Volume 58, Issue 6

For the comprehensive investigation of advanced high strength steel grades, like complex and dual phase steels, Atomic Force Microscopy (AFM) and Scanning Kelvin Probe Force Microscopy (SKPFM) have proven to be useful tools, especially for analysis of hydrogen permeability of the individual steel phases. However, for these studies a preparation route, exposing the microstructure of the steel, is necessary. Various sample preparation methods were examined, focusing on electropolishing and sputtering, and the selected route was optimized to guarantee reproducibility and stability of the prepared surface. Electropolishing was shown to be highly efficient to selectively reveal the individual steel phases without introducing strong topographical features disturbing the AFM measurements. A subsequent sputtering step was introduced to improve the stability and preservability of the surface up to several months. Finally, distinction of the steel phases via AFM/SKPFM, was complemented and compared with results from EBSD and XRD.

In this study, the effect of multipass welding on the microstructure, mechanical and corrosion properties of the UNS 32205 duplex stainless steels (DSS) is investigated. The UNS 32205 DSS is welded in 3 or 7 passes by flux-cored arc welding (FCAW) using E2209 T1 – 1/4 flux cored wire. The weldments are characterized by light optical microscopy (LOM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Feritscope analysis, Charpy impact tests and electrochemical corrosion tests. The results suggest that the multipass FCAW process induces the formation of γ 2 in the weld seam. The mechanical and the corrosion properties of the weld joints are affected by the heat input variation and the phase transformations. Especially, the formation of the γ 2 in the weld seam results in a decrease in the corrosion resistance of the joint samples.

In this paper, a failure analysis of a high voltage feedthrough of the Indus 2 synchrotron radiation source which brought about venting of the vacuum chamber is discussed. A detailed failure analysis demonstrated that the main cause responsible for the failure of the feedthrough was high humidity in the atmosphere surrounding the feedthrough assembly, which resulted in the corrosion of CuSil braze filler alloy at the ground terminal and also caused flashover between positive (+) and ground terminals. Thermal cycling of the insulator due to flashover resulted in spalling of glazed surface layer and assisted crack propagation from the flashover-generated surface defects. The remedial measures recommended to avoid similar failure include: thorough cleaning of feedthrough and its connectors before installation, use gasket-type sealing in the protective cover of the connector on the air side, use of metallic tubes in place of polyurethane cooling water tubes and design change of feedthrough to shift brazed joint to vacuum side.