Volume 52, Issue 11

In this study, AISI 316L series austenitic stainless steel sheets were joined by tungsten inert gas welding method in continuous and pulsed currents. Regarding microstructural investigation and hardness values of weld metal, samples were welded to investigate the effect of current type on grain structures of weld metal. Results showed that samples welded by using pulsed current had considerable different properties compared to the samples welded by using continuous current. While the weld metals of joinings obtained by using continuous current displayed a coarse-grained and columnar structure, weld metals obtained by using pulsed current had a finer-grained structure. It was also found that hardness values of samples, which were welded with continuous and pulsed current, were quite different.

The metastable austenitic steels 1.4307 and 1.4404 are characterized by a fcc lattice with paramagnetic properties which can be transformed by cold forming to ferromagnetic bcc martensite with acicular structures via a defect-rich intermediate stage. However, using classical wetchemical etching methods, defects and bcc martensite can visually not be clearly distinguished. Usually, magnetic or X-ray diffraction methods are used to quantify the ferromagnetic fractions. However, they do not allow for imaging the bcc phase distribution in the microstructure. Even with EBSD measurements in defect-rich phases, which, for process-related reasons, involve high investigation costs and long measurement durations, this is often only possible to a limited extent. The present study presents fast and simple contrasting by means of ferrofluids. It allows for a precise and spatially resolved differentiation between (ferromagnetic) martensite and (paramagnetic) defect structures on metallographic cross-section by optical microscopy. Furthermore, it is shown that, after a ferrofluid treatment of metallographic cross-sections made of duplex steel, ferromagnetic ferrite areas already show evidence of grain orientations.

Fastening brackets for ceramic heat shields of gas turbine engine combustion chambers were found to be grossly overheated after only 1 100 operating hours. They were made from the wrought nickel-base superalloy Nimonic alloy 90. The affected parts were disassembled from the engine and replaced. One bracket was selected for an exemplary metallographic examination at the laboratory of the gas turbine engine manufacturer. The bracket exhibited severe deformation, oxidation, and creep damage. The alloy was selected properly for this application. The cause of the thermal overload is unknown to the investigating laboratory. It was recommended to find and remove the reason for the excess thermal exposure.