Band 52, Heft 8

The interlamellar spacing of pearlite is a very important microstructural parameter for steels containing pearlite, and becomes more important as the pearlite content increases towards a fully pearlitic microstructure. As the amount of pearlite in ferrite-pearlite microstructures increases, so does the strength, but toughness and ductility decrease. For a fully pearlitic steel, as the interlamellar spacing becomes finer, strength, toughness and ductility all increase. Consequently, in structure-property correlations it is important to measure the interlamellar spacing. This paper reviews procedures for performing such measurements. Due to the fineness of the spacing, either SEM images or TEM images of replicas or thin foils can be utilized. The range of spacings in a given specimen will be much narrower if the pearlite in the steel was formed isothermally rather than transformed over a range of temperatures, as in as-rolled or normalized steels.

In order to provide a solid basis for the correlation of microstructure and hydrogen isotope retention in tungsten, reference samples with different microstructures were prepared from a single batch of polycrystalline tungsten by standardised polishing and heat treatment procedures. Representative samples were analysed by scanning electron microscopy and scanning transmission electron microscopy as well as by electron backscatter diffraction. We show that if the annealing temperature is increased from 1 200 to 1 500 K, practically only the density of dislocations and grain boundaries with very small misorientations of less than 2° is reduced, while for annealing at 1 700 and 2 000 K, also the density of high-angle grain boundaries is reduced due to grain growth. Furthermore, the dislocation density is reduced by nearly two orders of magnitude compared to tungsten annealed at 1 200 K. We also comment on two different textures on the front and rear side of the samples that were observed both by X-ray diffraction and EBSD.

One out of eight lifting lugs of a large gas turbine engine exhibited indications upon magnetic particle surface crack inspection (MT), that were later verified by ultrasonic testing (UT). The subject part was made of a low-alloy steel casting. It had already been used in service and was supposed to be re-used, together with the other lifting lugs of the set. In order to determine the cause of the indications, non-destructive metallographic examinations by means of the replica technique were performed. In that investigation, the indications were determined to have been caused by crack-like decohesions of material. In one of the examined areas, these imperfections were associated with small casting defects, namely micro shrinkage. This might have contributed to crack initiation. The described small surface cracks at the cylindrical part of the lifting lug itself were probably caused by bending stresses in service. Indications were found only on one out of the entire set of eight parts. There, only the upper side of the short spigot, were axial tension on the surface is to be expected in service when the bollards are in lifting action, was affected. In a second investigation, that same part was sectioned in the area with the largest UT indications. The microstructure of this steel casting is unremarkable. However, several rather large macrocracks were found in the sectioning plane. Scanning electron micrographs confirmed this. There is a moderate carbon segregation in the subject area that is unremarkable. The longest crack in the metallographic section was forced open for microfractography in the SEM. A small pre-existing crack that could not have been caused by service loads was found. Within that area, microfractographic features of transgranular hot tearing were detected, as evidenced by a dendritic appearance of the fracture surface. This is a hot cracking mechanism.