Results

 

Key exploitable results identified through the project

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​Several outcomes have been obtained from the TOPGEAR project activities, always considering the limitations in number of steels, thermal treatments and microstructures, conditions of surface heat treatments, limited amount of tests and so. However, some clear conclusions have been attained, and even bad or poor results might open new research lines in the near future.

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The four studied steels (27MnCr5, 20CrNiMo2, 40CrMoBi4 and 36MnVSi4) are forgeable and machinable. Annealed grades (27MnCr5 and 20CrNiMo2) showed higher soft machinability, but once hardened the result is the opposite. Bismuth addition caused a slight drop in the hot formability of the 40CrMo4 steel, but that drawback is compensated by the improvement of its machinability. The 36MnVSi4 microalloyed steel shows a good hot formability and takes advantage of its ferrite-pearlite microstructure to keep an acceptable machinability. Clearly hardness is the main factor influencing steel machinability.

It was extremely difficult to achieve the STELLANTIS specifications for the core and the case of the hardened specimens. That was particularly difficult for the nitriding/nitrocarburizing steels, as the core hardness depended on the supplied material and was not modified by the surface treatments. For instance, 36MnVSi4 was much softer (270 HV) than specified (300-330 HV). Adjustment of the hardness profile was also complicate for the LPC+WSQ 20CiNiMo2 due to its low hardenability, although the higher severity of the water spray quenching led to a satisfactory core hardness. These discrepancies might have influenced the later characterization.

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The comparison of the four steel grades (27MnCr5, 20NiCrMo2, 40CrMoBi4 and 36MnSiV4) and four hardening treatments (carburizing, including both reference LPC+HPGQ and novel LPC+WSQ, nitriding and nitrocarburizing) by bending fatigue and tribological testing showed the following outcomes:

• The microalloyed steel is inadequate for BEV gears due to its brittle behaviour after case hardening.

• Nitrocarburizing does not improve significantly the steel performance compared to the carburizing reference.

• The innovative LPC+WSQ surface hardening process needs further optimization to improve the profile of residual stresses, as it was much lower than expected.

• Nitriding can be an alternative to carburizing in some BEV gear applications.

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Unexpectedly, water spray quenching led to a poorer compressive residual stress profile than the other hardening treatments, even though surface and core hardness were much higher than gas quenching. The causes are not clear because the interaction between the quenching parameters and the steel phase transformations on the residual stresses is too complex. The advantages of this technology regarding cooling homogeneity and quench severity respect to water or gas quenching respectively are clear and may bring huge benefits in certain applications. Nevertheless, it is necessary to solve the residual stress issues before its industrialization. This should be a matter of deep research in a short term.

 

Weighting the different parameters, 40CrMoBi4 steel plus nitriding was chosen as an alternative to conventional carburizing for upscaling to gear testing.

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​​​​​Machining of testing gears was OK. Nitrided gears showed much lower distortions than carburized ones.

Gear tooth bending fatigue showed opposite results to previous bending fatigue tests, having 27MnCr5 carburized gears near a 20% higher fatigue limit than nitrided 40CrMoBi4 gears. On the contrary, 27MnCr5 carburized gears showed very early micropitting damage. Failure mode was also different, as 40CrMoBi4 gear presented pitting damage, while 27MnCr5 gear does not fail due to pitting, but to extensive micropitting. Scuffing test conditions were not severe enough to cause damage on both carburized and nitrided gears. The test rig should be modified completely to increase torque and promote earlier damage.

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The Life Cycle Assessment (LCA) was performed using steelmaking and gear manufacturing data used in the project. The comprehensive assessment across all process steps, including steel production, surface hardening, and gear fabrication, demonstrates that the novel 40CrMoBi4 route offers substantial environmental benefits. The novel route incurs slightly higher overall production costs.

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In addition to all the solutions developed, the following two main conclusions have been achieved in the TOPGEAR project:

• The gear failure modes at very high speeds differ from low-medium speeds and might lead to unexpected early failures. This is important because transmission technologies for battery electric vehicles are going beyond the evolution of gear testing capabilities.

• There are valid technical and economic alternatives to traditional carburizing processes for high-speed gears. With an adequate selection of the steel grade, an optimization of the manufacturing processes and the surface hardening treatments and a holistic approach, the performance and the costs might be similar or even better.

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