The mechanical property requirements such as fatigue and thermal fatigue resistance of advanced diesel injection components are steadily increasing due to the growing performance of combustion engines and emission limit values. For this purpose, materials are necessary that fulfil these requirements while having preferably low alloy content and associated costs. This thesis focuses on characterising the bainitic microstructure of a newly developed steel alloy for diesel injection components, which was jointly developed by the Materials Center Leoben (MCL) and the Robert BOSCH GmbH. To transfer this alloy onto commercial scale, detailed knowledge is required regarding microstructural changes caused by the different heat treatment steps. The investigations thus focus on characterisation of the bainitic microstructure and its formation kinetics, in addition some microstructure investigations were also performed after tempering. Light microscopye investigations pointed out that the microstructure partially consists of granular bainite, which is formed due to continuous cooling. The microstructure of the granular bainite could be examined in more detail, by means of correlative microscopy. In these investigations, ferritic areas were found which are mainly free of carbides. In other areas, carbide precipitations in lamellar form and parallel nanometre sized carbides were found, which were characterized by “energy-filtered-transmission electron microscopy” (EFTEM). Furthermore, micrometre sized carbides were detected, which were not or only partially dissolved during austenitizing (“primary carbides”). To obtain information about their chemical composition, energy dispersive X-ray spectroscopy (EDX) was employed. With electron diffraction in a transmission electron microscope (TEM), the primary carbides were identified to be mainly of M2C type. Some vanadium-and chromium-rich carbides were also detected without studying their crystal structure in detail. The retained austenite content was determined by means of X-ray diffraction to be about 11.5 wt. %. By using higher resolution imaging techniques, e.g. “Electron-Back-Scattered-Diffraction” (EBSD), the position and distribution of the retained austenite was investigated. It was found that the retained austenite mainly occurs in the areas of granular bainite. The amount of austenite decreases during tempering between 100 and 600 °C with increasing temperature. From tempering temperature above 550 °C, the retained austenite content is below the detection limit of 2 wt. %.
|Translated title of the contribution||Microstructure of a bainitic, carbide hardening steel|
|Award date||27 Mar 2015|
|Publication status||Published - 2015|
Bibliographical noteembargoed until 27-02-2020
- granular bainite
- continuous cooling
- retained austenite