**2. Rail degradation**

There are many kinds of loadings which can adversely affect the life of rails; amongst these, wear and plastic deformation induced by contact stresses can combine to cause unacceptable changes in the rail head profile. Rails are subjected to complex stress state. There are many stresses that operate in a rail and can influence rail defects and rail failure. As bending and shear stresses arised principally from the gross vehicle load, the rail is also subjected to contact stresses, thermal stresses and residual stresses. Residual stresses in rails are introduced by different mechanisms. Primarily they stem from the manufacturing process, namely from heat treatment and roller straightening (Schleinzer & Fischer, 2000; Schleinzer & Fischer, 2001). A special case of residual stresses is welding residual stresses at rail joints. Since the loading conditions at the tread of a wheel and at the running surface of a rail have a number of features in common the appearance of cracks will also be similar. Cracks may be induced at or below the surface. Surface cracks are initiated due to high traction forces at high speed rails and they will propagate under the influence of a lubricant in an inclined angle in the direction of the motion of the applied load for rails operated in one direction. Transverse branching may then lead to the complete fracture of the rail. Subsurface cracks are reported to initiate beneath the gauge corner 10–15 mm below the running surface and 6–10 mm from the gauge face (Clayton,1994). They seem to propagate towards the rail surface and to behave like original surface cracks after penetration.

Note, that cracks close to or at the surface are a rather new problem connected with high speed operating. In former times rails experienced enough wear to permanently remove the surface layer containing the new emerging cracks. In order to fulfil the increasing demands for higher axial and dynamic loads modern rail steels tend to exhibit much higher resistance to wearing with the disadvantage that the surface layer removed is not any more large enough to prevent small cracks from extending into the rail (with respect to the development of rail steels (Muders & Rotthauser,2000)).

A typical development of a rail crack is illustrated schematically in Fig. 1 (Ishida &Abe, 1996). Originating from a small surface or sub-surface crack, a dark spot is developing at the surface accompanied by crack growth in an inclined angle below the surface. At a certain point this crack branches into a horizontal and a transverse crack. The transverse crack will extend down into the rail and finally cause its fracture.

Today's rail failures can be divided into three broad groups as follows: those originating from rail manufacturing defects; those originating from defects or damage caused by inappropriate handling, installation and use and those caused by the exhaustion of the rail steel's inherent resistance to fatigue damage (Cannon et al.,2003).

properties of reparation welded layers in comparison to base steel. In repaired rail, maximal stresses are induced in newly deposited layer, i.e. new layer becomes area of future crack initiation, that in turn will delay its initiation and provide secure and reliable exploitation. This results open further possibilities for cheaper and reliable rail maintenance in future. Finally, this work shows clearly that repaired rails, due to improved microstructure and crack initiation resistance, have dominant mechanical properties in comparison to the

There are many kinds of loadings which can adversely affect the life of rails; amongst these, wear and plastic deformation induced by contact stresses can combine to cause unacceptable changes in the rail head profile. Rails are subjected to complex stress state. There are many stresses that operate in a rail and can influence rail defects and rail failure. As bending and shear stresses arised principally from the gross vehicle load, the rail is also subjected to contact stresses, thermal stresses and residual stresses. Residual stresses in rails are introduced by different mechanisms. Primarily they stem from the manufacturing process, namely from heat treatment and roller straightening (Schleinzer & Fischer, 2000; Schleinzer & Fischer, 2001). A special case of residual stresses is welding residual stresses at rail joints. Since the loading conditions at the tread of a wheel and at the running surface of a rail have a number of features in common the appearance of cracks will also be similar. Cracks may be induced at or below the surface. Surface cracks are initiated due to high traction forces at high speed rails and they will propagate under the influence of a lubricant in an inclined angle in the direction of the motion of the applied load for rails operated in one direction. Transverse branching may then lead to the complete fracture of the rail. Subsurface cracks are reported to initiate beneath the gauge corner 10–15 mm below the running surface and 6–10 mm from the gauge face (Clayton,1994). They seem to propagate

towards the rail surface and to behave like original surface cracks after penetration.

development of rail steels (Muders & Rotthauser,2000)).

extend down into the rail and finally cause its fracture.

steel's inherent resistance to fatigue damage (Cannon et al.,2003).

Note, that cracks close to or at the surface are a rather new problem connected with high speed operating. In former times rails experienced enough wear to permanently remove the surface layer containing the new emerging cracks. In order to fulfil the increasing demands for higher axial and dynamic loads modern rail steels tend to exhibit much higher resistance to wearing with the disadvantage that the surface layer removed is not any more large enough to prevent small cracks from extending into the rail (with respect to the

A typical development of a rail crack is illustrated schematically in Fig. 1 (Ishida &Abe, 1996). Originating from a small surface or sub-surface crack, a dark spot is developing at the surface accompanied by crack growth in an inclined angle below the surface. At a certain point this crack branches into a horizontal and a transverse crack. The transverse crack will

Today's rail failures can be divided into three broad groups as follows: those originating from rail manufacturing defects; those originating from defects or damage caused by inappropriate handling, installation and use and those caused by the exhaustion of the rail

original rails.

