Mechanism of Carbon Infiltration in Vacuum-Carburizing Process

Morita, Toshiyuki; Inoue, Koichiro; Hanyuda, Tomoki

doi:10.4262/denkiseiko.77.5

Cited by 10 publications

(8 citation statements)

References 3 publications

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“…... (2) where X C γ (i,tn) is the C concentration of the i-th grid-point at t = n·Δt, and DC γ is the diffusivity of C in γ, which is also retrieved form the MOB2 database. Equation 2 is the simple form applied for a constant grid spacing Δz.…”

Section: Calculation Of the Grain-boundary θ Fraction Distributionmentioning

confidence: 99%

“…In previous work, [1][2][3] the dependencies of both the surface C content and amount of θ on the steel composition have been interpreted based on the equilibrium with deposited graphite (Gr) from decomposed hydrocarbon. More specifically, the surface C content can be determined by thermodynamic calculation for steel consisting of γ and θ that is in equilibrium with Gr (3-phase equilibrium).…”

Section: Introductionmentioning

confidence: 99%

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Calculation of Microstructure in Vacuum Carburizing Incorporating Kinetics Modeling of Grain-boundary Cementite

et al. 2012

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In the vacuum carburization of steels, short-time carburization is usually followed by a diffusion period to eliminate the filmy cementite (θ) grown on austenite (γ) grain boundaries. The surface C content in γ was found to increase past the solubility of θ due to radical reaction with decomposed hydro-carbon gas. The thermodynamic condition was recognized as the metastable equilibrium of supersaturated γ with graphite. Based on these results, a calculation model for not only C concentration profiles but also quantitative evaluation of grain-boundary θ was developed.The existing model estimates the amount of θ with the equilibrium fractions for local C contents in a framework of the finite difference method (FDM). However, with the corresponding surface condition of equilibrium γ plus θ with graphite, it overrates the amount of θ observed after several minutes of carburization. In the developed model, a parabolic law was assumed for the thickening of θ and a rate controlling process was considered for partitioning of Si. The local Si concentrations at the γ /θ interface were determined from the C isoactivity condition with supersaturated γ. The change in the rate coefficient (αSi) was also validated using multicomponent diffusion simulations. Coupled with the CALPHAD software, a onedimensional FDM program calculates the increment of θ using the updated local equilibrium. Predictions of the C profile and θ volume fraction corresponded much better with the experimental results than existing models.

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Section: Calculation Of the Grain-boundary θ Fraction Distributionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Calculation of Microstructure in Vacuum Carburizing Incorporating Kinetics Modeling of Grain-boundary Cementite

et al. 2012

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“…In contrast, the C steel has higher amount of Cr and lower amount of Si. For comparison with the boundary condition proposed by Morita, [10][11][12] both the overall carbon content and the carbon content in γ phase for the equilibrium border of (γ+θ)/(γ+θ+Gr) at 1 223 K were also calculated by Thermo-Calc, as shown in Table 1. Figure 1 shows a Fe-C-Cr phase diagram and the meaning of the two carbon contents are explained as follows.…”

Section: Chemical Composition Effectsmentioning

confidence: 99%

“…Morita et al [10][11][12] proposed that the boundary condition of the surface during vacuum carburizing can be defined as the γ+θ equilibrium condition of the steel and the graphite generated by resolution of the carburizing gas (acetylene). The experimental results in Figs.…”

Section: The Boundary Condition Of At the Carburizing Surfacementioning

confidence: 99%

“…Morita [10][11][12] explained that the dependence of carbon content at the surface during vacuum carburizing by the equilibrium condition with the steel and the graphite that is decomposed from hydrocarbon gases. This means that the equilibrium carbon content at the surface coincides with the γ side border of the γ+θ+graphite(Gr) three phases area in the phase diagram and can be uniquely obtained from the chemical composition and carburizing temperature.…”

Section: Introductionmentioning

confidence: 99%

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Effect of Chemical Compositions of Case Hardening Steels for Distribution of Carbon and Cementite during Vacuum Carburizing

et al. 2012

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In order to develop the model to predict the microstructures of case hardening steels during vacuum carburizing, the effects of chemical compositions such as Cr and Si on carburizing behaviors were investigated using three steels based on Fe-1.1Cr-0.8Mn-0.25Si-0.2C (in mass%). The distributions of carbon near the carburized surface and grain boundary cementite (θ) were analyzed by Glow Discharge Optical Emission Spectroscopy (GD-OES) and image analysis of microstructures respectively. It was revealed that the distributions of carbon and the volume fractions of θ were affected by the chemical compositions i.e. they increased with increasing amount of Cr and decreased with increasing amount of Si. The results by X-ray Diffraction (XRD) and field emission electron probe microanalyzer (FE-EPMA) showed the carbon content in the retained austenite(γ) near the surface was around 1.4-1.6 mass% which was higher than the equilibrium condition.By the comparing the experimental and calculation results obtained using the commercial multi-elemental diffusion solver, DICTRA, it was confirmed that the carbon in the γ was supersaturated during vacuum carburizing and the surface condition was estimated to be equal to the carbon activity of the meta-equilibrium border of γ/(γ+graphite). Additionally the result of θ distribution suggests that the volume fraction of θ cannot be predicted by equilibrium calculations; rather, a kinetic model for θ growth is required.

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In Situ Observation for Abnormal Grain Coarsening in Vacuum-Carburizing Process

Yogo

Tanaka

2014

Metall Mater Trans A

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Mechanism of Carbon Infiltration in Vacuum-Carburizing Process

Cited by 10 publications

References 3 publications

Calculation of Microstructure in Vacuum Carburizing Incorporating Kinetics Modeling of Grain-boundary Cementite

Calculation of Microstructure in Vacuum Carburizing Incorporating Kinetics Modeling of Grain-boundary Cementite

Effect of Chemical Compositions of Case Hardening Steels for Distribution of Carbon and Cementite during Vacuum Carburizing

In Situ Observation for Abnormal Grain Coarsening in Vacuum-Carburizing Process

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