Morphology Evolutions of the Short Fatigue Crack Propagation of 1Cr18Ni9Ti Weld Metal

Abstract

Surface and depth morphology evodslutions of short crack propagation of 1Cr18Ni9Ti weld metal are investigated. In accordance with the previous effective short fatigue crack (ESFC) criterion, attention is paid on the formation zone of the dominant ESFC (DESFC), which resulted finally in the specimen failure, in micro-structural short crack (MSC) regime and then, the tip zone(s) ahead of the DESFC in physical short crack (PSC) regime. Results show that in MSC regime the surface ESFCs were imitated from the distributed randomly delta ferrite bounds separated from austenite matrix. The initiated ESFCs on surface propagated perpendicularly to loading axle. But in depth direction, the initiated ESFCs grew first similarly to the surface behaviour but lately, tended to be perpendicularly to the formation direction of the material columnar grain structure. When sizes of some longer ESFCs reached around the material maximum barrier size, coalescence occurred to form a true DESFC. In PSC regime the surface DESFC grew almost perpendicularly to loading axle. But in depth direction, it grew first perpendicularly to the formation direction of the columnar grain structure and then, tended to having 45 degree angle to the loading axle. Obviously, the evolutions were strongly affected by interactions between the load and the microstructures, especially, the columnar grain structure.