Abstract
An investigation has been made of the influences of non-metallic inclusions and structure on the fatigue properties of 0.45% carbon steels deoxidized with Al, Si-Mn, and Ca-Si, with alternate bending test.Results of this study indicate as follows:1) The nucleation site of a main crack is definitely associated with a larger inclusion than 20p on the specimen surface, regardless of its composition.2) The effect of inclusion on propagation rate of the fatigue crack seems to be more evident with decreasing tempering temperature of the steels.The crack propagation rate is independent on the kind of inclusions but is dependent on the cleanliness of the steels.3) The macroscopic crack growth rate is formulated as a function of stress intensity factor range 4K, that may be expressed as: d (2l)/dN= (ΔK) n/Mwhere n is a variant exponent and is different from the case of Paris'law.While the value n is a function of composition, structure and inclusion of materials.In the present work, it is found that n varies from 1.45 to 7-69, and the minimum n value is obtained in the steel tempered at a temperature of 400°C.4) The refinement of sub-grain due to the increase in fatigue cycles or stress amplitude is considered as the result of a process which involves smaller cell formation by multiplication and migration of dislocations.
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