Microstructural effects on the fatigue crack growth resistance of a stainless steel CA6NM weld

•The fatigue crack growth behavior of a martensitic stainless steel weld was studied.•Growth rates decreased in the heat affected zone from filler metal to base metal.•Crack tip tensile residual stresses prevent crack closure.•Crack deflection concepts relate the weld microstructure to crack growth resistance.•Retained austenite has no significant influence on the weld crack growth behavior. The fatigue crack growth behavior of a martensitic stainless steel CA6NM hydraulic turbine runner weld was investigated to unveil microstructural effects in the filler metal, heat affected zone and base metal. Knowledge of such effects is paramount for engineers who design these components for long fatigue lifetimes. Constant stress intensity factor fatigue tests in river water environment revealed crack growth rate variations between the three weld zones. Tensile residual stresses were identified as a crack opening mechanism. Crack deflection concepts were used to relate the fatigue crack growth resistance to the weld microstructure. The conclusion of this study is that the crack growth path is largely driven by the weld’s microstructural features, influencing the materials resistance to fatigue crack growth.