Some theoretical and experimental studies were carried out concerning the outer-layer differential thermal dilatation method that was contrived for precise measurement of the relativity between the transformation temperature and the solute concentration of specimens, such as carbonitrided specimens which have an inclination of the solute concentration in the case. The results in summary were as follows: (1) In such a study, we may proceed by substituting a long round solid bar with a layer on the outside in which the thermal expansion coefficient change gradually as a function of the position for a bar that has solute inclination in the surface layer. From calculation of the stress in this composite round bar at uniform heating, an equation of differential value which is measured according to the outer layer method was derived. ε
z−ε
z′=\dfrac2
Δbb(α
bt−ε
z′). In this equation, ε
z: unit linear strain of the composite bar with radius
b, ε
z′: unit linear strain of the composite bar with radius
b−
Δb,
Δb: skin thickness of the bar, α
b: thermal expansion coefficient of
Δb part,
t: uniform heating temperature. (2) Applying the outer layer method to carburized and nitrided specimens, it was experimentally ascertained that this method is reliable.
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