Steel Gears Research Articles

Enhancing tribological properties of 18CrNiMo7-6 through grain refinement incorporating Al2O3 particles

Wind turbine gears are critical components within the internal gearbox of the wind turbines. However, the vulnerability of gears to wear adversely affects their operational lifespan. Here, we propose a novel approach utilizing ultrasonic shot peening to incorporate Al2O3 particles onto the surface of the 18CrNiMo7–6, termed the USG treatment. The tribological test shows that the coefficient of friction and wear rate of the USG-treated sample decreased by 14.4% and 58.3% respectively, as compared to the untreated sample. Experiments and detailed analysis reveal that 18CrNiMo7–6 not only undergoes grain refinement on its surface but also forms a wear-resistant layer enriched with Al2O3. The findings here provide a new insight into the design of wear-resistant layers on gear steel surfaces.

The Effect of Cooling Rate on the Banded Structure of 20CrMnTi Gear Steel Based on C Diffusion and Partitioning Analysis

This study, through experimental analysis and numerical calculations, not only identifies the critical cooling rate for eliminating banded structures in 20CrMnTi gear steel but also analyzes the impact of cooling rate on banded structures, including the mechanisms related to element distribution and interface migration. The results indicate that the fundamental cause of banded structure formation is the presence of chemical segregation bands, and increasing the cooling rate leads to a reduction in the grade of ferrite/pearlite banded structures, with a critical cooling rate of 0.55 °C s−1. Bainite and martensite banded structures are observed in samples cooled at rates of 3 and 5.8 °C s−1, respectively. As the cooling rate increases from 0.07 to 1 °C s−1. The ferrite phase transformation time in the solute‐poor zone is shortened from 171 to 12 s within the ferrite phase transformation temperature difference (Δ) between the poor and rich solute zones. The concentration difference of carbon (C) on both sides of the α/γ phase interface increases from 0.350 mass% to 0.431 mass%, and the diffusion distance of C decreases from 34.38 to 7.57 μm. When the diffusion distance of C is less than half of the width of the solute‐rich banding (12.5 μm), the ferrite banded structure disappears.

Investigation on wear behavior for SUS420 steel gear based on discrete laser surface melting

Gears are critical components that ensure the stable operation of a gearbox. In the era of mechanical equipment advancing towards higher speeds and heavier loads, more stringent requirements have been set for the wear performance of existing gears. Drawing inspiration from observations of natural organisms, particularly scallops, which have developed unique microstructure and excellent wear properties through long-term evolution, this paper focuses on the application of discrete laser surface melting (DLSM for short) technology to prepare units on the surface of SUS420 steel gears, simulating the radial rib structures found on scallop surfaces. The laser processing was carried out with varying defocusing amount and distribution types of discrete laser surface melted (DLSMed for short) units. The study encompassed microstructure observations, microhardness and residual stress testing, and bench wear experiments on DLSMed gears. The experimental findings reveal that a crack-free and porous-free hardened layer has formed on the tooth surface. The hardened layer consisted of the melting zone, heat-affected zone, and substrate, resulting in a surface structure characterized by alternating soft and hard phases. Comparing the wear behavior of as-received gear to DLSMed gears, it is evident that the wear damage on DLSMed gears is less pronounced. The DLSM treatment had altered the wear mechanism of the gears. During the wear test, the quantity of wear debris and iron (Fe) concentration in DLSMed gears consistently remained lower than that of as-received gears, indicating superior wear resistance in DLSMed gears. Furthermore, an increase in the defocus amount led to a deterioration in the wear performance of DLSMed gears. Notably, transverse units exhibited better efficacy in enhancing the wear resistance of gears compared to longitudinal units. There exists a positive correlation between the percentage of DLSMed unit area on the gear surface and the wear behavior of the gear.

