Abstract
Sintered compacts of type 316L stainless steel, made by powder metallurgy, were resistant to corrosion in 5% sodium chloride solution if the sintering was carried out under conditions designed to reduce the formation of chromium nitride and chromic oxide. The compacts absorbed nitrogen rapidly at temperatures above 900° and the equilibrium nitrogen content decreased with increase of temperature. In an atmospheee of cracked ammonia, the equilibrium nitrogen content fell from 5 mg N2 per g steel at 1,000° to 1·3 mg N2 per g steel at 1,220°. Electron-probe and X-ray diffraction analyses showed that intergranular precipitation of chromium nitride occurred in compacts held at 830° in cracked ammonia. Corrosion-resistant compacts could be made in cracked ammonia only by quenching from the sintering temperature (1,150°–1,300°) at a rate of at least 100° per min. Quenching was not necessary if the compacss were sintered and cooled in atmospheres of hydrogen containing less than 50 ppm by volume of water vapour (dew point, —48°); such atmospheres permitted corrosion-resistant compacts to be made by cooling from 1,150° to 300° over 4 h. In atmospheres of hydrogen containing moie than 50 ppm by volume of water vapour, quenching from the sintering temperature was found to be necessary to obtain corrosion-resistant compacts. The deleterious effect of water vapour was ascribed tentatively to oxidation of chromium at temperatures above 750°.
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