**5.2 Microstructure**

240 Mechanical Engineering

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

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

1- specimen for toughness and crack growth resistance estimation

improvement of mechanical properties and wear resistance.

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

2- specimen for microstructural analysis

4- specimen for hardness measurements

3- tensile specimens

**5.1 Hardness** 

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 wasn't formated.

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 filler material).

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 locally zones of proeutectoid ferrite.

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 confirmed by other detail tests (Popovic,2006).

Surface Welding as a Way of Railway Maintenance 243

The tensile tests were conducted on a 2 mm thick specimens. The room temperature mechanical properties (ultimate tensile strength, UTS) of the surface welded joint are shown in Figure 8. The basic requirement in welded structures design is to assure the required strength. In most welded structures this is achieved with superior strength of WM compared to BM (overmatching effect), and in tested case this is achieved (Burzic & Adamovic,2008; Manjgo et al.,2010). The highest UTS is in weld metal of sample 2 (1210

Fig. 8. Ultimate tensile strength of the surface welded joints (Popovic et al.,2011).

higher (11-12 J) and is similar for both samples at all testing temperatures.

Impact testing is performed according to EN 10045-1, i.e ASTM E23-95, with Charpy V notched specimens, on the instrumented machine SCHENCK TREBEL 150 J. Impact testing results are given in Table 3 for base metal and HAZ at all testing temperatures. Total impact energy, as well as crack initiation and crack propagation energies, for weld metal of both samples at all testing temperatures (200C, -200C and -400C) are presented in Table 4 and in

The total energy of base metal is very low (5 J), due to very hard and very brittle cementite lamellae in pearlite microstructure (Popovic et al.,2011), while the toughness of HAZ is

**base metal** 5 3 3 **sample 1-HAZ** 12 11 10 **sample 2-HAZ** 11 10 9

Table 3. Instrumented impact testing results of Charpy V specimens for base metal and HAZ

**Total impact energy, Eu, J**  200C -200C -400C

MPa), due to solid state strengthening by alloying elements.

**5.3. Tensile tests** 

**5.4 Impact testing** 

at all testing temperatures.

Figure 9.

Fig. 7. Microstructure of all surface welded layers (Popovic et al.,2007).
