Abstract
Alternate bending fatigue tests were carried out using specimens containing FeO-type inclusions, containing Mn-silicate type inclusions and containing substantially no inclusions respectively. The effect of inclusions on endurance limit was discussed. Some specimens were carburized and heat-treated in order to intensify the effect of inclusions on fatigue properties. Main results obtained are as follows:
1) Inclusions in low carbon steels do not exert any effect on endurance limit.
2) Influence of inclusions in carburized steels (0.25% C) on the endurance limit increases with the decrease of tempering temperature. The trend is remarkable in specimens containing Mn-silicate type inclusions, when the axis is transverse to the rolling direction. Inclusions in steels as carburized, however, do not influence the endurance limit.
3) Inclusions have little effect on ultimate tensile strength, but have a considerable one on reduction of area.
4) On the assumption that inclusion does not adhere to matrix, rate of reduction of endurance limit by inclusions is expressed by W=i (A-n), where i is a coefficient depending on size, amount and shape of inclusions, n is the work hardening coefficient defined by σ=kεn and A is a constant. In tis experiment, A was nearly 0.16. When n is larger than about 0.16, it is concluded that inclusions do not affect endurance limit. The above equation can not be applied for pre-strained specimens, in which case the work softening controlls the fatigue process.
5) Heterogeneous microstructure has worse influences on elongation and reduction of area than on endurance limit.
6) Effect of inclusions on crack propagation could be estimated from disorder of striation around inclusions.
