Vacuum Carburizing Research Articles

The Influence of the Depth of Grinding on the Condition of the Surface Layer of 20MnCr5 Steel Ground with the Minimum Quantity Lubrication (MQL) Method.

This paper describes the research on abrasive machining conditions and their influence on microhardness and residual stresses distribution in the technological surface layer of 20MnCr5 steel. The roughness of ground samples was also measured. Samples underwent a vacuum carburizing process (LPC) followed by high-pressure gas quenching (HPGQ) in a 4D quenching chamber. Processes were realized with a single-piece flow method. Then, the flat surfaces of samples were ground with a Vortex type IPA60EH20VTX alumina grinding wheel using a flat-surface grinder. The samples were ground to three depths of grinding (ae = 0.01; 0.02; 0.03 mm) with grinding fluid supply using either flood method (WET) or minimum quantity lubrication (MQL) method. The condition of the technological surface layer was described using microhardness and residual stresses, as well as some selected parameters of surface roughness. The results obtained revealed that changes in microhardness as compared to microhardness of the material before grinding were lower in samples ground with grinding fluid supplied with MQL method. At the same time, the values of residual stresses were also better for samples ground using MQL method. Furthermore, the use of grinding fluid fed with MQL method produced lower values of surface roughness compared to the parameters obtained with WET method. It was concluded that for the tested scope of machining conditions, the MQL method can be a favourable alternative to the flood method of supplying grinding fluid into the grinding zone.

Influence of Nb Addition on Microstructure and Creep Property of Heat-Resistant Cast Steel in Vacuum Carburizing and Quenching

Influence of Nb Addition on Microstructure and Creep Property of Heat-Resistant Cast Steel in Vacuum Carburizing and Quenching

Effect of Grain Size on Carburization Characteristics of the High-Entropy Equiatomic CoCrFeMnNi Alloy.

In this study, the carburization characteristics of cast and cold-rolled CoCrFeMnNi high-entropy alloys (HEAs) with various grain sizes were investigated. All specimens were prepared by vacuum carburization at 940 °C for 8 h. The carburized/diffused layer was mainly composed of face-centered cubic structures and Cr7C3 carbide precipitates. The carburized/diffused layer of the cold-rolled specimen with a fine grain size (~1 μm) was thicker (~400 μm) than that of the carburized cast specimen (~200 μm) with a coarse grain size (~1.1 mm). In all specimens, the carbides were formed primarily through grain boundaries, and their distribution varied with the grain sizes of the specimens. However, the carbide precipitates of the cast specimen were formed primarily at the grain boundaries and were unequally distributed in the specific grains. Owing to the non-uniform formation of carbides in the carburized cast specimen, the areas in the diffused layer exhibited various carbide densities and hardness distributions. Therefore, to improve the carburization efficiency of equiatomic CoCrFeMnNi HEAs, it is necessary to refine the grain sizes.

Effect of Deformation Structure of AISI 316L in Low-Temperature Vacuum Carburizing

The effect of plastic deformation applied to AISI 316L in low-temperature vacuum carburizing without surface activation was investigated. To create a difference in the deformation states of each specimen, solution and stress-relieving heat treatment were performed using plastically deformed AISI 316L, and the deformation structure and the carburized layer were observed with EBSD and OM. The change in lattice parameter was confirmed with XRD, and the natural oxide layers were analyzed through TEM and XPS. In this study, the carburized layer on the deformed AISI 316L was the thinnest and the dissolved carbon content of the layer was the lowest. The thickness and composition of the natural oxide layer on the surface were changed due to the deformed structure. The natural oxide layer on the deformed AISI 316L was the thickest, and the layer was formed with a bi-layer structure consisting of an upper Cr-rich layer and a lower Fe-rich layer. The thick and Cr-rich oxide layer was difficult to decompose due to the requirement for lower oxygen partial pressure. In conclusion, the oxide layer is the most influential factor, and its thickness and composition may determine carburizing efficiency in low-temperature vacuum carburizing without surface activation.

