Carburized Gears Research Articles

Exploring Fatigue Crack Initiation in Helical Gears: Impact of Carburization and Frictional Influences

A computational model is presented to analyze the fatigue crack initiation period in helical gears, incorporating considerations for heat treatment via carburization and friction effects. The aim is to determine the number of stress cycles necessary for the initiation and growth of initial cracks, while investigating the impact of dynamic behavior. This study employs a dynamic gear model with two degrees of freedom in torsion, developed using MATLAB, and incorporates the Papadopoulos criterion for fatigue analysis. The computational results are benchmarked against the strain-life method. The findings underscore the significant influence of the friction coefficient between surfaces, heat treatment, and dynamic loads on the growth of initial fatigue cracks. For instance, with a friction coefficient (μ) of 0.5, the initial crack forms on the tooth surface (y=0) in both methods, with the (ε-N) method suggesting 28-65 cycles and the Papadopoulos criterion indicating 101-156 cycles. The latter accounts for the greater influence of residual stresses. Additionally, untreated gears exhibit a minimum number of loading cycles for initial crack appearance at 1.07 × 104 cycles. In contrast, carburized gears demonstrate an extended lifespan, requiring 1.45 × 105 loading cycles for crack initiation in our example. This emphasizes the efficacy of carburization in enhancing gear durability.

Fatigue reliability analysis methodology based on fatigue life and failure mechanism for carburized gear

Gears are one of the important components in mechanical products, and their fatigue reliability determines the safety performance of mechanical products. In this paper, the fatigue characteristics of carburized gears under different torque and constant rotational speed conditions are investigated by using a gear contact fatigue testing machine, and combined with the dynamic maximum contact stress distribution on the subsurface of the meshing gears, the very-high cycle fatigue P-S-N curves of carburized gears under a stress ratio of −1 is established. Local stress concentration in the surface or subsurface of carburized gears causes grain dislocation movement under the combined influence of maximum contact and residual stresses, and then causes dislocation pileup and transcrystalline rupture after being hindered by grain boundaries, ultimately leading to pitting and fatigue failure of gears. Based on the dislocation energy method and combined with the fatigue failure mechanism of gears, a life prediction model of gears with good prediction results is established by considering the interaction of factors such as dynamic load, grain size, initial crack length, residual stress, slip band length and width. A fatigue reliability analysis method of gears is established based on the life state equation of gears considering the life prediction model. Further analyses of the influence of rotational speed, grain size, residual stress, slip band width, slip band length and initial crack size on the fatigue life reliability index of the gears resulted in the conclusion that the fatigue reliability of the gears not only decreases with the increase of the above-mentioned parameters, but also shows decreasing trend with the increase of time. This is of significance for evaluating the fatigue reliability of gears under very-high cycle fatigue conditions.

Impacts of DLC and Cr-DLC coatings on noise level emitted during contact fatigue tests on AISI 4320 steel carburized gears

Impacts of DLC and Cr-DLC coatings on noise level emitted during contact fatigue tests on AISI 4320 steel carburized gears

AlN and Nb(C,N) Composite Precipitation Behaviors and Their Effects on Austenite Grain Growth in SCr420H High‐Temperature Carburized Gear Steel

The effects of nitrogen (N) and niobium (Nb) contents on the precipitation behaviors of AlN, Nb(C,N), and AlN‐Nb(C,N) precipitates and austenite grain growth during the pseudo‐carburizing process are investigated for the carburized steel SCr420H. The results indicate that an increase of N content from 0.011% to 0.019% leads to the emergence of fine and uniform austenite grain structure in the sample while being held at 930 °C for 6 h, which is attributed to a notable rise in the number density of AlN and a decrease in the coarsening rate of AlN. In the steel containing 0.023% Nb, the composite precipitates AlN‐Nb(C,N) play a primary role in the pinning effect on austenite grains growth. The isolated Nb(C,N) and AlN precipitates have minor pinning effects due to their very low volume fractions. Nb(C,N) cap forming on AlN substrate obeys the Shoji–Nishiyama orientation relationship. The favorable lattice matching between AlN and Nb(C,N) and accelerated coarsening rate of Nb(C,N) cap decrease hindrances to the nucleation, growth, and coarsening of Nb(C,N) on AlN substrate. Meanwhile, these simultaneous effects also suppress the same processes for the isolated Nb(C,N) in the matrix.

