On the Fatigue Strength of Ausformed Mn-Cr-R Spring Steel

Abstract

When Mn-Cr-B spring steel is ausformed in the stable austenite phase the deformation of the material will concern its fatigue properties. It is the aim of the present study to clarify what are the effects that the deformation will have with its temperature and at its rate on the fatigue properties of the material. The steel specimens were ausformed at 800°, 850° and 900°C up to nearly 50% reductions in thickness and tempered at 480°C for 1 hour. Then, their fatigue properties have been investigated under completely reversed plane bending stresses and compared with those of the specimens treated conventionally and processed by the thermomechanical treatment of cooling it continuously. The results obtained within this scope of experiment are summarized as follows.(1) The average endurance limit of the ausformed specimens within the range of 22-40% reductions shows an increase of nearly 29% over that of the specimen treated conventionally.(2) The fatigue lives of the ausformed specimens too are remarkably prolonged as compared with the conventionally treated specimens.(3) The fatigue properties at the higher rate of deformation are unreliable because of the slight improvement of the fatigue limit and the great variation of the data.(4) There is not much difference between the fatigue strengths of the ausformed specimens and those specimens produced by the thermomechanical treatment of cooling continuously at the equal rate of deformation, but the hardness levels of the former are by Rc 1-2 numbers lower than those of the latter.(5) The authors recommend the range of 850°-900°C as the appropriate deformation temperature and reductions in thickness up to 40% for this spring steel.