Abstract
Steels containing Tellurium are generally used for free cutting steels of precision engineering parts. But the effect of Tellurium on mechanical properties of these materials are not well known yet. This paper deals with the influence of Tellurium on fatigue strength. And rotating bending fatigue tests were carried out using specimens containing MnS, FeS, MnTetype inclusions and containing substantially no inclusions respectively. And these tests were run in order to resolve some of the problems involved in evaluating machined surface layer for a given residual stress and work hardened layer by on endurance limit was discussed.Main results obtained are as follows : (1) Endurance limit of Tellurium free cutting steel was high order compared with the based steel and Sulphur free cuttting steel. And the endurance limit were arranged in ascending order as follows.Sulphur free cutting steel <Based steel <Tellurium free cutting steel(2) Endurance limit by the existance of machined surface layer were high stress level 5-8 kg/mm2 compared with no machined surface layer.(3) At some stress amplitude, residual stress of machined surface of Sulphur free cutting steel and based steel was reduced almost to zero at the number of cycles about N=103. On the other hand, residual stress of machined surface of Tellurium free cutting steel decrease gradually after the number of cycles about N=103.(4) The decrease of the hardness in the work hardened layer of Sulphur free cutting steel and based steel is observed within the range at the number of cycles about N=0103 But the decreasing rate of it for Tellurium free cutting steel were relatively small compared with the other specimens.(5) It seems that the inclusions FeS, MnS shows a promoting type for crack propagation, and the inclusion MnTe shows a controlling type for it. And effect of FeS, MnS, MnTe on crack propagation could be estimated from disorder of striation around inclusions.
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