Results

Summary of the work performed during the 1st reporting period (Aug. 2021 - May 2023)

 

During the 1st reporting period, the following research activities were conducted:

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• The manufacturing of industrial casts of one reference carburizing steel grade and three other steels suitable for alternative surface hardening treatments, with the aim to produce gears working at rotating speeds as high as 15,000 rpm without suffering relevant fatigue and scuffing problems. Besides the reference 27MnCr5, an alternative carburizing steel for a new carburizing and quenching process (20NiCrMo2) and, for nitriding and nitrocarburizing, one Q&T steel (40CrMoBi4) and one F-P microalloyed steel (36MnSiV4) have been casted, hot rolled and characterized.​

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• In order to guarantee that the modification of the traditional route of gear manufacturing does not affect drastically the total cost of the component, the main process steps were evaluated. Hot forgeability has been assessed, showing a good performance for all the studied steels, although slightly lower for the Q&T steel 40CrMoBi4. Rough and hard machinability have been evaluated, showing opposite results due to the different hardness of steels for each condition of machining.​

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• Optimization of the surface hardening parameters has been done to achieve the specified surface hardness and case hardening depth (CHD) for each steel and heat treatment (conventional carburizing and HPGQ, LPC+WSQ, nitriding and nitrocarburizing). Besides the difficulties to achieve the narrow window for the Stellantis specification of CHD for the four steels, the optimization of the process LPC+WSQ has been longer than expected due to the random occurrence of quenching microcracks.​

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• A detailed metallurgical analysis of the hardened case has been carried out for the six combinations of steel grades and heat treatments. Fatigue and tribological tests have started, but they accumulate a delay of six months due to the drawbacks to optimize the surface hardening treatments.​​

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• Dissemination activities include the creation of a project website (www.topgear-project.eu), which is regularly updated with TOPGEAR events and outcomes.

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Project in brief

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Summary of the context and overall objectives of the project

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The gradual electrification is transforming the working conditions of automobile transmissions and consequently, the loading and the types of gear failure. Maximum gear speed is changing from 5,000 rpm in an internal combustion car transmission up to 20,000 rpm in a battery electric vehicle. These differences provoke important design differences. A BEV gearbox contains few gears than an ICE one, but their size is bigger. As the rotating speed is much higher and the torque is smaller, BEV gears show minor problems regarding surface and bending fatigue but, on the contrary, they present possible scuffing problems in the absence of lubrication due to high rotation speeds. So, BEV gears have greater diameter, width, and teeth height. Moreover, some designs avoid broaching the central hole and use a honed hole for BEV gears. Some preliminary trials, conducted at CRF, demonstrated that, in gears produced through the conventional process, when the rotating speed is increased above 8,000 rpm, a drastic augmentation of fatigue and scuffing problems take place causing the component catastrophic failure. Consequently, a new case hardening route should be developed to allow the fabrication of the gears for future HEV/EV.

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The main goal of the TOPGEAR project is:

• To develop new manufacturing routes, combining new steels and surface hardening techniques, which allow to produce the new generation of gears for future HEV/EV that must work at 15,000 rpm and actual torque values avoiding fatigue and scuffing failures.

• Besides, the following sub-objectives are addressed:

• To conduct an integral Life Cycle Assessment (LCA) combining the traditional approach with other innovative perspectives in order to evaluate all the relevant factors related to the industrial implementation of the conventional and new manufacturing routes of future gears for HEV/EV.

• To evaluate, for the first time, the effect of the new carburizing and quenching process (combining LPC with WMQ) on the formation of compressive stresses and, therefore, on the gears in-service performance.

• Generation of very useful information, not available in the current State of the Art, related to the performance of gears working at speeds of 15,000 rpm.

• Determination of the benefits achieved from the application of the proposed innovative steel solutions: the use of F-P steels in nitriding and nitrocarburizing to avoid the Q&T treatment or the Bi addition to Q&T steels to enhance their machinability.

• Deep analysis of the benefits that the implementation of the proposed fabrication routes would bring to the automotive industry.

