African Fusion November 2019

Flux-cored wires for gas pipelines

testingwelded sampleswill most probably be in the base metal. For the nickel-base 625‑T1, the fracture always occurred in the base metal. Table 5 shows the tensile test results for the 9% nickel joints. Here only the nickel- based625-T1 filler passed the requirements set by the standard. For the austenitic filler 17/15-T1, the failure occurred in the weld metal and the tensile strength was not met. This indicates that the austenitic manganese-alloyedwire is undermatching in strength and thus not an alternative for welding 9% steels. The results from the impact tests are shown in Table 6. The requirements were met with bothwires for the 5%nickel steel. For the 9% steel, the weld metal impact toughness was somewhat below the 80 J requirement for the 17/15-T1 wire. Figure 4 shows the hardness measure- ments on the cross-section of the welds in Figure 3. The lines in Figure 3c indicate where the hardness profiles were made. The hardness range was 157 to 304 HV, well below the maximum hardness of 450 HV10 set in ISO/TR 15608:2017 for the base materials. The base metal hardness was constant at around 230 HV, while the highest peakswere found in the fusion line. For 625-T1, the weld metal hardness was slightly lower than that for the base mate- rial, but 17/15-T1 showeda significant drop, especially when welding the 9% nickel steel. This supports the opinion that the tensile tests failed in the weld metal due to under matching strength. The mechanical properties verify that the requirements for cryogenic applica- tions can be met with both filler concepts when butt joint welding 5%nickel steels in the vertical-upposition. Thismeans that for joining these alloys, manganese-alloyed austenitic filler metals can potentially be used, providing a cost-effective alterna- tive to the nickel-base alloys established in the market. Increasedweldmetal strengthwouldbe needed, however, toensure that tensile test fracture occurs in the base metal for A645 Grade A. When welding 9% nickel steels, only the 625-T1 wire showed satisfactory strength and impact properties. This is in agreement with the recommendation that only nickel-based fillers should be used for welding 9% nickel steels [9]. Conclusions Two types of high-alloyed flux-cored wires have been evaluated for the suitability of welding 5 to 9%nickel steels in the vertical- up position for cryogenic applications: a

Figure 3: Micrographic examination of the weld metal: a) 5%Ni+17/15-T1; b) 9%Ni+17/15-T1; c) 5% Ni+ 625-T1; d) 9% Ni+625-T1.

for the 5% nickel joints. Both fillers meet the requirements for the base materials in the EN standard 10028-4:2017 and the ASME Boiler Pressure and Vessel Code ASME BPVC.II.A-2017. For the austenitic filler 17/15-T1, the failure occurred in the weld metal, but at a strength exceed- ing the limits in the standards. In this work, the 5% nickel steel X12Ni5 was of type A645 Grade A, which has higher strength than the typical X12Ni5 fulfilling EN 10028‑4:2017, but not ASME BPVC.II.A- 2017. For the standard European X12Ni5 alloy with lower strength than for A645 Grade A, the failure location when tensile

combination (Figure 2a) the etching time was increased to elucidate the weld bead layers. All samples showed normal weld shape and good sidewall fusion. Figure 3 shows the metallographically prepared and etched weld cross-sections. The morphology was homogenous with the typical dendritic solidification pattern. No large slag inclusions or unmolten flux particles could be found. At higher mag- nification, fine inter-dendritic precipitates were seen for the 17/15-T1 welds. Due to the high carbon content of this wire, these are suspected to be carbides. Table 4 shows the tensile test results

Filler metal

Yield strength [MPa] Tensile strength [MPa] Failure location

17/15-T1

513 ±8 498 ±9 Min 390

658 ±2 664 ±3

WM BM

625-T1

EN 10028-4:2017

530 to 710

ASME BPVC.II.A-2017 Min 450 655 to 795 Table 4: Transversal tensile test results of 5% nickel joints (average of three specimens) and requirements according to the material standards.

Filler metal

Yield strength [MPa] Tensile strength [MPa] Failure location

17/15-T1

527 ±9 587 ±13 Min 390

655 ±6 695 ±4

WM BM

625-T1

EN 10028-4:2017 680 to 820 Table 5: Transversal tensile test results of 9% nickel joints (average of three specimens) and requirements according to the material standard.

5% Ni at -140 °C

9% Ni at -196 °C

WM

FL

WM

FL

17/15-T1

75 ±3 J 81 ±2 J

50 ±4 J 52 ±3 J

17/15-T1

72 ±3 J 82 ±2 J

165 ±5 J 130 ±3 J

625-T1

625-T1

Requirement > 27 J Requirement > 80 J Table 6: Results of Charpy-V impact testing in the weld metal (WM) and along the fusion line (FL).

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November 2019

AFRICAN FUSION