Microstructural Changes in Low-carbon Steel Occurring by Heating in Mixtures of Iron, Graphite and Alumina Powders

抄録

Carbon and nitrogen are easily diffused into stainless steel and titanium, when these are embedded in mixtures of iron, graphite and alumina powders and held at high temperatures in a nitrogen flow. To promote an understanding of this new diffusion technique, which we call “iron-powder pack treatment”, we focused attention on the microstructures of low-carbon steel subjected to such a treatment. The diffusion of carbon into the steel was understood by evaluating area fraction of pearlite emerging in ferrite. A pearlite area in the steel, which was heat-treated at 1273 K for 3.6 ks using a mixture of iron and alumina powders, was larger than before heating, because the steel was carburized by carbon in the iron powder. On the other hand, an increase in the pearlite area was hardly observed in the case of a mixture of graphite and alumina powders. By replacing a part of graphite in this mixture with the iron powder, however, pearlite was most remarkably produced in the steel. Instead of the iron powder, the use of nickel powder did not exert a significant impact on the diffusion of carbon into the steel. These indicate that the iron powder acts as an important agent for enhancing the migration of carbon from a powder mixture to low-carbon steel. The relationship between a diffusion phenomenon of carbon and the generation of carbon monoxide gas in heat treatment is also discussed.