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TOPGEAR is the first project in considering nitriding and nitrocarburizing as alternatives to carburizing in the automotive gears manufacturing. Apart from these two techniques, a recently developed carburizing and quenching process is also evaluated for the first time. TOPGEAR is the first European project in evaluating the fatigue performance of gears surface hardened through different techniques alternatives to traditional carburizing. Besides, this fatigue assessment will be conducted at very demanding conditions (combining actual torque and very high rotating speeds), which will allow to reproduce the performance of gears for future HEV/EV, and thus to define the most adequate combination of steel and surface hardening technique to guarantee a safe in-service behaviour.

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Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

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During the reporting period, most of the planned tasks corresponding to WP1 to WP4 have been carried out as scheduled. The research activities are detailed below.

It was planned the manufacturing of industrial casts of one reference carburizing steel grade and three other steels suitable for alternative surface hardening treatments, with the aim to produce gears working at rotating speeds as high as 15,000 rpm without suffering relevant fatigue and scuffing problems. Besides the reference 27MnCr5, an alternative carburizing steel for the new carburizing and quenching process (20NiCrMo2) and, for nitriding and nitrocarburizing, one Q&T steel (40CrMoBi4) and one F-P microalloyed steel (36MnSiV4) have been casted, hot rolled and characterized.

In order to guarantee that the modification of the traditional route of gear manufacturing does not affect drastically the total cost of the component, the main process steps were evaluated. Hot forgeability has been assessed, showing a good performance for all the studied steels, although slightly lower for the Q&T steel 40CrMoBi4. Rough and hard machinability have been evaluated, showing opposite results due to the different hardness of steels for each condition of machining.

Optimization of the surface hardening parameters has been done to achieve the specified surface hardness and case hardening depth (CHD) for each steel and heat treatment (conventional carburizing and HPGQ, LPC+WSQ, nitriding and nitrocarburizing). Besides the difficulties to achieve the narrow window for the Stellantis specification of CHD for the four steels, the optimization of the process LPC+WSQ has been longer than expected due to the random occurrence of quenching microcracks.

A detailed metallurgical analysis of the hardened case has been carried out for the six combinations of steel grades and heat treatments. Fatigue and tribological tests have started, but they accumulate a delay of six months due to the drawbacks to optimize the surface hardening treatments.

Dissemination activities include the creation of a project website (www.topgear-project.eu), which is regularly updated with TOPGEAR events and outcomes.

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Progress beyond the state of the art, expected results until the end of the project and potential impacts

 

Awaited beneficial aspects are:

• Manufacturing of gears able to work at 15,000 rpm. The results of the project will be the baseline for the industrial implementation of a new manufacturing process through which the next generation of gears for HEV/EV can be produced. TOPGEAR will provide a solution to one of the main problems found by HEV/EV manufacturers: how to enhance the gears properties in order to assure a safe functioning at high rotating speeds.

• Minimization of the teeth distortions. The low T surface hardening processes (nitriding and nitrocarburizing) and the new carburizing and quenching process with single-layer-treatment proposed in the project will minimize the teeth distortions, which is awaited to allow the simplification of the finishing operation conducted to correct them, leading to notable economic savings.

• Avoidance of the Q&T before the nitriding/nitrocarburizing processes. The use of F-P microalloyed steels for nitriding/nitrocarburizing will avoid Q&T, which is an expensive and pollutant heat treatment, leading to a simplification of the manufacturing process. Furthermore, the machining costs will be reduced thanks to the better machinability of the F-P compared to that of the martensite.

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As reflected in the project main goal, the principal application of the results obtained in TOPGEAR is related to gears of future HEV/EV. Only considering this use and taking into account the forecasts regarding the augmentation of the fleet of HEV/EV and that in each of this vehicle at least two gears working at high speed will be found, the techniques considered in this project could be directly employed to produce more than 20 million of gears per year in 2025 and more than 112 million of gears per year in 2040.

Although the solutions (combination of steels and surface hardening techniques) proposed in this project will be specifically tailored for the aforementioned application, they could be directly applied to any other gear which works at elevated speeds (> 5,000 rpm), i.e., for the aerospace industry, or to any other surface hardened component whose surface properties must be improved. Therefore, the transfer of the knowledge generated in the project will be general, not being limited to only one specific sector or application.

A manufacturing route specific to produce gears for future HEV/EV will allow increasing the fleet of these ecofriendly vehicles essential to reach the strict CO2 emission targets established by the EU.

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