Carburized Steel Research Articles

Influence of the carburization time on the structural and mechanical properties of XC20 steel

This study focuses on the effect of carburization time on the structural and mechanical properties of low carbon XC20 mild steel (C. Wt.% <0.25). The XC20 steel was carburized with activated carbon with a carbon potential Cp1 = 1.1%, at 910 °C at different carburization times of 2, 4 and 6 h. The results obtained show that XC20 steel (non-carburized) has a ferrite-pearlitic structure with a hardness and a Young’s modulus of the order of (150 HV, 26 KN/mm2). After carburization, the structure of the carburized layer is transformed in martensite (Fe γ) in which cementite (Fe3C) is imbricated. The depth of the carburized layer and the amount of carbon on the surface gradually increase with increasing carburization time. In addition, the carburized XC20 steel becomes hard and brittle where the hardness and Young’s modulus have been increased for a high holding time until reaching maximum values ​​(845 HV, 48 KN mm−2) after 6 h of carburization . However, the toughness of XC20 steel has been reduced from 163 to 40 J cm−2.

Effect of low-temperature tempering on confined precipitation and mechanical properties of carburised steels

Surface-carburised steels are widely used in gears and bearings. The mechanical properties of the surface layers are crucial to improve contact fatigue and anti-wear performance. In this study, the effect of low-temperature tempering on confined precipitation and mechanical properties across surface layers in a high-alloy carburising steel (18Cr2Ni4W) were investigated. The steel was tempered at 240 °C with different tempering durations. Microstructure changes, including confined precipitation, and local mechanical properties were analysed layer-by-layer. During tempering, the morphology and size of martensite remained unchanged while confined precipitation occurred in martensitic matrix; coherent ε phase precipitates formed after 1.5 h, they then transformed to less coherent η phase within 3 h and subsequently evolved to non-coherent θ phase after 3 h of tempering. After tempering for 1.5 h, the strength and ductility of the surface layers reach a maximum value, and elongation of the surface layers with high carbon content is comparable to that of the core with low carbon content. It is revealed that the coherent ε precipitates are mainly responsible for the increase in strength; whilst ductility of the martensitic matrix increases due to a reduced carbon concentration, avoiding tempering brittleness. The results are valuable to the optimisation of surface properties of carburised steels using lower temperature tempering.

Effects of Pre- and Post-Carburizing Surface Modification on the Tribological and Adhesion Properties of Heat-Resistant KHR 45A Steel for Cracking Tubes.

In this study, the effects of ultrasonic nanocrystal surface modification (UNSM) technology on the tribological properties and scratch-induced adhesion behavior of a heat-resistant KHR 45A steel cracking tube, which is used for the pyrolysis process, were investigated. The main objective of this study is to investigate the effects of pre- and post-carburizing UNSM treatment on the tribological and adhesion performances of carburized domestic KHR 45A (A) steel and to compare the results with the existing carburized Kubota-made KHR 45A steel (B). A carburizing process was carried out on the polished and UNSM-treated KHR 45A steel substrates, which were cut out from the cracking tube, at 300 °C heat exposure for 300 h. The thickness of the carburizing layer was about 10 μm. UNSM technology was applied as pre- and post-carburizing surface treatment; both reduced the friction coefficient and wear rate compared to that of the carburized KHR 45A steel substrate. It was also found that the application of UNSM technology increased the critical load, which implies the improvement of adhesion behavior between the carburizing layer and the KHR steel substrate. The application of UNSM technology as pre- and post-carburizing surface treatment could help replace carburized Kubota-made KHR 45A steel (B) thanks to the improved tribological performance, enhanced scratch resistance, load bearing capacity, and adhesion of domestic KHR 45A (A) steel.

Thermodynamic and experimental study on CO2 injection in RH decarburization process of ultra-low carbon steel

In this work, carbon dioxide (CO2) was investigated as a lifting gas in the Ruhrstahl–Heraeus (RH) decarburization process of ultra-low carbon steel. The main challenge was to determine whether the local carburization reaction leads to the carburization of molten steel with ultra-low carbon content. Thermodynamic analysis of the reaction of the Fe-C-O melt with CO2 demonstrated that compared with traditional Ar injection, the decarburization rate can be improved during the early stage of the decarburization process. This occurs because oxygen that is present not only in molten steel but also produced by CO2 dissociation can react with dissolved carbon to remove the carbon from molten steel. However, it is difficult to determine whether the decarburization rate can be influenced during the later stage by injecting CO2. This is because the carburization reaction at the surface of CO2 bubbles in the up-snorkel and the decarburization reaction at other reaction sites can occur simultaneously. Industrial test results demonstrated that compared with Ar injection, the decarburization rate was not significantly affected by CO2 injection at the early stage, whereas it was reduced during the middle and later stages. Nevertheless, the average carbon content in molten steel after the RH treatment with CO2 injection was only 2.6 ppm higher compared with that in Ar injection. Therefore, Ar can potentially be replaced by CO2 in the RH decarburization process of ultra-low carbon steel based on economic and environmental considerations.

