Carbon Concentration Profiles Research Articles

Theoretical determination of histories and carbon concentration profiles in steel resulted from a thermochemical process at a frontier

Steel is used to manufacture auto parts due to its properties and the ability to modify them through solid-state thermal processing. In some cases, the treated alloy itself does not satisfy the requirements of combined properties in the integrity of a single part. In these cases, thermochemical treatments do apply, with which the chemical composition does alter in a localized way. The mechanism of atomic diffusion in the solid is the mean by which atoms of a chemical element can mobilize in the lattice of the crystalline structure and concentrate, depending on the distance penetrated. The mass transport equation adequately describes concentration gradients if the diffusivity property is precisely known. The applicability to the analysis of the carburization method of steel can do a micrometric scale in one dimension. The results obtained on a computer application calculating the carbon concentration profile in iron can be shown. The strategy consists of qualitatively evaluating the distribution of equilibrium phases by optical microscopy and associating it with the carbon concentration profile.

On the creep behaviour of HP40Nb alloy with high-temperature carburisation damage

The effect of carburisation damage on the creep behaviours of HP40Nb alloy was investigated. The carbon concentration profile and microstructural evolution were characterised for the carburised specimens with and without creep, respectively. The variations of creep properties and fracture mode in carburisation treatment were also revealed. The results indicate that massive Cr-rich secondary carbides are precipitated in the matrix after carburising for 5 h, leading to the prolongation of creep lifetime. The further carburisation promotes the coarsening of primary and secondary Cr-carbides and weakens the creep strength of HP40Nb alloy. In creep process, the outward diffusion of Cr atoms contributes to the formation of Cr2O3 oxide layer on the surface and the emergence of Cr-depleted zone on the subsurface, while the inward diffusion of C atoms causes the rapid rise and gradual fall in the carbon concentration for all creep-carburisation specimens. Additionally, the creep fracture mode changes from brittle fracture to ductile fracture with the prolonging carburising time and the increasing creep stress, and the continuous carburisation results in the transformation of creep fracture mechanism towards the viscoplastic cavity growth over the applied stress range.

Effect of soil organic matter on the fate of 137Cs vertical distribution in forest soils

Understanding the fate of the vertical distribution of radiocesium (137Cs) in Japanese forest soils is key to assessing the radioecological consequences of the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. The 137Cs behavior in mineral soil is known to be primarily governed by interaction with clay minerals; however, some observations suggest the role of soil organic matter (SOM) in enhancing the mobility of 137Cs. Here we hypothesized that soil organic carbon (SOC) concentration profile determines the ultimate vertical pattern of 137Cs distribution in Japanese forest soils. In testing this hypothesis, we obtained soil samples that were collected before the FDNPP accident at four forest sites with varying SOC concentration profiles and quantified the detailed vertical profile of 137Cs inventory in the soils roughly half a century after global fallout in the early 1960 s. Results showed that the higher the SOC concentration in the soil profile, the deeper the 137Cs downward penetration. On the basis of the data for surface soils (0–10 cm), the 137Cs retention ratio for each of the 2-cm thick layers was evaluated as the ratio of 137Cs inventory in the target soil layer to the total 137Cs inventory in and below the soil layer. A negative correlation was found between the ratio and SOC concentration of the layer across all soils and depths. This indicates that the ultimate fate of 137Cs vertical distribution can be predicted as a function of SOC concentration for Japanese forest soils, and provides further evidence for SOM effects on the mobility and bioavailability of 137Cs in soils.

Decarburization of Wire-Arc Additively Manufactured ER70S-6 Steel.

Decarburization is an unwanted carbon-loss phenomenon on the surfaces of a material when they are exposed to oxidizing environments at elevated temperatures. Decarburization of steels after heat treatment has been widely studied and reported. However, up to now, there has not been any systematic study on the decarburization of additively manufactured parts. Wire-arc additive manufacturing (WAAM) is an efficient additive manufacturing process for producing large engineering parts. As the parts produced by WAAM are usually large in size, the use of a vacuum environment to prevent decarburization is not always feasible. Therefore, there is a need to study the decarburization of WAAM-produced parts, especially after the heat treatment processes. This study investigated the decarburization of a WAAM-produced ER70S-6 steel using both the as-printed material and samples heat-treated at different temperatures (800 °C, 850 °C, 900 °C, and 950 °C) for different durations (30 min, 60 min, and 90 min). Furthermore, numerical simulation was carried out using Thermo-Calc computational software to predict the carbon concentration profiles of the steel during the heat treatment processes. Decarburization was found to occur not only in the heat-treated samples but also on the surfaces of the as-printed parts (despite the use of Ar for shielding). The decarburization depth was found to increase with an increase in heat treatment temperature or duration. The part heat-treated at the lowest temperature of 800 °C for merely 30 min was observed to have a large decarburization depth of about 200 μm. For the same heating duration of 30 min, an increase in temperature of 150 °C to 950 °C increased the decarburization depth drastically by 150% to 500 μm. This study serves well to demonstrate the need for further study to control or minimize decarburization for the purpose of ensuring the quality and reliability of additively manufactured engineering components.

