Effects of Rare Earth Element on Isothermal and Martensitic Transformations in Low Carbon Steels

Abstract

Rare earth elements (RE) may segregate at the grain boundaries of austenite, lead to form carbide and refine the austenite grain. In case of no change of grain size and carbon content of austenite, an addition of RE is beneficial to the hardenability of steels. In case of a marked refinement of austenite grain, addition of RE will deteriorate the hardenability. The incubation period of the proeutectoid ferrite can be expressed as a function of grain boundary energy, grain size, activation energy for growth and the driving force for transformation and the calculated results are in good agreement with the experimental data. RE may retard the isothermal pearlitic transformation, because RE diminishes the diffusion coefficient of carbon as well as tends to segregate at Fe₃C/α interface, showing a pinning effect on the transformation. RE reduces the lamella spacing of the pearlite owning to lowering the interfacial energy, e.g., from 0.7 to 0.53 J/m² in 0.27C–1Cr–RE steel. RE tends to segregate at ferrite/island interface in the granular bainite. In grain refined steel, at the earlier stage of bainite formation, the transformation rate is high while at later stage it becomes sluggish. The activation energies of pearlitic and bainitic transformations increase by the addition of RE. The segregation of RE at ferrite/island interface may act as a drag effect. A drag factor α is expressed as a function of transformation fraction and calculated in a 0.27–1Cr–RE steel. RE segregates at the grain boundary of austenite and this kind of distribution will not be changed during the martensitic transformation. It is reasonable to predict that RE will lower the martensite/austenite interface energy, resulting in the formation of a finer lath structure. RE lowers M_s, decreases the amount of the retained austenite and retards the autotempering process. It is emphasized that the amount of the retained austenite, γ, in quenched low carbon steel depends on not only the M_s and the temperature of quenching medium, T_q, but also the influence of alloying elements on the carbon diffusion during quenching. A general equation modified from the Magee's equation is derived as γ=exp{α(C₁–C₁)-β(M_s–T_q)} where C₀ and C₁ are carbon concentrations in austenite before and after quenching respectively, α and β are constants. RE decreases M_s but also lowers C₁ so as to reduce the amount of the retained austenite.