Carbon Diffusion Profiles Research Articles

Carburization of austenitic and ferritic stainless steels in liquid sodium: Comparison between experimental observations and simulations

Three steels were exposed in carburizing sodium at 600 and 650 °C. The kinetics and extent of carburization were characterized. Numerical simulations using the coupled thermodynamic-kinetic modeling software DICTRA were performed. It was proposed that the observed carbon diffusion profiles were induced by the combined diffusion of carbon in the grains and at grain boundaries coupled with the slow formation of carbides. The blocking effect of carbides on the carbon diffusion was observed to evolve as a function of time and microstructure. Acceptable agreement between experimental and simulated intragranular carbon profiles was achieved by optimizing the labyrinth factor and phases.

The effect of applied compressive stress on the diffusion of carbon in carbon supersaturated S-phase layer

In this study, hydrostatic compressive stresses are applied on the prior-carburized samples of 316 ASS to discover the relationship between interstitial diffusion process and the applied stresses. Carbon diffusion profiles are compared and concentration dependent diffusion coefficients are discussed to further advance scientific understanding of diffusion process in supersaturated S-phase. The experimental results demonstrate that the applied compressive stress can retard the fast diffusion process comparing to the unstressed one. Based on theoretical modelling, the effect of the applied hydrostatic compressive stresses on the diffusion of carbon in carbon supersaturated 316 austenitic stainless steel is discussed.

An Empirical Model for Carbon Diffusion in Austenite Incorporating Alloying Element Effects

The diffusion of carbon in austenite is important to the design and implementation of many steel heat-treating processes. The present study proposes a simple and computationally efficient equation for the diffusion of carbon in austenite for iron-carbon alloys, as well as for steels alloyed with a variety of elements. The proposed empirical model provides a pragmatic engineering approach to this important diffusion process. The model is shown to better match the carbon diffusion profiles of several carburized steel alloys, as compared to other equations that have been reported in the literature. These other equations were primarily obtained from experiments on iron-carbon alloys, and it is not surprising that they are less accurate when compared to the proposed equation that takes the full range of alloy compositions into account.

Evaluation of concentration dependant diffusion coefficients of carbon in a dissimilar joint of ferritic steels

When dissimilar weldments between 9Cr-1Mo and 2¼Cr-1Mo ferritic steels are exposed to high temperature, microstructural and microchemical modifications are observed near the weld interface. Diffusion of carbon driven by the activity gradient from low Cr to high Cr steel leads to the formation of carbon enriched ‘hard’ zone and carbon depleted ‘soft’ zone near the fusion joint. The present paper deals with the measurement of carbon diffusion profiles and the evaluation of concentration dependent diffusion coefficients of carbon across the interface at a temperature of 1023 K. Accurate carbon concentration profiles are generated using carbon calibration graph. The profiles are smoothened to reduce the experimental scatter and the concentration dependant diffusion coefficients are determined using Den Broeder’s method. In the base materials, where the concentration gradient is extremely small, D(c) values are determined using Hall’s method. Variation in D(c) across the weld interface is understood based on the microstructural and microchemical changes that take place during heat treatment.

Runner-up Simulation of Gas Carburising: Development of Computer Program with Systematic Analyses of Process Variables Involved

Numerous process parameters involved in different stages of gas carburising make the process difficult to control, which in turn makes predictions of properties difficult. In the present work, a computer program was developed to simulate the entire gas carburising process, focusing on the thermodynamics of the gas phase and thermochemical reaction kinetics at the gas/solid interface in endothermic atmospheres. By combining gas phase kinetics and carbon activities in gas and steel with the alloy dependent diffusivity of carbon in austenite, the program can predict accurate carbon diffusion profiles for both batch and continuous furnaces. The program has been designed to account for all the major process variables that affect resultant carbon diffusion such as steel alloy composition, part geometry, gas composition and basic treatment parameters with their possible variations during the process. It is also able to predict other important properties such as transformation temperatures and the as quenched and tempered hardness of carburised parts. Comparison with the results of the experiments performed in industrial carburising furnaces provided validation of the model predictions.