Modeling and Simulation of Vacuum Low Pressure Carburizing Process in Gear Steel

A combination of simulation and experimental approaches to optimize the vacuum carburizing process is necessary to replace the costly experimental trial-and-error method in time and resources. In order to accurately predict the microstructure evolution and mechanical properties of the vacuum carburizing process, a multi-field multi-scale coupled model considering the interaction of temperature, diffusion, phase transformation, and stress was established. Meanwhile, the improved model is combined with the heat treatment software COSMAP to realize the simulation of the low-pressure vacuum carburizing process. The low-pressure vacuum carburizing process of 20CrMo gear steel was simulated by COSMAP and compared with the experimental results to verify the model. The results indicated that the model could quantitatively obtain the carbon concentration distribution, Fe-C phase fraction, and hardness distribution. It can be found that the carbon content gradually decreased from the surface to the center. The surface carbon concentration is relatively high only after the carburizing stage. With the increase in diffusion time, the surface carbon concentration decreases, and the carburized layer depth increases. The simulated surface carbon concentration results and experimental results are in good agreement. However, there is an error between calculations and observations for the depth of the carburized layer. The error between simulation and experiment of the depth of carburized layer is less than 6%. The simulated surface hardness is 34 HV lower than the experimental surface hardness. The error of surface hardness is less than 5%, which indicates that the simulation results are reliable. Furthermore, vacuum carburizing processes with different diffusion times were simulated to achieve the carburizing target under specific requirements. The results demonstrated that the optimum process parameters are a carburizing time of 42 min and a diffusion time of 105 min. This provides reference and guidance for the development and optimization of the vacuum carburizing process.

Wear behaviors and high-temperature oxidation resistance properties of tantalum carbide layer

A TaC/Ta2C bilayer is obtained by vacuum carburizing technology and characterized by scanning electron microscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction, the nano-indentation, friction and wear test, high-temperature oxidation experiment. XPS analyses indicate that the carburized layer is composed of intermediate layer Ta2C and outer-layer TaC. The total thickness of the carburized layer is approximately 20 μm. The nano-indentation hardness of the carburized layer along the depth direction varies from 14.632 to 35.832 GPa. The formation of the hard phase in carbide considerably improves the wear resistance of pure Ta. The high-melting-point ceramic phase in carbide improves the high-temperature oxidation resistance of pure Ta, thereby making it serve in high-temperature air environment for a short time.

Recent Progress of Low and Medium-Carbon Advanced Martensitic Steels

This article introduces the microstructural and mechanical properties of low and medium-carbon advanced martensitic steels (AMSs) subjected to heat-treatment, hot- and warm- working, and/or case-hardening processes. The AMSs developed for sheet and wire rod products have a tensile strength higher than 1.5 GPa, good cold-formability, superior toughness and fatigue strength, and delayed fracture strength due to a mixture of martensite and retained austenite, compared with the conventional martensitic steels. In addition, the hot- and warm-stamping and forging contribute to enhance the mechanical properties of the AMSs due to grain refining and the improvement of retained austenite characteristics. The case-hardening process (fine particle peening and vacuum carburization) is effective to further increase the fatigue strength.

Investigation of the contact fatigue strength of high quality carburised steel

The parameters of contact fatigue strength during vacuum carburization and ion nitriding of high quality steel (HQS) HQS-7 and HQS-10 are considered. Information about the modes of chemical and heat treatment is given and the technological process of hardening for HQS-7 and HQS-10 steels is considered. The characteristics of the contact fatigue strength of steels in the statistical aspect at different levels of contact stresses are obtained. A graphical analysis of the obtained values after testing was carried out, the optimal stresses for research were determined and the optimal version of chemical heat treatment was selected, which determines the best indicators for mechanical characteristics and operational requirements.

Microstructure, Hardness, and Tensile Properties of Vacuum Carburizing Gear Steel

We investigated the effects of the austenitizing temperature on the microstructure, hardness, and tensile properties of case-carburized steel after vacuum carburization at 930 °C and then re-austenitization at 820–900 °C followed by oil quenching and tempering. The results show that fractures occurred early with the increase in the austenitizing temperature, although all the carburized specimens showed a similar case hardness of 800 HV0.2 and case depth of 1.2 mm. The highest fracture stress of 1919 MPa was obtained for the experimental steel when the austenitizing temperature was 840 °C due to its fine microstructure and relatively high percentage of retained austenite transformed into martensite during the tensile tests. We also found that the stress–strain behavior of case-carburized specimens could be described by the area-weighted curves of the carburized case and the core in combination. The strain hardening exponent was about 0.4 and did not vary with the increase in the austenitizing temperature. We concluded that the optimum austenitizing temperature was around 840 °C for the experimental steel.

Effect of Vacuum Carburizing Time on Microstructure and Mechanical Properties of Tantalum Carbide Layer

A (TaC/Ta2C) carbide bilayer is obtained by vacuum carburizing technology on the surface of Ta substrate at 1673 K for 4, 8, and 12 h. XRD, SEM and EBSD are utilized to investigated phase composition and the microstructure. The mechanical properties of the Ta and tantalum carburized materials are studied with Vicker’s hardness tester and nanoindenter, adhesion automatic scratch tester, reciprocating friction and wear testing machine. The results show that the outside surface phase composition of the carbide bilayer is all the TaC phase. With the increase of the carburizing time from 4 to 12 h, the average grain size from approximately 500 nm to 10 μm, the thickness of the carbide bilayer is from 11 to 20 μm. The microhardness increases from 104.1 to 322.5 HV, and the elastic modulus are from 466.6 to 615.3 GPa. Adhesive strength is best at 8 h, 49.1 N, compared to 19 N at 4 h and 36.5 N at 12 h. The friction and wear coefficient of Ta fluctuates significantly between 0.6 and 1.0, after carburizing treatment, the friction and wear coefficient fluctuates smoothly, and the wear resistance is well improved.