The Hybrid Process of Low-Pressure Carburizing and Metallization (Cr + LPC, Al + LPC) of 17CrNiMo7-6 and 10NiCrMo13-5 Steels

This paper presents the concept of modification of physicochemical properties of steels by simultaneous diffusion saturation with carbon and chromium or aluminum. The application of a hybrid surface treatment process consisting of a combination of aluminizing and low-pressure carburizing (Al + LPC) resulted in a reduction in the amount of retained austenite in the surface layer of the steel. While the use of chromium plating and low-pressure carburizing (Cr + LPC) induced an improvement in the corrosion resistance of the carburized steels. It is of particular importance in case of vacuum processes after the application of which the active surface corrodes easily, as well as in case of carburizing of low-alloy steel with nickel, where an increased content of retained austenite in the surface layer is found after carburizing.

Bending Fatigue Behaviour and Fatigue Endurance Limit Prediction of 20Cr2Ni4A Gear Steel after the Ultrasonic Surface Rolling Process.

To study the effect of the surface properties on the bending fatigue performance of heavy-duty gear steel, the authors of this paper used the ultrasonic surface rolling process (USRP) to strengthen 20Cr2Ni4A carburized gear steel. USRP is a novel technique in which the ultrasonic technology is incorporated into the concept of conventional deep rolling. In this study, we illustrated how the surface properties and cross-section mechanical property influence the three-point bending fatigue life of the samples before and after USRP treatment. At the same time, the predicted failure probability-stress-number of cycles (P-S-N) curve was drawn, and the fatigue fracture was analysed. The results show that the fatigue limit increased from 651.36 MPa to 918.88 MPa after USRP treatment. The fatigue source is mainly from the sample interior or surface scratches, and the fatigue performance is positively correlated with the results of the material surface roughness, surface residual stress and surface hardness. At the same time, combined with the change in the phase structure, dislocation structure, residual stress and hardness of the cross section of the material, it is found that the USRP process turns the steel into a gradient material with five layers. Finally, the coupling mechanism between the ultrasonic surface strengthening deformation layer and the carburized layer of 20Cr2Ni4A carburized gear steel is presented, and the grain structure distribution diagram of the section of the 20Cr2Ni4A model after surface strengthening treatment was simulated. The mechanism that influenced the fatigue performance after USRP treatment is explained from the perspectives of the surface and cross section of the samples.

Effect of shot peening intensity on surface integrity of 18CrNiMo7-6 steel

The surface integrity of components treated by shot peening is determined by many influencing factors, which are generally divided into shot peening process parameter, procedure parameter and machine parameter. To understand the influencing relationship of these parameters, and thereby obtain the appropriate peening intensity for the engineering practice, this study experimentally examined the effect of different shot peening parameters on the peening intensity, and further explored the influence of which on surface integrity parameters of carburized steel rollers. It is found that the intensity is mainly affected by the shot diameter (36.73%), air pressure (32.07%) and shot flow rate (23.11%). The maximum compressive residual stress and surface hardness increase linearly as the intensity increases, reaching −1250 MPa and 785 HV at the intensity of 0.55 mmA, respectively. Surface roughness parameter Sq can be used to predict the surface hardness due to the approximately linear relationship between them.

A novel predictive model for multiaxial fatigue in carburized bevel gears

AbstractThis publication focuses on the numerical stress prediction in case‐carburized bevel gears and on their fatigue assessment. Four gear sets are analyzed for the common fatigue failure modes of pitting, tooth root breakage, and subsurface fatigue. The proposed algorithm, enabling the prediction of the dominant failure type and region, relies on the previously published material model for carburized CrNiMo steels. It utilizes a 2D plane strain simplification as only the mean cross‐section is analyzed and evaluates the shear mean and amplitude stresses through the maximum rectangular hull method. A novel multiaxial fatigue criterion is presented and validated.

Isothermal Heat Treatment of the Low-Carbon Martensitic Steel

A study of the low-carbon steel with high hardenability was carried out. The steel contained the following alloying elements, wt. %: C – 0.20; Cr – 2.0; Mn – 2.0; Si – 1.04 Ni – 1.0; Mo – 0.3. The quenching – partitioning treatment of the studied steel was implemented. The microstructure of the steel consisted of the tempered martensite laths, bainite and martensite-austenite regions. The amount of the residual austenite and the carbon concentration in the residual austenite were estimated. The possibility of the quenching – partitioning treatment of the carburized steel was shown.

