Volume 51, Issue 8

The replica technique was applied to evaluate crack initiation and crack propagation behavior of welded joints on high strength structural steel with a minimum yield strength of 690MPa. Wöhler tests (cyclic loading tests) on these high strength welded joints, supported by the replica technique, show, that at a given particular stress amplitude, crack initiation occurs in several areas of the heat affected zone. At high stress amplitudes, the failure occurs in the intercritical zone, whereas stress amplitudes near the endurance limit lead to a failure within the coarse grained zone. This can be attributed to the crack initiation and propagation behavior within these individual zones. Evidence for this can very well be provided by means of the microstructure replica technique described in this article.

Oxidation protection coatings are required for thermally highly stressed components such as turbine blades in aircraft engines. Cyclic oxidation experiments were performed on a NiCoCrAlY protective coating of a nickel-based superalloy and hardness and modulus of elasticity (mechanical properties) were determined by nanoindentation before and after the experiments. Microstructure and chemical composition were characterized by means of scanning electron microscopy. Here, the focus is on the phase identification by combining electron backscatter diffraction and energy dispersive X-ray spectroscopy. Findings indicate that the chemical composition strongly influences the mechanical properties.

It is known from field experience that gas turbine vane fixing bolts, made from high-alloy chromium steel, got stuck in their mating bores and often caused problems upon disassembly. Many of them had to be drilled out rather than removed normally. Several metallurgical in­vestigations determined excessive surface oxidation to be the physical cause for the seizing of the bolts. It was recom­mended to change the bolt material from high-alloy steel to a nickel base superalloy with significantly higher oxidation resistance than steel in order to prevent surface scaling and seizure.