Studies on Vacuum Brazing (Report II) : Removal of oxide film from stainless steel surface and brazing alloy spreading mechanism(Welding Physics, Process & Instrument)

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Vacuum brazing of stainless steel was studied on both the mechanism of removal of oxide film from stainless steel (SUS304L, SUS3042B, SUS321) surfaces in vacuum heating and the spread mechanism of brazing alloy (BAg-8b) on stainless steel. The surface change of state with vacuum heating was observed by a high temperature optical microscope. The growth and removal of oxide on the surface with heating was clarified. The spreading phenomena were investigated by SEM, EDX, AES and Mass spectroscopy. The growth of oxide on the surface (SUS304L and SUS3042B) in vacuum heating from 550℃ to 750℃ was confirmed and the grown oxide was removed with more heating to 860℃, preferentially at the grain boundary part of the surface by reduction reaction of the carbon contained in steel. The phenomenon was confirmed by a striking increase in CO gas in vacuum atmosphere. The spreading of brazing alloy is due to the penetration of alloy into the grain boundary grooves produced by removing the oxide on the surface in vacuum heating. The spreadability was greatly influenced by heating speed in vacuum. Copper or copper-rich alloy diffuses primarily along the grooves without oxide and then liquid brazing alloy easily impregnats them and spreads on the surface. With SUS 321, the removal of oxide film and the formation of grain boundary grooves occur at a temperature of about 900℃ which is higher than that for SUS 304L.