Toshio Katagiri1 and Nobujiro Takamoto1
1The Faculty of Engineering, Tokyo Metropolitan University, Tokyo
Microscopic examination was made on the formation of the pore in the boride layer of pure iron by gas boronizing, as it is considered as a cause of its brittleness.
By boronizing for various periods at 1000°C, it was observed that a pore was found from the initial stage of boronization, and with treating for 10 min the pore was formed all over the boride layer. With further boronization, the pore was found mostly on the boundary between the boride layer and the matrix. At the temperatures below 900°C, the pore was observed slightly in the FeB layer and not found on the boundary between the boride layer and the matrix irrespective of the boronizing time. It was found that by diffusion annealing of the boride layer, the pore did not move or form newly. By chemical analysis, Fe2+ and Fe3+ were observed in the reaction tube after boronizing.
From the above-mentioned results it was considered that the formation of the pore was not caused by the Kirkendall effect or difference in the coefficient of thermal expansion between the boride layer and the matrix,but by some chemical reaction.
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