Influence of the processing route on the microstructure and mechanical behaviour of bainitic forging steels

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Abstract

This thesis deals with the development of high strength bainitic steels used for forged components. These steels should have enhanced mechanical properties and thus an improved service fatigue strength in comparison to the traditional used quenched and tempered steels. For this purpose two steel grades with a medium carbon content (0.20 wt.-%) are investigated which differ mainly in their chromium content. Depending on the applied process routes different bainitic microstructures will form because the transformation is very much dependent on the austenite state prior to transformation as well as on the temperature profile which is applied during the phase transformation. The different process routes are realized by laboratory tests and by industrial trials. In addition, tempering after the bainitic phase transformation shows an impact on the microstructure as well as on the mechanical properties. To derive this fundamental behaviour, dilatometric measurements of the phase transformation kinetics of different bainitic morphologies are realized. The microstructural analyses consist of light microscopy, scanning electron microscopy, transmission electron microscopy and X-ray diffractometry. The static mechanical properties, which result from the quasistatic tensile test and Charpy-V-notch tests are investigated after isothermal and continuous phase transformation performed in the laboratory as well as by real die forging. Furthermore, cyclic mechanical properties are carried out from die forged specimens by the means of Manson-Coffin testing. In this work, the T-t conditions to achieve an optimal combination of strength and toughness were analysed and it was found that this optimum is material dependent. The desired profile of requirements for bainitic steels can be achieved by fine homogeneous microstructure. This morphology can be realized by short transformation time at low isothermal temperatures or controlled continuous transformation within a constricted bainite phase field. It was found that the holding temperature close the martensite start temperature results in the best combination of strength and ductility. Higher transformation temperatures lead to a lower stability of the austenite. This results in a higher amount of martensite / austenite-constituents, hence the strength and ductility values are decreased. The concepts of new bainitic steels for forging parts exhibit an increased content of retained austenite. This higher amount of retained austenite, which forms due to the higher silicon content, transform into martensite under cyclic loading. Hence these new steel concepts show cyclic hardening effects compared to the reference quenched and tempered steels. This results in a higher fatigue strength and thus in a higher light weight potential. Furthermore, the strength potential can utilized mainly by controlled cooling from the forging heat. The optimal controlled cooling procedure includes an isothermal transformation at 400°C and holding time of 15 minutes. Thus, the parameters of a subsequent tempering process have to be controlled to achieve the optimal mechanical properties.

Details

Translated title of the contributionEinfluss der Prozessführung auf die Mikrostruktur und mechanischen Eigenschaften von bainitischen Schmiedestählen
Original languageEnglish
QualificationDr.mont.
Supervisors/Advisors
Publication statusPublished - 2017