**5. Experimental procedure**

238 Mechanical Engineering

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

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

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

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

(b)

generation, i.e. bainitic microstructure.

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

if not detected by ultrasonic inspection, result in rail breaks.

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 and mechanical properties are given in Table 1.


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

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


Table 2. Chemical composition of filler materials.

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

Surface Welding as a Way of Railway Maintenance 241

Microstructural analisys of all characteristical zones of welded layer has been done. Heat affected zone (HAZ) also has pearlitic microstructure, but with finer grain, than base metal (Figure 6), so its structure is improved and it is not a critical place in weldment. That is result of thermomechanical treatment of HAZ which is re-heated three times. Structural compatibility between deposite metal and base metal was achieved and martensitic layer

The greatest differences appear in first layer microstructure, Fig.7. First layer microstructure of sample 1 consists of ferrite, pearlite and bainite, what is result of mixing of low-alloyed filler material with high-carbon base metal. For first layer deposition of sample 2 is used low-carbon wire alloyed with Mn, as a function of buffer layer, so characteristical structure consist of great fraction of ferrite with relatively large primary grains. Beside proeutectoid ferrite, microstructure contains Widmanstatten and acicular ferrite (Popovic et al.,2007).

(a) 200x (b) 200x Fig. 6. Microstructure of a) base metal and b) HAZ of both samples (Popovic et al.,2007).

The second layer microstructure is the most important in surface welded joint, because it has the greatest influence on mechanical and technological properties and exploatation behavior of repaired parts. For this structure is characteristic larger fraction of bainite, consequence to the less mixing with base metal. In second layer of sample 2 occurs fine grain ferritic structure with low content of bainite. This structure has finer grain compare to first layer, what is result of heat treatment and chemical composition (presence of Mo in

The third layer of sample 1 has some coarser grain structure, with higher content of bainite, compare to previous layer, what is consequence of re-heating absence. For third layer of sample 2 is characteristical bainitic microstructure with small amount of martensite and

Though used filler materials are different type, alloying concepts, sort of protection, buffer layer, as final result is obtained desirable bainitic microstructure with superior properties compare to base metal (Popovic et al.,2007). Except metallography examination, this is

**5.2 Microstructure** 

wasn't formated.

filler material).

locally zones of proeutectoid ferrite.

confirmed by other detail tests (Popovic,2006).

1- specimen for toughness and crack growth resistance estimation


Fig. 4. Specimens from surface welded rail head (Popovic et al.,2010).

#### **5.1 Hardness**

Hardness measurements were performed using a load of 100Pa. Hardness profiles of surface welded joints are shown in Fig. 5. The lowest hardness is in the base metal (250-300 HV), being the hardness of naturally cooled standard rails(Lee & Polycarpou, 2005; Singh et al., 2001). In HAZ hardness increase is noticable in both samples, due to complex heat treatment and grain refinement (Popovic et al.,2010). In sample 2 comes to a sharp decrease of hardness in first surfaced layer, i.e. in buffer layer. The function of buffer layer is to stop the growth of initiated crack with its own plasticity and reduced hardness. The hardness of II and III welded layers of both samples are the highest and similar, due to influence of alloying elements in filler materials, which shift transformation points to bainitic region4. Maximum hardness level of 350-390 HV is reached in surface welded layers and it provides improvement of mechanical properties and wear resistance.

Fig. 5. Hardness profiles along the joint cross-section of samples (Popovic et al.,2011).
