Surface Layer Microstructure of Carburised and Bainitically Transformed Parts and their Mechanical Properties Part 3: Investigations of the microstructure and residual stresses of carbon graded samples after partial and full bainite transformation

• Published in: Journal of heat treatment and materials : HTM = Zeitschrift für Werkstoffe, Wärmebehandlung, Fertigung Vol. 74; no. 3; pp. 164 - 180
• Main Authors: Steinbacher, M., Hoffmann, F., Zoch, H.-W.
• Format: Journal Article
• Language: English
• Published: 14.06.2019
• ISSN: 1867-2493; 2194-1831
• DOI: 10.3139/105.110375

Abstract In many applications in the roller bearing industry, bainitic microstructures are considered superior to martensitic [1–3] due to their specific advantages such as compressive residual stresses at the surface of a though hardened steel component and the increased toughness for hypereutectoid steels. Especially tolerance to damage incurred in operation, manifested in such roller bearings by extreme flaking of the bearing surfaces, yet without failure in operation, constitutes a clear advantage compared to martensitic bearings, being prone to circumferential failure even after slight damage to the bearing surface. Bainitic transformed surface layers are hardly an issue for case-hardened components, for instance, toothed gearbox components that will typically be martensitic hardened after carburising. The reason for this is, among others, that compressive residual stresses can already be achieved after carburising and martensitic hardening and the advantage of residual compressive stresses in the surface layer of a bainitic hardened component to increase fatigue strength are therefore not in focus. Isothermal transformation into lower bainite of a carburised surface layer nevertheless promises advantages in terms of the mechanical characteristics of components treated in this way, due to the increased toughness compared to martensitic microstructures and the expected more favourable residual stresses that can be achieved. The combination of carburisation and bainitic transformation in the lower bainite temperature range has not yet been comprehensively and systematically examined and evaluated. The results of the examination shown in this paper are aimed at laying the foundations for wide applications of this combination of processes.