CTOD tests

Even after the impact tests the fracture strength of welded structure was still unambiguous. As some results were not within the limit of the standards, CTOD tests were deemed necessary. These CTOD test were done according to standard ASTM E1290-2. The first CTOD test was conducted on the welded structure while the other test first used Gleeble simulation (as reported in experimental investigations 6.6.) before continuing with CTOD testing.

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CTOD tests are trustworthy and give accurate measurements of material toughness. It is very important to clarify toughness in a welded structure, especially in the HAZ which is a critical area in relation to material toughness.

The CGHAZ of the HAZ has been reported (Shi & Han 2007, Lee & al. 1993, Güran & al. 2007) to be the most brittle area where toughness is at its lowest.

Depending on heat input, the CGHAZ can have different widths. Finding the CGHAZ during testing has proven to be quite difficult. Simulation has been developed to clarify the characteristics of different areas in the HAZ, and a Gleeble simulation was used in this research to clarify ductility in the CGHAZ.

If the weld is welded with many passes, then the ICCGHAZ has been observed (Liu at al. 2007, Hamada 2003, Li et al. 2001, Lambert et al. 2000, Matsuda et al. 1995, Davis & King 1993, Lee et al. 1993) to be the worst impact ductility zone between two CGHAZs. This LBZ has a very brittle structure where the M-A phase will destroy the impact ductility. This ICCGHM-AZ is narrow and discontinuous, and only 0.5 mm width (Davis & King 1993) depending on heat input. CTOD tests are better suited to find this kind of brittle areas than Charpy-V tests, but in this study test place was unfortunately too far from the fusion line and ICCGHAZ LBZs were not under investigation. In Gleeble made test bars only one heat input was used.

The very brittle microstructure proves that the CGHAZ is a weak area within the HAZ. In this situation, it is assumed that the CGHAZ is the weakest zone in welded structure. In real structures, there are many zones in the HAZ and the width of the CGHAZ is usually narrow. The total width of all zones in the HAZ depends on heat input. When the heat input is large, those zones are wider and the tensile strength and toughness properties of the structure go down. The microstructure of the CGHAZ can be composed of M-A constituents and this making the structure very brittle. A good example of this brittle structure is seen in fig. 57 which was taken of Gleeble simulated QT test bar. The main microstructure is martensite and the proportion of bainite is less than half.

Additionally, the coarseness of bainite is a metallurgical factor affecting the impact properties as Lampert et al. (2000) have also studied.

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CTOD test results from the welded structure and base material are presented in table 35 and in fig. 58. Overall, the base material has the lowest CTOD value.

Only steel H exhibited different behaviour, as the base material of steel H had the highest CTOD value and only decrease by its higher heat inputs. This result was one of the hypotheses of this study. As seen in fig. 58, the highest results from this CTOD test were 0.2 or more. Five of the eight tested HSSs reached this value when the heat input was 1.7 kJ/mm. Steel A also reached this value with a heat input of 1.0 kJ/mm, but the value was too high as the result of a measurement mistake which is not clear. There were big differences between the base material CTOD test values. The lowest values, near 0.05, were seen in steels A, C, D and G, whereas the highest value, 0.2, was seen in steels B, F and H. With the heat input at 1.0 and 1.3, the measured values were not so unambiguous because the measured HSS, like steel B, had a low value when the heat input was 1.0 (0.15) and a high value (0.3) when the heat input was 1.3 and 1.7kJ/mm. Steel F had good values with all the welded structures.

It is very difficult to find the weakest zone of the HAZ. It was expected that the CGHAZ would to be the weakest area, however, it is very difficult to find the CGHAZ from within the HAZ. The place of CTOD test was 2 mm from fusion line, the same measurement as was used in Charpy-V test. As shown in table 35, near all test results were higher than base materials results. This most likely means that these measurements were taken from a HAZ area other than the CGHAZ. In fig. 56 it can clearly be seen that the place of the test was not in the CGHAZ. Depending on heat input, this zone of the HAZ was so far from the fusion line that the test place was most likely in the ICHAZ or SCHAZ. When conducting the CTOD test on a welded structure made from HSS, the initial crack must not be more than 0.5 mm from fusion line. If this criterion is met, then the initial crack will be in the CGHAZ.

Table 36 shows the results from Gleeble tested pieces. These test pieces were made to clarify the features of the CGHAZ microstructure from the tested HSSs.

These CTOD test results are very low compared to CTOD test results from

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welded structure, which means that all the CTOD test results from simulated structures were very brittle, as seen in fig. 59. In fig. 59, it is clearly explained that all of the results of this CTOD test were very low and within close value proximity to one another. Only steel E had one value over 0.05, however, this value was very low when compared to the base material. Fig. 58 additionally explains that the base material CTOD test values in all the tested HSSs were higher than in the Gleeble simulated test bars.

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Figure 56. Broken QT steel CTOD test piece where the place of initial crack is well seen.

When welding using undermatched filler material as was used in this study, it is clear that the weakest zone is in the weld. The rate of undermatching has a significant role in fracture toughness. When the rate of undermatching is low, the HAZ can have lower toughness than the weld or base material. This can encourage the toughness of the welded structure to decrease. Pisarski and Dolby (2003) concluded that the worst case fracture toughness of softened HAZs occurred when the HAZ undermatched in strength both the weld deposit

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and the parent plate. In this study, the highest level of undermatching was in the weld, which makes the fracture toughness acceptable.

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Figure 57. The CGHAZ of Gleeble simulated and CTOD tested QT steel. Aspect ratio is 1:500.

Table 35. CTOD test values from the welded structure.

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Table 36. CTOD values (mm) of Gleeble simulated CGHAZ.

STEEL

Figure 58. Compared CTOD values (mm) of welded HAZ structure.

0 CTOD values of welded HAZ structure

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Figure 59. Compared CTOD values (mm) of Gleeble simulated CGHAZ.

When comparing the structure of a Gleeble made CTOD test bar and a welded test bar, the size and phase of microstructure of the CGHAZ is different. In Gleeble made test bars, the initial austenite grain size was greater, ranging in value from 3-4 (ASTM E112-10) than grain size of welded CGHAZ, ranging in value from 4-5 (ASTM E112-10). The same differences in size were observed in the initial austenite grains of both QT and TMCP HSSs. This is explained in more detail in 7.11.5. Additionally, the microstructure of Gleeble made test bar had more martensite than the welded CGHAZ. In this study, the fracture toughness between welded and simulated structure cannot be compared because the CTOD test of welded structure was not in the CGHAZ.