PSI - Issue 37

R.J. Mostert et al. / Procedia Structural Integrity 37 (2022) 763–770 Author name / Structural Integrity Procedia 00 (2021) 000 – 000

765

3

the longitudinal axis parallel to the rolling direction of the plate, using capacitive discharge spot welding. The selection of the sample orientation was forced by the specimen geometry. Table 1: Specified and actual chemistry of the ASME SA 302 Gr C base plate material (in wt.%) C Mn Si Mo Ni P S Al Fe 0.25 max 1.07 - 1.62 0.13 - 0.45 0.41 - 0.64 0.37 - 0.73 0.025 max 0.025 max Not stated Balance 0.18 1.32 0.26 0.48 0.53 0.006 0.001 0.02 Balance An ambient-temperature calibration procedure was performed through tensile testing using MTS Criterion 45 test system with both strain-gauge and a 50 mm gauge length contact extensometer connected to the surface opposite each other over the softer weld-metal to check the accuracy of the strain-gauge measurements (Pretorius et al. 2021). It was observed that a very good correlation exists between the extensometer and the strain-gauge strain measurements for a given applied stress with the elastic moduli falling well within the expected range of 190 to 210 GPa for both the extensometer and strain-gauge. Therefore, the strain gauge was used in all the strain measurements with high confidence (Pretorius et al. 2021). The instrumented samples were placed within an autoclave and the strain gauge leads were taken out through the pressure envelope. After performing a stress relief heat treatment, the specimen was exposed to hydrogen gas within the autoclave at 200 bar at a temperature of 500 °C , test conditions which were aimed at accelerating the HTHA process (Pretorius et al. 2021). Swelling was continuously monitored until the swelling rate reduced to zero as indicated by the flattening of the strain – time curve. The experimental set-up is described in more detail elsewhere (Pretorius et al. 2021). In addition to the results obtained for the C- 0.5 Mo weldment, published strain – time curves by other authors (Munsterman, Seijas, and Williams 2010), were also analysed and discussed. 3. Results and analysis The strain-time curve for the HTHA swelling in the coarse grained HAZ region for the instrumented autoclave test runs is presented in Figure 1 and the microstructure at the completion of the run is shown in Figure 2.

1000

900

800

700

600

R 2 = 0.996 (sigmoidal curve)

300 HTHA strain ( με) 400 500

200

y = 0.7495x R² = 0.9677

Experimental data points Fitted Linear and sigmoidal functions

100

0

0

200

400

600

800

Exposure time (h)

Figure 1: Swelling strain of coarse- grained HAZ zone of the C-0.5 Mo welded joint as a function of time at a hydrogen partial pressure of 200 bar at 500 °C, as measured with the encapsulated strain gauges.