鉄鋼のイオン窒化およびイオン軟窒化において希薄雰囲気へのNH<SUB>3</SUB>ガスの添加が窒化層の特性に及ぼす影響

Abstract

The study on the ion-nitriding (or ion-softnitriding) in the atmosphere of N2-H2 mixing gas (or N2-H2-C3H8 mixing gas) was previously carried out. In this report, NH3 gas was added to the above-mentioned atmosphere, and the degree of dissociation of NH3 gas, the thickness and composition of the compound layer and the depth of the diffusion zone were investigated (nitriding temperature: 550°C, nitriding time: 6 h, total gas pressure: 400 Pa (3 Torr), current density: 11.5 A/m2). The main results were as follows.(1) The degrees of dissociation of NH3 gas as a function of its molar fraction in the introduced gases (NH3-N2-H2 and NH3-N2-H2-1%C3H8) were measured when the molar fraction ratio of N2 to H2 gases in the exhaust gases was kept constant as 1/3. Using the above measured degrees of dissociation of NH3 gas, the molar fractions of NH3, N2, H2 gases in the introduced gases (NH3-N2-H2 and NH3-N2-H2-1%C3H8) were well calculated to keep the molar fraction ratio of N2 to H2 gases in the exhaust gases as 2/1.(2) When the molar fraction of NH3 gas in the exhaust gases was increased, the voltage was increased and the thickness of the compound layer was also increased; however, the depth of the diffusion zone was almost constant.(3) The volume ratio of the ε-Fe2–3N phase to the sum of the ε-Fe2–3N and γ′-Fe4N phases in the compound layer, cε⁄(cε+cγ′), was almost constant when the molar fraction ratio of N2 to H2 gases in the exhaust gases and the C3H8 gas content were constant. Here cε and cγ′ were the volume fractions of the ε-Fe2–3N and γ′-Fe4N phases in the compound layer.