Evaluation of the fatigue behavior of ductile irons with various matrix microstructures

Abstract

The stress-failure (S-N) curves for ferritic irons, pearlitic irons, and austempered ductile irons (ADIs) have been determined under tension-tension loading with a stress ratio of 0.1. The effects of Ti contents of up to 0.10 wt pct (resulting from the deliberate use of Ti-containing steel scrap) on fatigue behavior were investigated. It was found that ferritic and pearlitic ductile irons can contain up to 0.10 wt pct Ti without any adverse effect on fatigue behavior. In ADIs, fatigue properties deteriorate at such high Ti contents. Tests were also conducted to investigate the effects of microstructural features on fatigue properties. It was found that the effect of the graphite nodule count (the number of graphite particles on a unit area of a polished surface) on the fatigue limit is significant only in ADIs. Scanning electron microscope (SEM) analysis has shown that cracks usually initiate from surface dross-type defects. However, in ADIs, fatigue cracks can also initiate at shrinkage cavities and at surface or subsurface locations. An offset bilinear S-N curve behavior (the linear S-N curve at higher stress levels is separated from the linear S-N curve at lower stress levels) has been observed in ADIs. This is attributed to surface residual compressive stresses, which prohibit fatigue crack initiation from surface positions at lower applied stress levels. In ferritic and pearlitic ductile irons, the offset bilinear S-N curve behavior is not observed because of the rapid relaxation of the residual compressive stresses.

Microstructural Dependence of Tensile and Fatigue Properties of Compacted Graphite Iron in Diesel Engine Components

“…A fortiori, no model exists on the adhesion strength at the graphite/metal interface. However, there are some literature reports on the (fracture) mechanical properties of cast iron with different matrix structures. It is largely accepted that the metal matrix microstructure affects the micromechanisms of graphite debonding with ensuing influence on the crack tip stress distribution.…”

Effects of temperature and shot peening on S-N behavior of a PM Ni-base superalloy UDIMET 720

“…Under cyclic loading, residual stress will be gradually relaxed with fatigue and the effect of this residual stress relaxation on fatigue has to be considered in understanding fatigue behavior under residual stresses. [48] Note here that the "release" and "relax" of residual stress are distinguished to represent different meanings.…”

Fatigue and Fracture Resistance of Heavy-Section Ferritic Ductile Cast Iron

“…The microstructural control of DCI is of paramount importance as it greatly influences a wide range of mechanical properties. The typical microstructure consists of graphite nodules dispersed in a matrix that can be ferritic, ferritic and/or pearlitic depending on the alloy formulation, the casting control and the final heat treatment [1,2]. The ferritic matrix displays usually the lower tensile and fatigue strength but also a noticeable ductility, which have promoted the DCI in the production of thick walled parts undergoing in service low/medium levels of mechanical stresses.…”