Ultrafast Heating and Initial Microstructure Effect on Phase Transformation Evolution of a CrMo Steel, Metals 2019, 9, 72; doi: 10.3390/met9010072
Abstract
Main target of the present work is to elucidate the effect of both initial microstructure
and heating rate on phase transformations that occur during ultrafast processing. For
this purpose, two initial microstructures, a ferritic-pearlitic and a soft-annealed
microstructure were considered. We applied different heating rates (10oC/s, 200 oC/s,
300 oC/s) up to the peak austenitization temperature, θ 900oC. The evolving
microstructure is analysed via SEM and EBSD, whereas the carbide dissolution and
austenite formation is simulated with Thermocalc® and DICTRA software. Data
obtained in this research proves that, when the heating rate increases, the carbide
dissolution rate is disseminated. Compared to a conventional heating rate, where the
local chemical composition homogenizes as a result of diffusion, rapid reheating leads
to intense segregation of the substitutional atoms at the cementite/austenite interface
and turns diffusion to a sluggish process. This fact, combined to the infinitesimal time
for diffusion, forms an inhomogeneous carbon distribution along the microstructure.
This inhomogeneity is further enhanced by the presence of increased carbides’ size
present in the initial microstructure. Due to rapid heating, these carbides cannot be
decomposed since the diffusion distance of alloying elements increases and the
diffusion of alloying elements is impeded during ultrafast heating, thus, remain
undissolved at peak austenitization temperature. Their presence and effect in
heterogeneous ferrite nucleation restrict austenite grain growth. Consequently, fine
austenite grains in conjunction with their chemical heterogeneity lead to the
coexistence of fine martensite, bainite laths and undissolved carbides in the final
microstructure after quenching.