Peter Kaestner

Plasma-assisted boriding of pure titanium and TiAl6V4

Abstract Pulsed-DC plasma boriding in an Ar–BCl 3 atmosphere was performed for pure titanium and the titanium alloy TiAl6V4 in the temperature range of 700–900°C. The plasma boriding leads to the formation of TiB 2 and TiB at the surface for pure titanium as well as for TiAl6V4, depending on the process parameters. These phases were identified using X-ray diffraction (XRD). The amount of boron, oxygen and the alloying elements at the surface and the depth profiles were examined by glow-discharge optical emission spectroscopy (GDOS). The hardness profile was measured on metallographic sections after boriding. The high hardness of the layer is similar to the hardness known for titanium boride layers formed by plasma-assisted chemical vapor deposition (PACVD). Scratch tests indicate that the adhesion strength is relatively high, thus indicating a high potential for industrial applications under tribological conditions.

Improvement in the load-bearing capacity and adhesion of TiC coatings on TiAl6V4 by duplex treatment

Abstract It is shown that the use of nitrogen or carbon as alloying elements in plasma diffusion treatment of TiAl6V4 is beneficial for the load-supporting properties of the base material. The plasma diffusion treatment produces a compound layer consisting of nitrides or carbides and an inner diffusion zone, essentially characterized by the presence of nitrogen- or carbon-rich α-Ti crystals which are embedded in the metal matrix. This results in a hardness gradient from the interior to the surface. To avoid hydrogen embrittlement, plasma nitriding is performed in a pure nitrogen atmosphere. The formation of soot by carburizing could be avoided by optimizing the carbon flow during plasma carburizing. The influence of plasma nitriding and carburizing on the adhesion of the TiC coating depends on the plasma diffusion pretreatment and the conditions of the TiC coating. Plasma-nitrocarburizing pretreatment leads to the best adhesion of the TiC coating; the critical load of failure is 80% higher than in the case of TiC on base material. A critical load of approximately 33 N was found under the optimum parameters. We conclude that the combination of plasma diffusion treatment and TiC coating extends the tribological applicability of TiAl6V4 in many industrial sectors. Fretting fatigue and wear could be increased by a duplex plasma treatment combined with shot peening.

Metallurgical investigation of electron beam welded duplex stainless steel X2CrNiMoN22-5-3 with plasma nitrided weld edge surfaces

Abstract Duplex stainless steel is common in thick-walled components such as longitudinal welded pipes in the oil and gas industries and as parts of various machines in the chemical and food industries. Electron beam welding is a very suitable method for welding such components. Due to the high power density of the electron beam combined with the decreased evaporation temperature in a vacuum atmosphere, steel with a sheet thickness of up to 150 mm can be welded in one pass. In the case of the electron beam welding of duplex stainless steels, vacuum atmosphere in the working chamber causes a nitrogen effusion from the weld pool. The microstructure of the resulting weld is characterized by an unacceptable high ferrite content, which is the main reason for both low impact toughness and low pitting corrosion resistance. This work focuses on the influence of nitrided weld edge surfaces on metallurgical properties of the resulting welded joint. The aim here is to investigate the effect of increased nitrogen content at the weld edges on nitrogen loss during EB-welding. The welds produced within the experimental work were characterized by means of microstructural analysis and the use of optical and energy dispersive X-ray spectroscopy. It was shown that nitrided weld edge surfaces can compensate nitrogen loss from the weld pool and decrease the ferrite content in the resulting weld.