Influence of Carbide Morphology and Microstructure on the Kinetics of Superficial Decarburization of C-Mn Steels

Abstract

Decarburization is an important process during the heat treatment of steels. It can be divided into three separated events: dissolution of carbides, diffusion of carbon through the iron matrix, and surface reactions. The process is very sensitive to temperature. During intercritical annealing, austenite nucleates in the cementite-ferrite interface and grows at the rate determined by the diffusion of carbon in austenite. The presence of a decarburizing atmosphere during annealing guides the carbon diffusion in ferrite toward the surface, generating a flux of carbon from austenite toward ferrite, disturbing the austenite growth. In the presence of pearlite, the ferrite-austenite interface can be assumed to remain static until pearlite is completely dissolved, reducing then the carbon flux in austenite, consequently diminishing the austenite formation rate. At intercritical temperatures, the cementite-free ferrite layer at the surface reaches a greater width due to the combination of the thermodynamic fraction of austenite, dissolution rate of cementite, and the diffusivity of carbon in austenite and ferrite. In this study, an experimental investigation of the effects of the carbide morphology and distribution and the a À c phase transformation in the decarburization kinetics on hypo-eutectoid steels is presented. It is suggested that the change of the dissolution kinetics of the carbides due to its morphology will affect the austenitization kinetics. Thus, the distribution of the carbon in the microstructure may determine the rate of decarburization in combination with the carbon diffusion through the phases or the gas-metal reactions.

<i>In situ</i> Observations of Transformation Behavior upon Heating for a 1.5Mn-1.5Si-0.2C Steel -Comparison between Neutron Diffraction, XRD, EBSD and Dilatometry-

“…24) Another possible reason of carbon enrichment inside the specimen would be attributed to the ferrite transformation on cooling, which preferentially started from the surface. However, as was discussed by Alvarenga et al, 25) decarburization from the surface, particularly before the onset of austenite reversion, would seriously take place. If we did similar computation of Appendix for carbon, the diffusion length L for C is about 100 times larger than that for Mn.…”

Quantitative determination of carbon concentration profiles by GD‐OES for the study of decarburization in low‐carbon steels

“…a, the steel shows a degree of decarburization prior to annealing. This may be a result of the exposure of the steel surface to the air humidity during the homogenizing and subsequent hot‐rolling steps . After the decarburization annealing, a ferrite layer is formed near the surface as a result of the dissolution of cementite and the diffusion of carbon towards the surface during annealing.…”

“…The morphology of the cementite alters its interface area, consequently affecting their dissolution. The diffusion of carbon from the cementite towards the surface depends on the phases present in the material …”

“…Although there is a clear correlation between the carbon content and mechanical properties, the micro‐hardness technique does not unambiguously describe microstructural features such as the ferrite layer and presence of carbides. The difficulty to accurately measure small quantities of carbon can be circumvented by metallographic characterization of the ferrite layer formed near the surface, which is commonly associated with the decarburized region …”

“…The difficulty to accurately measure small quantities of carbon can be circumvented by metallographic characterization of the ferrite layer formed near the surface, which is commonly associated with the decarburized region. [10,14,[17][18][19] Glow discharge optical emission spectroscopy (GD-OES) emerges as a good candidate to measure carbon depth profiles of up to approximately 100 m in a fast and reliable way. [20] GD, especially the Grimm source, [21] have been widely used in analytical chemistry.…”