Since the 1970s, bismuth is widely used as an auxiliary ingredient in stainless steel flux-cored wires to improve slag detachability. But, for components subject to post-weld heat treatment (PWHT) and/or applications at high temperature, bismuth has been confirmed to have a negative effect on weld metal ductility. It has been suggested that this is due to grain boundary bismuth segregation, and it has been debated whether it is as bismuth or as bismuth oxide Bi 2 O 3 . There are also reports on cracks found in weldments after service at elevated temperatures. This has affected the specifications, and API RP 582 has included a maximum bismuth content of 20 ppm in the weld deposit when welding with austenitic stainless steel flux-cored wires for applications above 538°C, including PWHT. This demand required development of a range of flux-cored wires intended for overlay welding (cladding) of creepresistant steels and joining stainless steels for hightemperature applications. Standard E347, E309L and E308H wires have here been compared with bismuthfree versions in as-welded condition and after PWHT. All-weld metal has been subject to mechanical, hot ductility and Varestraint testing. Results show that bismuthfree wires have higher ductility, and this was confirmed also when welding in single V-butt weld joints. Electron microprobe analysis (EPMA) modified for high precision mapping is used to illustrate that bismuth has a particle-like distribution without any clear relation to oxygen.
Laboratory trials were performed in an induction furnace to study droplet formation during lance blowing. Oxygen was blown on a molten iron bath consisting of iron alloyed with carbon and silicon. Iron droplets were collected using a specially designed sampler. The average iron droplet composition and the oxide layer thickness were determined using scanning electron microscopy combined with energy dispersed spectroscopy. In addition, the concentration gradient of elements was determined using electron probe microanalysis. It should be noted that a special technique had to be developed in order to prepare the droplet sample. The size distribution and composition of the droplets were also determined using the microprobe. The carbon was found to be homogeneously distributed throughout the droplet independently of the size of the droplet. For the experiments using both carbon and silicon it was found that the silicon in most droplets could be found in the periphery of the droplets. It was also found that the tendency was that both the carbon content as well as the silicon content in the droplets decreased with a decreased droplet size. Thus, it was concluded that it is necessary to modify top blown decarburisation processes so that a maximum area between droplet and atmosphere is obtained.
In material science, there is an increased demand for mapping of microstructural components and their composition. EPMA (Electron Probe Micro Analysis) with WDS (Wavelength Dispersive Spectrometry) is known as having high spectral resolution and sensitivity, but in practice considered to be slow in mapping applications. The present work describes a development of EPMA including design of both instrumental hardware and software related to electronics and calibration.
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