Rates of nitriding and nitride composition in austenitic steels containing titanium

Abstract

Nitriding rates have been measured in austenitic steels with varying titanium and silicon contents using 95%N₂–5%H₂ gas at 945–1150°C. The parabolic rate of penetration and the activation energy (214± 8 kJ mol⁻¹) agree with control by nitrogen diffusion. Titanium additions decrease the nitriding rate owing to the increased quantity of nitrogen required to form TiN. Silicon additions also decrease the nitriding rate but this appears to be due to a slight decrease in nitrogen solubility with increasing silicon content. The TiN particles formed on nitriding were not of constant composition. The chromium content of TiN increased with distance from the nitriding front, at which it was about 8%in a 1.5%Ti alloy. The chromium content also increased with nitriding time so that weight gains could not be used to obtain the solubility of nitrogen in the matrix. The Nb/Ti ratio in ‘TiN’ particles was constant and depended only on the Nb/Ti ratio in the alloy. Cr₂N formed in all alloys nitrided at temperatures below 1150°C. The grain boundary penetration of Cr₂N appears to be associated with the ease of nucleation rather than grain boundary diffusion. A silicide (G-phase) was stabilized in the presence of zirconium or high silicon contents. This phase retained a small grain size but was converted to zirconium or titanium nitride during nitriding and was left as a coarse lump so that it did not contribute to the fine TiN dispersion