Fully ceramic versus steel gears: Potential, feasibility and challenges

The most common gear strength calculations performed in accordance with ISO/TC 60/SC 2 include surface durability and root bending strength. Typical geared power transmissions are characterized by high Hertzian contact stresses (surface durability) on the flanks – frequently in the order of 103 MPa – versus almost half an order magnitude lower first principal stresses on the tooth root (tooth bending strength). Therefore, the use of high strength special steel alloys is almost mandatory, especially in high-end applications, mainly to withstand the high contact stress values. Ceramic materials are typically characterized by high compressive strength versus very low flexural strength that at first glance makes them inefficient for gear transmissions since they are unlikely to withstand the tensile root stresses. Nevertheless, ceramic materials possess a number of advantages that makes them potential candidates for geared transmissions, such as their exceptional thermal properties, their low density (weight), their excellent wear characteristics and limited lubrication demands. The present work investigates the main challenges of using high-end ceramic gears and attempts to provide an insight on defining the design envelope of such gears to counter the main disadvantages. The increase of the normal pressure angle beyond the 25° limit set by ISO 6336 is primarily investigated, since in minimizes the tensile tooth root stresses, while also increasing the curvature of the contact surfaces without greatly increasing Hertzian stresses. Comparative assessment of the performance of fully ceramic gear designs versus their conventional steel counterparts is performed through quasi-static FEA simulations to set the basic design requirements for such gears and to promote further investigation of the feasibility of their usage in high power density applications.

Comparative tribological performance of a-C:H, ta-C and AlCrSiN coatings on 20CrMnTi gear steel under dry sliding condition

PurposeThis study aims to investigate the surface modification on 20CrMnTi gear steel individually treated by diamond-like carbon films and nitride coatings.Design/methodology/approachFor this purpose, the mechanical properties of a-C:H, ta-C and AlCrSiN coatings are characterized by nano-indentation and scratch tests. The friction and wear behaviors of these three coatings are evaluated by ball-on-disc tribological experiments under dry contact conditions.FindingsThe results show that the a-C:H coating has the highest coating-substrate adhesion strength (495 mN) and the smoothest surface (Ra is about 0.045 µm) compared with the other two coatings. The AlCrSiN coating shows the highest mean coefficient of friction (COF), whereas the ta-C coating exhibits the lowest one (steady at about 0.16). The carbon-based coatings possess excellent self-lubricating properties compared with nitride ceramic ones, which effectively reduce the COF by about 64%. The major failure mode of carbon-based coatings in dry contact is slight abrasive wear. The damage of AlCrSiN coating is mainly adhesive wear and abrasive wear.Originality/valueIt is suggested that the carbon-based film can effectively improve the friction-reducing and wear resistance performance of the gear steel surface, which has a promising application prospect in the mechanical transmission field.Peer reviewThe peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-05-2023-0129/

Effects of Optimal Tooth Microgeometry Modifications on Static and Dynamic Transmission Errors of Hybrid Spur Gear Drivetrains

A hybrid gear concept that combines a metallic outer rim of gear teeth with a composite web to reduce drivetrain weight was evaluated for impact of tooth microgeometry modifications on transmission error. Control of transmission error through tooth microgeometry modification is important for control of noise and vibrations generated by a drivetrain. The added flexibility of hybrid gears over steel gears brings to question the performance of hybrid over conventional gears relative to their dynamic transmission error and resulting noise levels. Previously developed drivetrain models featuring hybrid spur gears were used to determine optimal tooth microgeometry modifications that minimized peak-to-peak transmission error. Static and dynamic transmission errors were then calculated using the optimal microgeometries and compared to results for a similarly optimized all-steel drivetrain. From the results, it appears that the use of hybrid gears will not negatively affect vibration performance for low- and medium-speed applications, as hybrid gear models predicted similar transmission errors to their all-steel counterparts. At higher speeds, drivetrains featuring hybrid gears were predicted to have significantly different transmission errors, but whether this difference was an improvement or detriment is design and speed-dependent. Therefore, careful design is necessary for high-speed hybrid gears.

