Carbon Diffusion In Steel Research Articles

Decarburisation of 60Si2MnA in wet air at 1000 – 1280 °C

ABSTRACT The present study investigated the decarburisation behaviour of a Si-containing spring steel 60Si2MnA in wet air containing 24.8%H2O at 1000–1280°C. It was found that after the steel was oxidised for 20 minutes, the depth of the total decarburisation zone increased with increased temperature up to 1150 °C but then decreased with increased temperature to 1170 °C. At 1170–1280 °C, no decarburisation was observed. An oxide layer comprising Fe2SiO4 + FeO and porosity formed between the FeO layer and the steel. Thermodynamic calculation indicated that, if the interface equilibrium was determined by 2Fe + 4CO2 + (Si)→ Fe2SiO4 + 4CO, the interface carbon activity could be significantly higher than that when the interface equilibrium was determined by FeO + CO → Fe + CO2 and afforded a good protection against decarburisation. However, the experimental results indicated that the latter reaction prevailed at 1000–1150 °C, leading to severe decarburisation. Calculation of carbon diffusion in steel at 1000–1150 °C indicated that the diffusivity data given by Krishtal for quaternary Fe-C-Si-Cr steels allowed a good prediction to the decarburisation tendency of the steel, indicating that the combined effect of Si and Cr was significant. At temperatures above 1170 °C, the molten oxide phase formed on the steel surface had a shielding effect against decarburisation. This was attributed to the impermeable nature of the molten oxide to CO and CO2 gases as the potential decarburisation reaction products. However, molten oxide formation at the interface led to rapid consumption of the steel by encapsulating and eroding the steel piece by piece into the scale.

Multi-step Simulation of Vacuum-carburizing Reactor based on Kinetics Approach

A novel and efficient simulation technique for the purpose of optimization of vacuum-carburizing process was proposed. This method consists of three steps: calculation of gas convection and diffusion, calculation of only gas diffusion, and calculation of carbon diffusion in steel. The first step provides the gas convection velocity that is employed in the second step. Adsorption rate of carbon on the steel surface is obtained in the second step, and carbon concentration in the steel is calculated in the third step based on the adsorption rate of carbon. Experiments were conducted to verify the proposed method in both laboratory- and industrial-scale reactors. Comparison of the computational predictions to the experimental data revealed that the proposed technique enabled accurate prediction of the adsorption rate of carbon on the steel surface at various temperature conditions, the amount of carburized carbon at each operating time, and the profile of carbon concentration in the steel; in other words, the carburized depth. In addition, the calculation of the industrial-scale reactor, whose simulation model consisted of approximately seven million computational meshes, was completed within about two days. Therefore, the proposed simulation technique could be used to control and optimize the process in industrial vacuum-carburizing reactors.

Modeling of vacuum pulse carburizing of steel

A review on carburization process modeling is presented and a new model for carbon diffusion in steel subjected to heat treatment by vacuum pulse carburizing is proposed. Such a process is carried out by repeating consecutively a carburizing stage (the boost), when the carburizing gas is supplied into a carburizing chamber, and the diffusion stage at vacuum conditions, when the carburizing gas is exhausted and only the diffusion in steel takes place. The model is based on Darken bi-velocity method which comprises two velocities: the diffusion velocity, various for various components, which depends on the diffusion potential gradient and is independent of the choice of the reference frame, and the drift velocity which is equivalent to the lattice velocity, and is common for all components. The model allows predicting the kinetics of carbon transfer at various processing conditions and has been applied in computer simulations. The numerical calculations were performed for varying carburizing and diffusion intervals and the results were compared to the experimental data On the basis of the series of computer experiments some findings, which are important for the carburizing process design, have been formulated.

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.

Simulation of Carbon Diffusion in Steel Driven by a Temperature Gradient

The basis of thermomigration in multicomponent alloys is summarized, and the general equations are given and implemented in the DICTRA software. Experimental information from Okafor et al. is analyzed with the new simulations and it is concluded that steady-state conditions was not established during their experiment. A heat of transport $$ Q_{C}^{ * } = - 44000\;{\text{J/mol}} $$ , almost four times larger than the value given by Okafor et al., was found to give a satisfactory representation of the experimental information.

Study on the softening in overlapping zone by laser-overlapping scanning surface hardening for carbon and alloyed steel

Softening in overlapping passes by laser-overlapped scanning surface hardening is a difficult problem of laser surface modification. Despite the advantage of laser quenching, softening in overlapping zone limits its practical application. In this paper, 45, 9Cr2Mo and W18Cr4V steel were hardened by laser-overlapping quenching. Softening occurred in all overlapping zones. The results of hardness testing indicated the softening width of 45 steel was the broadest and that of W18Cr4V steel was the narrowest. Different mixed microstructures composed the overlapping zone for three steels, i.e., tempered sorbite and a little tempered martensite in 45 steel, tempered martensite, tempered sorbite and a small amount of carbides in 9Cr2Mo steel, tempered martensite and a little carbide in W18Cr4V steel. The effect of activation energy of diffusion for carbon in steel and cooling rate on the decomposition of martensite have been investigated by developing a diffusion model based on the principle of carbon diffusion in martensite. The results indicated that action energy for diffusion of carbon in steel plays a main role in hindering decomposition of carbide and cooling rate has a limited action in reducing temper softening during laser-overlapping scanning.

