The effect of nonmetallic inclusions on the fatigue strength of quench-hardened steel

Abstract

1. The current metallographic method of evaluating the degree of contamination of steel does not give a sufficiently accurate indication of the nonmetallic inclusions content, particularly in high-grade, electroslag- and vacuum-melted steels. This is mainly because this method fails to take into account the very fine nonmetallic inclusion particles. 2. Since very small nonmetallic inclusions have the same harmful effect on the fatigue strength of high-purity vacuum- and electroslag-melted steels as microporosity, voids, and other crystallization defects, the density of steel will serve as a criterion of its degree of contamination and hence its quality. 3. Studies of hydrogen embrittlement of steels under review prove that they contain large proportions of fine nonmetallic inclusions and crystal defects, which are not revealed by the standard method of evaluating the purity of steel, but are reflected in its density. 4. In the absence of surface stress risers, the fatigue strength of steel depends upon its purity. As the concentration of nonmetallic inclusions and crystallization defects decreases, i.e., as the density of the steel increases, its fatigue limit increases. 5. The total content of nonmetallic inclusions and crystallization defects in electroslag-melted steel is lower than in vacuum-melted steel; thus the former has a higher fatigue limit. 6. So far density has never been included in the properties specified for type ShKh steel. In view of the experimental evidence cited above, this property is an important criterion of the purity of steel and, as such, should be included in specifications relating to the fabrication of high-purity, electroslag- and vacuum-melted steels.