Volume 59, Issue 7

Power density demands for gearboxes are steadily increasing. Higher gear steel qualities can help to meet these demands. Steel manufacturer, as well as customers, usually examine the quality of gear steel batches. For this purpose, ultrasonic immersion testing is a fast and non-destructive evaluation method for the determination of the macroscopic degree of cleanliness. The evaluation follows certain standards and the common influence factors are usually well known. However, the state of knowledge shows that there are still many open questions. Some of these open questions can lead to differences between the results of different laboratories. In the framework of a research project, the macroscopic degree of cleanliness of ultra-clean gear steels had to be determined. However, first results showed significant differences between the results of the laboratories. For this reason, an interlaboratory test was initiated. The same samples were used for the characterizations at the different laboratories. The aim of the investigation was to determine the influencing factors that led to the different results. Based on the interlaboratory test, countermeasures should be defined to ensure a reliable determination of the macroscopic degree of cleanliness of (ultra-clean) gear steels and to ensure comparable results from different laboratories in the future.

State-of-the-art modelling algorithms allowing for prediction of macroscopic properties of martensitic steels are primarily based on microstructural parameters such as prior austenite grain size as well as martensite packet-and block size distribution. The latter are usually obtainable via electron backscatter diffraction (EBSD) measurements. However, determination via light microscopy would present a more cost-effective determination method. This work presents a python-based multi-step image processing algorithm capable of separating the grain boundaries and the martensitic substructure from etched micrographs. Additionally, the viability of a characteristic mean free path parameter λ of the martensitic substructure for comparison of different martensitic microstructures is tested. To this end, a microstructure variation of PH15-5 was performed using different heat treatments and the specimens were analyzed using EBSD and electrolytically etched micrographs.

Several high-pressure lift oil pump failures were reported from the field. These assemblies are used to inject lubrication oil into the sliding bearings of heavy rotating turbomachinery equipment, such as steam and gas turbine rotors, often weighing in excess of a hundred tons. This ensures that the rotor shaft “floats” on a lube oil film even at low rotational speeds, when hydro-dynamic lubrication conditions have not yet been fully established. A fractured spline shaft, which was the driven shaft of one of the failed pumps, was received from the client for the determination of the metallurgical root cause of failure. The subject spline shaft failed due to torsional overload. This cracked the hardened case and initiated high cycle fatigue (HCF) cracking as secondary damage. The main fatigue cracks were nucleated at overload fractures in the hardened case, in the fillet radii at the base of the teeth of the spline shaft. No evidence of any material defects that could have contributed to the failure or could even have been causative for it was found.