α鉄からの窒化物の析出速度におよぼす合金元素の影響

抄録

Aging curves for the precipitation of Fe₄N and Fe₁₆N₂ from α-iron were measured in a temperature range between 373 K and 623 K by internal friction technique, and effects of alloying elements (Co, Mo, Ni, W) on the precipitation process were studied. The results are discussed in terms of the C-curves for 50% precipitation of Fe₄N and Fe₁₆N₂.
Experimental results are summarized as follows:
(1) The C-curve in an unalloyed iron depends on the concentration of nitrogen; the nose of the C-curve shifts to shorter times and higher temperatures as the nitrogen concentration increases.
(2) Precipitation of Fe₄N. The addition of 0.8 and 1.2%W has no effect. The addition of Co and Mo shifts the upper part of the C-curve to the right, i.e., the precipitation is delayed, but has little effect on the lower part. The addition of Ni enhances the precipitation rate considerably. In an unalloyed iron, Fe₄N precipitates only above 473 K, but in 4.7%Ni and 4.9%Co alloys it precipitates even at 423 K.
(3) Precipitation of Fe₁₆N₂. The addition of W shifts the whole C-curve to the right. Other alloying additions (Co, Mo, Ni) shift the upper part of the C-curves to the right and have little effect on the lower part. The effect of Co is the most pronounced.
(4) The upper part of the C-curve is related to the rate of precipitate nucleation, and it has been found to be affected by the alloying. The lower part, which is related to the diffusion of nitrogen, is not affected appreciably by the alloying. The diffusion rate of nitrogen is not affected by the alloying either. Hence, it is considered that the alloying affects the rate of nucleation. This effect is discussed in terms of the change in the solubilities of nitrides by the alloying; the change in the degree of supersaturation of nitrogen by the alloying is in a very good correlation with the change in the nucleation rate represented by the nose-time of C-curves.