Volume 59, Issue 1

Since it affects, among other things, the mechanical properties, the creep behavior, but also the cyclic strength, the grain size is a key quality criterion in the manufacture of engine disks made of superalloys. Large quantities of microstructure samples are produced and a metallographer manually determines their grain size with the aid of standardized image series tables. In the aviation sector, for instance, it has become established to indicate the grain size according to the ASTM grain size class as per ASTM E 112. In order to improve the efficiency of the process, a specific object-based image analysis was developed based on which a superalloy’s grain size was analyzed. The results were compared with manually determined grain size classes. In addition to that, the grain size was measured by electron backscatter diffraction ( EBSD ) and used as reference.

In this study, dissolution of the Mg 17 Al 12 phase carried out at 413 °C within 24 h and process followed by cooling the samples in different media with various cooling rates. Some samples were directly aged and others were held in liquid nitrogen for 24 h prior to aging. In this study for investigation of microstructure, the specimens were studied using a Meiji optical microscope and Scanning Electron Microscopy and Energy Dispersive Spectroscopy analysis was used to characterize the chemical composition of the phases. Results showed that at low cooling rates, a large amount of discontinuous precipitations were formed and by increasing the cooling rate, their amount reduced and their number and sizes decreased. Application of the deep cryogenic treatment with aging causes the dimensions of the secondary phases to be slightly larger than that of the non-deep cryogenic treatment state. Deep cryogenic treatment especially in samples with high cooling rate causes reducing the amount of Al, compared to usual stoichiometric value by formation of shrinkage stress in two stages (cooling after solutioning and deep cryogenic treatment). Deep cryogenic treatment in all of the samples increased the hardness, compared to the case without deep cryogenic treatment, by increasing the amount of Al out of the structure and providing conditions for the formation of more continuous and discontinuous phases.

Supposedly simple cases of failure are most often best suited to communicate the principles of component failure analysis in the field of materials engineering to a wide readership, especially to those peers in the specialist community who are just beginning to familiarize themselves with the subject. The present case of failure relates to components that failed as early as during the assembly, and more specifically, during the final assembly stage of combustion chamber components for heavy-duty gas turbine engines. Hence, they lost their functionality (in fact, the common definition of component failure). At tightening torques of the nuts opposite of the tapered heads as low as below 25 Nm, so-called twistoff bolts which, when welded into combustion chamber sheets, take on the function of stud bolts, sheared off. By way of exception, a materialographic failure analysis could show that the primary cause of the failure was not the component’s design, but the disregard of the drawing specifications during final assembly. However, on a secondary level, design deficiencies had to be mentioned, as untempered welded joints in martensitic chromium steels invariably act as metallurgical notches. If the respective part is subjected to dynamic loads, as is the case in virtually all turbo machinery, they are thus to be avoided.