Hydrogen content and outgassing of air-baked and vacuum-fired stainless steel

Abstract

The hydrogen content of stainless steel samples, treated by 400 °C heat treatment for 2, 4, 8, 16 and 24 h in ambient air, was determined by the 1H( 15N, αγ) 12C nuclear resonance reaction, and a standard extraction method. Two reference samples were also analysed, one virgin sample which was not heat treated at all, and one which had been vacuum fired (950 °C, 1 h). The results show that the heat treatment in ambient air results in an outgassing rate comparable to vacuum fired material although the amount of hydrogen in the samples was only lowered by approximately 50%, by the air baking, compared to a reduction to < 10% by vacuum firing. The decrease of the outgassing rate is therefore not governed by a reduction of the hydrogen content, the transport of hydrogen from the bulk to the vacuum phase is kinetically limited. The origin of the reduced outgassing rate is therefore assigned to deep trapping of hydrogen in the bulk material and/or formation of diffusion barriers in the near surface region and not to reduction of the hydrogen content of the material.