Carbon diffusion in steels: A numerical analysis based on direct integration of the flux

Carbon diffusion in steels: A numerical analysis based on direct integration of the flux

Analytical study on the Fe/Ti interface in titanium‐clad steels

AbstractThe precipitates on the Fe/Ti interface in a rolled titanium‐clad steel was investigated by the potentiostatic etching with non‐aqueous electrolyte (SPEED method) followed by SEM observation and chemical analysis using a computer‐aided X‐ray microanalyser (CMA). Titanium‐clad steel (CP‐Ti/0.01% C steel) was annealed between 700 and 1000 °C for 0.5–100 h in vacuum. The cross‐section of the annealed clad steel was etched by the SPEED method and the morphology and the chemistry of the precipitates were investigated in relation to the interface by using SEM and CMA.Film‐like titanium carbide, intermetallic compunds (Fe2 Ti, FeTi) and β‐Ti containing Fe were observed on the Fe/Ti interface.The shear strength depends on the quantity of film‐like titanium carbide on Fe/Ti interface but intermetallic compunds and β‐Ti have slight influence.Thus it is considered that the embrittlement of the Fe/Ti interface was caused by (1) diffusion of carbon in steel towards the titanium layer, (2) precipitation of film‐like titanium carbide at the interface, and (3) fracture at the film.

Selection of parameters of laser heating of carbon steels for obtaining the specified hardening depth

1. The applicability of the unidimensional model of heating a homogeneous semi infinite body for selecting the parameters of laser hardening without fusion of the surface of carbon steels with different carbon content was experimentally confirmed. 2. Upon transition from the regime of heating with fusion of the surface to the regime of heating without fusion, the shape of the laser track changes. 3. The distribution of microhardness across the thickness of the zone of laser hardening is not the same in steels with different carbon content: with 0.2% C the microhardness of steel decreases monotonically, with 0.8% C it decreases stepwise. Such a change of the microhardness profile upon change of carbon content may be connected with the different path length of carbon diffusion in steels with different amounts of hypoeutectic ferrite.

Correlation of the quantitative characteristics of the microstructure and the wear resistance of gray cast iron hardened by laser radiation

3. The distribution of microhardness across the thickness of the zone of laser hardening is not the same in steels with different carbon content: with 0.2% C the microhardness of steel decreases monotonically, with 0.8% C it decreases stepwise. Such a change of the microhardness profile upon change of carbon content may be connected with the different path length of carbon diffusion in steels with different amounts of hypoeutecti c ferrite.

Carburizing of steels

Carbon diffusion in steels was studied. Low carbon steel and austenitic 304 stainless steel were carburized in the range 900–1000 °C using solid or liquid carburization techniques. Carburization was carried out at 25 °C intervals in this temperature range in order to determine the effect of temperature on the carbon diffusion. A metallographic technique was used to measure the case depth under different experimental conditions. The activation energies of carbon diffusion in low carbon steel and in 304 stainless steel are 127.95 kJ mol -1 and 205.57 kJ mol -1 respectively for solid carburization and 180.59 kJ mol -1 and 218.58 kJ mol -1 respectively for liquid carburization.

Effect of thermomechanical treatment on carbon diffusion in steel produced by dynamic hot pressing

Effect of thermomechanical treatment on carbon diffusion in steel produced by dynamic hot pressing

The diffusion of carbon in steel obtained by hot pressing

1. It was found that carbon diffusion is greatly slowed down in steel U10 produced by hot pressing at 800° as compared with the standard steel of the same composition. 2. The slowing down of the diffusion is affected by the hot pressing temperature and the annealing time of the sample. An increase of the hot pressing temperature and the annealing time reduces this effect.

On the Kinetics of Hydrogen Attack of Steels

Experimental data for hydrogen attack of carbon steels at elevated temperature are discussed. In general the specific gravity of steels decreases when exposed to hydrogen at increased pressures and elevated temperatures. This decrease is caused by the expansion of fissures resulting from the formation of methane. The kinetics of this process is analyzed in terms of a model which assumes a steady rate of methane formation at the surface of pre-existing spherical voids and the creep expansion of these voids as a result of the methane pressure. Evaluation of experimental data shows that this model satisfactorily describes the kinetics of intermediate stages of attack. The activation energy for the rate of methane formation coincides with that of carbon diffusion in steel. Résumé On examine ici les résultats expértmentaux de l'attaque d'aciers au carbone par l'hydrogène à haute température. En général, la densité de l'acier décroit à l'exposition à l'hydrogene pour des pressions et températures élevées. Cette diminution est causée par l'expansion de fissures résultant de la formation de méthane. On analyse la cinétique de ce processus par l'intermé diaire d;amp;#x0027;un modèlem, supposant un taux uniforme de formation de méthane ;amp;#x00E0; la surface de pores sphé rique pr;amp;eacute existants ainsi que l'expansion en fluage de ce s vides par suite de la pression du méthane. L' étude des r;amp;eacutesultats expérimentaux montre que ce mod;amp;#x00E8;le décrit de manlère satisfaisante la cinétique des stades intermédiaires d'attaque. L'é nergie d'activation pour le taux de formation du méthane coincide avec celle de la diffusion du carbone dans l'acier.