Determination of Nitride and Dissolved Nitrogen in Steel by Hydrogen Hot Extraction

Abstract

A coulometric nitrogen analyser for the fractional determination of metallurgically dissolved nitrogen and various nitride precipitates in steel by hydrogen hot extraction has been developed. Nitrogen as NH₃ in a hydrogen stream is successfully determined continuously by every 0.5μg and printed integrally by the analyser. Blank values of the determination are less than 0.5μg N.
Aluminium-killed steel with dissolved nitrogen and precipitated nitrogen is heated in a hydrogen stream through a wide range of temperatures from 50°C to 1000°C, with the combined application of the newly developed nitrogen analyser and isochronous heating, in place of both photometric analysis which yields the only one determination of total extracted nitrogen and isothermal heating that has been adopted by many investigators, and the extracted nitrogen as NH₃ provides separate and quantitative determinations of dissolved and precipitated nitrogen.
Dissolved nitrogen is extracted at about 265°C, which corresponds to the reduction temperature of oxide film on specimens by hydrogen stream. Precipitated nitrogen as AlN is extracted mainly at about 800°C, and partially at the temperature between 425 to 950°C depending on the particle size and stability of the precipitates. Variations of extraction temperatures, therefore, also provide separative determinations of unstable fine precipitates and stable coarse nitride particles. Above mentioned experimental results are confirmed electron-micrographically and supported by the considerations of the reaction mechanism and the free energies of oxide film and nitride in the steel.