CarboBain: Case Hardening by Carbo-Austempering–a short Introduction to Transformation Kinetics, Microstructure and Residual Stresses*

Abstract

Abstract The beneficial strength properties of bainitic microstructures have been perceived since several years. The transformation kinetics and different morphologies of bainite were analyzed scientifically in depth and with increasing understanding the application of bainite is focus now. But still, bainitic microstructures are mainly utilized with chemically homogeneous steels. An application of bainitic transformed carburized samples is rather rare up to now. Therefore highly stressed geared components today typically are carburized and martensitically hardened. The microstructure in the case is generally composed of martensite with finely dispersed retained austenite and/or carbides. For gear wheel application during the past 50 years research was focused on topics like microstructure optimization within the bounds of martensite with no more than 25 % of retained austenite. For gears only very limited information on heat treatments with bainitic transformation of the case is available. In the USA few suppliers are propagating carbo-austempering heat treatment for case hardening applications [1]. In the literature a combination of carburization and austempering is discussed with only scarce data on material and microstructure performance [2–6]. To close the gap a systematic approach was made in a public funded project, analyzing transformation kinetics, microstructure properties and mechanical performance of two typical case hardening steels in carburized state being bainitic transformed. The heat treatment method using case hardening by carburization and adjacent austempering, the transformation behaviour, microstructure development and mechanical properties will be discussed.