Volume 74, Issue 3

Surface hardening and compressive residual stresses are keys to superior part performance in numerous applications. In this context the development of advanced laser surface treatments for new materials and complex sample shapes is a time consuming process. Eventually, determination of residual stress states by means of X-ray diffraction in the whole surface and subsurface region, respectively, is one of the main time consuming facors in terms of characterization. In many applications the provision of an adequate distribution of compressive residual stresses, however, is needed for approval of parts. In the present work it is shown that micrograph analysis can be used to provide the zero transition zone of residual stresses revealing the relevant penetration depth of the laser treatment conducted. One single surface stress scan employing standard techniques only is needed to verify the sign of residual stresses induced by the treatment. In fact, the screening approach introduced in this study enables time- and cost-efficent development processes for studying new sets of laser hardening parameters.

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.

During manufacturing of metallic components, thermo-mechanical loadings induce surface layer states like topography or residual stresses, which influence the component properties like fatigue strength. In order to optimize the component properties, a mechanical surface treatment can be carried out after the machining process. In this work, the influence of the process parameters processing velocity and penetration depth on the resulting tool wear during external longitudinal turning of AISI 4140q&t by Complementary Machining is analyzed. The process strategy Complementary Machining (CM) combines machining and mechanical surface treatment using the cutting tool. The mechanical surface treatment takes place after the machining in the opposite machining direction. The results of this study show that the process variables have an influence on the tool wear and thus directly influence the resulting topography. The fatigue analysis shows that the fatigue strength after Complementary Machining is comparable to that of shot peening. Furthermore, the concept of local fatigue strength is used to show the extent that residual stress reduction as a result of cyclic loading affects the fatigue strength.

The heat treatment method Internal Quenching constitutes an alternative method to increase the surface strength of internally loaded high pressure steel components like common rail parts, pipelines, or valves. With the prototypic Internal Quenching device, which was built up at IAM-WK, it is possible to generate hardened surfaces and influence residual stresses in the inner surface of through-drilled parts. This is reached through an austenitisation followed by an internal quenching step, which leads to a martensitic microstructure and compressive residual stresses. Furthermore, it is possible to heat the outer surface by inductive heating during the quenching process which helps to avoid a through hardening of the parts. Due to that various heat treatment strategies can be realized and the residual stresses in the inner surface could be tailored to the load situation.

In this study, two initial conditions of the steel 16MnCrS5 are considered: the industrially hot-rolled pearlite structures in their ferritic matrix and a specifically adjusted microstructure in the lab condition. Based on the experimental investigations and quantitative microstructural analyses, an empirical model for the prediction of pearlite banding within a broad range of annealing durations could be derived. Both, experiment and model, agree that pronounced pearlite bands in the initial state almost disappear after 25 h of spheroidization annealing. On the other hand, a marginal degree of pearlite banding in the initial state increases slightly during annealing. This fact could be explained by inhomogeneous cementite formation inside and outside the primary segregation regions of manganese.