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

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 carburized at 880 ℃ and 900 ℃ 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

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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.

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.

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.

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.

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.

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.

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

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Optimizing the low-pressure carburizing process of 16Cr3NiWMoVNbE gear steel

Compared with the traditional atm ospheric carburization, low-pressure carburization has the benefits of producing no surface oxidation and leaving fine, uniformly dispersed carbides in the carburized layer. However, the process param eters for low-pressure carburization of 16Cr3NiWMoVNbE steel have yet to be optimized. Thus, we use the saturation-value method to optimize these parameters for aviation-gear materials. Toward this end, the m icrostructure and properties of 16Cr3NiWMoVNbE steel after different carburization processes are studied by optical microscopy, scanning electron microscopy, transm ission electron microscopy, and electron probe microanalysis. Considering the saturated austenite carbon concentration, we propose a model of carbon flux and an alloy coefficient for low -pressure carburization to reduce the carbon concentration in austenite and avoid the surface carbide network. At the early stage of carburization (30 s), the gas-solid interface has a higher concentration gradient. The averaging method is not ideal in practical applications, but the carbon flux measured by using the segm ented average m ethod is 2.5 times that measured by the overall average method, which is ideal in practical applications. The corresponding carburization tim e is reduced by 60%. By using the integral average method, the actual carburization time increases, which leads to the rapid form ation of carbide on the surface and affects the entire carburization process. Nb and Wcombine with C to form carbides, which hinders carbon diffusion and consumes carbon, resulting in a sharp decrease in the rate of C diffusion in austenite (the diffusion rate is reduced by 52% for 16Cr3NiWMoVNbE steel). By changing the diffusion coefficient model and comparing the hardness gradient of different processes, the depth of the actual layer is found to be very similar to the design depth.

Ultrahigh hardness of carbon steel surface realized by novel solid carburizing with rapid diffusion of carbon nanostructures

In this study, a novel rapid solid carburizing process with a large diffusion depth using nano-diamonds(NDs) was conducted for low carbon steel. Changes of annealed NDs were obtained by Raman spectroscopy and transmission electron microscopy(TEM), and the results suggested that the NDs experience a stripping process before a special solid-reaction with surface iron atoms from steel substrate. Onionlike carbon(OLC) derived from the annealed NDs provided broken graphitic ribbons as carbon sources that accelerated the rate of adsorption and diffusion. Examination of the surface layer at equilibrium using TEM and X-ray photoelectron spectroscopy(XPS) also revealed the special state of carbon, and an ultrafine mixed phase microstructure was obtained by rapid solid-phase transformation. As a result, a surface hardened layer with ultrahigh hardness and a smooth transition region were realized. We believe that these kinds of diamond or graphitic structures with high activity states have an important influence not only on adsorption and diffusion but also on this special solid-phase transformation.

Effect of surface Cr/C infiltration on microstructure,mechanical properties and wear resistance of high chromium cast iron

A new type of high chromium cast iron(HCCI)was prepared,and its microstructure,mechanical properties,and abrasion resistance were investigated systematically.Results showed that after surface carburizing and chromizing,the microstructure of HCCI mainly consists of martensite,boride(M_(2)B),and carbide(M_(7)C_(3)),accompanied with a large amount of secondary precipitations M_(23)C_(6).Moreover,the morphology and hardness of the carbide and boride in HCCI change little,while the volume fraction of carbide and boride increases from 16.23%to 23.16%.This effectively increases the surface hardness of HCCI from 64.53±0.50 HRC to 66.58±0.50 HRC,with the result that the surface of HCCI possesses a better abrasion resistance compared to the center position.Furthermore,the wear mechanism of HCCI changes from micro-plowing to micro-cutting with the increase of surface hardness.

Transformation Plasticity─The Effect on Metallo-Thermo-Mechanical Simulation of Carburized Quenching Process

Theory of metallo-thermo-mechamics and the developed CAE code offer a powerful tool to simulate residual stresses and distortion during heat treatment processes, in which the transformation plasticity is one of factors to be considered being coupled with stress/strain and metallic structure. It is pointed out in this paper that, especially in the case of carburized quenching, transformation plasticity plays very important role on the distortion, which is verified by axisymmetric finite element, employing heat treatment simulation code "HEARTS". Simulated results with the careful consideration on the effect of transformation plasticity reveal to improve remarkably the accuracy of prediction of the displacement and the mode of distortion, compensating the discrepancy between experimental and calculated results. Attention is also paid on the difference in transformation plasticity and conventional plasticity in simulating the volume fraction, stress and strain in ring-shaped specimen during quenching. Moreover some discussions are made on practical use of the effect, and recent experimental results on the coefficient of transformation plasticity are presented.

Stress Field Analysis of Carburized Specimen and Its Numerical Simulation

Residual stress field of carburized specimens was determined experimentally and simulated by using a finite element method (FEM). The experimental results show that the compressive residual stress is formed in carburized layer of the specimen. The peak value of compressive stress appears inside of carburized layer. The calculated results are tallied with the measured ones quite well, which proves the FEM model used for simulation is correct. According to this model, the stress field of carburized specimen during quenching process was also analyzed in this paper.

