Influence of matrix structure on fatigue strength of nodular iron with high tensile strength.

Abstract

The rotating fatigue tests were carried out on nodular irons with 7 types of matrix structures prepared by heat treatments, and effects of matrix structure on the fatigue limit σw were examined. In order to study the crack initiation and growth, the fracture surface was observed by SEM. The results obtained were as follows : (1) The effect of matrix structure on σw was remarkable. (2) Among irons with ferrite-pearlite duplex structures, the iron with the finest spheroidal graphite prepared by the metal mold had the highest fatigue limit. (3) Among irons with hardened matrix structure prepared by cooling of high rate, the iron with upper bainite had the highest fatigue limit. (4) The matrix structure influenced the metallurgical factor that initiated the crack, because increasing Hv of the matrix structure induced the high notch sensivity of micro-porosity. This fact caused the effect of matrix structure on σw.