Volume 53, Issue 10

After performing an alternating load test for several monobloc tubular shafts (so called MTS) a fractographical evaluation with subsequent metallographic examination as well as a review of the heat treatment according to drawing requirements were carried out on a test component which had reached the lowest number of cycles. An example is used to present a typical examination procedure in a quality laboratory which helps to ensure high product quality in a automotive supplier company and is supposed to highlight the close connection to and support for elaborate bench tests through the instrument of metallography. Besides the fractographical determination of the damage origin, an evaluation of heat treatment and microstructural development of the components is prerequisite for assessing the examination results.

The present article demonstrates the importance of metallography as an indispensable tool for process control as well as for establishment of basic principles for the process development based on a concrete practical example from the field of brazing of gas turbine components. This example serves to identify and prove the effects of differences in the thermal expansion behavior of non-similar materials on the braze joint by means of metallographic examinations. The knowledge acquired is used to realize braze connections of respective material composites.

Under certain conditions, Nb-containing stainless steels are susceptible to hot cracking. Such conditions include low melting phases on the grain boundaries, a coarse-grained microstructure such as cast structures, microstructure orientations towards the main tensile direction and high processing temperatures. The case of damage was characterized using metallographic and microanalytical methods. In the laboratory, the critical temperature range for the formation of hot cracks could furthermore specifically be localized under mechanical stresses by means of a dilatometer aiming at clearly verifying the cause of the damage, namely “hot cracks”.

The embrittlement of metals triggered by liquid zinc, has been known for about 140 years. William H. Johnson [1] stated that iron loses its forming capacity when dipped in melted zinc. Even today, this phenomenon still causes significant damages to hot-dip galvanized steel parts. Here, close attention should not only be paid to the composition of the molten zinc but also to the composition of the steel.