Microstructure of Surface Layer Formed at Low Temperature and High Carbon Concentration Carburizing with Rare Earth Element

A suitable carburized microstructure with fine granular dispersed carbides in hypereutectoid zone,ultra fine martensite in matrix and recrystallized austenite to be refined to the grain size of 12～14 has been obtained by a new process,which is a high carbon concentration carburizing with rare earth element at low temperature(860～880℃)in a discontinuous gas carburization furnace.There was not much difference for the microstructure in eutectic zone between this and conventional process.Forming mechanism of granular carbides has been also studied in this paper.

Numerical Simulation on Carburizing and Quenching of Gear Ring

The carburizing process of the gear ring was simulated by taking into account the practical carburizing and quenching techniques of the gear ring and by solving the diffusion equation. The carbon content distribution in the carburized layer was obtained. Based on the results, the quenching process of the gear ring was then simulated using the metallic thermodynamics and FEM： it was found that the carburization remarkably affects the quenching process. Microstructures and stress distributions of the gear ring in the quenching process were simulated, and the results are confirmed by experiments.

Effect of Rare Earths on Diffusion Coefficient and Transfer Coefficient of Carbon during Carburizing

The diffusion coefficient of carbon in surface layer of steel-20 rare earth carburixed at 880 degreesC and 900 degreesC for 8 h was calculated by substituting the measured layer depths into the diffusion equation. The mathematical model of the transfer coefficient of carbon was deduced based on the kinetics of weight gain during gas carburizing. The calculated results show that the main reason why the gas carburizing process is accelerated is due to the obvious increase in the diffusion coefficient and transfer coefficient of carbon resulted from the addition of RE.

Observation and Analysis of the Microstructure in CarburizedSurface Layer of Steel 20Cr2Ni4A Treated with ConventionalAnd Rare Earth Carburizing Processes

Observation and analysis with TEM show that the fine granular dispersed carbides in hypereutectoid zone of steel 20Cr2Ni4A are distributed in the matrix of large number of lath martensite after rare earth carburizing. But while treating by conventional carburization and double quench hardening the retained carbides are finer and more dispersive, and its matrix is perfectly twin martersite. The different micrcotructures of matrix around carbide are formed with different kinds of carburization processes.

Accelerating Effect of Rare Earth in Steel on Its Surface-Carburizing Process

The carburization of steel type 20 with and without RE addition was investigated. The results show that RE in steel can accelerate carburizing process at 850 and 910 ℃. The optimum RE content in steel is about 0 032%. The mechanism of enhancing effect of RE on carburizing process was discussed.

Double Glow Plasma Hydrogen-free Carburizing on Commercial Purity Titanium

A carburized layer with special physical and chemical properties was formed on the surface of commercial purity titanium by a double glow plasma hydrogen-free carburizing technique,High-purity netlike solid graphite was used as a raw material and commercial purity titanium was used as the substrate material.Argon gas was used as the working gas.The carburized layer can be obviously observed under a microscope.X-ray diffraction indicates that TiC phase with higher hardness and dissociate state carbon phase was formed in the carburized layer.The glow discharge spectrum（GDS） analysis shows that the carbon concentration distributes grodiently along the depth of carburized layer.The surface hardness of the substrate increases obviously.The hardness distributes gradiently from the surface to inner of carburized layer.The friction coefficient reduces by more than 1/2,the ratio wear rate decreases by above three orders of magnitude.The wear resistance of the substrate material is improved-consumedly.

Modeling of numerical simulation and experimental verification for carburizing-nitriding quenching process

A model considering quantitative effects of diffused carbon and nitrogen gradients and kinetics of phase transformation is presented to examine metallo-thermo-mechanical behavior during carburized and nitrided quenching. Coupled simulation of diffusion, phase transformation and stress/strain provides the final distribution of carbon and nitrogen contents as well as residual stress and distortion. Effects of both transformation and lattice expansion induced by carbon and nitrogen absorption were introduced into calculating the evolution of the internal stress and strain. In order to verify the method and the results, the simulated distributions of carbon and nitrogen content and residual stress/strain of a ring model during carburized and nitrided quenching were compared with the measured data.