Effect of local equivalent stress on fatigue life prediction of carburized Cr-Ni alloy steel based on evaluation of maximum crack sizes

The fatigue assessment methods considering surface strengthening process were proposed through experimental investigation of fatigue properties of carburized Cr-Ni alloy under stress ratios of 0 and 0.3. Interior-failure is caused by continuous debondings of refined grains due to stress concentration around interior-inclusion, while surface-failure is caused by grain dislocation slip around surface-inclusion. In view of the effect of local equivalent stress (residual stress, mean stress and stress concentration around the inclusion), the PRx-S-N curves of carburized Cr-Ni alloy steel were constructed based on the prediction of crack characteristic sizes by Generalized Extreme Values (GEV) and Generalized Pareto (GP) distributions under different cumulative probabilities, and the comparison shows that GP distribution can predict the fatigue strength better than GEV distribution. The fatigue life prediction models can be established by GP distribution for surface and interior failures, and the predicted results are in good agreement with the experimental results.

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.

Low-temperature carburized high-alloyed austenitic stainless steels in PEMFC cathodic environment

Austenitic stainless steels are promising materials for bipolar plates in polymer electrolyte membrane fuel cells (PEMFC). In this study, we explore the possibility to apply low-temperature carburized high-alloyed (high Cr, Ni, Mo) steels as bipolar plate materials. The electrochemical properties and the microstructural characteristics of the modified surfaces were analyzed by optical microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and Auger electron spectroscopy. The carburizing treatment (Kolsterising®, Bodycote) induced the formation of a carbon-stabilized expanded austenite layer. The amount of carbon dissolved in the metal matrix was dependent on the alloy composition, in particular on the amount of Mo. Additionally, surface carbides were found at the surface of low-Ni containing steels. The interfacial contact resistance (ICR) measured in a simulated PEMFC was reduced after carburizing as a consequence of thinner oxide layer and change in oxide composition. Potentiodynamic tests revealed nobilitation of all the steels when carbon was present in solid solution. The occurrence of surface carbides, however, was highly detrimental to the corrosion properties. The ICR of carburized materials in aged condition was higher than the corresponding untreated alloys. The presence of localized corrosion in carburized materials could be responsible for this effect.

Precipitation Criterion for Inhibiting Austenite Grain Coarsening during Carburization of Al-Containing 20Cr Gear Steels

AlN precipitates are frequently adopted to pin the austenite grain boundaries for the high-temperature carburization of special gear steels. For these steels, the grain coarsening criterion in the carburizing process is required when encountering the composition optimization for the crack-sensitive steels. In this work, the quantitative influence of the Al and N content on the grain size after carburization is studied through pseudocarburizing experiments based on 20Cr steel. According to the grain structure feature and the kinetic theory, the abnormal grain growth is demonstrated as the mode of austenite grain coarsening in carburization. The AlN precipitate, which provides the dominant pinning force, is ripened in this process and the particle size can be estimated by the Lifshitz−Slyosov−Wagner theory. Both the mass fraction and the pinning strength of AlN precipitate show significant influence on the grain growth behavior with the critical values indicating the grain coarsening. These criteria correspond to the conditions of abnormal grain growth when bearing the Zener pinning, which has been analyzed by the multiple phase-field simulation. Accordingly, the models to predict the austenite grain coarsening in carburization were constructed. The prediction is validated by the additional experiments, resulting in accuracies of 92% and 75% for the two models, respectively. Finally, one of the models is applied to optimize the Al and N contents of commercial steel.

Spectroscopy analyses of tribofilm and surface microcracks in carburized SCM420 steel under high-pressure rolling contact fatigue

The tribofilm and surface microcracks developed on a carburized SCM420 steel through a two-roller-type pitting test were characterized. A streaky 100- to 200-nm-thick tribofilm was generated on the specimen surface after 1.0 × 105 cycles. The tribofilm was identified to be mainly composed of calcium phosphate Ca3(PO4)2 and iron phosphate Fe3(PO4)2. Microcracks having lengths of a few tens of micrometers were detected on the contact surface without the tribofilm after the test. The number of cycles corresponded to a transition of the tangential force between two rollers decreasing to a stick-slip phenomenon as a steady state. A friction modifier additive (molybdenum dialkyldithiocarbamate) was added to a lubricant to improve rolling contact fatigue (RCF) pitting resistance. This lubricant containing the friction modifier additive exhibited a lifetime that was a factor of 10 greater than the RCF lifetime for the lubricant base oil. Therefore, the tribofilm may affect the tangential force and RCF lifetime. The contact surfaces and subsurface structure, especially in the vicinity of the microcracks, were dynamically reconstructed by severe friction and damage accumulation. Oxygen was detected on the surface of the microcracks.

