Volume 52, Issue 3

The ongoing trend towards miniaturization in various fields of material science requires the capability to investigate the local mechanical properties of the concerned structures by miniaturized mechanical experiments. Besides nanoindentation, miniaturized experiments such as micro-compression, micro-tension, micro-bending, or micro-fracture tests were employed frequently in recent times. A major challenge for these experiments is the fabrication of specimens. Therefore, we present different approaches to prepare miniaturized testing objects in a site specific way, using strategies that employ chemical etching, broad beam ion milling, and focussed ion beam milling. Depending on the required sample size and precision, the typical strategies for sample fabrication will be outlined, and the benefits and drawbacks of the techniques are discussed. Finally, applications of specimens produced by the different procedures are presented.

AISI type 316L stainless steel was used as a heat exchanger tube material in an inter-cooler column. After less than a year of operation, severe corrosion failures occurred and a transverse opening leakage was observed on one of the heat exchanger tubes. The failed tube was carefully analyzed using various metallurgical laboratory equipments. The root cause of the tube leakage was believed due to the presence of horizontal micro and macro pores as a hydrogen gas entrapment during casting of the parent ingot. The overlapped and gaping pores formed notch on the shell side of the tube surface, and it increasingly evident when the use of a high-energy water-jet and metal brush as cleaning procedure results in an establishment of pitting type local-action corrosion cells penetrated the tube wall. As a result, corrosive fluid in the tube side dissolved into the cooling water, accelerating the corrosion process.