Investigation of anti-corrosion property of hybrid coatings fabricated by combining PEC with MAO on pure magnesium

Novel hybrid coatings on pure magnesium were prepared by combining plasma electrolytic carburizing(PEC)with micro-arc oxidation(MAO)to further enhance the anti-corrosion property in this paper.Scanning electron microscopy(SEM)was used to observe the microstructure of the coatings,meanwhile,energy dispersive spectrometry(EDS)and X-ray diffraction(XRD)were separately used to investigate the elemental as well as phase compositions of the coatings.The anti-corrosion property of the coatings was evaluated by potentiodynamic polarization curves as well as electrochemical impedance spectroscopy(EIS).The results show that PEC process is closely related with the effects of adsorption as well as diffusion of the activated carbon atoms,and it can provide a favorable pretreatment surface with predesigned chemical composition to obtain a new kind of phase,namely Si C with superior corrosion resistance and chemical stability,in the following PEC+MAO hybrid coatings.Meanwhile,PEC preprocessing also can afford an excellent micro-structure to increase the coating thickness as well as to improve the compactness of the PEC+MAO hybrid coatings.During the fabrication process of the PEC+MAO hybrid coatings,an overlapping phenomenon in regard to coating thickness can be observed instead of heaping up layer by layer.Compared with both single PEC surface modification layers as well as single MAO coatings,the PEC+MAO hybrid coatings exhibit more superior anti-corrosion property.Especially,the EIS data reveal that the PEC+MAO hybrid coatings can act as an effective protection system to provide relatively excellent long-range anti-corrosion protection.Note also that employing same MAO technique for both single MAO treatment as well as PEC+MAO combining procedure is the key to this research.

Study on the Mechanism of Heterogeneous Catalysis (1) —The Measurement of Equilibrium Pressure of Reaction BaCO₃+ C = BaO + 2CO, the Chemical States of Fe in Reaction Tank, and Chemical Reaction Model Catalytic Cycle Mechanism (CRMM)

The results on the equilibrium pressure measurement of BaCO3 + C = BaO + 2CO reaction and the demonstration of chemical state of iron proved that decomposition-generation cycle of BaCO3 and oxidation-reduction cycle of Fe in that reaction condition case were impossible to occur. The Chemical Reaction Model Catalytic Cycle Mechanism-CRMM does not exist at all for BaCO3, Fe, Noble metal catalysts. To call CRMM as a “principle” was obviously inappropriate. The TOF (Turnover Frequency—TOF: unit: s-1) was used to measure the activity size and life of catalyst, and it is clearly out of nothing.

A Study on Wear Resistance, Hardness and Impact Behaviour of Carburized Fe-Based Powder Metallurgy Parts for Automotive Applications

In order to study the mechanical and triboloical properties of powder metallurgy (PM) parts under different process parameters, the specimens were used in pack carburizing processes. These specimens made from industrial test pieces were carburized in a powder pack for about two to five hours at a temperature of about 850?C - 950?C. The effects of austenitization and quenching are investigated on some specimens. Also the wear tests are performed by means of a pin-on-disc tribotester using roll bearing steel as the counterface material. The results indicate that by appropriate selection of process parameters, it is possible to obtain high wear resistance along with moderate toughness. It is concluded that surface treatments increases the wear resistance and performance of PM parts in service conditions. By increasing the role of PM in industry which resulted from their ability to produce the complex shapes, high production rate, and dimension accuracy of final products, they need to be heat treated. Carburizing method was selected as a surface hardening method for PM parts. Results of wear and hardness show considerable enhancement in mechanical properties of PM parts.

INTERGRANULAR EMBRITTLEMENT IN PLASMA CARBURIZED LAYER

This paper deals with the cause of intergranular fracture occurred in the retained austeniticregion in plasma carburized layer.The results show that the presence of retained austenite,which has a good effect on the impact toughness,has no relation to this embrittlement.Analy-sis by Auger electron spectroscopy shows that the impurities S and P segregate at the grainboundaries is the main reason of the intergranular embrittlement in carburized layer.However,the segregation of P and S can be removed by reheating and quenching treatment.

Heat treatment process optimization for face gears based on deformation and residual stress control

In this paper, based on the principle of heat transfer and thermal elastic-plastic theory, the heat treatment process optimization scheme for face gears is proposed according to the structural characteristics of the face gear and material properties of 12Cr2Ni4 steel. To simulate the effect of carburizing and quenching process on tooth deformation and residual stress distribution, a heat treatment analysis model of face gears is established, and the microstructure, stress and deformation of face gear teeth changing with time are analyzed. The simulation results show that face gear tooth hardness increases, tooth surface residual compressive stress increases and tooth deformation decreases after heat treatment process optimization. It is beneficial to improving the fatigue strength and performance of face gears.