and internal defects which can cause failure. Track installation requires that the steel is to be weldable and that procedures are to be developed to enable its maintenance and repair. Success of the rail steel is judged from the material and life–time costs.
Rails are subject to very high stresses and are made of very high quality steel. A railway wheel rolling along a rail gives rise to high surface stresses in the normally elliptical contact area between wheel and rail. Besides the vertical stress component, traction load locally produces additional transverse stresses beneath the wheel tread and the running surface of the rail respectively. Such stresses lead to local plastic deformations of the materials in the surface area if the yield strength of the rail steel is exceeded. Recurrent plastic deformation of the rail surface induces residual compressive forces. Plastic deformation causes strain hardening to occur on the surface, moreover, that leads to the yield strength being raised locally. As a result, the material is initially able to absorb further strains by purely elastic means.
Rails not only wear but they break also. The inherent toughness of rail steel is poor as a result of the presence of the brittle carbide phase. Fracture can occur from relatively minor stress concentrating features inside the rail or on the surface due to the manufacturing defect or handling damage. The rail breakage has a high replacement cost and can be very disruptive to the railway network.
There are several kinds of loadings which can adversely affect the life of the rails. These include the wear and plastic deformation induced by contact stresses which can combine to cause unacceptable changes in the rail head profile. The system of contact stresses is dependent upon the relative motions of the wheel and rail within a small contact zone of about one square centimeter. The motions lead to lateral and longitudinal surface tractions and a spin moment. The rate of rail degradation depends also on the location with the rail head erosion is at a maximum in regions where the track curves.
In recent years, on railway tracks, trains are running with heavier axle loads, at higher speed and the rail wagons have different characteristics. So, the requirements from rail steel are to make the rails stronger and more wear and defect resistant, in order to minimize rail maintenance costs and maximize asset life. The increase in strength of the rail steel needed mainly to cater for the heavy axle loads. Rail steel of around 1,300 MPa to 1,400 MPa is now being used in large quantities. This steel is finding increasing use in tight curve / high wear situations. This high strength is being achieved by making the spacing between the pearlite lamellae finer by controlling the growth rate of pearlite. Rails are also to meets the requirements of track safety and high availability of the rail track.
For enabling faster rail transport of heavier loads at shorter service intervals, the use of rails with better wear resistance is of great importance. There has been a continuous tendency in the development of rails toward increasing the wear resistance. This is based on the knowledge that the wear resistance of rails with natural mill hardness is directly related to their strength. Substantially higher strength values than 1,100 MPa are not attainable in rails with natural mill hardness since the rails are required to have a pearlitic structure in all their parts and the formation of martensite is to be avoided under all circumstances. In the case of alloyed rails the chemical composition of the steel is adjusted so that during self cooling of the rail on the cooling bed no martensite is formed, not even in the web and the base of the rails. Hence, a limitation exists in the strength attainable in this manner in the rail head.
Rail steel with higher strength also has a higher resistance to rolling contact fatigue. It is possible with pearlitic rail steels to either by reducing the lamellar spacing in the pearlite through heat treatment (head-hardened rail), or by adding of alloying elements, or by raising the carbon content in the steel (hypereutectoid steels). All these measures lead to an increase in material strength and hence of resistance to wear and rolling contact fatigue. One alternative pursued in recent years is the use of bainitic rail steels. In conjunction with the appropriate alloying additions and, if need be, a suitable form of heat treatment, these steels can attain even greater strengths than the pearlitic steels.
In addition to the wear resistance, all pearlitic rails experience surface damage with time under heavy axle loads. This surface damage is a form of ‘cold work’ where the pearlitic microstructure deforms and aligns parallel to the running surface. The higher the hardness and strength, the more resistant the rail is to this cold work condition. This shallow region eventually delaminates and shallow surface cracking occurs. The phenomenon is called rolling contact fatigue. If this layer is not removed by grinding, it begins to spall and generates deeper cracks which can develop into transverse defects. For this reason railways lubricate the rails in curves and inspect and grind the rails periodically to remove surface damage.
With the advent of head hardening processes in the last 50 years, rail producers are now able to increase the hardness and strength of rail well beyond that of standard strength ambient air cooled rail while maintaining adequate ductility. Presently, the head-hardened rails are normally produced by using an additional heat treatment. After reheating the head of rail (normally by induction heating), accelerated cooling is used with a mixture of water and air. By this process, in principle, good results are achieved with regard to structure, strength, and wear characteristics. For economical reasons, however, a heat treatment directly from the rolling temperature is preferred. Hence, the head hardening process is carried out in the head hardening facility which is normally in-line with the rail mill. This means that the as-rolled hot rails proceed directly to the facility in order to maintain the rail in the austenite state.
The water-spray head hardening system is ideally suited for developing refined pearlitic microstructure and properties. Once in the head hardening facility, the rails are tracked with pyrometers where exact control of the cooling path is maintained. Cooling is achieved by adjusting the water flow to the spray nozzles in independently controlled zones. Adjusting the water flow to these zones (i) allows the rail to cool quickly to begin the pearlite transformation temperature at the lowest temperature possible for fine pearlite, (ii) continues the pearlite transformation at this low temperature, (iii) removes the extra unwanted heat of transformation by additional cooling, and (iv) continues cooling to obtain greater depth of hardness in the rail head. After leaving the facility, the head-hardened rails are normally sent to a packing bed where they are subsequently placed in control cooling boxes to further lower hydrogen level.
