Abstract
Atomic-scale investigation was performed on 51CrV4 steel, isothermally held at different temperatures within the bainitic temperature range. Transmission electron microscopy (TEM) analysis revealed three different morphologies: lower, upper, and inverse bainite. Atom Probe Tomography (APT) analysis of lower bainite revealed cementite particles, which showed no evidence of partitioning of substitutional elements; only carbon partitioned into cementite to the equilibrium value. Carbon in the bainitic ferrite was found to segregate at dislocations and to form Cottrell atmospheres. The concentration of carbon remaining in solution measured by APT was more than expected at the equilibrium. Upper bainite contained cementite as well. Chromium and manganese were found to redistribute at the cementite-austenite interface and the concentration of carbon in the ferritic matrix was found to be lower than the one measured in the case of lower bainite. After isothermal treatments close to the bainite start temperature, another austenite decomposition product was found at locations with high concentration of Mn and Cr, resembling inverse bainite. Site-specific APT analysis of the inverse bainite reveals significant partitioning of manganese and chromium at the carbides and at the ferrite/martensite interfaces, unlike what is found at isothermal transformation products at lower temperatures.
Highlights
The formation of bainite has been attracting scientific interest for almost a century
Atom Probe Tomography (APT) analysis of lower bainite revealed cementite particles, which showed no evidence of partitioning of substitutional elements; only carbon partitioned into cementite to the equilibrium value
The results indicate that the carbon content in ferrite of upper bainite is 0.35 at.%, higher than the equilibrium value resulting from the extrapolated α/γ line in the phase diagram at the selected temperature, calculated by Thermocalc, which is 0.12 at.%
Summary
The formation of bainite has been attracting scientific interest for almost a century. What makes it attractive is the fact that it combines characteristics of fundamentally different phases, which are martensite and Widmanstätten ferrite; this circumstance is the reason why the mechanism of formation of this phase, a mixture of ferrite and cementite, is still object of controversy. When formed at low temperatures, lower bainite shares common characteristics with martensite and it is sometimes even impossible to distinguish them microscopically. This experimental finding suggests that the mechanism of formation of bainite is similar to that of martensite and might be diffusionless in nature. An increasing number of experimental and theoretical studies support the similarity between bainitic ferrite and proeutectoid ferrite with Widmanstätten morphology and the idea that bainite growth is determined by carbon diffusion
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