Carbon in silicon: Modeling of diffusion and clustering mechanisms

Carbon often appears in Si in concentrations above its solubility. In this article, we propose a comprehensive model that, taking diffusion and clustering into account, is able to reproduce a variety of experimental results. Simulations have been performed by implementing this model in a Monte-Carlo atomistic simulator. The initial path for clustering included in the model is consistent with experimental observations regarding the formation and dissolution of substitutional C–interstitial C pairs (Cs–Ci). In addition, carbon diffusion profiles at 850 and 900 °C in carbon-doping superlattice structures are well reproduced. Finally, under conditions of thermal generation of intrinsic point defects, the weak temperature dependence of the Si interstitial undersaturation and the vacancy supersaturation in carbon-rich regions also agree with experimental measurements.

The contribution of vacancies to carbon out-diffusion in silicon

Diffusion of carbon is mostly assumed to be governed by carbon interstitials via the kick-out mechanism. Carbon in-diffusion experiments are associated with thermal equilibrium concentrations of point defects, whereas in the case of carbon out-diffusion a remarkable undersaturation of Si self-interstitials may develop provided the carbon concentration is several orders of magnitude over its solubility value. New carbon out-diffusion experiments demonstrate that this model qualitatively describes the observed carbon diffusion profiles. However, we demonstrate that an accurate description of the experimental profiles is only possible if the Frank–Turnbull mechanism, involving vacancies, is additionally taken into account. Detailed investigations of carbon and boron profiles in the same sample can be used to determine the splitting of the known vacancy component of the silicon self-diffusion coefficient into the vacancy diffusion coefficient and the vacancy thermal equilibrium concentration at 900 °C.

Carbon diffusion in silicon

Carbon diffusion in silicon has been investigated by using a superlattice structure of carbon spikes (10 nm-wide, carbon concentration >1019 cm−3, spikes spaced 100 nm apart) grown epitaxially by Si molecular beam epitaxy. Samples were annealed in the range between 680 and 850 °C. The diffusive behavior of carbon was monitored by secondary ion mass spectrometry. Carbon diffusion profiles observed at temperatures above 800 °C show highly nonregular behavior. The diffusion results are interpreted in terms of the kick-out mechanism.

An examination of carbon diffusion profiles in steel specimens by means of the nuclear microprobe

Carbon distributions through steel specimens 100–400 μm thick have been examined by nuclear microprobe techniques. The analytical determination was based upon the measurement of protons emitted by the 12C(d,p)13C reaction when samples were irradiated with 1.3 MeV deuterons. Beams of deuterons a few micrometers in diameter were used to irradiate small volumes of the sample, and spatial variations in carbon content were followed by moving the position of the deuteron beam and following the proton yield with beam location. The position of the ion beam on the sample was established by simultaneous measurement of emitted X-rays.Special steps were necessary to ensure that the overall carbon background of the accelerator was kept to within acceptable limits, and samples were calibrated by comparing emitted proton yield with that of standards. Repeat-scanning over the same region of the sample demonstrated good reproducibility. Microprobe results compared well with chemical determinations carried out by removing small quantities of the sample surface by micro-machining and subsequently measuring carbon in the removed material by combustion. The technique is illustrated by scans obtained during the examination of a number of stainless-steel samples. Experiments show that the method may be extended to the analysis of irradiated steel specimens.

Kinetics of the carburization-decarburization process of austenitic stainless steels in sodium

A mathematical analysis for the carburization-decarburization kinetics of austenitic stainless steels in high-temperature flowing sodium has been developed that incorporates 1. (1) the thermodynamic and kinetic information for carbon in the alloys, 2. (2) the thermal-mechanical treatment of the material (solution-annealed versus cold-worked) which influences the microstructure, and 3. (3) the carbon concentration in sodium and its dependence on sodium-system parameters. Carbon concentration-distance profiles in Types 304 and 316 stainless steel were generated as a function of time, temperature, and carbon concentration in sodium and compared with experimental data. The analysis was used to evaluate the carburization-decarburization behavior of Type 316 stainless steel fuel cladding exposed to sodium and to develop carbon-diffusion profiles in Type 304 stainless steel intermediate heat-exchanger piping upon exposure to primary and secondary system sodium for periods to 30 y.