PSI - Issue 68

Sakari Pallaspuro et al. / Procedia Structural Integrity 68 (2025) 802–808 Pallaspuro S. et al. / Structural Integrity Procedia 00 (2025) 000–000

805

4

3. Results and Discussion Fig. 1 shows the weld seam (WS), HAZ, BM, and the UCI hardness mapping results. The microstructure of WS (Fig. 1a) at the centreline and middle thickness of the specimen (= crack tip region) is large-grained martensite (Fig. 1b), which is significantly coarser than the base material matrix (Fig. 1d). Fig 1c shows the locally nearly undistinguishable fusion line and the approximately 250 μm wide coarse-grained heat-affect zone. XRD reveals that the retained austenite fraction (RA) of the as-welded weld seam is 3.2 ± 0.2 vol.%. With the austenite lattice parameter of 3.593 Å, this would translate to an average carbon concentration of 0.67 wt.%, threefold to the nominal bulk content. This is also higher RA fraction than with the direct-quenched DQ condition at ≲ 1 vol.% (Somani et al., 2018). The PWHT treatment at 275 °C results in partial decomposition of RA with the retained austenite fraction lowering to 1.8 ± 0.1 vol.%, a/b/c = 3.609 Å, and estimated remaining average carbon concentration of 1.02 wt.%. This implies that the remaining retained austenite should be stable, but also that a fraction has decomposed into carbon-rich martensite and/or cementite during the PWHT treatment. The hardness distributions show how the PWHT treatment suppresses the hardness levels and widen the softened zone within HAZ (Fig. 1 e-h). This softening is in a magnitude of ~20 HV 1 based on the line-scans over the weld seam as reported by Pallaspuro et al. (2022), peak values for the HAZs, and average values for the base materials and WS. Due to the PWHT treatment, cementite formation can be expected within the hardened region in WS and HAZ, and up to 50 % of the macroscopic residual stresses can be relieved in tempering at 275 °C (Lohe & Vohringe, 2002). The fracture toughness results are summarised for the key parameters in Table 1, and the Master Curve plots are presented in Fig. 2 for the base materials and the weld seam, and in Fig. 3 for the fusion line / coarse-grained heat affected zone notch-position. All the specimens have straight crack propagation, i.e., cracks have not deflected to different zones, but exact HAZ region of the specimens notched at the FL/HAZ were not identified in this study. Note that the Master Curve -fitted results in Fig. 2 and Fig. 3 are scaled to 25.4 mm (1”) specimen thickness, K Jc(1T) , while Table. 1 reports the actual measured values. The specimens tested at room temperature fall outside the T 0 ± 50 °C range, and are not included in the Master Curve fits. These single specimens show extensive ductile tearing beyond the censoring limits, achieving K Jc levels between ~140 MPa√m to ~200 MPa√m. Considering also the expected temperature dependency (Figs. 2 and 3), these levels appear low when comparing against the nearly similar toughness levels at -40 °C. However, keeping in mind the lowest yield strength of 1130 MPa (DQ&P BM) within the sampled materials, these steels can even outperform the best bcc-fcc and austenitic materials reported recently by Javaheri et al. (2023). In this comparison, the room temperature properties of the as-welded and PWHT samples appear excellent, K Jc(1T) being well above 100 MPa√m. Before further claims, more tests are needed to gain statistical certainty to support these findings. Table 1. Fracture toughness test results: room temperature K Jc (RT), low-temperature K Jc (-40 °C), fracture toughness reference temperature T 0 (or provisional T 0Q ), and with comparison to impact toughness transition temperatures T 28J reported by Pallaspuro et al. (2022).

K Jc (RT) [MPa√m] K Jc (-40 °C) [MPa√m] T 0 or T 0Q (*) [°C] T 28J [°C]

DQ (BM)

~173 ~197 ~145 ~152 ~168 ~179 ~176

114 ± 16 111 ± 28 124 ± 20 106 ± 10 157 ± 25 127 ± 40 148 ± 20 141 ± 42

-30 -41 -37

-104 -103

DQ&P (BM)

EB WS

-66 -33 -86 -39 -78 -39

EB WS + PWHT

-25 *

DQ HAZ

-58

DQ HAZ + PWHT

-44 *

DQ&P HAZ

-55

DQ&P HAZ + PWHT ~161

-51 *