Steel Gears Research Articles

The influence of Λ ratio and surface roughness on the initiation and progression of micropitting damage

Micropitting is a major failure mode in gears and rolling bearings. Despite its practical importance, there currently exist no universally accepted design guidelines for its prevention primarily due to the great number of influencing parameters. In the absence of an established criterion, the Λ-ratio (the ratio of lubricant film thickness to surface roughness) is often used as a simple way to assess the risk of micropitting. In this paper we present new data to establish and decouple the individual effects of two of the most important influencing parameters in micropitting: the surface roughness amplitude and Λ-ratio. The experiments are conducted on a triple disc contact fatigue rig and carefully designed to decouple the effect of roughness from that of the Λ-ratio. To isolate the influence of the Λ ratio, we use specimens with the same and very tightly controlled surface roughness and then vary the film thickness by blending different viscosities of the same PAO base oil while keeping all other influential parameters constant. To isolate the influence of surface roughness, we keep the Λ ratio the same using the same PAO oils with appropriate viscosities while changing the RMS roughness of the specimen and keeping other roughness parameters as constant as possible. All tests use case carburised 16MnCr5 gear steel specimens. Results show that higher Λ ratio at fixed roughness produces less micropitting as may intuitively be expected. More importantly, at a fixed Λ ratio the amount of micropitting damage was extremely sensitive to the actual surface roughness, with lower roughness both lengthening the micropitting incubation phase and decreasing the rate of material loss in the micropitting wear phase. This shows that Λ ratio on its own cannot be used to assess the risk of micropitting because it is not able to account for the effects of different roughness levels on micropitting, despite including roughness in its definition. Furthermore, in relative terms micropitting was more sensitive to surface roughness than to Λ. In addition, we show that there exists a sufficiently low surface roughness below which micropitting is unlikely to occur even at extremely low Λ ratios. The results presented here can help in design of gears and other mechanical components to mitigate against micropitting.

Numerical and experimental analysis of the transmission error in hybrid metal-composite gears

The aim of this work is to provide a numerical analysis and an experimental characterization of the meshing behaviour of hybrid metal-composite gears, manufactured by press-fitting a carbon fibre reinforced polymer (CFRP) web into a steel gear structure. Hybrid gears have been recently proposed as an answer to the quest for lighter mechanical transmissions, with improved meshing performance in comparison to conventional lightweight gears. Numerical and experimental studies at single-gear level as well as gear-pair testing demonstrated a huge potential for such a technology to contribute to system-level weight reduction.In this paper, the meshing behaviour of a hybrid gear, manufactured by interference fitting and engaging with a conventional steel gear, is analysed numerically by using a hybrid gear contact model that combines, in a multibody simulation environment, an analytical description of the non-linear contact phenomena with a Finite Element (FE) formulation of the overall gear compliance. An experimental characterization of the Static Transmission Error (STE) is also given by using a high-precision gear test-rig, where quasi-static tests have been carried out under different load conditions. Important indications on how the manufacturing process must be implemented to prevent from poor meshing performance are illustrated.

Challenges in the determination of the macroscopic degree of cleanliness determined by ultrasonic immersion testing of (ultra-clean) gear steels

Abstract Power density demands for gearboxes are steadily increasing. Higher gear steel qualities can help to meet these demands. Steel manufacturer, as well as customers, usually examine the quality of gear steel batches. For this purpose, ultrasonic immersion testing is a fast and non-destructive evaluation method for the determination of the macroscopic degree of cleanliness. The evaluation follows certain standards and the common influence factors are usually well known. However, the state of knowledge shows that there are still many open questions. Some of these open questions can lead to differences between the results of different laboratories. In the framework of a research project, the macroscopic degree of cleanliness of ultra-clean gear steels had to be determined. However, first results showed significant differences between the results of the laboratories. For this reason, an interlaboratory test was initiated. The same samples were used for the characterizations at the different laboratories. The aim of the investigation was to determine the influencing factors that led to the different results. Based on the interlaboratory test, countermeasures should be defined to ensure a reliable determination of the macroscopic degree of cleanliness of (ultra-clean) gear steels and to ensure comparable results from different laboratories in the future.

