Research and Development Project - Shot peening of gears treated with the three most common methods of Improvement (cementing, nitriding, carbonitriding)

Dear readers, I’m glad to meet you again on our blog this New Year 2021.

More and more vehicles are using automatic transmissions, gears are still the most important part in transmitting rotation and this is not going to change for a long, long time. So it is smart to learn more about them and also about the surface treatment on gears and pinions.

In the previous posts, we learned about our research and the results achieved on cemented gears, where we saw how quickly a mistake can happen unwillingly – to professionals as well.

The shot peening surprise from the cemented gears did not repeat with the nitrided and carbonitrided gears – we got useful, very good results, which are roughly presented below. Feel free to contact us for details.

Carbo-nitrided gears

Carbo-nitrided gears have the thinnest layer of improvement. A suitable shot peening was performed, which did not cause a significant change in surface roughness. Shot peening with S110 steel shots improved the surface quality (Ra = 0.3 µm), while there was a slight increase (Ra = 0.6 µm) when using S330 shots. In the case of carbo-nitrided gears, the shot peening treatments with the largest shots  (S330 and S330 + Z150) resulted in cracks in the surface layer, which can act as an initiator of damage under dynamic gear loads. The hardness distribution and the effect of shot peening in the case of carbo-nitrided gears are shown in Figure 1. In all cases, shot peening causes an increase in the hardness of the surface layer (by ~20%), with a noticeable hardening effect to a depth of 0.3 mm.  In the case of carbo-nitrided gears, shot peening with S330 and S110 + Z150 is reflected in the improvement of root strength, especially the combination of steel and ceramic shots  S110 + Z150 (Figure 2). In the case of shot peening with S110 + Z150, we achieve significant improvement in the dynamic root strength in both cases, and with S330 in the range of + 30% (Figure 1).

Nitrided gears

In the case of nitrided gears, shot peening led to a thinning of the surface layer, the thickness of which decreased from 8 µm to ~6 µm. Also, in this case, shot peening with larger shots (S330 and HT-S330) led to the formation of surface cracks, and there was also an increase in surface roughness. In the case of S110 steel shots roughness only up to the minimum (Ra = 0.22 – 0.23 µm), and in the case of S330 and hot shot peening to a much higher level (Ra = 0.3 – 0.6 µm). In contrast to cemented gears, in nitrided gears, both cold as in hot shot peening resulted in hardening the surface layer to a depth of 0.25 mm.

The highest hardenings were achieved with the S330 and S330 + Z150 procedures, followed by the small shot  peening (S110 and S110 + Z150) and hot shot peening. In the case of nitrided gears, all shot peening treatments have a positive effect on the dynamic root strength (Figure 3). Treatments with a combination of steel and ceramic shots  (S110 + Z150 and S330 + Z15) have the greatest impact, where the latter significantly improves the dynamic root strength at lower loads.

Even in the case of hot shot peening, this leads to an improvement in the dynamic root strength of the nitrided gears, especially at lower load levels. The improvement ranges between 30 – 40% and an achieved improvement of more than 5x (Figure 4). The most significant improvement is again achieved with the combination of steel and ceramic shots HT-S110 + Z150.

The implementation of an appropriate double peening process can induce a larger compressive residual stress layer (CRSL), and slightly reduce the surface roughness.

Conclusion:

• Almen intensity was set-up on the following values: 7A for S110, 14A for S330, and 11N for Z150 (N type of strips was used to measure intensity in dual-peening with ceramic beads). In the case of cemented gears, hot shot peening leads to a decrease in the hardness of the gear surface, while in the case of nitrided gears it increases hardness of the surface to a lesser extent than cold shot peening.
• Shot peening also increases the surface roughness of the gears, especially in the case of hot shot peening and the use of larger steel shots . On the other hand, shot peening only leads to surface damage in the case of peening with larger steel shots (S330), most likely due to excessive energy input.
• A comparison of the dynamic root strength of gears shows that the highest strength is achieved by cemented gears, followed by carbonitrided gears, while nitrided gears achieve only a tenth of the dynamic strength of cemented gears. On the other hand, in the case of cemented gears, all subsequent cold shot peening treatments, with the exception of S110 + Z150, are reflected in up to a 50% deterioration in dynamic root strength.
• Shot peening treatment with S110 + Z150 on cemented gears reflected in significantly better root strength, especially at lower loads, as the teeth can withstand over 300,000 loading cycles without breakage. In hot shot peening, the deterioration of the dynamic root strength is up to 2 times.
• In the case of carbonitrided gears, the shot peening processes S110 + Z150 and S330 lead to an improvement in root strength, especially the combination of the use of steel and ceramic shots , where several improvements are achieved.
• In contrast, with nitrated gears, with all shot peening procedures, both cold and hot, a significant improvement in the dynamic root strength of the gears is achieved. Again, they have the greatest impact on processing, where shot peening with steel shots is followed by shot peening with ceramic shots (S110 + Z150 and S330 + Z150).
• The combination of shot peening and subsequent surface polishing does not have a significant effect on the dynamic root strength of the gears, which applies to all three chemo-thermal treatment processes. A better effect is achieved with subsequent shot peening with ceramic shots (S330 + Z150). These results were expected, since the polishing does not affect the hardening of the material, but is welcome as less friction means easier sliding and a softer, and above all, a smoother and quieter run of the transmission. The only downside is the slow polishing process and its’ associated costs.
• From the results we can conclude that the most optimal treatment is a combination of cold shot peening with steel and ceramic shots (S110 + Z150), which, regardless of chemo-thermic treatment, is reflected in the improvement of the dynamic root strength of the gears. On the other hand, the shot peening process has the greatest effect on improving root strength in the case of nitrided gears, and in the case of cemented and carbonitrided gears in most cases it is slightly negative. This shows that the thinner and harder than the surface layer after heat or. chemo-thermic treatment, a more positive effect is achieved with the shot peening procedure.

The perspectives of shot peening in the future show us that great improvements are expected in the field of classical shot peening of gears. We’re almost there today, and will certainly be tomorrow. In particular, Japanese studies show great results with 50-150 micron and 140-250 micron of microspherical peening shots  on crack length with a 9% improvement. As mentioned before, our company is already implementing the latest studies and tests with this ultra-fine steel (Winoa) and ceramic (Saint Gobain) microspherical peening shots and will publish the results in second part of this year.

Ultrasonic peening processes are getting closer to industrial use, but there are still many questions about very long peening time and the possibility of direct tool access to the peening site. Anyhow, some specific use are not excluded anymore.

Laser peening promises an unprecedented breakthrough in this technology in the next decade. It works several times deeper into the material and only moderately increases the surface roughness compared to the classic peening with shots. Of course, there are still problems with surface masking and the process of laser peening into liquids with individual pulses, but with today’s remarkable development of laser units and more affordable prices, these limits are also changing quickly.  I am very happy to be living in this century of unimaginable progress!

And now – just for the end – the most relevant question: What did we learn and what did we obtain from this study about shot peening of gears? And what did you, my dear readers, gain from my blog?

The greatest added value of this project is the acquired knowledge for our readers and especially for our younger generation and their direct participation in the physical implementation of all measurement procedures, shot peening process and data comparison, as well as exchanging of knowledge and views and in the end, coming to important conclusions.

And what does this mean for me personally? With the realization of each such and similar project at FerroECOBlast, which our younger generation completely plans and implements themselves, my colleagues became more experienced and trustworthy experts – and I can calmly and gradually retire…

Thank you for being so patient with me and my blog during these two months. I hope we’ll read each other again soon!