Fatigue crack initiation and propagation of different heat affected zones in the presence of a microdefect

Abstract

The research focuses on the mechanical properties and fatigue behaviour of individual parts of Heat Affected Zones (HAZ) in a weld joint on 18CrNiMo7-6 steel. In real welds, the HAZ is heterogeneous and narrow. Therefore, specimens having a desired HAZ microstructure uniform over the whole specimens‘ volume were prepared artificially by water quenching: The Coarse Grain HAZ (CG HAZ), with average grain size of 200 µm, Fine Grain HAZ (FG HAZ) with average grain size of 10 µm, and CG-FG HAZ with average grain size of 25 µm. Comparison with a real weld revealed that microstructures of specimens corresponded very well to the microstructures in HAZ in a real weld joint. Fatigue crack growth and the threshold for long crack propagation ΔKthR were determined on HAZ microstructures. A weld toe and its stress concentration due to weld cap were simulated by a U shaped groove on the fatigued specimens. Real HAZ often contains small defects where a crack usually initiates. Therefore, a defect was simulated with a Vickers impress at the bottom of the groove in all three HAZ-microstructures. Investigation revealed that the threshold for long crack propagation ΔKthR of the FG HAZ was the lowest, not much higher was the threshold in CG HAZ, while the threshold ΔKthR in CG-FG HAZ was significantly higher. On the other hand, long crack propagation in CG HAZ is faster in comparison to the long crack propagation in FG HAZ. In all three cases, a Paris curve and its coefficients were determined for R ratios −1 and 0.1.Fatigue tests revealed that some cracks, which had been initiated below the fatigue limit in CG-FG HAZ, stopped propagating due to strong grain boundary barriers and high toughness, but, in the cases of CG HAZ and FG HAZ, the cracks propagated without stopping until the breakage of the specimen. The fatigue limits were also determined for all three HAZ-microstructures in the presence of the defect.