Improvement of the wear resistance of 20CrMnTi steel gear by discrete laser surface melting

This paper studied the effect of discrete laser surface melting on the wear resistance of gears. The radial ribs on the surface of intertidal shellfish had the function of anti-abrasion. The discrete laser surface melted (DLSMed) units were processed on the gear surface, and they looked like radial ribs.The microstructure and wear morphology of the DLSMed unit were observed under a scanning electron microscope. The phase composition and residual stress of the DLSMed unit were analyzed through a X-ray diffractometer. The hardness of the DLSMed unit was measured with a microhardness tester. The roughness of the DLSMed unit was measured with a surface topography meter. The wear test of gear was carried out with a Forschungsstelle für Zahnräder und Getriebebau (FZG) testing machine. The lubricating oil analysis technology was used as the evaluation basis of gear wear performance. The experimental results showed that discrete laser surface melting could refine the grain on the gear surface, improve the hardness of the gear, and form residual compressive stress on the gear surface. The cross-sectional area of the DLSMed unit gradually decreased as the laser scanning speed increased. As the cross-sectional area of the DLSMed unit decreased, the wear resistance of the gear also deteriorated. Finally, the wear mechanism of the DLSMed gear was discussed from many aspects.

Variation of hydrogen content in gear steel after carburizing and heat treatment and its effect on fatigue properties

In this study, the variation of hydrogen contents in gear steel 17Cr2Ni2MoVNb after gas carburizing, low-pressure carburizing and heat treatment was investigated by means of thermal desorption spectrometry (TDS) analysis. The results showed that there was an additional peak at around 255 ℃ in the TDS curves for the specimens after gas carburizing and quenching in comparison with that for the specimen after low-pressure carburizing, which was absorbed presumably from the high temperature environmental gas. The results of fatigue tests showed that the fatigue limits for the specimens after gas carburizing with the additional hydrogen desorption peak and after low-pressure carburizing without the additional hydrogen desorption peak were both about 980 MPa. It could be attributed to the fact that the absorbed hydrogen content in the gas-carburized specimens was decreased from 0.084 mass ppm to 0.057 mass ppm after fatigue tests, which was too small to have evident effect on the fatigue properties of the steel.

A computational model of wear evolution for shot peened surfaces of gear steel

In mechanical engineering, shot peening is frequently used to reinforce the surfaces of some crucial components. Currently, there are no models available which can accurately predict the long-term wear behavior of surfaces treated with shot peening. This research proposes a computational model and technique for simulating the evolution of wear on gear steels with shot-peened surfaces in a ball-on-flat contact. Based on the fundamental Archard’ s equation, a wear equation is derived from the perspective of a contacting point on the static flat to represent the wear loss. The model takes into account the effects of abrasive particles and variations in hardness along the vertical direction by introducing a scaling factor in the wear equation and stratification of the flat material, respectively. The proposed model and developed method are validated through experiments and can be utilized to predict the wear evolution of shot peened surfaces on gear steels under the application of various loads and friction duration. Furthermore, the model and method can be easily extended to other materials as well.

Fine Blanking of Austenitic Stainless Steel Gears Using Carbon-Supersaturated High-Speed Steel Tools

Austenitic stainless steel gears were fabricated via the fine blanking process that can be used for mass production. A carbon-supersaturated (CS)-matrix high-speed steel punch was prepared to minimize the adhesive and abrasive wear damage. Its edge profile was tailored and finished to control the local metal flow around the punch edges and edge corners. This CS punch was utilized in fine blanking the AISI304 austenitic stainless steel gears. Ball-on-disc (BOD) testing was first employed to describe the frictional behavior of the CS tool steel disc against the AISI304 stainless steel balls. SEM-EDX analysis on the wear track revealed that a free-carbon tribofilm was formed in situ in the wear track to prevent adhesive wear via galling on the tool steel disc. No significant adhesive or abrasive wear was detected on the punch edges and punch edge corners after continuously fine blanking with 50 strokes. AISI304 gears were produced to have fully burnished surfaces. Their pitches, widths and circles were measured to evaluate their gear-grade balancing during the fine blanking process. The stabilized gear-grade balancing in JIS-9 to JIS-10 grades was attained for these as-blanked AISI304 gears without finishing processes.

