13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- A fatigue crack growing in a weld can be under the influence of tensile or compressive residual stresses, depending on the geometry of the joint and the position of the crack tip. Under tensile residual stresses, the crack tip is submitted to higher maximum and minimum loads resulting in a higher effective load ratio. This can also affect the effective stress intensity factor range as it can prevent closure, often resulting in higher fatigue crack growth rates (FCGR) [4-6]. On the other hand, compressive residual stresses can accentuate crack closure and be beneficial if they reduce the effective stress intensity factor range and thus the fatigue crack growth rate [5]. The main objective of this study was to characterize the effect of residual stresses on the fatigue crack growth behavior of a flux-cored arc weld consisting of alloy CA6NM and filler metal 410NiMo. Intra-specimen fatigue crack growth rate variations between the filler metal (FM), heat affected zone (HAZ) and base metal (BM), as well as the effect of post-weld heat treatment on fatigue crack growth behavior were studied. Microhardness profiles and metallographic observations were realized on as-welded (AW) and heat treated (HT) specimens encompassing the three zones (FM, HAZ and BM) in order to position the fusion line and determine the width of the heat affected zone. Fatigue crack growth tests were carried out at constant stress intensity factor ranges in order to highlight the effect of residual stresses on the fatigue crack growth behavior. The results are discussed based on the effect of the residual stresses as well as crack closure. 2. Experimental procedure 2.1. Materials The materials used in this study are base metal martensitic stainless steel alloy CA6NM, and the matching filler metal 410NiMo. The chemical composition and mechanical properties of the CA6NM alloy used are given in Table 1 and Table 2, respectively. Using a fully automated flux-cored arc welding (FCAW) process, 40 mm of weld was built up at the surface of a 50 mm thick CA6NM plate. The welding parameters are given in Table 3. The welded plate was cut in two parts, one of which underwent a post-weld heat treatment at 600°C for two hours. Specimens were prepared from both plates for microhardness measurements and fatigue testing. Table 1. Chemical composition of base metal CA6NM (weight %) [7]. Material C Mn Si S P Cr Ni Mo CA6NM 0.02 0.66 0.59 0.008 0.031 13.04 4.07 0.53 Table 2. Mechanical properties of base metal CA6NM [7]. Yield strength Tensile strength Young’s Modulus Elongation Reduction Area 763 MPa 837 MPa 206 GPa 27.0 % 58.8 % Table 3. Welding parameters. Process Shielding gas Wire diameter Voltage Current Speed Heat input FCAW 75% Ar, 25% CO2 1.6 mm 27.5 V 260 A 5 mm/s 1.4 kJ/mm
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