Fractography on Fatigue Fracture Surface of Steel Castings

Abstract

Thin plate specimens with 1mm thickness were cut out from the different micro structures of a large casting and subjected to fatigue tests under a condition so as to minimize the effects of defects such as porosity, shrinkage and non-metallic inclusions. The influence of casting structure on fatigue behavior was experimentally examined through these fatigue tests and fractographical investigation on the crack initiation and propagation behavior of steel castings.
The results of fatigue life tests on the unnotched and V notched specimens showed that the fine grain material had a higher fatigue limit as compared with the coarse grain material and the influence of cast structure on crack initiation was observed. On the other hand, the influence of micro structure on the crack propagation rate was not observed above the crack propagation rate of 10^-6mm/cycle.
In the crack initiation area of the fine and coarse grain materials, cast structure sensitive fracture surface was predominantly observed. Striation appeared clearly at K>12.4MN·m^-3/2. The crack propagated preferentially in the ferrite phase and slowed down when the crack tip met with the pearlite phase.