Decarburization in Laser Surface Hardening of AISI 420 Martensitic Stainless Steel.

Decarburization deteriorates the surface mechanical properties of steel. It refers to the loss of carbon from steel's surface when exposed to an open-air environment in elevated-temperature conditions. Despite the short interaction time and fast thermal cycle of the laser surface-hardening process, decarburization may still occur. This paper investigates if decarburization occurs during the laser surface hardening of AISI 420 martensitic stainless steel. For comparison, surface-hardening results and decarburizations in a conventional air furnace-heated hardening process (water-quenched and air-cooled) of the same steel material were also investigated. Decarburization seems to have occurred in the laser surface hardening of AISI 420SS. However, the decarburization might not be significant, as the hardness of the steel's surface was increased more than three times to 675 HV during the laser surface hardening, and the hardness drop due to decarburization was estimated to be only 3% with the decarburization depth of 40 μm. Simulations using ThermoCalc software to get the carbon concentration profiles along the depth for both laser-hardened and furnace-heated samples were also investigated.

A physics-based life prediction model of HP40Nb heat-resistant alloy in a coupled creep-carburisation environment

This paper reports the development of a physics-based life prediction model that incorporates three micro-damage processes. Both the particle area fraction and size, as determined using scanning electron microscopy, are used as the model input to predict the precipitate coarsening damage. Moreover, the carbon concentration profile, as measured using electron probe micro-analyser, are used to derive the carburisation damage distribution index. Compared to the Kowalewski-Dyson model, the present model can predict the stress dependent creep fracture strain and rupture time. The model, compiled into finite element via the user subroutine, can predict the heterogeneous damage accumulation of the cracking furnace tube made of HP40Nb alloy in a coupled creep-carburisation environment. The predicted total damage is comprised of precipitate coarsening and carburisation as the predominance, while the creep cavitation contributing much less due to the low internal pressure. The distribution of the precipitate coarsening is relatively homogeneous through the thickness, whereas the carburisation causes a steep increased damage value close to the inner surface. With regard to time, the precipitate coarsening damage is predominant in the initial stage, while the total damage is governed by the carburisation process with further exposure. Eventually, the total damage reaches the critical value at 59600 h, with the thickness of carburised layer exceeding 50% of the tube thickness, and the tube fails in the inner surface which agrees with the operational experience. The parametric sensitivity study reveals the importance of determining the Cr-rich carbide area fraction for the life prediction.

Influence of Plasma Power and Oxygen-Containing Process Gases in Active Screen Plasma Nitrocarburizing with Carbon Solid Source*

Abstract Plasma nitrocarburizing by means of active screen technology using an active screen made of carbon fiber-reinforced carbon was carried out by varying the power at the active screen and using oxygen-containing fresh gas components (O2, CO2) in the N2:H2 plasma using the example of the quenched and tempered steel AISI 4140 (42CrMo4). The investigations focused on the analysis of the process gas by means of laser absorption spectroscopy, the evaluation of the produced compound layers with regard to structure and phase composition, as well as the resulting properties. It was shown that by varying the process gas atmosphere, the structural composition of the compound layer and the concentration profiles of nitrogen and carbon can be specifically influenced. The high concentrations of carbon-containing compounds in the process gas resulted in complete suppression of γ’-Fe4N formation, but cementite was detected in the lower part of the compound layer. The addition of oxygen-containing fresh gases and the resulting change in process gas composition suppressed cementite formation. The results suggest that, in particular, high powers at the carbon active screen and the simultaneous addition of oxygen-containing gases results in the generation of nitrogen-rich, single-phase ε-compound layers.