Tooth root stress reduction of plastic gears with biological inspired shape optimization

The geometry of plastic gears used today is usually based on conventional steel gears, which are bound to the restrictions of the machining production of gears. The injection molding process provides more design freedom. This paper presents an approach to reduce the tooth root stress of plastic gears using a biologically inspired shape optimization method called CAO (Computer-Aided Optimization). CAO imitates the biological growth behavior from nature, such as bones or trees, which adapt to the load. It strengthens more heavily loaded areas of the surface and weakens less heavily loaded areas, resulting in an increase in load-bearing capacity. The simulation method is based on finite element analysis and considers the complex material behavior of short fiber reinforced plastics as well as the inhomogeneous fiber orientation. Object of the study are polymer gears made of polyamide 46 with and without short fiber reinforcement. By using the method presented here, it is possible to reduce the tooth root stress by up to 24% compared to a trochoidal tooth root with maximum rounding.

Study on electrochemical machining of involute internal spline of Cr-Co-Mo-Ni high-hardness gear steel

Study on electrochemical machining of involute internal spline of Cr-Co-Mo-Ni high-hardness gear steel

Study on the Influences of Adding Rare Earth Ce on the Precipitation Behaviors of TiN Inclusions in 20CrMnTi.

The morphologies and sizes of TiN inclusions in gear steel 20CrMnTi have a significant impact on its service performance. This paper selects rare earth Ce to modify TiN inclusions in 20CrMnTi. The inclusions are analyzed by SEM (scanning electron microscope), EBSD (electron back-scattered diffraction), EDS (energy disperse spectroscopy), and OTS statistical software, and Thermo-Calc software is used to calculate the inclusion formations. The inclusions of MgAlO4-Ce and CeAlO3 can be formed when rare earth Ce is added into 20CrMnTi, which becomes TiN nucleation core after precipitation. Without the addition of rare earth Ce, square TiN inclusions ranging from 2 to 5 μm account for 60% of the total inclusions in 20CrMnTi. After adding rare earth Ce, the TiN inclusions in 20CrMnTiCe account for 36.7% of the total inclusions. Due to the new phase formations of MgAlO4-TiN and CeAlO3-TiN with sizes less than 2 μm, the titanium-containing inclusions are refined. Fatigue tests are conducted on the steels before and after the addition of Ce. The average fatigue lives of 20CrMnTi do not reach 107 times, and the deviations between the maximum and minimum fatigue lives are great. Large-sized TiN are the main inclusions that affect the fatigue performance of 20CrMnTi. The average fatigue lives of 20CrMnTiCe exceed 107 times, and the deviations of the fatigue lives are smaller than those of 20CrMnTi.

Methodology for Determining the Effective Thickness of the Cemented Layer of Steel

Highly loaded transmission gears are cemented and hardened. An important parameter of the hardened cemented layer is its effective thickness hef . Metal banding and the unavoidable instrumental error in hardness measuring have a great influence on the reliability of hef determination. The purpose of this article was to develop a methodology to improve the reliability of determining of the effective thickness hef of the hardened layer in steel after carburizing and quenching.The value of hef is the distance h from the surface of the product to the hardness zone of 50 HRC. The article substantiates that approximation of hardness change from the distance h to the product surface will allow to obtain a more reliable dependence of hardness change in the investigated area when making hardness measurements in a wider range of distance h. Therefore, to increase the reliability of hef determination, results of the HV0.5 hardness measurement in an extended range of changes in h in the vicinity of the analyzed zone were used. The HV0.5 measurement results are converted to HRC hardness values using the formula recommended by the international standard. The HRC(h) distribution of HRC hardness values in the measurement area is interpolated by a second-degree polynomial which physically correctly reflects the change in metal hardness in the analyzed area. The resulting polynomial is used to determine of the distance hef at which the hardness takes on a value of 50 HRC. The methodology was used to determine the hef of an 18KhGT steel gear wheel after carburizing and quenching. It is shown that results of two independent measurements of the hef sample differ from each other by 0.003 mm. This is significantly less than the permissible error of 0.02 mm of the hef determination according to the standard technique. The error of hef determination is reduced by extending the range of variation of h and statistically valid interpolation of the monotonic change in hardness with the distance from the surface of the item in the measurement area. The developed method of determining the effective thickness hef of the hardened steel layer consists in determining the distribution of its hardness in the expanded vicinity of the hef area, approximating the obtained dependence by a polynomial of the second degree and solving the square equation obtained with its use. The technique provides a significant reduction in the influence of the structural banding of the metal and the inevitable error in measuring hardness on the result of determining the hef . Its application will allow to optimize the cementation regimes of gear wheels to increase their service life.