Computer Predictions and Experimental Verification of Residual Stresses and Distortion in Carburizing-Quenching of Steel

In this paper, the carburizing-quenching process of carbon steel is analyzed with computer simulation which is based on the metallo-thermo-mechanical theory and finite element analysis coupled temperature, phase transformation and stress/strain fields. The residual stresses and distortion of a steel cylinder during carburizing quenching process were predicted and compared with experimental data. From the prediction results, improvement of hardness and strength of the cylinder component in carburizing-quenching process was verified.

Thermodynamic Simulation on the Change in Phase for Carburizing Process

The type of technology used to strengthen the surface structure of machine parts,typically by carbon-permeation,has made a great contribution to the mechanical engineering industry because of its outstanding advantages in corrosion resistance and enhanced mechanical and physical properties.Furthermore,carbon permeation is considered as an optimal method of heat treatment through the diffusion of carbon atoms into the surface of alloy steel.This study presented research results on the thermodynamic calculation and simulation of the carbon permeability process.Applying Fick’s law,the paper calculated the distribution of carbon concentration in the alloy steel after it is absorbed from the surface into the internal of the sample.Using the SYSWELD software,an analysis was performed on the carbon permeability process to determine the distribution of carbon concentrations in 20CrMo steel that was then followed by a detailed analysis of the microstructure of the sample post the carburizing process.According to the calculation results,the surface carbon content was 0.9%and steadily decreased into the core.After 3 hours,the depth of the absorbent layer was measured at 0.5 mm for both the cylindrical and cubic samples.By analyzing the phase,the distribution of martensite phases such as ferrite/pearlite and residual austenite was also determined after the carburizing process.

The advantages of Low Pressure Carburizing in the Heat Treatment Subcontracting Business

Low Pressure Carburizing (LPC) was introduced in the 90’s in the western Europe Heat Treatment business, mainly for in-house applications where it was especially appreciated for Carburizing of transmission parts. However the success of the LPC units installed for gears Carburizing in the automotive industry has hindered the development of the process in other fields - like subcontracting business -, where its advantages deserve to be enlightened. After a brief review of the principle of the process, the interest of its classical application to transmission parts is described, underlining peculiarly the reduction of the distortion observed when LPC is associated with high pressure gas quenching. Then the less-known advantages of the LPC process, like the high accuracy and reproducibility of the results, the modeling possibility and the simulation easiness, the case-depth uniformity and the full flexibility of the units are considered, showing how they can be beneficial to subcontracting business.

The Influence of Carburizing Parameters on Carbon Transfer Coefficient

Definition of coefficient of carbon transfer in European Standard (EN 10052) is presented as: "Mass of carbon transferred from carburizing medium into the steel, per unit surface area per second for a unit difference between the carbon potential, and actual surface carbon content". In this paper, a model is presented of carbon transfer from endothermic atmospheres to carbon steel. The carbon transfer coefficient values were determined experimentally by the foil technique and on specimens, taking into account the following parameters: chemical composition of atmospheres, carbon potential, temperature and time of the carburizing process. Some examples of the variation of the carbon transfer coefficient for two steps of the carburizing process, including soaking before quenching, are given, based on results obtained. The effect of carbon transfer coefficient on carbon content at the steel surface is given.

Acetylene Vacuum Carburizing

Almost 30 years has passed since the publication of materials on vacuum carburizing technology, and is attracting a great deal of attention as a technology capable of being used as a substitute for gas carburizing technology. However, the technology was not popular except in specific fields. The main reason for this is due to a variety of harmful influences accompanying the sooting problems caused by CH4 or C3H8. We have succeeded in that the occurrence of sooting was suppressed by utilizing acetylene, at extremely low pressure for carburizing (below 1 kPa). This process is now showing the excellent quality and prospects for this technology in terms of quality, economy and safety. At present almost 70 practical mass production furnaces are used in production lines, in Japan and abroad. At this time, we will report summary of the present acetylene vacuum carburizing process and the actual results obtained by these acetylene vacuum carburizing furnaces for mass production.

