13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- 3. Results 3.1. Microhardness measurements and metallographic observations The microhardness profiles of the as-welded and heat treated specimens are shown in Fig. 2a and b, respectively. Metallographic observations were realized to identify the position of the fusion line as shown in Fig. 3 for an as-welded joint. The microstructure of the heat treated joint is visually similar and has been omitted. The heat affected zone extends from the fusion line to the microhardness plateau characterizing the base metal, as indicated in Fig. 2. The shape of the microhardness profile is similar for both specimens. The microhardness has its highest value in the filler metal and decreases from the fusion line down to characteristic values of the base metal. The heat affected zone extends up to about 5 mm from the fusion line. The main difference between the two profiles is the microhardness of the filler metal, which is 360 HV on average in the as-welded condition and 325 HV following the tempering heat treatment. These results are in good agreement with what was measured by Thibault and al. from a FCAW CA6NM joint using filler metal 410NiMo [9]. Figure 2. Microhardness profiles a) As-welded specimen b) Heat treated specimen, 2 hours at 600°C Figure 3. As-welded joint microstructure near the fusion line revealed by etching (50X). a) b) FM HAZ
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