過共析鋼における炭化物のオーステナイトへの拡散

Abstract

When hypereutectoid steels are heated above Acm line, carbides do not diffuse instantaneously into austenite. In the heat-treatment of tool steels, the normal quenched structure is gained only after the adequate normalizing practice to dissolve the carbide into austenite.Here, the author has studied the diffusibility of carbides into austenite, using the specimens of the high carbon steel and Cr bearing steel. These specimens are previously heat-treated with the following methods to get the desired forms of carbide, for the diffusibility depends on the form of carbide, such as pearlitic laminar, sorbitic fine or globular. (1) Pearlitic—furnace cooled from 920°, (2) sorbitic—air cooled from 920°, (3) globular—annealed at 760° after air quenched from 920°. These various specimens kept at the given temperatures above Acm line for some time are quenched in water. The time required of the carbides to be perfectly dissolved into austenite can be known by the microstructure and hardness of the quenched specimens. Most carbides are dissolved into austenite at the early stage of the holding time and it requires much time to dissolve a little residual carbides. Fig. 2 and 3 show that the pearlitic carbide is the most easily diffused into austenite and the globular carbide the most sluggish to diffuse. The diffusion constants of these carbides into austenite are quantitatively known from the experiment in which the iron electroplated specimens are heated at the given temperature for some time and the diffused carbon content in the surface layer is chemically analysed (Table 4).