Band 54, Heft 3

A failure in the form of leakages caused by crack formation occurred after approximately 25 years of operationn on a type 1.4301 stainless steel container thermally insulated with polyurethane. From the appearance of the damages it was clear that the failure was caused by stress corrosion cracking. It was shown that chloride release by the ageing of the PU foam was responsible for the corrosion. The damages were documented via 3D digital microscopy and metallography. To avoid these kinds of damages, insulation materials free of halogen compounds should be used. Regular checks of old stainless steel reactors insulated with PU foam and operated at elevated temperatures are recommended.

Semi-solid processes of aluminium alloys have been a focus of interest for many years. Thixoforging is a kind of forging process which is carried out in the semi-solid region of the alloy. Ingots used in thixoforging process cannot be produced by conventional ways; they must be prepared with different processing routes. In this study, low superheat casting (LSC) technique was used for ingot fabrication in thixoforging process. LSC can be summarized as the pouring of molten metal at a temperature very close to liquidus followed by rapid solidification. Low heat input combined with sudden cooling suppresses dendritic growth and the conventional casting structure converts into a more spherical structure. A356 and A380 are well-known casting aluminium alloys which are commonly used in sand and die casting processes. These alloys are appropriate for semi-solid process and there are many applications that researches have been carried out in time. In view of that, A356 and A380 alloys were selected as subject of this comparative study and final structures were evaluated together. A LSC combined thixoforging process was successfully applied to both alloys and a spherical microstructure was obtained. High sphericity with a simple microstructure is the characteristic of the A356 alloy. A380 shows relatively lower sphericity with more complex microstructure which has numerous different intermetallic compounds.

Delayed brittle fractures of high-strength bolts of the strength class 10.9 are presented, taking the example of three damage cases. The respective damage mechanisms could be attributed to hydrogen induced stress corrosion which was caused, in turn, by hydrogen absorption during operation. The examples were chosen with a particular focus on the material condition‘s susceptibility which explains the cause for the occurrence of the damage mechanism. However, in only one of the three cases the susceptibility was evident and could be explained by violations of normative specifications and an unfavorable material choice. Whereas in the two other examples, only slight or no deviations from the standards and/or regulations could be found. The influencing parameters that caused the damage, those that further promoted the damage, as well as possible corrective actions are discussed taking into account the three exemplary damage cases.