PSI - Issue 5

Lars Sieber et al. / Procedia Structural Integrity 5 (2017) 1011–1018 Sieber et al. / Structural Integrity Procedia 00 (2017) 000 – 000

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Fig. 3. Cutting plan of the sample TFT for two welding seams with highlighted welding defects

3.5. Metallographic investigations

In order to analyze both the structure of the produced weld seams as well as the changes in the grain structure of the base materials, metallographic investigations were carried out. Figure 5 shows an example of a micro-section. The etching with 3% alcoholic nitric acid visualizes the grain structure and the heat-affected zones (HAZ). Due to the low carbon content of the old steel, the HAZ is only weakly visible. In contrast to the linear ferritic-perlitic structure of the new structural steel the grain structure of the old steel is predominantly ferritic with linear inclusions. Due to its higher purity the new mild steel obviously shows less metallic inclusions. Therefore, a failure during mechanical testing on this side of the weld is not probably. On the other hand, the old mild steel is considerably weakened by its inclusions and impurities. Due to the very low carbon content a nearly pure ferrite structure is found in the old steel. Thus, grain growth was expected during manual welding and subsequent slow cooling. Contrary to the expectations, a significant reduction of grain size can be seen in the old mild steel HAZ. Starting from the base grain size (Figure 4a) this effect is visible throughout the HAZ (Figure 4b) up to the fusion line (Figure 4c). A grain boundary analysis revealed an accumulation of inclusions and thin pearlite phase structures. It was concluded that grain growth during and after  -  -change was probably inhibited by high-melting inclusions. This assumption is currently analyzed by means of SEM investigations. It would explain the overall smaller HAZ grain size. As a result, strength and ductility could even benefit from this effect.