Effect of grain size on the static strain aging of a ULC-bake hardening steel

Abstract Bake hardenability in ultra low carbon bake hardening (ULC-BH) steels is primarily influenced by the (i) solute carbon content in the matrix, (ii) dislocation density and (iii) grain boundaries. Grain boundaries in fine-grained steels significantly change the dislocation distribution and carbon-concentration profiles from grain interior to the grain boundary and correspondingly, the bake hardening response may change. The present study examines the effect of different grain sizes (66–15 µm) on the bake hardening behavior of a ULC-BH steel (total carbon content 24 wt.ppm) as a function of tensile strain (2–5%) and aging temperatures (50–170°C). The carbon concentration profiles across the grains of different sizes have been evaluated both through modeling calculations and experimental measurements. Results showed that the bake hardening decreases with decreasing grain size in general, but there is a critical fine grain size corresponding to a specific amount of tensile strain, at which the bake hardening increases significantly due to the contribution of grain boundary carbon and increased dislocation density adjacent to the grain boundaries.

Experimental determination of carbon diffusion in liquid iron at high pressure

Abstract The transport properties of liquid iron alloys at high pressure (P) and temperature (T) are essential for understanding the formation, composition, and evolution of planetary cores. Light alloying elements (e.g., Si, O, S, C, N, H) in liquid iron are particularly relevant due to the density deficit of Earth’s core, yet high P-T experimental diffusion studies involving such alloys remain scarce, with large uncertainties on the P and T dependence required for extrapolation to core conditions. In this study, we measured the chemical diffusion of carbon in liquid iron over a P-T range of 3–15 GPa and 1700–2450 K using a multi-anvil apparatus. Diffusion couples consisting of pure Fe and Fe-2.5 wt%C cylinders were placed end-to-end in an MgO capsule in a vertical orientation. Carbon concentration profiles were measured by electron microprobe and modeled numerically to correct for non-isothermal diffusion that occurred prior to reaching the peak temperature. Carbon diffusion coefficients range from 6 × 10−9 m2·s−1 to 2 × 10−8 m2·s−1, with global Arrhenian fit parameters D0 = 1.4 ± 0.5 × 10−7 m2·s−1, ΔE = 43 ± 6 kJ/mol, and ΔV = −0.06 ± 0.19 cm3·mol−1. A negligible P effect is consistent with previous studies of oxygen diffusion in liquid iron and high-T calculations but differs from larger ΔV values previously reported from carbon self-diffusion experiments for liquid Fe3C and simulations for an Fe-Ni–C alloy. Carbon diffusion coefficients determined here are approximately three times faster than those reported from Fe-Ni-C liquid simulations, which highlights the potential significance of compositional effects on mass transport properties of liquid iron alloys and the need for expanding the P-T experimental diffusion data set currently available in the literature to more complex and geologically relevant compositions.

Determination of residual stress gradient in a Ti-stabilized austenitic stainless steel cladding candidate after carburization in liquid sodium at 500 °C and 600 °C

Non-destructive residual stress profile measurement with a micrometric depth resolution within the depth of carburized Ti-stabilized stainless steel cladding candidate was carried out by high-energy X-ray diffraction. The samples were carburized in carburizing nuclear liquid sodium at 500 °C and 600 °C for 1000 h. The full residual stress tensor profile was determined thanks to the estimation of the strain-free lattice parameter evolution using the carbon concentration profile measured by electron probe microanalysis and thermodynamic simulation. For the sample carburized at 500 °C, the residual stress genesis was governed by the carbon concentration within the steel and the formation of expanded austenite. For the sample carburized at 600 °C, the residual stress profile in the austenitic matrix depended on the precipitation of M23C6 carbide. Stress relaxation was observed in the intragranular carburization zone. For the two temperatures, compressive residual stresses developed in the carburized zone and tensile stresses developed in the rest of the sample.

THERMODIFFUSION SATURATION OF THE SURFACE LAYERS OF α-TITANIUM BY OXYGEN, NITROGEN, CARBON

The purpose of the study is to analytically assess the depth of the gas-saturated zone in the case of a single-component diffusion saturation of alpha-titanium with nitrogen, oxygen and carbon from a rarefied controlled gas environment. Results. Based on the analysis of the literature data, the work schematically shows the interaction of alpha titanium with the elements of implementation and presents the processes with the corresponding parameters that characterize. It is shown that the surface impurity concentration is equal to the equilibrium concentration and is established instantly and does not depend on time. Consequently, with the proposed generalized nonstationary boundary condition in the absence of diffusion of impurities into the volume of the metal, the time dependence of its surface concentration is given, determined by the intensity of surface processes. The dependence of the relative change in the microhardness in the diffusion zone of titanium due to dissolved nitrogen (without taking into account the contribution of nitride inclusions) is presented. Analytically calculated concentration profiles of nitrogen generally correlate well with the distribution of the corresponding relative changes in microhardness in the surface layer. Analytical calculations of the concentration profiles of oxygen, nitrogen and carbon in titanium at a saturation temperature of 700 °C are presented. Practical value. The results obtained will make it possible to preliminarily estimate the size of the fortified near-surface layer depending on the parameters of chemical-thermal treatment and select the optimal parameters of thermal diffusion treatment to ensure the formation of reinforced layers on products made of alpha-titanium based on elements of interstitial in order to increase the functional properties.

