Transitional behavior of fatigue crack growth in welded joint of 25Cr2Ni2MoV steel

• Transitional behavior of FCG curves is observed at different parts of welds.
• Transition behavior is influenced by both microstructure and load ratio.
• Roughness distribution of crack surface shows a valley at the transition point.
• Transition is dominated by both ΔK and Kmax which depends on the load ratio.
• Estimation of ΔKt is proposed and agrees fairly well with experimental data.

Transitional behavior of fatigue crack growth is observed for initial notches located at base metal, weld metal and heat-affected zone of 25Cr2Ni2MoV steel welded joint. Results indicate that both microstructures and load ratio R contribute to the transition. Greater roughness at both weld metal and base metal indicates a more pronounced roughness-induced crack closure in the near-threshold regime than that at heat-affected zone, especially at lower R. This transition is dominated by both ΔK and Kmax which depends on R. An estimation of ΔK at transition point is proposed, which agrees fairly well with the experimental data.