Integral Approach and Numerical Improvement to Calculate Carbon Concentration Profiles in Carburising

The carbon diffusion in steel, where the carbon diffusivity varies with the carbon content, was solved with the integral methods under the third boundary condition. The variation of carbon diffusivity in steel with the carbon content was described with two different functions, linear dependence and exponential dependence. The integral approximation for both cases was improved with the numerical computation to more accurately predict the carbon profiles. The integral solution is more accurate than the formulation based on the assumption of a constant diffusivity or those based on the assumption of a constant diffusivity and/or constant carbon content at part surface. It is also more easily used in practice than the numerical method to describe the carburising process and predict the carbon content at steel surface and carbon profiles in treated layer.

Calculation of Gas Carburizing Kinetics from Carbon Concentration Profiles based on Direct Flux Integration

Initiated by the need of industry for gas carburizing process control and optimization, this paper focuses on understanding the effect of the time, temperature and carbon potential on the mass transfer coefficient and carbon diffusivity in Austenite. A method for direct flux integration has previously been proposed to calculate these kinetic parameters from the experimental carbon concentration profiles. AISI 8620 steel discs were gas carburized at different levels of atmosphere carburizing potential for selected austenizing temperatures. Analyses of the carburized parts included experimental measurement of weight gain, surface carbon concentration and carbon concentration profiles. The time-dependent weight gain and surface carbon content measurements allowed calculation of the time average mass transfer coefficient, while carbon concentration profiles were used to calculate the concentration dependent carbon diffusivity for selected process parameters. Excellent agreement was found between the calculated mass transfer coefficient and carbon diffusivity values and those reported in the literature. The calculated values served as input in the previously developed carburizing model validating the predicted results by comparison with the experimental concentration profiles.

Molecular Dynamics Investigation of Carbon Diffusivity in Metal Nanoparticles During CVD-Based Nanotube Fabrication Process

In this paper, molecular dynamics technique is utilized to investigate carbon molecules diffusivity into iron oxide nanoparticles such as FeO and Fe2O3. Such nanoparticles can be used during carbon nanotube (CNT) synthesis process in a chemical vapor deposition (CVD) system. Several simulations are conducted in the atmospheric pressure condition and for different temperatures ranging from 500 to 1100°C, which is common range for multi-walled and single-walled CNT (MWCNT and SWCNT) fabrication. The mean square displacement (MSD) diagrams and their corresponding diffusivity of the carbon molecules into the nanoparticles are then plotted. The results are compared with those obtained after adding silicon molecular structure to the each nanoparticle as substrate. The results From NPT/NVE (pressure constant-temperature constant/velocity constant-energy constant) simulations show that for each nanoparticle, the diffusivity increases as higher temperatures considered in the simulation. However, the corresponding diffusivity rate doesn't necessarily increase. In the diffusivity diagrams, three distinctive temperature regions are observed. In the temperature regions where MWCNT and SWCNT are commonly produced, decreasing in diffusivity rate is observed, while in transition region where CNTs change from multi-walled to single-walled, the diffusivity increases for both simulated metal nanoparticles. Finally, by investigating and comparing diffusivity diagrams of each nanoparticle, before and after adding silicon as substrate, it is observed that the effects of silicon layer decreases the amount of carbon molecules diffusivity. This decrease is more considerable in higher temperatures. The obtained information from this study helps to understand the growth rate and formation mechanism of MWCNT/SWCNT and their relationship with carbon diffusivity in metal nanoparticles in great detail which are important keys in controlling nanotube properties.

Parameters Influencing the Bake Hardenability of Microalloyed ULC Steels

Ti-V and Ti-Nb bake hardenable Ultra Low Carbon (ULC) steels are used to produce hot dip zinc coated steels for automotive applications. An important factor influencing the bake hardenability in such microalloyed ULC steels is the level of solute carbon available to diffuse for pinning dislocations during baking. The level of solute carbon must be controlled carefully during annealing of the steel in the ferritic region. Therefore, this paper summarizes highlights of research conducted to study the effects of chemical composition and annealing temperature (in the ferrite region) on the precipitation (or dissolution) of NbC and VC using a variety of Ti-Nb and Ti-V ULC steels. Carbon diffusivity is another factor that could also influence the bake hardenability through controlling the aging and baking kinetics. Therefore, the paper presents highlights of internal friction measurements performed to assess effects of microalloying elements (Nb,V) and some commonly used solid solution strengthening elements (Mn, P) on carbon diffusivity measured using the internal friction technique.