RESEARCHES ON THE SPECIAL CAST STEEL

ON TAE CHANGE OF MACROSTRUCTURE BY DIFFUSION ANNEALING TREATMENT

Abstract

The change of macrostructure on the diffusion of dendritic segregation were studied by means of normal annealing and high temperature normalizing treatment. Repeated normadizing test were applied to the various specimens, among which were contained Si, Ni Mn and Cr-Mo Cast steels (1% Cr, 0.25-0.35% Mo).
The dendritic segregation is not affected by the normal annealing or quenching at just above the Ac3 transformation point, but is diffused and destroyed by the normalizing treatment at higher temperature.
Though the rate of diffusion of dendritic segregation increases rapidly as the normalizing temperature elevates, it has some difficulties in diffusion according with the degree of coarseness or refinement of it.
In general the addition of Si to Cr-Mo cast steel gives a considerable difficulty upon the diffusion of dendritic segregation. The larger the Si content, in these cast steels, the more diffiicult will be the diffusion, on the contrary the addition of Ni to these steels does not shows any significant change.
Though the diffusion are easy in a finely dendritic structure; it shows a remarkable difficulty, When the dendrites are developed. In such case the high temperature normalizing must be carried out, though it is undesirable, lest grains should grow and toughness should rapidly decreases. Therfore repeated normalizing is preferable to high temperature normalizing.