3.1 Visual and Radiographic examination
As briefed in Table 7, all mechanical and Non-destructive testing (NDT) was performed in accordance with relevant EN standards. Visual and NDT testing of radiography was performed to 100% extent on each sample of proximity distance. Due to spray mode in MAG+FCAW welding process weld B had wider and irregular cap weld bead in contrast to smooth weld cap bead of weld A, which has welded with TIG process as shown in figure 4. As per acceptance criteria of ISO 10675-2016, both joints were accepted in accordance with NDT radiography testing as shown below in figure 5.
3.3 Hardness testing
Vickers hardness testing on each case of weld proximty distance of 5, 10 & 15 mm was peformed as per NS-EN ISO 9015-1:2011 as per positions marked shown in figure 7. The indentation marks shown in figure 7 can be seen clearly seen in macro graphs of each weld promxity case in figure 6a-c. As proximity distance was area of concern, hence hardness meassurement points between adjacent weld toes at ‘weld cap level’ for 5 mm proximity is shown in figure 8.a. It can be observed that high hardness values is noticed at proximty distance (PD-A&B) between two weld metal (WM-A&B) and their respective adajacent HAZ-A&B in constrast to parent metal PM as shown in figure 8a. This proximty distance or area called as ‘alien metal’ has experienced microstructural changes due to high restraint and succesive thermal cycles of multipass welds as noticed in [20] for proximty welded joints K -brace joint of offshore jacket structures.
A maximum value of 210HV was observed close to HAZ value of 225HV on either side for proximty distance 5mm welds as shown in figure 8a. Cold cracking chances increases if hardness values are between 350-400HV. This region may be prone to cracking as it has accumlated high stresses and chances of martensitic formation have been sustained due to succesive thermal cycle of heating and cooling[20]. Similar hardness points profiles for weld proximity distance of 10 & 15 mm have been shown in figure 8b & c. As proximity distance is increasing, drop in hardness values can be seen and comparable to PM in case 10& 15mm case. In case of 10 mm proximity distance, highest value of 186 HV was observed at alien metal and 167 HV in case of 15 mm distance which is almost equivalent to parent metal.
3.4 Charpy testing
Charpy testing was performed in accordance with KV8, NS-EN ISO 148-1:2016; ISO 9016:2012. As discussed previously in figure 3, due to limited space for FL+5mm samples only six specimens were available for 5 mm proximity distance whereas FL +5 mm was possible for 10- & 15-mm proximity distance as shown in figure 9. Sample size was kept as 5x10x55 mm as thickness of pipe was 8.2 mm hence a conversion factor of 2/3 was applied, when calculating energy values as per code EN 1614-2017.
It can be observed from figure 10 that the least energy values of 55J at proximity region i.e., weld A-FL+2 mm for 5mm proximity distance was observed in contrast to 10 & 15mm case. To draw fair comparison, FL+2 mm Charpy values are highlighted in figure 10 and values of 62 J & 66 J was observed i.e., between weld A-B for 10 & 15mm case. Due to high hardness and high restraint in 5mm proximity case, drop in energy values can be observed. This drop can be attributed to high hardness values observed in section 3.3. However, microstructure graphs shown in section 3.6 later, also substantiates, that high hardness values at proximity regions for changed morphology of grain size in proximity region.
3.5 Tensile & Bend tests
Tensile tests were performed for all three weld proximity cases (5, 10 & 15mm) in accordance with ISO 6892-1:2016. Table 8, summarize values of all tensile test data along with its fracture location. It was interesting to note that values of tensile strength for 5 mm weld proximity case was found to somewhat lower than 10- & 15-mm cases, Fracture location could not provide any conclusion as it changed from outside of weld A or B sufficiently far from HAZ and proximity region. It would have been interesting if failure would have occurred between welds for 5mm case. However, in case of 15mm proximity case fracture occurred between welds in one case. It can be inferred that in 15 mm distance is sufficiently far and fracture location can be considered as close to parent metal.