The prescribed requirements for this are (i) heat treatment of the rails is to take place directly from the rolling temperature without causing pollution. (ii) a fine pearlitic structure is to be obtained, as far as possible, in the entire head area, but at least over a depth of 20 mm, (iii) the coolant is required to have a homogeneous effect over the entire length of the rails, remaining unaffected by contaminations of the installation, (iv) it is not to be toxic or flammable, and (v) the chemical composition of the rail material is to ensure perfect weldability.
The microstructure of rail steel (Fig 2) consists fully of pearlite which comprises a mixture of relatively soft ferrite and hard, brittle iron carbide called cementite. Ferrite and cementite takes the form of roughly parallel plates in a lamellar structure. Due to this structure rail steel achieves a good resistance to wear because of the hard carbide and some degree of toughness as a result of the ferrite’s ability to flow in an elastic / plastic manner. The hardness and tensile strength of steel are determined by the proportion of pearlite in its microstructure. They are also determined by the ‘fineness’ of the pearlite structure. The fineness of the pearlite structure (inter lamellar spacing) of rail steel is controlled by the rate at which the hot rail cools. Heat treatment methods are available to control the cooling rates and control the microstructure which in turn controls the hardness and strength of the rail steel.
Fig 2 Micro-structure of rail steel
It is well known that there is an inverse relationship between pearlite inter-lamellar spacing and rail hardness. The shorter is inter lamellar spacing the higher are the hardness, wear resistance and tensile strength of the rail steel. The hardness increases as the inter-lamellar spacing decreases or as the fraction of cementite within the pearlite is made larger. Pearlite presumably achieves a high resistance to wear because of the hard cementite and its containment by the more plastic ferrite. The very cementite particles which confer hardness are also brittle. The situation is made worse by the fact that each pearlite colony is a bi-crystal. It is the size of the colony, rather than the inter-lamellar spacing, which defines the length scale of fracture.
In addition to pearlite spacing, alloy additions to head hardening rail steel contributes to hardness and strength. Silicon contributes solid solution strengthening and vanadium contributes precipitation hardening of the ferrite in the pearlite. Vanadium is added along with chromium (0.2 % ro 0.25 %) to the head hardening rail steels. Alloying elements are added to rail steels to improve properties. Alternatively, the rail can be cooled quickly to reduce the time available for diffusion. The rail can also be heat treated. Combination of alloying and heat treatment is possible and a range of grades can be produced.
Pearlitic steels are not tough. Fracture can occur from relatively minor stress–concentrating features inside the rail, or on its surface, as a result of manufacture or subsequent handling damage. Although such fractures are rarely dangerous when actively managed, they entail a high replacement cost and can be disruptive to the rail network.
Rails with a pearlitic micro-structure have proven to provide the best wear resistance under severe wheel-rail interaction in heavy haul application. The hard iron carbide (cementite) phase of the pearlitic microstructure, imbedded in the soft iron (ferrite) phase, is the reason for this special attribute. However, controlling the spacing of the iron carbide phase in the pearlite lamellar microstructure and minimizing the formation of grain boundary networks of either ferrite or excess cementite is crucial to maintain the balance between rail hardness and ductility (safety).
The 0.86 % to 1 % C level of head hardened rail makes the steel hypereutectoid. Hypereutectoid rail steels can develop continuous or semi-continuous cementite on the prior austenite grain boundaries during cooling. Continuous cementite networks theoretically reduce the ductility and fracture toughness of the rail. However the combination of silicon and vanadium additions of the head hardened rail coupled with the cooling path during head hardening, there are around 40 % fewer grain boundary cementite deposits than other widely used hypereutectoid rail steels
As a next generation of rails, the bainitic steel rails have attracted a great deal of attention. With a specific alloy design, the bainitic steel rails attain strength beyond that of heat-treated pearlitic steel rails. Accordingly, the bainitic steel rails have a potential to provide unique characteristics different from those expected in the development of conventional pearlitic steel rails. Bainitic structure has generally higher wear than pearlitic structures because pearlitic structure consists of carbide particles finely spread over the matrix of fine ferritic structure. Carbide causes particles shelling away from ferritic matrix during run over bainitic rails. This accelerated wear removes fatigue damaged surface layer out of the top of the rail. Rolled low-alloyed rail steel with bainitic structure, has lower strength limit because of fixed ferritic matrix and roughly dispersive particles of carbides.
For similar hardness levels the bainitic grade shows a reduction in rate of the wear of the rail track as well as the reduced wear of the wheel. Further presently the formation of very fine surface cracks by a phenomenon of rolling contact fatigue is of increasing concern. The new bainitic grade rail steel has shown dramatic improvements in comparison to the traditional pearlitic rail in this respect. A whole wheel / rail test done on two types of rail steels has shown crack initiation at two hundred and twenty thousand cycles on pearlitic rail, whereas after one million cycles the bainitic rail steel was crack free when the test was terminated. The bainitic steel has considerable potential in terms of life in track, reduced maintenance, increased safety, with anticipated reduction in rail fractures.
However bainitic steels with appropriate chemical composition, and appropriate thermo-mechanical treatment are considered as materials applicable for heavy load rails. Problems with wear of bainitic steels can be solved with production of bainitic structure rails prepared with addition higher percentage of chrome or other alloying elements that can provide demanded high strength. Alloying elements are not only expensive, but also forms hard and brittleness martensitic structure in rails welds and resurfaces. General phenomena observed from the comparison of characteristics of bainitic steel rails and pearlitic steel rails are summarized as (i) generally, pearlitic steel rails have superior wear resistance to bainitic steel rails, (ii) bainitic steel rails have higher fatigue strength than pearlitic steel rails, (iii) bainitic steel rails have fracture toughness almost double that of pearlitic steel rails, and (iv) no significant difference in fatigue crack propagation behaviour appears between bainitic steel rails and pearlitic steel rails.