Study on Carburizing Kinetics of Low-carbon Steel at High-temperature and Short-term

In this paper, the carburizing kinetics of low-carbon steel at high-temperature and short-term in liquid cast-iron were studied by metallographic microscope, chemical analysis and so on, and the microstructure of carburized layer was also analyzed. The results show that the carburizing rate of low-carbon steel at high-temperature and short-term is so fast, and the microstructure of carburized layer possess higher carbon content, and cementite, pearlite and ferrite exist in carburized layer structure simultaneously. Besides, the kinetic equations of permeating layer forming have been presented, and the carburizing mechanism was preliminary discussed also.

Evaluation of Process Capability in Gas Carburizing Process to Achieve Quality through Limit Design Concept

Steel is the most important metallic material used in industry. This is because of the versatility of its engineering properties under different conditions. In one condition it can be very mild, soft and suitable for any forming operation. In another condition the same steel can be very hard and strong. This versatility is made possible by the different heat treatments that the steel can be subject to. One such treatment is Gas carburizing. This is the most widely used process for surface hardening of low carbon steels. In this method the surface composition of the steel changes by diffusion of carbon and or nitrogen and result in hard outer surface with good wear resistance properties. A striking feature of Gas Carburizing process is that in this process the original toughness and ductility remains unaffected even after heat treatment. 3% nickel chromium case hardened low carbon steels are widely used for critical automotive and machine applications such as rack and pinion, gears, camshaft, valve rocker shafts and axles which requires high fatigue resistance. Fatigue behaviour of case carburized parts depends to a great extent on the correct combination of Hardness Penetration Depth (HPD) and the magnitude of hardness at the surface and beneath the surface with low size and shape distortion. In order to reduce the manufacturing costs in terms of material consumption and elimination of the number of processing steps, the effect of Gas carburizing parameters on the fatigue behaviour should already be considered in the parameter design stage. Therefore it is of importance to optimize the gas carburizing process variables to attain quality products with respect to hardness and case depth. In the present paper, the evaluation of process capability was carried out through a Limit Design Concept called orthogonal array design of experiment. To optimize the process variables the influence of several parameters (Holding time, Carbon potential, Furnace temperature and Quench time) of the gas carburizing process on the micro hardness, total case depth, effective case depth and level of distortion of AISI 8620 steel were discussed.

Study on Carburizing and Temperature Rising of the Semi-Steel Melt with Plasma Heating

Experiments were carried out on carburizing and temperature rising of the semi steel melt in a plasma induction furnace.Influence of many factors, such as power supply mode,position of the plasma torch and bottom blown gas stirring,on heating efficiency and melt temperature distribution was studied. Melt temperature could be effectively controlled by plasma heating,and carbon content of semi steel melt increased from 1.92 % to 4.58 %, and the utilization rate of carbon reached up to 61.57 % during carburizing of the melt.

Layer Structure Analysis of Low-Carbon Steel Containing Rare Earth by High-Temperature Carburizing of Liquid Cast-Iron

The layer structure of low-carbon steel containing RE by high-temperature (T>1200 ℃) carburizing of liquid cast-iron was studied and the diffusion activation energy of carbon was calculated by metallographic microscpe, chemical analysis etc. The result shows that the technology of carburizing in liquid cast-iron can expedite caburization distinctly and changes the carburizing layer structure. The carburizing rate is 60～80 times of that of the traditional technology, and there is about 43% decrease in the activation energy compared with gas-carburization. In outer structure layer, cementite is formed simultaneously both on the crystal boundary reticularly and inside the crystal grains stripedly. In inner carburizing layer, there is undissolved blocky ferrite in reticular cementite. Besides, rare earth element can expedite carburization process.

Development of microalloyed steels for high temperature carburizing

The potential for use of microalloy additions to suppress abnormal austenite grain growth and produce steels with enhanced bending fatigue resistance after high temperature vacuum carburizing was investigated in a series of Ti-modified SAE 8620 steels with w(niobium) additions up to 0.1%.Results are considered from a series of papers at the Advanced Steel Processing and Products Research Center on the effects of Nb content,heating rate, rolling history,and processing temperature on the evolution of austenite grain structures in carburizing steels. Emphasis is placed on understanding the effects of alloying and processing on each stage in the annealing process including the as received laboratory rolled conditions,during the onset of carburizing after annealing at different heating rates,and after annealing for various times at carburizing temperatures up to 1 100℃.Heating rate to the carburizing temperature was shown to be an influential variable and suppression of abnormal grain growth was dependent on the development of a critical distribution of fine NbC precipitates,stable at the austenitizing temperature.The importance to industrial carburizing practice of heating rate effects on precipitates and austenite grain size evolution are discussed and correlated to selected data on fatigue performance.

