オーステナイト系ステンレス鋼の窒化速度の異常温度依存性

抄録

A sudden decrease of the nitriding rate at about 873 K and a markedly promoted nitriding rate below this temperature were investigated. SUS304, 309 and 310 austenitic stainless steels were nitrided in ammonia gas in the temperature range 661∼1123 K for 0.9∼25.2 ks. In this paper, an explanation for understanding of the abnormal phenomena is attempted on the basis of a fact that the standard free energy of formation of chromium nitrides (Cr₂N and CrN) intersects at about 703 K. The experimental results were as follows: (1) A discontinuity of the nitriding rate was observed at about 693∼713 K near the intersection temperature of the free energies, and the promotion of nitriding began above the temperature of the discontinuity. (2) A supersaturated solid solution (fcc, a₀\fallingdotseq0.406 nm), containing approximately 33 atomic percent of nitrogen estimated from its lattice paramater, was detected from the surface of SUS304 steel nitrided by X-ray diffractometry, and transformation of this phase into γ′-(Fe, Ni)₄N and CrN was observed in the temperature range where the nitriding has been promoted. (3) The structure under the surface of SUS304 steel nitrided varied from [γ′-(Fe, Ni)₄N+CrN] to [austenite+CrN] between 923 and 973 K. (4) All the nitriding rates of SUS304, 309 and 310 steels showed a sudden decrease at about 873 K irrespective of the compositions. From the above results, it is concluded that the sudden decrease of the nitriding rate is a phenomenon as a result to caused upon termination of the effect of nitriding at approximately 873 K.