Volume 50, Issue 5

Sintered hard metals like tungsten carbide compounds are a challenge for TEM sample preparation due to the diverging properties of the individual components. Especially when both very thin as well as large areas are required, for example for nanoanalysis investigations, conventional preparation techniques like ion milling and FIB preparation are often incapable of delivering samples of necessary quality. In this study, the preparation of a hard metal consisting of a tungsten carbide/cobalt compound via a technique called “Wedge Polishing” is described, along with the results of concomitant TEM investigations. The impact of the preparation procedure on the crystallographic structure of the material is discussed, and a post-treatment process to dispose of these artefacts is presented.

The experimental results show that a small dead region is seen in front of the rake face during cutting with a rounded-edge cutting tool. Although this fact was accepted, metallographic analysis of the dead metal zone had never been conducted. A quick stop device was used to obtain chip root samples that are representative of the deformation taking place during dynamic (actual) cutting conditions. A metallographic analysis was performed on selected specimens to identify the microstructure and formation. The photomicrographs were analyzed and an effort made to explain the dead metal zone microstructure.

As part of an existing research project [1] the fatigue behaviour at room temperature of the austenitic steel X6CrNiNb18–10 was investigated. From a combination of interrupted low cycle fatigue tests and EBSD-measurements in the scanning electron microscope (SEM) the deformation induced transformation of austenite into α'-martensite occurring was able to be observed over the life of the specimen. From TEM-investigations carried out on thin metal foils prepared from the fatigue specimens, the presence of body centred cubic (bcc) α'-martensite was confirmed within the austenitic matrix. This phase transformation affects the crack initiation mechanism. Fatigue cracks often form within the microstructure at the phase boundary between the austenite and the martensite and in the martensitic areas where they grow into the martensitic phase formed at the crack tip.

Multiple cracking was observed in disc springs made from an austenitic stainless steel and used in large stationary gas turbine engines. It was determined that they failed in service by bending overload. The metallurgical cause of the cracking was found to be grain boundary embrittlement by secondary chromium carbide precipitates. They formed upon heat treatment during manufacture of the components. It was recommended to either alter heat treatment parameters to avoid embrittlement, or to select a different spring steel that does not require heat treatment.