Failure mode mapping for rolling/sliding contacts under low lambda conditions

Tribological failures come in different forms, and there are many parameters (e.g. speed, pressure, sliding, roughness, viscosity, etc.) that affect which failure mode will be realized in testing. It is not uncommon to have two apparently identical tests produce different failure modes. In this study, the failure modes of pitting, micropitting, and abrasive wear were evaluated for rolling/sliding contacts and mapped at low lambda conditions (λ < 0.5). These tests were conducted with a disc-type machine using carburized steel specimens with a circumferential lay direction and oil with anti-wear additives under variable conditions typical in rotating machinery. The surface condition of the specimens was evaluated periodically throughout testing using non-contact profilometery. The data demonstrate that a transition point exists where the primary failure mode switches from abrasive wear/pitting to micropitting, driven primarily by the lambda ratio. The data further shows that (1) increasing film thickness, and thus reducing asperity contact, does not necessarily reduce the likelihood of micropitting in low lambda conditions, and (2) when the lambda ratio is less than 0.1, there exists a sensitivity where one cannot predict the resultant failure mode, most likely due to competing failure modes between abrasive wear and pitting. Additionally, some recommendations will be discussed for replicating gear tests with a disc test stand.

ST37 Steel Carburization with Coconut Charcoal

The carburization process is a surface hardening process where carbon is added to the surface without changing the core properties of the material. This process is carried out at the austenite temperature so that the carbon can diffuse into the phase. This process can only be done on low carbon steels with content below 0.25%. This research was conducted on ST37 steel, which is steel with low carbon content with 0.18% carbon content. This type of steel is surface hardened with a carburizing temperature of 850 C with a long lasting time of 1 hour, then it is carried out under moderate cooling with outside air media. And change its mechanical properties from the comparison of the initial mechanical properties of the specimen. The highest hardness value occurs in the carburizing process of coconut shell charcoal, but this hardness value occurs not because of the carburization process but because of the enlarged grain size caused by heating at temperatures below 723⁰ C, thus reducing the elongation properties of the material. Carburizing with battery stone media is more efficient than coconut shell charcoal at temperatures below 723 C. Because the temperature is below the austenite temperature, the absorbed carbons cannot diffuse as happened in the carburization process, but the absorbed carbons can bind the grain boundaries and change their hardness by 4%. In the microstructure research that occurs in this process nothing can change its phase because the temperature does not reach the austenite temperature. However, there are differences in the microstructure between the carburization process with coconut shell charcoal media.

The impact of the retained austenite in the case-hardened steels on the crater wear formation of the PcBN tools

The finishing operation of transmission components in the automotive industry is widely performed by the hard part turning process, using polycrystalline cubic boron nitride (PcBN) as cutting tool material. Carburizing steels are used in shafts and gears, and after the heat treatment processes, the components have typically 10-30 vol% of retained austenite in the carburized layer. The hard-turning is essentially a process involving high temperatures. The temperature at the tool-chip interface typically reaches the austenitization temperature range of the machined steel. In carburizing steels, the consequence is the generation of crater wear in PCBN tools by the chemical wear mechanism. In this context, this research aims to determine the impact of the retained austenite in the carburized steels on the crater wear formation in the PcBN tools. The 18CrNiMo7-6 steel was used to manufacture the workpieces. During the heat treatment, two carbon potentials (0,64% and 0,72%) and two tempering temperatures (160 °C and 180 °C) were applied to reach two levels of hardness (710 HV and 740 HV) and amount of retained austenite (20% and 27%). To study the wear, hard-turning tests were performed, applying industrial cutting parameters. The PCBN inserts were analyzed by a focus variation microscope (FVM) and a scanning electron microscope (SEM). For the same level of hardness, an increased phase fraction of the retained austenite in the steel increases the crater wear of the PcBN tools. Two complementary hypotheses were framed to explain the results: i. the high fraction volume of retained austenite increases the overall oxidation potential of the iron on the steel, accelerating the chemical degradation of the cBN grains; ii. the high amount of retained austenite reduces the formation of the protective layer on the chip tool interface. The in-depth investigation of chemical degradation is a largely unexplored subject on the wear evaluation of the ultrahard cutting tool.

