Influenza della microstruttura sulla propagazione di cricche di fatica nelle ghise sferoidali

摘要

Ductile irons show well-known useful properties, such as good castability and satisfactory fatigue crack growth control. The effect of graphite shape on cast iron fatigue has been already considered in the literature. The purpose of this work is to compare the fatigue behaviour of four ductile irons with different ferrite and perlite content, ranging from 100% ferrite to 95% perlite. According to the ASTM E647 standards, CT specimens (B = 11mm) have been tested by means of an oleodynamic 1NSTRON 8501 machine in air at 20 Hz, in con-slant load amplitude conditions. Three different load ratio conditions have been chosen (R = 0.1, 0.5 and 0.75) for each composition (figg. 1-4). The threshold values have been measured only for the latter loading condition. The experimental results have been discussed in terms of Paris-Erdogan fatigue crack propagation model (stage Ⅱ), with the help of metallographic and fractographic evidences. The pivot points for the three different loading conditions have been calculated. Different fracture morphologies have been reported (figs. 11-14). The matrix micro structure seems not to play a primary role in fatigue crack growth control except for the higher R and ΔK values. The presence of spheroidal graphite thus implies an homogeneous fatigue crack growth behaviour for the lower stress intensities. Crack tip plasticity induced and graphite spheroids wedging mechanism are responsible of this behaviour. Differences in crack propagation resistance among investigated cast iron are evident only for higher R and ΔK values, because of the different propagation niicromechanisms in ferrite and perlite. For these loading conditions, the best behaviour is connected to GS500-7 (50% ferrite - 50% perlite) cast iron, because of the distribution of ferrite shells around graphite spheroids in perlite matrix, and the different ductile behaviour of perlite and ferrite constituents that implies an increasing of closure effect on ferrite grains.