The Influence of Method of Carburizing and Nitrocarburizing on the Microstructure and Properties of Tool Steels.

The paper presents the results of wear and metallography tests of tool steel grades: ASTM A681, Chl2M and CH12FS per COST, AMS 6437E i BS X46Crl3, all surface hardened by the Carbo process (carburizing) and by the NiCar process (nitrocarburizing). The thermo-chemical treatment was conducted in powder pack for a duration of 6 h (carburizing) and for 4 h in the case of nitrocarburizing. Factors investigated were: morphology, depth and microhardness of the cases obtained, their microstructure, as well as phase composition. Wear tests were conducted by the three cylinder-cone method. Wear velocity was 0.58 m/s, unit load was 50 MPa and 400Mpa, wear path was 3470 m. Oil SAE30 was applied at the rate of 30 drops per minute.

Study On the Tribological Behavior of the Arc-added Glow Discharge Plasma Non-hydrogen Carburization On Titanium Alloy Surfaces

This paper introduces a new titanium alloys surface strengthening treatment by using the arc-added glow discharge plasmas non-hydrogen Carburization technique. High purity and high strength graphite is selected as cooling cathode arc source for supplying carbon atoms and particle, which migrate to the titanium alloy(Ti6A14V) surface and form modified layer. Thus, the hydrogen embrittlement is avoided while the tribological behavior of the titanium alloy surface is improved in the respects of anti-friction and anti-wear ability.The tribological behavior of the modified layer under dry sliding against SAE52100 steel was evaluated on a ball-on-disc test rig. The results showed that the modified layer obtained a thickness of 30u,m at 980°C, 30minutes. The microhardness of the Ti6A14V alloy surface attained 936 HV, which was much larger than that of the T16A14V alloy. The TJ6A14V alloy was characterized by adhesion wear and scuffing under dry sliding against the steel, while the surface modified layer experienced much abated adhesion wear and scuffing under the same testing condition. This could be attributed to the carbon element with different modalities exists in the modified layer. The modified layer showed good friction-reducing and fair anti-wear ability in dry sliding against the steel. Using the SEM, XRD and XPS, the phase structure and morphology of the Carburization modified layer was analyzed.

Heat Treatment Simulation for Low Pressure Hyper Carburizing Process

Carburizing heat treatment is well used to increase the strength of gears and automobile drivetrain parts.When very high contact fatigue strength is required,hyper carburizing or carbonitriding is well used to improve the surface hardness and tempering softening resistance.Over saturate carburizing as called hyper carburizing can be used in a conventional ordinary carburizing furnace and investment of new gas supply equipment is unnecessary,while there are problems such as controlling precipitation of cementite and securing hardenability of the surface,etc.Also,problems such as secondary precipitation to control the material micro structure,its quality prediction method becomes important.In recent years the low pressure carburizing method has been widely used and the possibility of making hyper carburizing in low cost has been expanding.However,when hyper carburizing is performed by low pressure carburizng,reticulated cementite occurs at the edge of the part,there is a problem such as local increase of carbon concentration distribution and toughness.In this study,we tried to formulate precipitation when super carburizing was performed on steel parts using a low pressure carburizing furnace,formulated the surface hardness distribution,and tried to calculate the hardness and cementite precipitation size distribution.CALPHAD method is used to obtain the physical property values,graphite of an intermediate product was precipitated from the low pressure carburizing atmosphere on the surface layer,and it was assumed that carburization occurred therefrom.Estimating supersaturated carbon precipitates as cementite,the growth amount of the precipitate is calculated.The deposition rate was estimated by comparing the actual measurement with the calculated value.Estimating hardness is determined by the sum of the hardness of the precipitate and the hardness of the parent phase part,the hardness distribution was verified.As a result,the hardness distribution at the time of high concentration carburization in the test piece shape was predicted.The method was also applied to the actual part shape,and the quality difference between the edge part and the flat part of the gear and the heat treatment deformation quantity were estimated.