13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Residual stress effects on the propagation of fatigue cracks in the weld of a CA6NM stainless steel Alexandre Trudel1,*, Myriam Brochu1, Martin Lévesque1 1 Department of Mechanical Engineering, École Polytechnique de Montréal, Montreal, Canada * Corresponding author: alexandre.trudel@polymtl.ca Abstract The fatigue crack growth behavior in an automatic flux core arc weld was investigated for stainless steel alloy CA6NM. As-welded and heat treated compact tension specimens were prepared with the crack propagation plane perpendicular to the weld. Constant stress intensity factor range tests were performed at a load ratio R = 0.1 to reveal intra-specimen fatigue crack growth rate variations and differences between the fatigue crack growth behavior of as-welded and heat treated specimens. Comparing the fatigue crack growth rate of as-welded and heat treated specimens revealed that the crack propagates at a faster rate in the as-welded specimens where indications point to the existence of tensile residual stresses acting on the crack tip by inhibiting crack closure. As for the heat treated specimens, when the crack is in the base metal, the fatigue crack growth rate decreases accompanied by an increase in crack closure levels. Tensile residual stresses that may have remained after the post-weld heat treatment and their subsequent relaxation with crack growth could explain this behavior. Keywords Welding, Stainless Steel, Fatigue Crack Growth, Residual Stresses, Crack Closure 1. Introduction Cast martensitic stainless steel alloy CA6NM has been used to manufacture hydraulic turbine runners for many decades. Its high strength, weldability and resistance to corrosion and cavitation damage make it a prime candidate for this application. Turbine runners are manufactured by welding cast blades to a cast core using a matching filler metal such as 410NiMo. During service, the cyclic loads acting on the runners can lead to fatigue failures [1]. A damage tolerance approach for the stress analysis of turbine runners can be an efficient strategy to account for the discontinuities, such as casting and welding defects as well as partial penetration weld joint designs, found in these components. A good knowledge of material resistance to fatigue crack growth is therefore needed. During welding, residual stresses develop and are believed to have a significant influence on the fatigue crack growth behavior of CA6NM welds. Weld-induced residual stresses arise from the elastic interactions between the constraining base metal and the weld metal shrinkage upon solidification and cooling. In stainless steel CA6NM, the volumetric expansion resulting from the austenite to martensite transformation, in conjunction with the low transformation temperature, contributes to the buildup of residual stresses during the welding process [2]. Previous studies have shown that significant tensile and compressive residual stresses can be found in as-welded CA6NM welds [2, 3]. These studies have also shown that post-weld heat treatment (tempering) can reduce the residual stress level. Thibault and al. obtained a 75 percent reduction of the maximum tensile residual stress measured in a five beads V-preparation weld (534 MPa to 136 MPa) [2]. Moisan and al. also showed that the maximum average tensile residual stress in a multi-pass T-joint was reduced after post-weld heat treatment by an amount of 73 percent (204 MPa to 56 MPa) [3].
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