Isothermal Transformation of Spring Steels and Their Mechanical Properties

Abstract

The study of isothermal transformation of plain carbon (0.56% C, 0.82% Mn), Si-Mn (0.57% C, 1.65% Si, 0.87% Mn) and Mn-Cr-B steel (0.57% C, 0.84% Mn, 0.75% Cr and 0.003%B) was carried out to establish the relation between the microstructure and the mechanical properties.The results obtained are summarized as below.
1) The effect of Si is observed in the microstructure where a great amount of ferrite (or probainitic ferrite) precipitates in the structure after isothermal transformation.This effect of Si results mainly from a notable retardation of carbide formation, therefore, a bainite that is formed at low temperature contains super-saturated carbon atoms, so that ductility decreases remarkably.
2) The effect of Cr on retardation of transformation curve is remarkable, as compared with that of plain C steel or of Si-Mn steel.Moreover, the additon of a small quantity of B contributes to greater retardation of bainite transformation.In this steel, the hardness change, as a function of isothermal transformation temperature, is characterized by the transformation mechanism.
These are due mainly to a carbide density of structure.The lower bainite structure of this steel represents a high ductility even at high strength level, so that it may be concluded that a reduction of area of isothermal transformation product is independent of the strength level, and that it depends mainly on the microstructure.The isothermal transformation of this Mn-Cr-B steel has a beneficial effect on its mechanical properties.