An Empirical Model for Carbon Diffusion in Grey Iron/Stainless Steel Bimetallic Composites Incorporating Eutectic Carbides

The mechanism of carbon diffusion in the as-cast and heat-treated grey iron and 304 stainless steel bimetallic composites was investigated in this study. A simplistic empirical model for the interface carbon diffusion was proposed and has been validated against the experimental data. A liquid-solid compound casting technique was used to fabricate the bimetallic composites. For the graphitizing annealing heat treatment, the bimetallic composite specimens were heated to 940 °C and held for 120 min and 240 min in an electric heating furnace. The proposed empirical model is based on the presence of eutectic carbides chill zone near the interface that formed during solidification of the liquid grey iron above the solid stainless steel substrate. It was found that the calculated carbon concentration profiles based on the proposed diffusion model were in good agreement with the experimental results.

Enhanced Performance of Automotive and Industry Precision Components by Advanced Carbonitriding Technology

Due to their enhanced mechanical properties, gas carbonitrided steel components containing martensite and nitrogen stabilized austenite are currently widely used for highly loaded and severe application conditions. Carbon and nitrogen (C–N) concentration profiles developed during the gas carbonitriding have significant effect on the final properties of the steel. To achieve consistent properties and increase the reliability of processes, simulation of the C–N profile and evolving precipitates during the carbonitriding is essential. Generally, it is observed that the surface nitrogen content developed in the low-alloyed bearing-grade steels is much higher compared to the nitrogen potential in the furnace atmosphere during gas carbonitriding. The formation of nitrides/carbonitrides is one of main reasons for this difference. Thus, diffusion equations cannot be directly applied to calculate the C–N profile, as they do not include precipitation. To solve this problem, a mathematical approach is developed in this work. Thermodynamic data from MatCalc and experimental data are used to formulate equations to calculate the precipitated fraction of C–N during carbonitriding. Furthermore, these equations are integrated with diffusion equations to predict C–N profile that includes precipitates and the developed model is validated with experiments.

Modelling of the degradation of martensitic stainless steels by the Boudouard reaction

Numerical modelling of the degradation of P92 and VM12 martensitic stainless steels in gaseous (i) 70% CO2-30%H2O and (ii) Ar-50%CO2 at 550 °C is reported, emphasising the formation of duplex oxide scale and internal carburisation. A 1D numerical model is presented for the evolution of multiphase regions, subject to a Robin-type boundary condition representing a Boudouard surface reaction. The surface reaction kinetics at the spinel/substrate interface and the carbon transport kinetics in the substrate result in unusual carbon concentration profiles, measured here by glow-discharge optical emission spectroscopy (GDOES). Analysis of experimental data indicates that moisture decreases the rate of the surface reaction, despite higher carbon activity of the gas.

Experimental and Numerical Investigation of the Catalytic Gas Carburization of Steel

The carbon surface saturation process of the steel plates during the catalytic gas carburization was under consideration. The rate of the carburization was determined, as well as the hardness of the diffusion layer, after quenching. A mathematical model based on experimental and numerical modeling data had been obtained. The carbon concentration profile of the surface layer was estimated depending on the carburization process parameters. A simplified simulation technique of the steel carburization was proposed.

Simulation of the Carbon Diffusion Saturation Gear from 15Cr2 Steel in the Cementation Process

The studies on cementation focus exclusively on the carbon’s movement. It is described by diffusion equations, often with constant coefficients and without regard to the liaising with temperature. It does not allow to have regard to the further carbon diffusion into the workpiece with the lower temperature range. The most accurate prediction of carbon concentration profiles depending on the parameters of the carburization regime and the chemical composition of steel is possible with the mathematical models using. However, most models show good results for Fe-C austenite without affecting the effect of alloying substitution elements. Taking into account the influence of alloying elements leads to complex empirical dependencies with difficult selected coefficients. It makes their use difficult. The study describes the simulation using the finite element method for the process of austenite’s diffusion saturation Fe-C-Cr system with carbon during cementation. Here is an example of a steel gear 15Cr2 with the temperature influence. The COMSOL Multiphysics program is used to solve the problem numerically. It is found that the model of carbon diffusion in unalloyed austenite for the single-stage cementation regime is in good contact with the experimental data for the Fe-C-Cr austenite of 15Cr2 steel. For a two-stage process, the calculation of the carbon concentration in the surface layer has a slightly greater deviation from the experimental data than it is at a greater depth.