Effect of Post-Plasma Nitrocarburized Treatment on Mechanical Properties of Carburized and Quenched 18Cr2Ni4WA Steel

Under extreme conditions such as high speed and heavy load, 18Cr2Ni4WA steel cannot meet the service requirements even after carburizing and quenching processes. In order to obtain better surface mechanical properties and tribological property, a hollow cathode ion source diffusion strengthening device was used to nitride the traditional carburizing and quenching samples. Unlike traditional ion carbonitriding technology, the low-temperature ion carbonitriding technology used in this article can increase the surface hardness of the material by 50% after 3 h of treatment, from the original 600 HV0.1 to 900 HV0.1, while the core hardness only decreases by less than 20%. The effect of post-ion carbonitriding treatment on mechanical properties and tribological properties of the carburized and quenched 18Cr2Ni4WA steel was investigated. Samples in different treatment are characterized using optical microscopy (OM), scanning electron microscopy (SEM), optimal SRV-4 high temperature tribotester, as well as Vickers hardness tester. Under two conditions of 6N light load and 60 N heavy load, compared with untreated samples, the wear rate of ion carbonitriding samples decreased by more than 99%, while the friction coefficient remained basically unchanged. Furthermore, the careful selection of ion nitrocarburizing and carburizing tempering temperatures in this study has been shown to significantly enhance surface hardness and wear resistance, while preserving the overall hardness of the carburized sample. The present study demonstrates the potential of ion carbonitriding technology as a viable post-treatment method for carburized gears.

The Prediction and Experimental Study of Bending Fatigue Life of Carburized Gears

The Prediction and Experimental Study of Bending Fatigue Life of Carburized Gears

Morphology, properties and stress-strain state of carburized gear made of steel 12KhN3A after high-frequency currents quenching

The results of the study of the effect of cementation with subsequent induction quenching by high-frequency currents on the physical properties of a gear made of 12HN3A steel are presented, the influence of technological factors (temperature and time) on the cementation layer is studied. The study was carried out using a well-known pasty carbonatesoot carburetor and in a pasty carburetor with composition optimization through the synthesis of a di-ammonium salt of 2-monoethanolamine and citric acid, in terms of carbon saturation activity of steel not inferior to melts of cyanide salts. The structure and chemical composition of 12HN3A steel before and after cementation were studied. In the course of the work, the following mode of chemical-thermal treatment of 12HN3A steel was used: cementation due to exposure to highfrequency currents temperature T = 960 °C, medium — air; induction hardening due to exposure to high-frequency currents temperature T = 860 °C, medium — aqueous solution, tempering low temperature T = 190 °C, medium — aqueous a solution that provides an increase in the margin coefficient for fatigue strength to 86.1 MPa and yield strength to 820 MPa while maintaining a sufficient level of plastic properties. The stress-strain state of a gear wheel made of 12HN3A steel is estimated before and after cementation of its working surfaces with subsequent induction hardening by high-frequency currents.

Experimental investigation of the relation between the surface integrity and bending fatigue strength of carburized gears

Experimental investigation of the relation between the surface integrity and bending fatigue strength of carburized gears

Study on the relation between surface integrity and contact fatigue of carburized gears

Surface integrity is critical for gear contact fatigue performance. The relation between gear surface integrity and contact fatigue remains unclear, which is a challenge for gear anti-fatigue design. This study investigates the relation between surface integrity and contact fatigue of 18CrNiMo7-6 carburized gears through fatigue experiments and data-driven modeling. A series of gear contact fatigue tests, with approximately 110 × 106 running cycles in total, has been conducted. P-N curves and fatigue limits of the tested gears are investigated for four typical manufacturing processes: carburizing and grinding, shot peening, barrel finishing, and barrel finishing after shot peening. The influence of different surface integrity components on contact fatigue is explored with a Pearson correlation coefficient analysis and a random forest algorithm. Formulae of gear contact fatigue life and fatigue limit considering surface integrity are proposed. Results show that a high surface integrity state with surface hardness of 686.5 HV, maximum compressive residual stress of −1162 MPa, and surface roughness Sa of 0.36 μm, exhibits the highest gear contact fatigue limit, which is 15.1% higher than the carburizing and grinding state, indicating the benefits of improving surface integrity. For both the gear contact fatigue life and fatigue limit, the most significant surface integrity components are surface hardness, maximum compressive residual stress, and surface roughness. The proposed formulae considering surface integrity illustrate reasonable prediction accuracy, with 1.5 times dispersion band for the predicted fatigue life and a maximum relative error of 2.1% for the predicted fatigue limit.

