Volume 52, Issue 7

In order to be able to carry out reliable measurements using a Raman Spectrometer it is important to know what stresses and localised changes have been induced in the specimen by the preparation process. To investigate this, a trial was set up to investigate the changes which occur in the structure of the material as a result of the specimen preparation process, thus leading to an optimum method of preparation for Raman Spectroscopy.

Self-tempering effects can be observed in steels with relatively high martensite start temperatures. After the formation of the first martensitic laths, carbon is able to diffuse in these laths during cooling, which can be attributed to sufficiently high temperatures. This effect cannot be observed in laths formed at lower temperatures. In steels containing up to 0.2 m.-% carbon, up to 90 % of the carbon atoms in the martensite segregate to dislocations during quenching. Due to its atomic resolution and sensitivity with respect to light elements, atom probe tomography is very well suited for the investigation of this phenomenon. In this study, the self-tempering effect in a quenched and tempered steel 42CrMo4 with a martensite start temperature of 310 °C is investigated by means of atom probe tomography.

Carbide-free bainitic steels are composed of bainitic ferrite laths, which are separated by films and larger islands of austenite, which is stabilized by carbon enrichment. Due to their multi-phase microstructure, the preparation of such steels for a characterization by means of electron backscatter diffraction (EBSD) represents a challenge. Attention must particularly be paid to prevent the transformation of the retained austenite to martensite during the preparation. This study compares the sample preparation by vibratory polishing using different suspensions to the electrolytic preparation. During vibratory polishing, it must be ensured that very little force is applied in order to prevent the metastable austenite from transforming. No influence of the different suspensions could be found. Electropolishing with a voltage of 40 V for 10 s at 23 °C is well suited for microstructural investigations. However, the surface relief is too pronounced for EBSD analyses. OP-U polishing for 15 min subsequent to electropolishing accomplishes the best results.

The failure of a coupling in a two-feet light railway occurred when the engine began to move out of a station. According to the machinist who operated the steam engine at the time of failure, a number of railcars were coupled to the engine. The railcars carried an unusual high number of passengers but were not over-loaded. The crack was driven by a mode that was cyclic in nature. The crack propagated in a fully reversed bending mode. It could be ascertained that the crack propagated in a low cycle fatigue (LCF) mode.