JOURNAL OF THE JAPAN WELDING SOCIETY

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Heat treatment of steel is one of the most important matters in productive engineering. Many authorities have already made an effort to establish the theory of transformation of steel. But these theories are not completely able to explain the phenomena of phase change of steel in both cases of quenching and slow cooling.
The author has endeavoured to certify the transformation of austenite→martensite and austenite→ferrite + cementite from different points of consideration. As a result he has proposed the following two important theories concerning the transformation.
1. A homogeneous γ solid solution is stable to Ar" point in cooling but carbon atoms do not move to aggregate locally or to settle in compounds. In this case the austenite-martensite transformation is concluded as a single component system and an iron-carbon alloy of a certain composition is transformed from austenite to martensite at a certain temperature.
2. If it is assumed that carbon atoms in γ solid solution are allowed to move freely, they distribute themselves to keep the equal relative stability in crystals even in a transient state of transformaton. In this case it is explained as the phase change of two component Fe-C system. The equiliorium diagram shows the ideal case of this change.
Theory 1 and 2 are confirmed by experimental facts. And in the case of slow cooling, carbon atoms in γ crystal move previous to a lattlice change of γ→a or separation of Fe₃C from γ solid solution. In quenching, carbon atoms can not be transferred and only a lattice change takes place at Ar" point. This change is considered a partial equilibrium change of transformation neglecting the movement of carbon atoms whose migration is very slow compared to lattice change.
After quenching the secondary change of transformation in tempering or aging is
M→a+Fc₃C
and reversible change
γ M
is not realized in ordinary conditions.
Dr. K. Honda has an opinion that the change of A₁ transformation is always passed through the following steps :
γ→M→a+Fe₃C
and that cementite is to be separated after lattice change
γ→M
According to the author's theory the carbon atom is to move in a solid solution before the Iattice change and therefore the change of
γ→α
and separation of Fe₃C from γ are direct ones as follows:
γ→α
α→Fe₃C
Dr. K. Iwase's theory that the locus of Ar" points concerning the composition of steel, is presumed as the extension of A₃ solidus line GP, is not considered to be correct.

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