**3. Rail steels**

Choice of material for rail steels is of fundamental importance. This is because the rail's behaviour in service depends critically on the properties of the metal. Much effort and a considerable amount of research has already been undertaken in the search for the ideal rail steel (Pointner & Frank,1999). In recent years rail steel production has improved as manufacturers have developed steels with increased hardness and better wear resistance.

There are many criteria which determine the suitability of a 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 localised plasticity and by

(a) Damage of the inner edge of rail head (b) reparation of rail head

A few different fracture control concepts are applied in railway systems, and one of them is damage tolerance concept (Zerbst et al., 2005). Within the frame of this concept, the possibility of fatigue crack growth is basically accepted. The aim is to prevent the crack to grow to its critical size during the lifetime of the component, i.e. to estimate number of cycles to critical crack size. In fatigue, crack extension is expressed as a function of stress intensity range *∆K* and the crack extension rate, *da/dN*, whereby *da* denotes an infinitesimal crack extension due to an infinitesimal number of loading cycles *dN*. The basic idea is that the largest crack that could escape detection is presupposed as existent. After that, the initial crack can extend due to various mechanisms such as fatigue, stress corrosion cracking, high temperature creep, or combinations of these mechanisms. Such a failure process is visible, and catastrophic rail failure can be prevented by regular examination of the top surface of the railhead. Maintenance methods (lubrication and grinding) help combat the wear and rolling contact fatigue phenomena referred to in local parameters. By applying these methods appropriately, maintenance costs can be reduced (Vitez et al.,2005). Rail grinding prolongs rail service life by preventing the emergence of defects or by delaying their development, preventive grinding to improve the quality of the running surface of newlylaid rails and corrective grinding to remove rail defects that have already developed by

Choice of material for rail steels is of fundamental importance. This is because the rail's behaviour in service depends critically on the properties of the metal. Much effort and a considerable amount of research has already been undertaken in the search for the ideal rail steel (Pointner & Frank,1999). In recent years rail steel production has improved as manufacturers have developed steels with increased hardness and better wear resistance. There are many criteria which determine the suitability of a 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 localised plasticity and by

Fig. 2. Rail head degradation (Popovic et al., 2006).

reprofiling the rail to optimize wheel/rail contact.

**3. Rail steels** 

**2.1 Fracture control concepts** 

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

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

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 & Polycarpou, 2005).

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 tensile and fatigue strengths and performed well in service.

Surface Welding as a Way of Railway Maintenance 239

drawbacks. Namely, the use of buffer layer significantly slows down surface welding process, due to replacement of wires and settings of other welding parameters. Since, as already noted, for surface welding are mainly in use semi-automatic and automatic processes, it significantly extends the working time. The new classes of flux-cored and selfshielded wires are recently developed, and it is possible to achieve the requested properties

The material used in present work is pearlitic steel, received in the form of rails, type UIC 860 S49, what is the most common rail type on domestic railroads. It's chemical composition

Chemical composition, % Tensile

0.52 0.39 1.06 0.042 0.038 0.011 0.006 680-830 ≥14

Table 1. Chemical composition and mechanical properties of base metal.

Wire diam. mm

Ac (%) C Si Mn P S Cu Al

The surface welding of the testing plates was perfomed by semi-automatic process. As the filler material, the self-shielded wire (FCAW-S) and flux-cored wires (FCAW) were used, whose chemical compositions and mechanical properties are given in Table 2. The plates were surface welded in three layers; sample 1 with FCAW-S without buffer layer; sample 2

(self-shielded wire) 1.6 0.15 <0.5 1.1 1.0 0.5 2.3 1.6 30-40

Heat input during welding was 10 kJ/cm and preheating temperature was 2300C, since the CE equivalent was CE=0.64 (Popovic et al.,2010). Controlled interpass temperature was 2500C. Sample 1 is surfaced with one type of filler material (self-shielded wire), while for surfacing of sample 2 were used two types of wires, but both flux-cored: one for buffer layer and the second one for last two layers. As shieleded gas for welding of sample 2, CO2 was used. To evaluate the mechanical properties, specimens for further investigation were cut

strength Rm (N/mm2)

Chemical composition Hard-

HRC C Si Mn Cr Mo Ni Al

1.2 0.05 0.35 1.4 - - - - -

1.6 0.12 0.6 1.5 5.5 1.0 - - 37-42

Elongation

ness,

of welded joints without buffer layer (Popovic et al.,2011).

and mechanical properties are given in Table 1.

with FCAW with buffer layer (according to Table 2).

Filtub 12B (flux-cored wire)

Filtub dur 12 (flux-cored wire)

Table 2. Chemical composition of filler materials.

from surface welded rail head, according to Fig.4.

No. Wire designation

1.layer (buffer layer)

2. and 3. layer

OK Tubrodur 15.43

**5. Experimental procedure** 

Sample

Sample 1

Sample 2

Fig. 3. Optical microstructures of rail steels: (a) pealite; (b) bainite (Aglan et al.,2004).