Table 8
Examination and tensile testing data for all weld proximity cases
Weld proximity (mm) | No of samples | Test indent | Thickness (mm) | Width (mm) | Area (mm2) | Tensile (Mpa) | Elongation % | Fracture |
5 | 2 | Cross weld 1 | 8.61 | 24.93 | 214.6473 | 480 | 14.71 | Basemetal B side |
Cross weld 2 | 8.16 | 24.91 | 203.2656 | 489 | 12.93 | Basemetal A side |
10 | 2 | Cross weld 1 | 8.41 | 24.91 | 209.4931 | 493 | 16.31 | Basemetal A side |
Cross weld 2 | 8.12 | 25.04 | 203.3248 | 516 | 17.36 | Basemetal A side |
15 | 2 | Cross weld 1 | 8.2 | 25.09 | 205.738 | 487 | 12.10 | Basemetal between welds |
Cross weld 2 | 8.58 | 24.92 | 213.8136 | 498 | 15.59 | Basemetal A side |
Figure 11 illustrates the tensile plots for all weld proximity cases tested under monotonic loading. Tensile results sufficiently meet the required strength of 460-620 MPa, however all values were found to be less than pipe measure value of 553 MPa referred from material test certificate mentioned in Table 5. Elongation values in all cases were found to be less than 22% indicating the high restrain and residual stresses caused by welds placed at proximity.
Bend tests were also performed as per EN ISO-5173:2010 at designated location of root and face as mentioned in NORSOK M-101 & EN 15614-2017. For thickness less than 12 mm, two samples each for face and root bend are tested. During testing for all weld proximity samples of 5, 10 & 15 mm did not reveal any one single flaw greater than 3 mm in any direction. Flaws appearing at corners of test specimen are ignored as mentioned in EN 15614-2017.
3.6 Microstructure characterization
Due to high values in hardness and drop in Charpy values for 5mm weld proximity case a need to further investigation of microstructure characterization was initiated. Optical microstructures across the weld interface starting from parent metal to HAZ of weld A to proximity region to HAZ of weld B was performed on macro specimen samples of weld proximity 5mm specimen as shown in figure 12a-d. The microstructures at HAZ showed acceptable weld interface without any defects however different orientation of grains was observed in region identified as proximity region/alien metal i.e., between two welds as shown in figure 12c.
Microhardness results at designated location of HAZ and proximity area between welds (PA) corresponds to values of 225 HV close to 210 HV respectively as shown in figure 8a. Based on this information, microstructure in HAZ regions can be mainly composed of composed of ferrite (50 to 80%), bainite (0 to 30%), pearlite (0 to 20%), martensite (0 to 20%) corresponding to Vickers hardness values for steel S355[20] referred from Table 9 [21]
Table 9
Microstructures and the corresponding Vickers hardness ranges of a low-alloyed steel[21]
Microstuctures | Average Vickers Hardness (approximately |
Ferrite | 84 |
Austenite | 263 |
Perlite (granular) | 211 |
Perlite (lamellar) | 316 |
Cementite | 632 - 684 |
Martensite | 421 - 948 |
The optical images seem to confirm regions of perlite (P) and ferrite (F) in parent metal as shown in figure 12 a. Acicular ferrite (AF-large light areas) with grain boundary ferrites (GBF) can be observed in weld metal A& B. HAZ depending upon temperature range is divided intocoarse-grained HAZ (CGHAZ), fine-grained HAZ (FGHAZ), and the inter-critical HAZ (ICHAZ) [8]. In HAZ, structure is formed upon temperature attained between Ac1 and Ac3 i.e., upper & lower critical temperatures as shown in figure 12 b & d. As proximity area/ alien metal corresponds to fine grained structure in contrast to CGHAZ & FGHAZ hence it can be identified as a region close to ICHAZ that’s is subjected to successive cycle of heating and cooling between Ac1 and Ac3 temperatures i.e., 725-915°C with finer grain as confirmed form thermocouple data.
Martensite presence is difficult to identify and quantify its fraction however sufficient evidence of martensite -austenite (M-A) islands have been identified in inter-critically reheated coarse-grained heat affected zone (ICCGHAZ) undergoing partial austenisation and forming austenitic-martensitic (M-A) phases which are brittle and known as local brittle zones (LBZ) [22]. However, their presence needs further investigation by performing scanning electron microscopy (SEM) analysis.