In situ investigation of the influence of varying load conditions on tooth deformation and wear of polymer gears

A new method for in situ deformation and wear measurement of polymer gears has been developed at the LKT and validated for polyamide-66 (PA66) gears at constant loading torque and rotational speed. This contribution contains a more comprehensive validation of the newly developed test method by examining polybutylenterephtalate (PBT) gears under varying loading conditions. The deformation test method is based on measuring and analysing the timing differences between the index pulse signals of rotary encoders on the input and output shaft of the test rig. Since the total tooth deformation is a combination of different effects, such as elastic and plastic deformation, thermal expansion and wear, different testing modes with a low and a high torque level are implemented to separate the effects of elastic deformation on the one hand and plastic deformation and wear on the other hand. As a consequence, the new test rig design allows a deeper understanding of the wear and deformation behaviour of polymer steel gear sets over time. This potential is used to analyse the interactions of different loading conditions on the time-dependent deformation of plastic gears. The influence of both, different transmitting torques and rotational speeds is examined.The test method shows good correlation with well-established ex situ measurements for different combinations of rotational speeds and loading torques and thus could be validated. Long-term gear tests under varying rotational speeds and loading torques show increasing wear and deformation at higher speeds and torques confirming the state of the art described in the literature. In addition, the time dependent deformation behaviour at different load conditions due to superposition of wear and plastic deformation could be analysed in detail.

Effects of the Cooling Rate and Recovery Temperature on the Growth of AlN Precipitates in Gear Steel

During the continuous casting and rolling process of gear steel, AlN precipitation often causes surface cracks on the cast slab. Rapid cooling of the slab surface is a new concept to inhibit AlN and improve the cracking resistance, but it requires further investigation for practical application. Here, the effects of the cooling rate and recovery temperature on AlN growth behavior in gear steel are investigated by designing five cooling conditions. A mathematical model to describe the growth characteristics is constructed based on Ostwald ripening theory. The experimental results show that the cooling rate significantly influences the size and quantity of AlN. When the cooling rate increases from 3.0 to 5.0 °C s−1, the average diameter of AlN decreases from 133.6 to 75.8 nm and the number density of AlN decreases from 6.8 × 105 to 2.1 × 105 cm−2. The calculated results show that AlN has maximum size, which significantly decreases with decreasing temperature. When AlN reaches the maximum size, it stops growing. AlN growth during the cooling process mainly occurs above 800 °C. Therefore, a high cooling rate and a recovery temperature lower than 800 °C are key to inhibiting AlN precipitation and improving the cracking resistance.

Numerical simulation of a magnetic induction coil for heat treatment of an AISI 4340 gear

In manufacturing industry, heat treatment is a fundamental requirement for improving the material quality of readily manufactured products. Induction heating technology is repeatable and easily controlled by the advantage of having an electronical control unit. Nowadays, numerical methods have gained so much importance that it become as a reference for the induction heating industry. Experimental methods are costly and time demanding procedures. However, making use of finite element method software, induction heating simulations of a steel gear can be performed relatively cost effective and in a short time. In this paper, induction heating simulation of an AISI 4340 steel gear using FEA software is performed. The effect of variation of inductor frequency and gear workpiece-inductor coil distance on the hardening depth of the gear surface is investigated. The temperature profile of the workpiece is obtained. From the temperature distribution on the steel gear workpiece, the regions of the gear at which the austenitizing temperature (Ac3) - responsible for martensite phase formation- are observed. From the numerical results, hardening profile and hardening depth of the gear is interpreted.