Analytical study of tooth flank fracture in case-hardened gears considering non-metallic inclusion

Tooth flank fracture (TFF) is a critical fatigue failure mode in case-hardened gears, especially in large-scale and heavy-duty carburized gears. In this study, a TFF failed gear detached from wind turbine gearbox is examined. A contact model incorporating inhomogeneous inclusion and hardness gradient is developed based on semi-analytical method (SAM) and equivalent inclusion method (EIM). The risk of TFF is evaluated by considering a wide depth range of the material gradient properties and the site of inclusion by using material exposure theory. Results indicate that the increase of mean shear stress dominate the stress concentration around the inclusions. The risk of TFF rises dramatically once the inclusions present near the case-core interface. The higher core hardness is more effective to resist the stress concentration caused by inclusions.

Failure analysis of the carburized 20MnCr5 gear in fatigue working condition

This paper analyzed the reason of the fatigue failure of the carburized 20MnCr5 gear and proposed optimized carburizing process. The number of load cycles of the failure carburized gear was 2.4 million times. The carburized layer was 0.52 mm and dissatisfied the requirements. The number of load cycles of the carburized gear with optimized carburizing process was 8.4 million times. The depth of the carburized layer was 0.75 mm. The shallow depth of carburized layer, low hardness and less retained austenite in the microstructure caused by insufficient carburization were main reasons of the low fatigue life of the failure carburized gear.

Evaluation of pre-shot peening on improvement of carburizing heat treatment of AISI 9310 gear steel

Effective hardening depth (EHD) and surface hardness (SH) achieved by carburizing heat treatment play vital roles in guaranteeing the fatigue performance of heavy load carburized gears, which are often subjected to fatigue failure in the engineering practice. In this study, a strategy of combining the pre-shot peening process and subsequent carburizing heat treatment was proposed to improve the efficiency of carburizing heat treatment of AISI 9310 gear steel. Characterizations of hardness, material microstructure, surface roughness and micro-strain of treated roller samples were conducted with helps of a microhardness tester, optical microscopy, scanning electron microscope, white light interferometer and X-ray diffraction, respectively. The underlying mechanism was explored in terms of discovering the relationship between EHD and microstructure, plastic deformation, surface roughness. Results indicated that the efficiency of carburization heat treatment of AISI 9310 gear steel has been greatly enhanced by the proposed strategy, leading to an increasing in EHD and an increase of about 36.4 HV in SH, respectively. Additionally, the importance of shot peening intensity and coverage to EHD was analyzed by the random forest (RF) algorithm.

Very High Cycle Fatigue Properties of 18CrNiMo7-6 Carburized Steel with Gradient Hardness Distribution

As research of the high cycle fatigue of carburized gear steel could not meet the status quo of longer and longer service lives, research of very high cycle fatigue (VHCF) performance has become the focus of current research. The VHCF properties of case-hardening steel 18CrNiMo7-6 after being carburized with gradient hardness distribution were investigated by means of ultrasonic fatigue tests. The results showed that the carburized specimens with a case hardness of 705 HV and core hardness of 530 HV showed VHCF phenomenon, and the fatigue lives continuously increased to even 109 cycles as the stress amplitude decreased to about 500 MPa. Observations of the fracture surfaces of the fatigue specimens showed that the fatigue crack initiation sites were located in the transition area with the hardness at about 580 HV. It was found that the transition area had low VHCF properties, since the core did not show VHCF phenomenon, and the case had a higher hardness. A fine microstructure was observed in the granular bright facet (GBF) area, and the stress intensity factor ΔKGBF was measured to be 3.04 MPam−1/2. The 109 cycles fatigue life was predicted based on the inclusion size, and the 1010 cycles fatigue life was 490 MPa based on the prediction model.

Simulation of Fixture Hardening for Tool and Process Design Optimization

Abstract Fixture hardening is the process of choice when thin-walled, carburized gears need to be heat treated with minimal distortion. Only a few publications have studied and described the phenomena taking place in carburized gears during a fixture hardening process. An integrated approach is proposed to improve the design of hardening fixtures and introduce new strategies and instrumentation to better understand and control the process. The proposed approach is to tightly integrate the knowledge in metallurgy, numerical simulation, process control, and machine design areas to mutually compensate and overcome the current limitations in each single area. An instrumented fixture is designed for a reference gear and tested on an industrial quenching press. The signals from the fixture are combined directly with the signals from the quenching press, recorded, and used to validate and improve the numerical simulation models. Metallurgical simulations are used to predict temperature distribution, phase transformations, distortion, and to identify new process strategies.

The effect of surface integrity on contact performance of carburized gear

The effect of surface integrity on contact performance of carburized gear

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.