Applicability of classic analytical calculation approaches for the design of plastic gears

AbstractPlastic gears are produced cost-effectively in high volume but mainly for applications under low load. Current trends are leading to the design and application of plastic gears for more demanding operating conditions. This requires finding the optimum macrogeometry for the application, which is determined in practice using iterative design processes. Numerical and analytical approaches are available for the design of gears. Due to their speed, only the latter are suitable for an iterative design process. However, the approaches currently used in the calculation programs are designed with a focus on steel gears. Their applicability to gears made of plastics, which material properties differ drastically from those made of steel, is therefore only partially possible.The paper presents an investigation into the applicability of the classic analytical calculation approaches of Weber/Banaschek and Schmidt for contact analysis and the load-carrying capacity calculation of plastic gears. For this purpose, the results from analytical calculations are compared with those from numerical calculations. In addition, a comparison is made with typical standards (DIN3990, ISO6336) and guidelines (VDI2736).As a result, the paper provides an initial statement on the transferability of results from steel-focused analytical approaches to gears made of plastic. In addition, calculation results are identified in which particularly large differences occur between the various calculation approaches and which must be considered in the design phase.

Role of Precipitates on the Grain Coarsening of 20CrMnTi Gear Steel during Pseudo-Carburizing

The carburizing period for tool steel could be significantly shortened by operating at a higher carburizing temperature. However, grain coarsening happens during the carburizing process, and then results in the deteriorated surface properties in 20CrMnTi gear steel, especially at an elevated carburizing temperature. The relationships between grain coarsening and the precipitates in the developed 20CrMnTi gear steel during pseudo-carburizing were established by microstructure characterization, precipitate analysis and in-situ observation to clarify the coarsening mechanism. The results manifested the Baker–Nutting orientation relationship between the (Ti, Mo)(C, N) particles and the matrix, and then testified to the redissolution and ripening of the (Ti, Mo)(C, N) precipitates pre-formed in the α phase during the carburizing. Coarsening in austenite grain during the carburizing process was mainly caused by the rapid redissolution and ripening of the (Ti, Mo)(C, N) precipitates, although this occurred in a very short pseudo-carburizing time. The area density of the dispersed unripe (Ti, Mo)(C, N) particles markedly decreased from 0.389% in as-hot rolled gear steel to 0.341%, and then from 0.279% in carburized steels at 970 and 980 °C, respectively. Additionally, the redissolution and ripening of the (Ti, Mo)(C, N) precipitates were accelerated by the elevated carburizing temperature of 980 °C, at which time the growing rate in austenite grains was 2.34 μm/min during the prior 1 min (0.79 μm/min during the prior 3 min at 970 °C). The temperature then decreased to 0.003 μm/min in the subsequent carburizing process. The results obtained our current work reflected that the particles with excellent thermal stability should play important roles in the limitation of grain coarsening during the carburizing process.

Contact fatigue performance and failure mechanisms of Fe-based small-module gears fabricated using powder metallurgy technique

Gears fabricated using powder metallurgy technique (PMT) have the potential to replace traditional wrought gears due to their very high forming efficiency and relatively low cost. However, their practical applications are restricted due to the lack of understanding of their fatigue properties and failure mechanisms. This work investigates the contact fatigue performance and corresponding failure mechanism of a typical Fe-based small-module gears fabricated by PMT. The fatigue tests were conducted under grease lubrication condition, using a power-opened small-module gear testing rig. The experimental results show that the current PMT fabricated small-module gears exhibit contact fatigue properties comparable to those of traditional wrought steel gears. It has been found that the subsurface cracks induced from pores are the dominant cause of spalling of PMT fabricated small-module gears during rolling contact. The cracks tend to propagate between adjacent pores, playing a vital role in the overall fatigue life. Thus, this study suggests that PMT can be a successful method for fabricating Fe-based small-module gears with excellent contact fatigue property if formation of pores during fabrication is well inhibited.