Laboratory Study on the Formation of Endogenous CaO‐Containing Inclusion in Gear Steel SCr420H during the Simulated LF Refining Process without Ca Treatment

The individual effect of Ca‐containing ferrosilicon, MgO crucible, and slag–steel refining on the formation of endogenous CaO‐containing inclusion in steel is investigated by contrast experiments. Experiments using Ca‐containing ferrosilicon are compared with the effect of high basicity slag and MgO crucible on inclusions compared to experiments with high purity Si. It is found that the influence of Ca‐containing ferrosilicon has a noticeable impact on endogenous CaO‐containing, while slag–steel reaction and MgO refractory mainly increase the (MgO) in inclusion. When Ca‐containing ferrosilicon is added after Al deoxidation, Al2O3 and MgO–xAl2O3 inclusions with low MgO content are modified into CaO‐containing inclusions preferentially, and [Mg] supplied by MgO refractory reacts mainly with (Al2O3) in inclusions in the subsequent refining process. Ca‐containing ferrosilicon addition before Al deoxidation is helpful in reducing Ca content in the steel. The dissolution of refractory in slag has a significant effect on slag–steel reaction by changing slag composition. Under the effect of slag and MgO refractory, MgO‐saturated spinel and MgO form in steel during the whole process. Herein, Ca‐containing ferrosilicon is a significant contributor to endogenous CaO‐containing inclusions during the simulated ladle furnace refining process without Ca treatment.

Predictions and Experiments on the Distortion of the 20Cr2Ni4A C-ring during Carburizing and Quenching Process.

This paper focuses on the effect of gear steel on distortion due to phase transformation in carburizing and quenching. The carburizing and quenching process of C-rings under suspension was studied by using the finite element method based on the thermo-mechanical theory, considering phase transformation. The phase transformation kinetics parameters, depending on different carbon contents, were measured by Gleeble-3500. The distortion behavior of the carburized C-ring during the cooling stage was analyzed, as well as the carbon concentration distribution and martensite volume fractions. The accuracy of the simulation was also verified by comparing the experimental data with the simulated result of the distortion and microstructure. A reliable basis is provided for predicting the distortion mechanism of gear steels in carburizing and quenching.

Acicular martensite induced superior strength-ductility combination in a 20Cr2Ni2MoV steel

The super combination of ultrahigh strength and good ductility is in high demand for the gear steels. Here we improved the mechanical properties of a micro-alloyed gear steel through the fabrication route composed of carburizing and re-quenching. The results showed that the different re-quenching temperatures significantly affect not only the sizes and morphologies of precipitates and martensite but also the ratio of low-angle grain boundaries to high-angle grain boundaries as well as dislocation densities. The maximum ultimate tensile strength up to 2420 MPa was obtained after re-quenching at 780 °C, while the steel re-quenched at 820 °C exhibited the highest yield strength of 1500 MPa despite of the decreased ultimate tensile strength of 2145 MPa. Especially, a good ductility of 8.5% was maintained in the carburized steel processed after 780 °C re-quenching. The spherically nanosized precipitates together with the high ratio of low-angle grain boundaries were beneficial for the simultaneously improved strength and ductility.

The effect of TiN precipitated particles on the austenite grain and hardenability of 20CrMnTi gear steel

ABSTRACT To study the effect of TiN precipitated particles on two end-quenching specimens of 20CrMnTi steel with different hardenability characteristics, the peripheral hardness of the samples is tested. The microstructures, austenite grain structures, characteristics of TiN particles are analyzed. The larger the grain size, the higher the martensite fraction and the higher the hardness. More uniform and more refined grains reduce the maximum peripheral hardness difference of end-quenched samples from 9.1HRC to 3.3HRC. The precipitation of as much fine TiN as possible in the solid phase plays an important role in obtaining more refined and more uniform austenite grains. TiN precipitates less during solidification and more in the solid phase in the sample with better hardenability, promoting grain refinement and more uniform peripheral hardness. In production, with a specific mass fraction of Ti, effectively inhibiting the precipitation of coarse TiN during solidification is the key to control narrow hardenability bandwidth for 20CrMnTi gear steel.