Effect of Stages of Vacuum Carburizing on Deformations in Splines of Steels 16MnCr5, AMS6265 and 17CrNiMo7-6

Deformations of straight-sided splines (changes in the width of the hubs) of steels 16MnCr5, AMS6265 and 17CrNiMo7-6 are studied after different treatments, i.e., a hold at 1050°C without carburizing and with slow cooling to room temperature, vacuum carburizing with subsequent slow cooling to room temperature, and vacuum carburizing with subsequent oil quenching. The sizes of the deformations are shown to depend on the grade of the steel. The lowest deformations are obtained for steel 16MnCr5.

Control of Phase Composition of Complexly Alloyed High-Temperature Steels Under Vacuum Carburizing and Quenching

A process of hardening of steel 16Kh3NVFMB-Sh involving vacuum carburizing and subsequent hardening heat treatment is studied. It is shown that in order to obtain an optimum phase composition and, as a consequence, high operating properties, the steel should be subjected after carburizing to high tempering for formation of a fine ferrite-carbide structure. The treatment should be finished with quenching after the second heating, cold treatment and low tempering.

Change of Geometric Accuracy of Structural Steels after Carburizing in Gas

Nowadays, carburizing at low pressure or vacuum carburizing is often used, which is a modification of carburizing in gas, while carburization is performed at pressures well below the level of atmospheric pressure (101 kPa). The advantage of this process is that no internal oxidation occurs; carburizing has a higher rate and uniformity of carburization, good reproducibility of the process and lower deformation. Carburizing is mainly used for parts subjected to abrasion (eg gears). However, even with this process, the geometric accuracy and surface texture of the parts change. The changes of geometric accuracy of ground samples from carbon steel C45, low-alloy Mn-Cr carburizing steel 16MnCr5 and medium-alloyed Ni-Cr carburizing steel 14NiCr14, which were vacuum-carburized in a CO/CH4 atmosphere, were evaluated in this work. Geometric accuracy was evaluated by measurement using a 3D coordinate measuring machine, surface texture using a coherent correlation interferometer. After the carburizing process, changes in the dimensions of the samples were found in all steels. For C45 steel, the sample dimensions were reduced by 0.028 mm ± 0.004 mm, the surface roughness expressed by the parameter Ra decreased by 0.28 µm, which is 20%. In case of carburizing steels there was an increase in the dimensions of the samples, especially in the case of 14NiCr14 steel and a smaller difference in surface roughness. The same parameters of the carburizing process lead to different behaviour of carburized parts in different steels, in terms of changes in dimensions and surface texture.

Crack Propagation Behavior of Impact Fracture in Case Hardening Steel Subjected to Combined Heat Treatment with Excess Vacuum Carburizing and Subsequent Induction Hardening

Crack Propagation Behavior of Impact Fracture in Case Hardening Steel Subjected to Combined Heat Treatment with Excess Vacuum Carburizing and Subsequent Induction Hardening

Mathematical Modeling of Carbon Flux Parameters for Low-Pressure Vacuum Carburizing with Medium-High Alloy Steel

Low-pressure vacuum carburizing adopts a pulse process mode to improve the carburizing efficiency and reduces gas and energy consumption. Carbon flux is the key to accurately control the time of strong infiltration and diffusion in each pulse. In order to obtain the carbon fluxes with various materials under diffident carburizing process conditions, an evenly segmented carbon flux method is proposed. A systematic study with each model using different materials (12Cr2Ni4A, 16Cr3NiWMoVNbE, and 18Cr2Ni4WA represent different initial carbon concentrations and different alloy compositions), carburizing temperatures, and carburizing pressures to determine the effect of these conditions on carbon flux is conducted. Compared with traditional segmented carbon flux method, an evenly segmented carbon flux method can predict the actual carbon flux more precisely and effectively in order to finely control the pulse carburization process. The paper also indicates that carbon fluxes increase with the increase of pressure. The optimal carburization pressure for low-pressure vacuum carburization is 300 Pa. Raising the carburization temperature to 980 °C instead of 920 °C can increase effective carbon flux by more than 30%. Among the material compositions, alloy content has the biggest impact over the carbon, initial carbon concentration the second, and saturated carbon concentration the third biggest impact.

Microstructure and hardness of Pyrowear 53 steel after low-pressure vacuum carburizing at 921°C

Abstract The present paper studies and analyzes the low-pressure vacuum carburizing of Pyrowear 53 steel. The carburizing was performed at 921°C. The results after the completion of the treatment process are presented, i.e. microstructure of the surface layer and hardness. The results confirm that carburizing can be effectively used in hardening of the steel.

