The following section presents the results of the investigations. Outcomes are divided into three subsections. In addition to toughness testing, the microstructure and the hardness of the welds were also investigated. The results of the Charpy impact test for the 20-mm samples and the 100-mm samples are presented in separate subsections.
Prior to toughness testing the microstructure of the welds was investigated. In Fig. 4, half of the FZ and the HAZ is displayed. The seam width in the 20-mm weld sample is approximately 2 mm. A fine grained heat affected zone can be recognised next to the fusion line with a width of less than half a millimetre. The 100-mm sample shows a FZ with approximately 6-mm width. In this case, a fine grained heat affected zone of about 2 mm can be recognised. A coarse grained zone could not be identified. The dendritic grains in the FZ in the 20 mm seam are significantly smaller than the grains in the 100 mm seam. The average dendrite length in the 100 mm FZ is 630 μm, compared to 140 μm in the 20 mm FZ.
Since Charpy impact tests were carried out on the as welded and in post weld heat treated conditions, a Vickers hardness test was performed in both conditions. Hardness lines were measured across the FZ and the HAZ, at half seam depth. Spacing between the imprints was set to 0.7 mm. Figure 5 shows the impact of the PWHT. Hardness and consequently the overmatching, between base metal and weld seam, could be decreased. The seam hardness after the PWHT was below 350 HV, as required by ISO 15614-1:2004. The missmatch ratio M is calculated as M = H
. Average mismatch ratio between the base material and the FZ in the as welded condition was M
=1.31 and for the PWHT condition M
The microstructures of the TIG and FC-MAG welds did not show any irregularities and appear like typical multi-pass arc welded microstructures. The hardness of the FZ in the TIG welds was measured with 315 HV10 and with 308 HV10 in the FZ of the post weld heat treated FC-MAG welded cross sections.
Toughness of 20 mm welds
The average impact toughness of the 20 mm welds is displayed in Fig. 6. Values which are not valid according the standard (specimen is not fully broken; fracture deviates out of the FZ) are marked with an asterisk.
For specimens in AW conditions with standard notch (AW-Std), no regular values could be measured due to fracture path deviation (see Fig. 1). Toughness values are extraordinarily high and exceed the values of the base material. Lateral expansion exceeded 1.5 mm for all specimens in this group.
The SN12 specimen with the side notches (AW-SN12) did not return valid results either. Samples did not fully fracture and inner FPD (fracture plane is curved within the notches, see Fig. 7) occurred. The fracture path actually crosses HAZ and base material. The value for the AW-SN12 group also exceed the value of the base material.
Specimen with standard 10 x 10 mm geometry and side notches (AW-SN) returned valid values. Some of the specimen also show this inner FPD but others broke with a flat brittle looking fracture. The absorbed energy reached 80 % of the side notched base metal specimens (BM-SN). Lateral expansion did not exceed 0.5 mm.
The situation for the post weld heat-treated specimen is more obvious. For all tested groups (Std, SN12 and SN), valid results could be measured. The PWHT-Std specimen fractured with a straight fracture plane with 100 % crystalinity; lateral expansion was 0.5 mm. Both side notched geometries (SN and SN12) fractured also in a brittle manner but with smaller lateral expansion (<0.5 mm).
Toughness of 100 mm welds
The average impact toughness of the 100 mm welds is displayed in Fig. 8. Values which are not valid according to the standard are marked with an asterisk.
The standard specimen in AW conditions offer no valid result since no sample broke fully. The fracture is clearly ductile with distinctive shear lips and the fracture path did not leave the FZ (see Fig. 9). Lateral expansion was measured with up to 2 mm.
The specimen with the side notches in AW conditions (AW-SN12 and AW-SN) returned valid results according the standard; all samples broke fully and no FPD occurred. The fracture planes are flat with a fully crystalline appearance and show a lateral expansion of approximately 0.2 mm.
In the PWHT-Std group, two samples did not break fully (values around 180 J, lateral expansion >1.5 mm, noticable shear lips). The other PWHT-Std specimen experienced fracture, with a rough crystalline surface. Small shear lips can be recognised. The lateral expansion was 0.9 mm.
All side notched specimen (SN12 and SN) in the 100 mm PWHT group behaved comparable to the PWHT side notched specimens in the 20 mm group. All specimen return valid values. The fracture surface appears brittle and flat with lateral expansion around 0.4 mm. No shear lips were found.
Comparison EBW to GMAW
For comparison, the results of the 100 mm EB welds were used, because in this tests the fracture path remained in the FZ (see Fig. 9b). To compare the maximum toughness values of the different processes, the EB and TIG welds were tested in as welded conditions, whereas the FC-MAG welds were tested in PWHT conditions. Results are displayed in Fig. 10.
The TIG welds offer a superior toughness for all specimen geometries. Neither the Std nor the SN and SN12 specimens broke fully (values marked with an asterisk). All specimens showed fully ductile behaviour. Shear lips appeared only on the Std specimens. Lateral expansion was measured on all specimen with at least 0.7 mm.
The absorbed energy of the FC-MAG welds is lower compared to the EBW welds. The Std FC-MAG specimen broke fully, showing noticeable shear lips. Lateral expansion was below 0.5 mm. The fracture surface appeared dull (ductile) with some crystalline (brittle) areas. Charpy behaviour was as expected from former tests. The SN12 and SN specimen did not show shear lips or lateral expansion. The fracture surface appeared equal to the Std specimens. FC-MAG Charpy values do not show a high decrease of absorbed energy when using side notched specimens for testing.