Cr-Mo浸炭鋼の変態特性に関する研究

抄録

Isothermal transformation and continuous cooling transformation diagrams were plotted for a 0·2% C-Cr-Mo steel and the steels carburized to 0·5, 0·7 and 1·0%C respectively.
In order to investigate the effect of carbon content on the transformation behaviour, hardenability, hardness, etc., a carburizing method was employed to increase the carbon content without any change of other alloying elements.
Acicular ferrite forms in the upper range of bainite transformation for steels containing carbide forming elements, such as chromium and molybdenum, and it accelerates pearlite transformation. Thus, the mechanism of these transformations was discussed from the viewpoint of kinetical theory.
Steels having the same carbon content were prepared by both carburizing and melting methods. Differences between both steels were studied on the transformation behaviour and mechanical properties.
The results are summarized as follows:
(1) In general, pearlite transformation is accelerated by an increasing carbon content. It is markedly affected by the formation of proeutectoid products. Proeutectoid cementite has a much greater effect on the acceleration of pearlite transformation than proeutectoid ferrite.
(2) In the upper bainite range, initiation of pearlite reaction is markedly accelerated. It is considered that the main factor of acceleration is the increasing carbon content, caused by the formation of bainitic ferrite, and kinetic data of pearlite reaction conform to the equation of nucleation and growth.
(3) Hardness of the structures formed isothermally is increased with an increasing carbon content in pearlite range. But it is less affected by the carbon content in the bainite range. On the other hand, hardness of steels which were transformed under continuous cooling conditions is much more affected by the cooling velocity and carbon content in the range of bainite formation than in that of pearlite formation.
(4) Effect of carburization treatment on the transformation behaviour is primarily the annealing effect, with exception of increasing carbon content. Homogenizing effect of carburizing process narrows an interval between beginning and end of transformation. No difference could be found between mechanical properties of carburized steels and melted steels under isothermal transformation.