**4. Weldability of rails and types of filler materials**

Main problem in welding of pearlitic steels is their poor weldability, i.e. susceptibility to welding defects, due to its high carbon equivalent. Since the rail is produced from this type of steel and subjected to complex strain state, leading to its degradation, surface welding is presently the dominant maintenance way to prolong exploitation life. Damaged parts produced from pearlitic high-carbon steel can be surface welded, in spite of their poor weldability, and by properly choice of welding technology, it is possible to get improved structure with dominant properties comparing to the original part (Popovic et al.,2010). To achieve that, it is necessary that obtained morphology corresponds to the new steel generation, i.e. bainitic microstructure.

For surface welding are mostly in use semi-automatic arc welding processes, with flux-cored and self-shielded wires. Basic difference between them is the first requires an external shielding gas, and the second does not. In both cases, core material acts as a deoxidizer, helping to purify the weld metal, generate slag formers and by adding alloying elements to the core, it is possible to increase the strength and provide other desirable weld metal properties (Lee,2001; Sadler,1997). These processes have replaced slowly MMA process and they almost ideal for outdoors in heavy winds. The key strength of these processes lies in the replacement of those aspects of the conventional MMA process that often results in variability in the quality of the repair with automatic and more controlled operations. Although the MMA process is used many industries, it is heavily reliant on the competence of the welder, is time consuming, and is prone to internal defects such as porosity that subsequently grow through fatigue, and if not detected by ultrasonic inspection, result in rail breaks.

The result of flux-cored wire application is higher quality welds, faster welding and maximizing a certain area of welding performance (Popovic et al.,2010). The number of layers in surface welded joint depends of the damage degree, most frequently it's three, sometimes with buffer layer. The buffer layer is applied at the crack sensitive materials, what high carbon steel certainly is (high CE). The function of buffer layer is to slow down the growth of initiated crack with its own plasticity. Constructions, like railways, are exposed to cyclic load and wear in exploatation life, so crack must be initiated. Sometimes in these cases it is necessary to use buffer layer, what besides the good affects, has some 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 of welded joints without buffer layer (Popovic et al.,2011).