Study on micro-mechanism of carbon atom diffusion in surface nanocrystallization low carbon alloy steel using molecular dynamics simulation

Molecular dynamics (MD) simulation was successfully used to explore the mechanism of carbon atom diffusion in supersonic fine particle bombardment (SFPB) pretreated low carbon alloy steel during vacuum carburization. Results of experiment and simulation demonstrated that the plastic deformation was occurred and high surface energy achieved on the surface of low carbon alloy steel during the process of SFPB. The defects were formed inside the crystal including vacancies, 1/2 <111>, <100> and <110> types of dislocations, and the number of defects gradually increased with the increase of bombardment time and speed. Compared with defect-free materials, the large number of defects increase the diffusion coefficient by several orders of magnitudes. The internal defects of the material, including vacancies, dislocations and grain boundaries induce the rise of carbon atom diffusion coefficient, accordingly. While the effect of dislocation types was not obvious to diffusion coefficient. The carburizing temperature has a significant effect on the carbon atom diffusion rate of SFPB pretreated samples, when comparing with that of non-pretreated ones.

Technological bases for increasing the durability of aviation parts by chemical heat treatment

The paper presents studies of the structure and properties of low-carbon alloy steels after carburizing. A comparison of four types of carburizing (batch carburization, conveyor carburization, fluidized bed carburization and ionic carburization) is made. The influence of technological modes of chemical-thermal treatment on the phase composition and amount of carbide phase in a carburized layer is considered. It is shown that the type of carburizing affects the operational properties of parts of aircraft mechanisms. The best results were shown by the details after ion vacuum carburizing. The increase in wear resistance and contact fatigue is associated with the formation of a high-quality surface layer having a higher hardness and a higher proportion of the carbide phase. Such properties cannot be obtained after conventional types of carburizing in a shaft or conveyor furnace. In this case, ionic carburizing and carburizing in a fluidized bed make it possible to halve the technological regime of carburizing in comparison with two other methods.

Carburizing Behavior of AISI 4115 Steel According to the Flow Rates of Acetylene in Vacuum Carburizing

The vacuum carburizing behavior of AISI 4115 steel according to acetylene flow rates (1, 5, 10, 20, 30, 40, and 50 sccm) and temperatures (900, 950, and 1000 &lt;sup&gt;o&lt;/sup&gt;C) was quantitatively investigated through microstructure, carbon concentration and hardness analyses. As the acetylene flow rate increased, the penetration rate of carbon into the surface of the samples (surface-carbon flux) increased, resulting in an increase in the solute carbon concentration at the surface up to the solubility of carbon in austenite. At the same time, the amount of carbon diffusion into the interior of the specimen and overall carburizing depth increased. When the carbon concentration in the surface of the carburized specimen exceeded the solubility of carbon in the austenite above a certain flow rate, cementite precipitated in the austenite grain boundaries. Accordingly, the surface hardness decreased and the amount of carbon diffusion into the specimen became constant as the solute carbon concentration in the austenite matrix at the surface of the specimen became constant. As a result, the overall solute carbon concentration and the effective case depth no longer increased. Meanwhile, the tendency of vacuum carburizing behavior according to temperature was the same, but as temperature increased, the surface-carbon flux and rate of carbon diffusion into the interior of the sample increased, resulting in increased carburizing depth. The appropriate flow rates (10, 20, and 30 sccm) and the corresponding surface-carbon fluxes (62.3, 135.7, and 216.7 g/m&lt;sup&gt;2&lt;/sup&gt;h) were derived according to carburizing temperatures (900, 950, and 1000 &lt;sup&gt;o&lt;/sup&gt;C), respectively.

Surface nanocrystallized structural steel with enhanced tribological properties under different sliding conditions

The surface nanocrystallization layer induced by supersonic fine particle bombardment (SFPB) was obtained on a structural steel. SFPB was applied as pretreatment on the steel prior to the vacuum carburization, for enhancing the tribological properties under various sliding conditions. Varying duration of pretreatment was performed to investigate the influence on microstructure, surface hardness, and tribological behavior of the carburized substrates. Results suggested that 240s of SFPB pretreatment induced finer grain boundaries and higher hardness. A decrease in weight loss and friction coefficient under unlubricated and lubricated sliding conditions indicated enhanced tribological properties of the 240s SFPB nanocrystallized steel after vacuum carburization. Notably, the lowest friction coefficient was achieved under sea water sliding condition, whereas the highest weight loss was obtained. Rolling contact fatigue behavior of the carburized steel with SFPB 240s pretreatment was discussed and Weibull life distribution was proposed in the study.