**5.4 Impact testing**

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 Figure 9.

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


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

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Values of total impact energy of samples 1 and 2 at room temperature are significantly higher (29 J and 34 J) than in base metal (5 J), as a consequence of appropriate choice of alloying elements in the filler material. The presence of Ni, Mn and Mo promotes the formation of needled bainitic microstructure and grain refinements, and increases the strength and toughness also(Popovic, 2006). By analyzing the impact energy values of sample 1, a change of toughness in continuity is observed, with no marked drop of toughness, and for all tested temperatures, crack initiation energy is higher than crack propagation energy. This is the reason for the absence of significant decrease of toughness. The highest value of total impact energy was found for the sample 2 at room temperaure (34 J), which is the only case when the initiation energy is lower than propagation energy (12 J and 22 J, respectivelly). This shown practically the buffer layer function. Namely, the initiated crack during propagation comes to plastic buffer layer, which slows down crack further growth. For this reason, the crack propagation energy is the largest part of total impact energy. However, at -200C, significant drop of total impact energy is noticable (14 J) due to losing of buffer layer plastic properties at lower temperatures. The low-carbon wire (0.05%C i 1.4%Mn) has excellent toughness, but and marked rapid drop on S-curve (dependence toughness vs. temperature). Transition temperature of this material above - 200C is confirmed by the obtained impact toughness results. The use of buffer layer is reasonable if the exploatation temperature is above -50C; on the contrary, at lower temperatures, buffer layer is losing its function and the toughess is decreased (Popovic et

Diagrams force-time, obtained by instrumented Charpy pendulum, are given in Figure 10. As can be seen, for the sample 1 the character of diagrams force-time changed little by lower temperature. Namely, this material at room temperature has diagram with marked rapid drop, as consequence of unstable crack growth. After the maximum load, a very fast crack growth is started, and it is confirmed by the low value of crack propagation energy(Grabulov et al.,2008). On the contrary, on the sample 2 diagram at room temperature, the presence of buffer layer is clearly shown. The initiated crack, during its growth, comes to buffer layer which temporary stops the further crack growth and changes crack growth rate. The obtained experimental diagram doesn't belong to any type, according to standard EN 10045-1. This leads to toughness increase, primarily crack propagation energy, and it is also here the only case when the crack initiation energy is lower than crack

t, 0C sample 1 sample 2 (BL)

al.,2011).

propagation energy.

200C


Table 4. Instrumented impact testing results of Charpy V surface weld metal specimens at all testing temperatures.

Fig. 9. Dependence total impact energy, crack initiation and crack propagation energy vs.temperature for (a) weld metal of sample 1 and (b)weld metal of sample 2 (Popovic et al.,2011).

**Total impact energy, Eu, J** 29 23 17 34 14 11 **Crack initiation energy, Ein, J** 20 16 15 12 10 10 **Crack propagation energy, Epr, J** 9 7 2 22 4 1 Table 4. Instrumented impact testing results of Charpy V surface weld metal specimens at

Fig. 9. Dependence total impact energy, crack initiation and crack propagation energy

vs.temperature for (a) weld metal of sample 1 and (b)weld metal of sample 2

all testing temperatures.

(Popovic et al.,2011).

**sample 1-WM sample 2- WM 200C -200C -400C 200C -200C -400C**  Values of total impact energy of samples 1 and 2 at room temperature are significantly higher (29 J and 34 J) than in base metal (5 J), as a consequence of appropriate choice of alloying elements in the filler material. The presence of Ni, Mn and Mo promotes the formation of needled bainitic microstructure and grain refinements, and increases the strength and toughness also(Popovic, 2006). By analyzing the impact energy values of sample 1, a change of toughness in continuity is observed, with no marked drop of toughness, and for all tested temperatures, crack initiation energy is higher than crack propagation energy. This is the reason for the absence of significant decrease of toughness. The highest value of total impact energy was found for the sample 2 at room temperaure (34 J), which is the only case when the initiation energy is lower than propagation energy (12 J and 22 J, respectivelly). This shown practically the buffer layer function. Namely, the initiated crack during propagation comes to plastic buffer layer, which slows down crack further growth. For this reason, the crack propagation energy is the largest part of total impact energy. However, at -200C, significant drop of total impact energy is noticable (14 J) due to losing of buffer layer plastic properties at lower temperatures. The low-carbon wire (0.05%C i 1.4%Mn) has excellent toughness, but and marked rapid drop on S-curve (dependence toughness vs. temperature). Transition temperature of this material above - 200C is confirmed by the obtained impact toughness results. The use of buffer layer is reasonable if the exploatation temperature is above -50C; on the contrary, at lower temperatures, buffer layer is losing its function and the toughess is decreased (Popovic et al.,2011).

Diagrams force-time, obtained by instrumented Charpy pendulum, are given in Figure 10. As can be seen, for the sample 1 the character of diagrams force-time changed little by lower temperature. Namely, this material at room temperature has diagram with marked rapid drop, as consequence of unstable crack growth. After the maximum load, a very fast crack growth is started, and it is confirmed by the low value of crack propagation energy(Grabulov et al.,2008). On the contrary, on the sample 2 diagram at room temperature, the presence of buffer layer is clearly shown. The initiated crack, during its growth, comes to buffer layer which temporary stops the further crack growth and changes crack growth rate. The obtained experimental diagram doesn't belong to any type, according to standard EN 10045-1. This leads to toughness increase, primarily crack propagation energy, and it is also here the only case when the crack initiation energy is lower than crack propagation energy.

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The behaviour of welded joint and its constituents should affect the change of curve slope in validity part of Paris law. Materials of lower fatigue-crack growth rate have lower slope in the diagram d*a*/d*N vs.* K. For comparison of the properties of surface welded joint constituents the crack growth rates are calculated for different values of stress-intensity

factor range K.

Fig. 11. Diagram da/dN vs. K for base metal.

Fig. 12. Diagram da/dN vs. K for sample 1 and sample 2.

Fig. 10. Diagrams force-time, obtained by instrumented Charpy pendulum for sample 1 and sample 2 (Popovic et al.,2011).