Prediction of carbon concentration profile within carburised-carbon steel 1024 using Alternating Direction Implicit (ADI) method

Pack carburisation is recognised to improve the carbon surface content of carburised-carbon steel 1024 that enhance the surface hardness and further can be used in automobiles, form implements, machines, gears and springs. The transient two-dimensional diffusion equation that represents the carbon concentration profile within the carburised-carbon steel 1024 is discretised using Alternating Direction Implicit (ADI) technique. A semi-analytical method is used to verify the ADI method and the numerical algorithm is developed in MATLAB software. To materialise the pack carburisation process, the boundary condition, initial condition and step time are encoded to signify the experiment conditions from the literature. The simulation results successfully predicted the carbon concentration profile within the carburised-carbon steel 1024 with variation of carburisation time (2 hours to 60 hours) and temperature (900°C - 1000°C). The effect of carburisation time and temperature are evaluated and the relation of both effects on the carbon development is explained. Simulation results show that carburisation extended to 60 hours only increases the carbon content at the core to 0.3wt % from 0.24 wt% of its original content. The small increment is satisfactory to maintain its ductility by hardening the surface.

Carburizing Heat Treatment of Selective-Laser-Melted 20MnCr5 Steel

As a novel manufacturing technology, additive manufacturing (AM) has many advantages such as energy saving, reduced material waste, faster design-to-build time, design optimization, reduction in manufacturing steps, and product customization compared to conventional manufacturing processes. Heat treatment is widely used to improve the properties of conventional manufactured steel parts. The response of additively manufactured steel parts to heat treatment may be different from conventionally manufactured steel parts due to variations in the as-deposited alloy microstructure. An understanding of heat treatment processes for additively manufactured steel parts is necessary to develop their heat treatment process parameters. In the present work, 20MnCr5 steel was selected to investigate the carburizing heat treatment of additively manufactured parts. These parts were fabricated by selective laser melting (SLM) for the carburizing study. It was found that the AM parts fabricated by the SLM process show the microstructure of tempered martensite, while the microstructure of as-received wrought part is ferrite and pearlite. It was also experimentally found that the SLM process decarburizes the entire SLM part. Before carburizing, a normalization process was conducted on both SLM and wrought 20MnCr5 parts to reduce the effect of the pre-carburizing microstructure. The objective of this project is to determine the carburizing performance of additively manufactured steel parts. The results for the SLM parts in terms of carbon concentration and microhardness profiles are compared with the results for the wrought steel. It was found that the carburized SLM part in the present work has higher carbon concentration near the surface, deeper case depth, and higher total carbon flux than the carburized wrought part.

Effect of sample thickness on the oxidation and carburization kinetics of 9Cr-1Mo steel in high and atmospheric pressure CO2 at 550 °C

The influence of 9Cr-1Mo sample thickness on oxidation and carburization kinetics was investigated. State of the art characterization and modeling techniques were employed to investigate the oxidation and carburization behavior. Within the tested exposure times, the sample thickness did not affect oxidation kinetics, products, or the carburized microstructure. The carburization profile measured by various techniques were in good agreement. From these measurements, a quantitative carburization kinetic law is given for 9Cr-1Mo steels allowing the prediction of carbon concentration profile and carbon concentration at the surface for any exposure time and sample thickness.

Effect of low-temperature surface hardening by carburization on the fatigue behavior of AISI 316L austenitic stainless steel

The influence of low-temperature gaseous carburization on the fatigue behavior of AISI 316 L austenitic stainless steel was investigated. Tension-compression axial fatigue tests were performed under ambient conditions on untreated and carburized AISI 316 L. The results show that the carburized AISI 316 L has a 22% higher endurance limit compared to untreated AISI 316 L. Fractography investigations with scanning electron microscope (SEM) reveal that for the untreated AISI 316 L fatigue cracks initiate at the surface regardless of the applied stress level. For the carburized AISI 316 L fatigue cracks initiate at the surface for relatively high-level stresses; for relatively low-level stresses fatigue cracks initiate at inclusions beyond the carburized case. After carburization, the ductility in the outmost 10 μm of the carburized case has significantly reduced, leading to micro-crack occurrence during fatigue tests and associated relaxation of compressive residual stress in this region. Beyond this surface-adjacent region, no evident stress relaxation occurs due to the enhanced yield strength of the carburized case. The enhanced fatigue performance is mainly ascribed to the compressive residual compressive stress profile introduced by the carbon-concentration profile over the case. Moreover, solid solution strengthening by interstitially dissolved carbon contributes to improve the fatigue performance.