Failure mechanism in the pearlite structure with lamellar graphite due to temperature

Abstract

Critical operating temperature can be determined for the machine components that form the key functional part of the device. This article describes a change of the failure mechanism in the material of an engine, where the material structure consists of fine pearlite layers with lamellar graphite equivalent to grey cast iron. The mechanical properties of this material influence not only the layers formed by the perlite, but also the lamellar graphite acting as a stress concentrator. To assess the failure mechanism in the material, a fractographic analysis of the fracture areas was performed after the impact test. The range of test temperatures at which the impact test was performed on the test specimens with notch ranged from -80°C to +180°C. The occurrence of brittle fracture was detected at the low temperatures by cleavage along and over the planar layers of the lamellar graphite. The brittle fracture initiation at the higher temperatures was observed by cleavage in the basic layer of the perlite. Depending on the failure mechanism of the test specimens fracture area, critical temperatures were determined, in which the measured value of the notch toughness decreased. Regression of temperature dependence of notch toughness allows very precise quantification of this decrease.