Correlative cross-sectional characterization of nitrided, carburized and shot-peened steels: synchrotron micro-X-ray diffraction analysis of stress, microstructure and phase gradients

Mechanical properties of case modified steels depend decisively on the near-surface gradients of residual stresses, microstructures, phases and chemical composition, which are generated by the empirically well-established case-hardening techniques. Currently, however, to obtain the correlation between near-surface structure–property gradients, applied hardening process parameters and steels’ overall performance is a very challenging task. In this work, high-energy synchrotron cross-sectional X-ray diffraction (CSmicroXRD) using a pencil beam cross-section of 20 × 500 μm2 and complementary analytical techniques are used to characterize the surface-to-bulk gradient of (i) a plasma nitrided steel W300, (ii) a carburized case hardening steel (grade 18CrNiMo7-6) and (iii) a shot-peened high strength steel, type 300M. CSmicroXRD analysis reveals complex gradients of martensite and austenite phases, residual stresses in both phases, crystallographic texture and the evolution of diffraction peak broadening with a spatial resolution of ~20 μm. These parameters are correlated with the gradients of hardness, morphology-microstructure and with the changes in N and C concentrations and/or retained austenite formation/depletion in all three model samples. Finally, the correlative micro-analytics approach indicates the complexity of near surface structure-property relationships as well as the importance of innovative cross-sectional characterization, which allows for assessing gradual near-surface physical and/or chemical changes accompanying thermo-chemical and mechanical surface treatments.

Wear behavior of nanostructured carbo-austempered cast steels under rolling-sliding conditions

Carbo-austempered steels with a nanobainitic microstructure in the case are established as an exciting alternative in applications demanding high stability and reliability against wear and fatigue. In this work, the effect of heat treatment conditions, i.e. austenitizing and isothermal transformation temperatures, on the wear behavior under rolling/sliding conditions of a carburized cast steel with a nanobainitic structure in the surface was examined. The results show that wear damage is mainly due to contact fatigue, with a small extent of oxidative wear, abrasion and adhesion. The results were compared with the conventional tempered martensite plus retained austenite microstructure obtained by means of carburizing followed by quenching and tempering. The nanobainitic microstructures show a better wear resistance than the quenched and tempered microstructures. Also, the specific wear rate in carbo-austempered steels was lower as the isothermal heat treatment temperature decreased. Results are explained in terms of the microstructural differences, hardening behavior and plasticity of the materials under study.

Influence of normalizing post carburizing treatment on microstructure, mechanical properties and fracture behavior of low alloy gear steels

&lt;abstract&gt; &lt;p&gt;Steel is a versatile metal, got a wide range of applications in all the fields of engineering and technology. Generally, low carbon steels are tough and high alloy carbon steels are hard in nature. Certain applications demand both properties in the same steel. Carburization is one such technique that develops hard and wear resistant surfaces with a soft core. The objective of this work is to study the influence of post carburizing treatment (normalizing) on three grades of steels (EN 3, 20MnCr5, and EN 353). Post carburizing treatments are necessary to overcome the adverse effects of carburization alone. Here carburization was carried out in the propane atmosphere by heating the gas carburizing furnace to 930 ℃ for more than a day. Normalizing was carried out at 870 ℃ for 1 h and cooled in air. Tensile, hardness, Charpy impact tests along with SEM (scanning electron microscopy) and EDAX (energy dispersive X-ray analysis) were conducted to analyze the phase transformation, failure mode analysis in all the samples. Carburized steels displayed the formation of ferrite, pearlite, and sometimes bainite phases in the core and complete coarse pearlite in the case regions, whereas in the post carburized steels, increased amount of ferrite, fine pearlite, and bainite in the core and fine pearlite with traces of bainite in the case region was observed. Normalizing also refines the grain with increased UTS (ultimate tensile strength), hardness, and impact resistance. EN 353 showed higher UTS among the steels with 898 MPa after carburization and 1370 MPa after normalizing treatment. Maximum hardness of 48 HRC was observed in 20MnCr5 and toughness was superior in EN 3 with energy absorbed during test i.e., 8 and 12 J before and after normalizing treatment. Based on the fracture surface analysis, in EN 353 steel, a finer array of dimples with voids and elongated bigger clustered dimples containing ultrafine dimples array are observed in the core and case respectively during carburizing whereas, more density of river pattern and cleavage failure (brittle) are observed in the core and case respectively after post carburizing (normalizing) treatment. There is a reduction in the ductility of the steels after post carburizing treatment. It was observed that normalizing treatment produces superior mechanical properties in the carburized steels by grain refinement and strong microstructures like bainite. Normalizing as post carburizing treatment can be recommended for engineering applications where ductile core and hard surface are of great importance.&lt;/p&gt; &lt;/abstract&gt;