Optimization of laser irradiation process parameters for 40Cr steel based on back propagation neural network and genetic algorithm

40Cr is a significant material in the manufacturing of gears. However, this process is plagued by uneven distribution of quenching layers and excessive residual stress, leading to a decline in the precision of the workpiece, deformation, and cracking. Relying solely on the trial-and-error method in experiments is inadequate for effectively revealing the evolution mechanism of the laser quenching process. This approach is detrimental to improving research and development efficiency. In this study, a multi-field coupled numerical model for the multi-track overlapping laser quenching of the 40Cr gear steel is established. This model quantitatively reveals the coupling evolution laws of the temperature field, stress field, and phase transformation field during the laser quenching. The focus is on calculating the size of the tempering zone under different overlapping rates and conducting correlation analysis between parameters. Based on the artificial neural networks and genetic algorithms, the width and depth of the tempering zone are selected as target values for parameter optimization. The aim is to find the nonlinear relationship between the laser power, spot diameter, scanning speed, laser overlapping rate, and the size of the tempering zone, which can accurately predict the size of the tempering zone. The material microstructure of 40Cr gear steel was characterized by the scanning electron microscope, microhardness tester and friction and wear tester. The results show that the training error and testing error of the output parameters in genetic algorithm optimized back propagation network can accurately predict the size of the tempering zone. It is found that the higher the lap rate, the more uniform the hardness of the phase transformation hardening layer surface, and the tempering softening zone becomes larger. The parts surface is easy to produce defects, which can provide a theoretical basis for optimizing the laser quenching process parameters.

Biomimetics Design of Tooth Root Zone at Cylindrical Gears Profile.

During the last few decades, the requirements for modern machine elements in terms of size reduction, increasing the energy efficiency, and a higher load capacity of standard and non-standard gears have been very prevalent issues. Within these demands, the main goals are the optimization of the gears' tooth profiles, as well as the investigation of new tooth profile designs. The presented design idea is based on the optimal solutions inspired by nature. Special attention is paid to the new design of the tooth root zones of spur gears in order to decrease the stress concentration values and increase the tooth root fatigue resistance. The finite element method is used for stress and strain state calculations, and the particular gear pair is modeled and optimized for these purposes. For tooth root strength analysis, the estimations are based on the theory of critical distances and the stress gradients obtained through finite element analysis. The obtained stress gradients have shown important improvements in the stress distribution in the transition zone optimized by biomimetics. An analysis of the material variation influence is also performed. Based on the investigations of a particular gear pair, a significant stress reduction of about 7% for steel gears and about 10.3% for cast iron gears is obtained for tooth roots optimized by bio-inspired design.

Effect of La Content on Inclusion Morphology in Automobile Sulfur‐Containing Gear Steel SCr420H

The inclusion morphology of sulfur‐containing gear steel with different La addition (0, 0.08, 0.5, and 1%) is investigated in laboratory experiments. The La and S concentrations in steel melt significantly decrease with the holding time after La addition increases, leading to different types of inclusions with specific shapes. Inclusion transformation routes with the holding time increased under different La addition are summarized as follows: 1) 0%La addition: cluster‐like Al2O3 + cluster‐like MnS → irregular block‐like MgAl2O4 + cluster‐like MnS; 2) 0.08%La addition: cluster‐like LaAlO3 + spherical‐like La‐Al‐O‐S‐(MnS) → spherical‐like La‐Al‐O‐S‐(MnS) + spherical‐like La‐Al‐O‐S‐(MgO)‐(MnS) + cluster‐like MnS; 3) 0.5%La addition: flower‐like La‐O + spherical‐like La‐O‐S → spherical‐like La‐O‐S; and 4) 1%La addition: flower‐like La‐O + spherical‐like La‐O‐S → spherical‐like La‐O‐S + cubic‐like La‐O‐S‐N‐As. The results of hot compression suggest that La‐containing inclusions are less prone to the shape change than MnS; moreover, microcracks are formed at the surface of La‐containing inclusions. The nanoindentation test demonstrates that the hardness of La‐O‐S is higher than MnS. The high La addition has a remarkable effect on refractory degradation. For 0.5% and 1% La, dissolved La reacts with the MgAl2O4 phase in the MgO‐MgAl2O4 dual‐phase refractory and changes into LaAlO3.