Static and vibration analysis of functionally graded gears

In this study, static and vibration analyses of functionally graded gears (FGGs) have been performed using the FE-based numerical method. The gear body is divided into uniform thickness homogeneous sub-domains conforming to the gear tooth profile. Material properties vary in the radial direction according to power-law gradation with metal at the innermost and ceramic at the outermost surface. Three FG gear cases are studied, assuming the gradation for the entire body, radially outer half, and the outer quarter of the gear. Parameters like root stress, weight reduction, tooth stiffness, and natural frequencies of FG and bi-material gears are studied and presented with steel gear as a reference. For FGGs, root stress increases and the weight of the gear reduces with an increase in gradient index. It has been observed that the maximum difference in the fundamental frequencies of FGGs and steel gears is 12% compared to 25% for bi-metal gears.

Dynamic response analysis of functionally graded gears

In this study, dynamic response analysis of functionally graded gears (FGGs) has been performed using a 6-degree of freedom dynamic model. The pinion and gear are divided into homogeneous sub-domains of uniform thickness, conforming to the gear tooth profile. The material composition varies radially according to power-law gradation with metal at the innermost and ceramic at the outermost surface. Mesh stiffness and transmission error of FGGs have been evaluated using a finite-element-based numerical method employing contact analysis. Results show that for considered values of gradient index (GI), FGGs show a 10% to 50% reduction in mesh stiffness with a 30% to 60% reduction in weight compared to steel gears of exact specifications. Also, FGGs show a 4% to 12% reduction in dynamic factor and a 9% to 17% reduction in peak-to-peak displacement amplitude than steel gears over the selected values of GI.

Microstructure Characteristics and Wear Performance of a Carburizing Bainitic Ferrite + Martensite Si/Al-Rich Gear Steel

In this paper, a new low-carbon alloy gear steel is designed via Si/Al alloying. The carburizing and austempering, at a temperature slightly higher than the martensitic transformation point (Ms) of the surface and much lower than the Ms of the core, for different times, were carried out on the newly designed gear steel. After heat treatment, a series of different microstructures (superfine bainitic ferrite + retained austenite, superfine bainitic ferrite + martensite + retained austenite, and martensite + retained austenite) were obtained on the surface, whilst the low-carbon lath martensitic microstructure was obtained in the core. The microstructure of the surface was examined using optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The phase composition was analyzed using X-ray diffraction (XRD). The hardness and wear resistance of the surface as well as the hardness distribution of carburizing layer of the samples with different microstructures were studied. The results show that the Si/Al-rich gear steel, after carburizing and austempering at 200 °C for 8 h, not only has excellent mechanical properties but also has high wear resistance, which meets the technical requirements of heavy-duty gear steel. The research work in this paper can provide a data reference for the application of carburized steel with mixed microstructures of bainitic ferrite and martensite in the design of heavy-duty gear.

Primary precipitates with four-layer structure in Mg-Ca treating gear steel

The behavior of primary phases precipitating during solidification in Ti-S containing gear steel was studied, when Mg-Ca treatment was applied to control the sulfide shape. Primary phases with four-layer structure were found in billet, and the characteristics and generating mechanism were studied by calculation of software FactSage and Thermo-Calc. The results showed that the core in the four-layer-structure phase was MgO wrapped by pure CaS both of which were generated in liquid steel before solidification. The outer phase in order was TiN and MnS which precipitated during solidification process succesively. The improvement effect of sulfide by Mg-Ca treatment would be significantly effected by the primary nitride.

Comparison of Methods for Calculating the Heating of Plastic Gears

Accurate determination of heating is significantly more important for plastic gears, even in simpler applications than for steel gears. These gears are often used without lubrication or with a single, lifetime lubrication. The load capacity of materials is significantly affected by temperature, even in ordinary cases. Even today, heating is often calculated for different heating cases according to the calculation method of VDI 2545 or the modified VDI 2545, although these methods have been proven to be obsolete. Since the main direction of our research is to explore the design features of drive units equipped with small plastic gears, we tried to assess in advance, using a series of calculations, how much difference there is between the results obtained with obsolete and modern calculation methods. In this article, we present the basic characteristics of the calculation methods and compare the results obtained for some m>1 mm and m<0,5 mm gears.

