Abstract
Fatigue behaviour of nodular cast irons is described in specialized literature quite well but the majority of fatigue tests in high cycle region has been made at symmetrical bending or at symmetrical tension-compression loading. The role of loading cycle asymmetry is very important because the fatigue limit of nodular cast irons is substantially influenced by mean static stress. Unfortunately, the number of papers describing the influence of mean static stress is quite low. The aim of the presented paper consists in presenting the results of long-term research of nodular cast irons at high cycle region at different loading cycle asymmetry. Several heats with various heat treatments leading to various structures of matrix were used for fatigue tests performed mainly at symmetrical tension-compression loading and at repeating tensile loading. From S-N curves the fatigue limits for 107 cycles were determined and then used for the calculation and construction of the Haigh and the Smith diagrams. Nodular cast irons seem to be more sensitive to mean static stress than structural steels. It was found that, in contrast to many papers, the dependence of upper stress on mean stress is not linear but parabolic with the exponent lower than 1 whose value is linear function of UTS.
Highlights
Besides excellent technological properties its advantage consists in favourable shape of graphite particles which, in comparison with other cast irons, does not decrease substantially the mechanical properties of metal matrix
Various heat treatments are applied for nodular cast iron as for usual structural steels and its mechanical properties can be changed in a wide range of strength and plasticity
Variability in the microstructure of cast iron matrix is even substantially wider than in the microstructure of steels because the graphite particles can work as stocks of carbon and, in one heat of cast iron wide range of ferritic-pearlitic mixtures can be obtained from purely ferritic to nearly purely pearlitic matrix
Summary
Nodular cast iron and its high-strength type called ADI (austempered cast iron) is very prospective structural material with worldwide increasing production. Besides excellent technological properties its advantage consists in favourable shape of graphite particles which, in comparison with other cast irons, does not decrease substantially the mechanical properties of metal matrix. Various heat treatments are applied for nodular cast iron as for usual structural steels and its mechanical properties can be changed in a wide range of strength and plasticity. Variability in the microstructure of cast iron matrix is even substantially wider than in the microstructure of steels because the graphite particles can work as stocks of carbon and, in one heat of cast iron wide range of ferritic-pearlitic mixtures can be obtained from purely ferritic to nearly purely pearlitic matrix
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