13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- the micrographs is that the etched micrograph becomes brighter in the transformed phase with an increasing pre-heating temperature. This indicates an increasing amount of bainite in the martensitic zone caused by a lower cooling rate during the phase transformation. Verification of this observation can be given by a micro-hardness indentation only, which is shown in Fig. 5 a-b). a) pre-heating temperature 300°C b) pre-heating temperature 600°C Figure 5. Micro hardness indentations on 300°C and 600°C pre-heated flange shafts Pre-heating the flange shaft with a temperature of 600°C decreases the hardness gradient in the flange as expected. As presented in Fig. 5 b), the difference in the Vickers hardness between transition zone and the martensitic zone becomes marginal compared to a 300°C pre-heated flange shaft, given in Fig. 5 a). c) deformed base material b) martensite Figure 6. SEM-images from the microstructure of a 500°C pre-heated flange-shaft Due to the high degree of deformation combined with the high temperature during the forming process a very fine grained and complex microstructure exists in the flange shaft. SEM micrographs show that the grain size is of the order of 1 micron, see Fig. 6 for a 500°C pre-heated flange shaft. The deformed base material, shown in Fig. 6 a), still has its ferritic-pearlitic microstructure, but the original lamellar structure of the pearlite is mainly destroyed. The former cementite lamellae now mostly have a globular shape while the remaining ones are highly deformed. The martensite, shown in Fig. 6 b), is characterized by small carbides in the martensitic needles. It has been observed, that with an increasing pre-heating temperature the amount of carbides increases as well. This observation may support the assumption of a hardness decreasing tempering effect which is caused by significantly slower cooling rate after the phase transformation [7].
RkJQdWJsaXNoZXIy MjM0NDE=