The Influence of Slack Quenching Structures on Internal Fatigue Fracture of a Shot Peened Spring Steel

Abstract

In order to understand internal fatigue fracture of a shot-peened spring steel, origins of fish-eyes formed on fracture surfaces were observed and discussed.
Most origins of fish-eyes were not nonmetallic inclusions, but were microstructural defects, which shapes on fracture surfaces were polygonal facets or narrow bands. From results of hardness tests and microstructure observations, the defects were considered to be slack quenching structures formed flatly along prior austenitic grain boundaries. The fatigue crack initiated at an interface between the defect and the matrix of tempered martensitic structure.
When the square roots of area of the defect of the origin √area _DEFECT were smaller than 15μm where the shapes of defects were narrow bands, fatigue life became more than 10⁷cycles and some areas with rough surface called ODA were observed around the origins. On the other hand, when the square roots of area of the defect were bigger than 15μm, where the shapes of defects were polygonal facets, fatigue life became less than 10⁷cycles and ODA were not observed around the origins.
The shape of the origin of the fish-eye on the fracture surface were considered to depend on the relationship between the direction of loading axis and the normal direction of the flat defect of the origin.