**2. Rail degradation** 

Fig. 1. Typical development of a rail crack (schematically) (Ishida &Abe, 1996).

Rolling contact fatigue (RCF) is likely to be a major future concern as business demands for higher speed, higher axle loads, higher traffic density and higher tractive forces increase. Head checks, gauge-corner cracks and squats are all names for surface-initiated RCF defects. They are caused by a combination of high normal and tangential stresses between the wheel and rail, which cause severe shearing of the surface layer of the rail and either fatigue or exhaustion of ductility of the material. The microscopic crack produced propagates through the heavily deformed (and orthotropic) surface layers of steel at a shallow angle to the rail running surface (about 10◦) until it reaches a depth where the steel retains its original isotropic properties. At this stage the crack is a few millimetres deep into the rail head. At this point the crack may simply lead to spalling of material from the rail surface. However, for reasons still not clearly understood, isolated cracks can turn down into the rail, and, if not detected, cause the rail to break. These events appear to be rare, but are highly dangerous since RCF cracks tend to form almost continuously at a given site. Fracture at one crack increases stress in the nearby rail, increasing the risk of further breaks and disintegration of the rail (Cannon et al.,2003).

RCF initiation is not normally associated with any specific metallurgical, mechanical or thermal fault; it is simply a result of the steel's inability to sustain the imposed operating conditions. The problem is known to occur in most of the rail-steel types in common use today.

While wear has been reduced, rolling contact fatigue defects have become more prominent on busy routes where the rails are highly stressed. Although its wear reserve may not be used up, rail may have to be replaced because such defects quickly become critical for safety (Pointner & Frank, 1999). The relationship between RCF and mechanical wear is not well undersood, as for example zero (or minimal) mechanical wear leads to significant microcrack propagation and thus RCF failure. On the other hand, excessive mechanical wear eliminates RCF but leads to unrealistically short rail life (Kapoor et al., 2001).

The rate of rail degradation depends also on the location; rail head erosion is at a maximum in regions where the track curves. In Fig.2 is shown damage of the inner edge of rail head, caused by centrifugal force which tends to expel vehicle towards the outside of the track. Such damage can be repaired by surface welding, Fig 2b.

Surface Welding as a Way of Railway Maintenance 237

excessive wear. All of these depend on interactions between engineering parameters, material properties and the environment. The track material must obviously be capable of being manufactured into rails with a high standard of straightness and flatness in order to avoid surface and internal defects which may cause failure. Track installation requires that the steel should be weldable and that procedures be developed to enable its maintenance

Since steel has one of the highest values of elastic modulus and shows superb strength, ductility and wear resistance, most modern rails have pearlitic microstructures and carbonmanganese chemistries similar to those produced in rails in 1900. Ordinary rail steels contain about 0.7 wt% of carbon and are pearlitic. Pearlite consists of a mixture of soft ferrite and a hard, relatively brittle iron carbide called cementite, Fig. 3a. Pearlite presumably achieves a high resistance to wear because of the hard cementite and its containment by the more plastic ferrite, but pearlitic steels are not therefore tough. In pealite, altering lamellae of iron and iron carbide are aranged, and lamella spacing has a large effect on hardness. Naturally cooled standard rails have coarse lamella spacing and relatively low values of about 300 Brinell hardness (HB). Control-cooled premium rails have finer lamella spacing

Raising carbon content and refining pearlite spacing increases the hardness of pearlitic steel, and this has been shown to lead to improved wear resistance. Hence rail manufacturers have worked to produce pearlitic steels with higher carbon contents (now achieving approximately 1 wt%) and finer structure (using head-hardening processes). Even though hardness generally has a positive effect on rail wear, there is a limit to the hardness that can be reached with pearlitic steels, and this hardness has been reached in modern rails (Lee &

There has been considerable effort devoted to nding alternatives to the pearlitic rails, but with alterable results. In an attempt to develop rail steels with higher hardness and alternative microstructures, several types of bainitic steel were developed. While pearlitic steels obtain their strength from the fine grains of pearlite, bainitic steels (Fig. 3b) derive their strength from ultra-fine structures with a lot dislocations which are harmless but confer high strength (Aglan et al.,2004). Bainitic steel is easy to be cast, welded and inspected by ultrasonic methods. The new generation of bainitic steels achieved higher

and repair. Commercial success depends also on material and life time costs.

and thus higher hardness of 340-390 HB (Lee & Polycarpou, 2005).

tensile and fatigue strengths and performed well in service.

(a)

Polycarpou, 2005).

(a) Damage of the inner edge of rail head (b) reparation of rail head Fig. 2. Rail head degradation (Popovic et al., 2006).
