Abstract: In-situ observation of short fatigue crack growth is conducted based on scanning electron microscopy and micro-tensile module. The initial cracks are located in heat-affected-zone(HAZ) of a NiCrMoV steel welded joint, i.e., fully quenched-tempered zone, partially quenched-tempered zone and tempered zone. Results show that the physically short cracks grow discontinuously though still driven by K in the HAZ, and the fatigue crack growth direction is influenced by the strength distribution. Fatigue crack propagation resistance in the fully quenched-tempered zone is the highest due to the barrier effects of the lathy tempered martensite and larger grain size leading to more tortuous crack propagation path. Whereas in the tempered zone the grain size and strength are relatively lower, and thus inferior fatigue behavior. Nevertheless, the lowest fatigue performance locates in the partially quenched-tempered zone, where the crack growth is tended to be intergranular due to the large amount of carbide precipitates around grain boundary.

Key words: Fatigue crack growth, Heat-affected-zone, In-situ test, Short crack, Welded joint