Microstructure impact on the machining of two gear steels. Part 1: Derivation of effective flow curves

A multiscale approach is presented here to investigate the effect of the ferrite-pearlite microstructure after annealing on the subsequent machining process of steel gears. The case-hardening steel 18CrNiMo7-6 and a cost efficient alternative with reduced Cr and Ni content have been studied. After detailed microstructure characterization, three different scales are defined: the nano-scale with pearlite, built of ferrite-cementite bi-lamellas, the micro-scale, which corresponds to a RVE of the ferrite/pearlite microstructure and the macro-scale. In order to derive the effective flow behaviour of pearlite, virtual uniaxial tensile and shear tests of the ferrite/cementite bi-lamella are performed at the nanoscale. The flow behaviour of the ferrite phase is described there by an extension of the Kocks-Mecking law suitable for large machining strains. Moreover, at the nanoscale, the effective flow curve of the ferrite matrix having either small MnS or NbC inclusions is determined. At the microscale, effective flow curves for both steel grades are derived from virtual tests on 3D RVE's of both steel microstructures and compared with experimental measurements.

On the Chemical Composition of Non-Metallic Inclusions in Ultra-Clean Gear Steels

Abstract Fisheye failures due to non-metallic inclusions can lead to the reduced endurance fatigue limit of case-hardened, shot-peened gears. Various approaches for preventing crack initiation at non-metallic inclusions are conceivable. As one approach, steel manufacturers have invested much effort in the steelmaking process to produce ultra-clean gear steels. Such ultra-clean gear steels were investigated in a gear research project. For each steel batch, different measures were taken during steel production to avoid or bind off non-metallic inclusions in order to reduce or even suppress the harmful effect of the non-metallic inclusions. However, the influence of the measures taken on the chemical composition of non-metallic inclusions in such ultra-clean gear steels is not investigated in the gear industry in detail so far. Therefore, in the framework of this publication, the chemical composition of non-metallic inclusions in ultra-clean gear steels is investigated. Furthermore, a comparison of the chemical analysis with the crack-initiating inclusions from the experimental investigations is performed.

Improving the wear resistance of heavy-duty gear steels by cyclic carburizing

In this work, we adopted a cyclic carburizing method to improve the wear resistance and antifriction properties of 17CrNiMo6 heavy-gear steel. The possibility of achieving cyclic carburization is described in terms of diffusion thermodynamics and carburization kinetics. The results indicated that the phase transmation during cyclic carburization can significantly refine the microstructure, and the change of carbon concentration provides a guarantee for cyclic carburization to obtain a high hardness, dense and thick hardened layer. As the number of cycles increases, the friction coefficient of surface wear of the carburized layer at room temperature gradually decreases, while the wear rate after surface removal of 50 µm is significantly diminished and wear rate was the minimum (0.69 ×10-14 mm3N-1m-1) after 4 cycles carburization. At 100 ℃, the oxidation of wear scars on the surface of carburized layer increases. In the case of oil lubrication, the friction coefficient of the cyclic carburized layer was clearly declined. Wear analysis revealed that the wear mechanism of the circulating carburized layer was mainly abrasive wear and oxidation wear with a small amount of adhesive wear.

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.

Evaluating data-driven algorithms for predicting mechanical properties with small datasets: A case study on gear steel hardenability

Data-driven algorithms for predicting mechanical properties with small datasets are evaluated in a case study on gear steel hardenability. The limitations of current data-driven algorithms and empirical models are identified. Challenges in analysing small datasets are discussed, and solution is proposed to handle small datasets with multiple variables. Gaussian methods in combination with novel predictive algorithms are utilized to overcome the challenges in analysing gear steel hardenability data and to gain insight into alloying elements interaction and structure homogeneity. The gained fundamental knowledge integrated with machine learning is shown to be superior to the empirical equations in predicting hardenability. Metallurgical-property relationships between chemistry, sample size, and hardness are predicted via two optimized machine learning algorithms: neural networks (NNs) and extreme gradient boosting (XGboost). A comparison is drawn between all algorithms, evaluating their performance based on small data sets. The results reveal that XGboost has the highest potential for predicting hardenability using small datasets with class imbalance and large inhomogeneity issues.