Evaluation of contact fatigue risk of a carburized gear considering gradients of mechanical properties

Carburized gears are widely used in geared machines such as wind turbines. Contact fatigue problems occur in engineering practice, reducing reliabilities of machines. Contact fatigue failures are related to many factors, such as gradients of mechanical properties of the hardening layer. In this work, an elastic-plastic contact model of a carburized gear is developed based on the finite element method to evaluate contact fatigue failure risk, considering variations in hardness and strength. The Dang Van multiaxial equivalent stress is calculated via Python coding within the Abaqus framework. The gradient of yield strength along the depth from case to core is considered. The concept of local material fatigue failure risk is defined to evaluate the probability of pitting failure. The effects of design factors, such as the case hardening depth (CHD), surface hardness, and contact pressure on fatigue failure risk, are studied. As the CHD increases or the surface hardness decreases, the risk of deep spalling failure reduces. The increase in surface hardness leads to a decreased risk of pitting failure, while the variation in CHD hardly affects the pitting failure risk.

Analytical approach for low and high cycle bending fatigue life prediction of carburized gear steel specimens

Bending fatigue is one of the most common failure modes in spur gears. In carburized spur gears, subsurface failure initiation is characteristic for the high cycle fatigue regime, which is rather hard to detect and may cause rapid crack growth and complete failure of the tooth. Experimental testing of specimens made of the same material under comparable loading conditions is often used to gain insight into actual fatigue behavior of the component. Furthermore, in the absence of definite fatigue data of the specimens, various methods are used for estimating fatigue parameters. In this paper, analytical model for bending fatigue life prediction of carburized gear steel specimens based on strain-life approach is proposed. Multilayer method and hardness method are used for estimating strain-life fatigue properties. Rule of mixture is employed to find average cyclic stress-strain curves of carburized specimens. An approach for conversion of axial to bending fatigue data by utilizing Neuber’s rule and modifying factors is suggested. Initial validation of the analytical model is carried out against experimental results from the literature of two types of specimens with different carburizing depths. Good correlation between the predicted and experimental data is observed for both types of specimens, with nearly all data points falling within the scatter factor of three.

A microstructure sensitive contact fatigue model of a carburized gear

Rolling contact fatigue (RCF) issues of carburized gears are not fully understood and relate to many factors, such as gradients of mechanical properties, the multiaxial stress state and the complex stress-strain response due to the polycrystalline anisotropy. In this work, a numerical model for RCF performance evaluation of a carburized gear is proposed by considering the anisotropy of carburized material microstructure and the hardness gradient. The crystal plasticity framework is adopted by using a user material subroutine. The effects of mechanical property gradients induced by carburizing are considered and introduced on the grain level. The Fatemi-Socie fatigue criterion is implemented to capture the RCF performance on crystallographic slip systems during the gear meshing process. The RCF performance of carburized gear is evaluated and compared with a non-carburized gear. The simulation results reveal that the influence of microstructure anisotropy on the maximum contact pressure is the most significant at the pitch point meshing position. The plasticity accumulates at a certain depth of non-carburized gear, and this accumulated plasticity leads to more scattered and larger values of a fatigue indicator parameter. Several sets of random grain orientations are developed, and the result shows that the RCF indicator parameter of non-carburized gear is more sensitive to the grain orientation.

Modeling of contact fatigue damage behavior of a wind turbine carburized gear considering its mechanical properties and microstructure gradients

Carburized gears are commonly used in heavy-duty machines such as wind turbines, ships and high-speed rails. The gradient characteristics of mechanical properties and microstructure from the case to the core and the complex subsurface stress state in service make it very challenging to understand the contact fatigue performance of the carburized gears. In this study, a numerical model considering the effects of mechanical properties and microstructure gradients is developed to investigate the contact fatigue damage behavior of a wind turbine carburized gear. The hardness gradient in the case depth is modelled using Thomas empirical equation. The microstructure gradient is considered in terms of the grain size variation adopting the technique of Voronoi tessellation. The damage-coupled elastic-plastic material constitutive relations are developed to capture the intergranular mechanical response and the progressive fatigue damage under repeated gear meshing. The results indicate that the shear stress reversal is not uniformly distributed along the grain boundaries of the same depth. The critical subsurface depth of crack initiation and the fatigue damage evolution obtained from the calculation model compare well to the experimental and numerical results readily available in literature. With the developed framework, the influences of normal load and case carburization on the fatigue damage behavior of the carburized gear are also investigated and discussed in detail.