Fatigue crack growth in the heat affected zone of a hydraulic turbine runner weld

• The fatigue crack growth behavior of a stainless steel weld HAZ was studied.
• The HAZ has similar fatigue characteristics than base metal at high load ratios.
• Crack tip tensile residual stresses inhibit crack closure in the as-welded HAZ.
• Post-weld tempering is beneficial at low load ratio but has no effect at high load ratio.
• Yield strength mismatch causes the crack to deviate towards base metal.

The fatigue crack growth behavior of a CA6NM weld heat affected zone (HAZ) was investigated. Fatigue crack growth tests in river water environment were conducted on as-welded and post-weld heat treated specimens at load ratios R = 0.1 and R = 0.7. For a fully open crack, i.e. at R = 0.7, the HAZ fatigue behavior was similar to that of the base metal. When crack closure occurred, i.e. at R = 0.1, the HAZ showed a lower near threshold crack growth resistance. The post-weld heat treatment was beneficial at R = 0.1 by relieving tensile residual stresses, while its effect was negligible at R = 0.7. The crack trajectory was influenced by the weld’s yield strength mismatch which promoted deviation towards the soft material (base metal). The main conclusion of this study is that the HAZ does not represent a weak link when its high load ratio fatigue behavior is considered. This confirms the validity of currently used fatigue assessment methods of hydraulic turbine runners.