13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- 2.2 Microhardness measurements and metallographic observations As-welded (AW) and heat treated (HT) specimens encompassing the three zones (FM, HAZ and BM) were sampled for microhardness measurements and metallographic observations. A rough polish was performed with abrasive paper of grit sizes ranging from 240 to 600, followed by a final polish using polycrystalline diamond suspension of sizes 6 μm, 3 μm and 1 μm. Vickers microhardness profiles were taken across the as-welded and heat treated specimens using a force of 100 gf and a dwell time of 15 seconds. The indentation spacing was 100 μm and measurements were taken on a length of 12 mm starting in the filler metal up to the unaffected base metal. A modified Fry’s reagent was used to chemically etch the specimens and metallographic observations were realized to position the fusion line. 2.3. Fatigue testing Compact tension (CT) specimens having a width of 50.8 mm and a thickness of 12.7 mm were machined from heat treated and as-welded plates in accordance with ASTM E647 [8]. The notches were aligned perpendicularly to the welding direction to allow for the crack to grow through each zone of the joint (FM, HAZ and BM). As seen in Fig. 1, the crack tip was positioned in the filler metal at 7 mm from the fusion line (FL). Constant stress intensity factor range tests in river water environment were performed at a constant load ratio R = 0.1 and at a frequency of 20 Hz using a 100 kN MTS servo-hydraulic machine. As-welded and heat treated specimens were tested at stress intensity factor ranges of 8 MPa∙m½ and 20 MPa∙m½. The crack length was continuously monitored using the compliance method with a crack mouth clip gauge. The load and crack opening displacement (COD) were recorded for every cycle and used to determine the closure loads. The closure stress intensity factor (Kcl) was defined for a compliance offset of 2 percent [8]. The effective stress intensity factor range (ΔKeff) was calculated by the difference between the maximum applied stress intensity factor (Kmax) and the closure stress intensity factor (Kcl). Figure 1. Compact tension specimen sampling layout. Starter notch aligned perpendicularly to the welding direction. Crack tip positioned at 7 mm from the fusion line in the FM. Filler metal 410NiMo Base metal CA6NM Welding direction CT specimen Fusion line Heat affected zone
RkJQdWJsaXNoZXIy MjM0NDE=