Sizing of Defects and Fatigue Behavior of Nitrided Nodular Cast Irons

Abstract

The paper presents and discusses the influence of structural characteristics such as content of effective ferrite, size of graphite particles and of microshrinkages on fatigue life of gas nitrided nodular cast irons (NCIs). A classical NCI melt and two synthetic melts obtained with addition of SiC to the liquid metal were produced. The surface of a NCI part may be modified for fatigue-critical applications by thermo-chemical surface treatments, such as nitriding with a corresponding increase of the fatigue limit. To quantify this increase, the rotating bending testing of smooth specimens was used because with this type of loading the material near the surface is subjected to the maximum stress. The quality of the nitrided layer, which has to be hard, elastic and compact, was controlled by measurement of the dynamical nanohardess according to Vickers. Results of fatigue behavior on the nitrided NCIs show a significant increase in fatigue limit and a different trend in the stress amplitude versus number of cycles to failure (S/N) curves in dependence of structural characteristics. Typically, fatigue endurance is reduced when the size of porosity increases. Therefore, the quality of the castings is related to porosity control. Optical microscopy was used to analyze the microstructure and defects of material after fatigue testing. The statistical method proposed by Murakami, based on the largest extreme value distribution (LEVD) was used to evaluate the porosity size population and to correlate with the fatigue data.