Comparative analysis of the Shot peening effect on the dynamic root strength of gears, treated with different parameters in different heat treatment methods [Part 3]

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

In the previous two blogs, we already said that we made a series of test gears from 18CrNiMo7-6 material. After rough mechanical manufacturing by milling, all gears were annealed for 3 hours at a temperature of 630–650 ° C and gradually cooled in an oven to remove any internal stress after machining.

The gears were then fabricated to their final geometric dimensions and all teeth were carefully inspected and registered for subsequent verification of possible deformations during the thermal improving and surface treatments.

One-third of the gears were thermally improved by the cementation process, to a depth of hardening of the diffusion zone (EHT) of about 0.7 mm;

The second third of the gears was carbonitrided to a hardening depth of max. 0.6 mm, and the last third of the gears were nitrided on a depth of 0.2 -0.3 mm.

Thermal Improvements and quenching procedures were performed in a vacuum furnace and in a controlled atmosphere to prevent atmospheric influences.

Measurements of the surface hardness and micro-hardness of the tooth profile (HV0.1) were performed on each individual gear, and metallographic investigations were performed for each group of gears separately.

We measured tooth roughness before and after each heat treatment, as well as before and after each phase of peening, i.e. in double peening twice.

Needless to say, all these measurements were performed on at least three identical gears, and at least 3 different teeth in order to provide a representative sample of measurements and to be able to rely on the averages obtained in this way.

The following table (Table 1) presents very simply and clearly, all the performed shot peening procedures and types of abrasives, as well as the influence of individual peening on the surface roughness:

Shot peening mediaCemented gearsCarbonitrided gearsNitrided gears
Reference gear (not peened)0,100,510,16
S1100,310,300,22
S110 + Z1500,400,460,23
S3300,450,570,52
S330 + Z1500,650,610,31
S330 + Polish0,490,510,48
HT S1100,790,37
HT S110 + Z1500,680,57
HT S3301,470,57
Table 1: Influence of shot peening parameters on the change in surface roughness (Ra)

Shot peening reduces the risk of catastrophic failures and reduces parts weight or maintenance cost

Shot peening of gears was performed according to the standardized procedure SAE J2441-201506 with certified abrasive materials in dedicated peening machines, which operate in the range of the closest tolerances. The peening machine automatically records and stores all operating parameters (compressed air pressure, effective peening time, amount / density of peening media), while other values such as nozzle diameter, rpm rate, nozzle distance and angle of incidence are pre-set, do not change and are considered as constant.

Table 1 shows that we were peening gears with steel balls of sizes S110 and S330 (SAE-AMS 2431/5 standard), which are also mostly used for classic shot peening. An additional peening process was repeated on the second half of the gears from both series. On this gears we made Double Peening Process with ceramic balls for Peening Z150, defined by the SAE standard AMS2431/7B – 2013-02-07.

To determinate the right impact power on the surface of gears, we installed several holders for Almen test measurement strips, oriented parallel to the tooth involute and tooth root. The impact of the peening medium was accurately measured and set to the prescribed Almen value. Saturation (200%) was firstly checked roughly with a pencil and ultraviolet light, and accurately by optical comparison with a magnifying glass and etalons.

I know you’re already getting very curious about the results we got, right? So here they are:

Let’s start with Cemented Gears:

The influence of various shot peening parameters on the microstructure and hardness of cemented gears is shown in Figures 7 and 8. As can be seen from the obtained measurements, the S330 + Z150 and HT-S330 treatments damaged the tooth surface during Shot peening, which is also reflected in a significant increase of surface roughness (Table 1). In the case of shot peening with S110 steel balls, the surface roughness increased to 0.3 – 0.4 microns, using S330 steel balls, Ra increased to 0.45 microns, and in combination with ceramic balls (S330 + Z150) up to 0.65 microns.

When shot peening at an elevated (HT) temperature of 180°C, the surface roughness increased to 0.7 – 0.8 microns in the case of S110 steel balls, and up to 1.5 microns in the case of S330 balls.

Figure 7 shows that the highest surface hardnesses are achieved on gears processed with S330, S330 + Z150, and S330 + P (larger steel balls), followed by processing with S110 and S110 + Z150. In all peening processes performed at room temperature, there was an increase in the hardness of the surface layer to a depth of 1 – 1.2 mm.

On the other hand, shot peening performed at the elevated temperature (HT = 180°C) results in a decrease in hardness of the cemented layer, which falls below 700 HV in all three Improvement processes and below 600 HV at a depth of 0.8 mm and 30 – 50% lower dynamic root strength.

Figure 7: Hardness distribution in cemented gears after Peening process
Figure 8: The effect of shot peening on the dynamic root strength of cemented gears

And here comes a surprise – take a look at Figure 8!

If you have followed the data carefully, you’ll have already noticed that something is not right here.

What went wrong with the shot peening process on cemented gears?

Apparently, we overestimated the preferred peening parameters and processed gears with too much force, breaking the cemented layer instead of just micro-hardening it. We would achieve better results with finer balls and less peening pressure. This is an example of what often happens when technicians want to shorten the time of the peening process! So be really, really careful and precise on every single step and perform shot peening with full attention on each detail, please.

This kind of “surprises” happen from time to time, but they are helpful to understand how close we are to the “scrap”.

Our research confirms the thesis of how important careful adjustment of the intensity of peening, especially for the gears of module 3 or less is, and how much more favorable and welcome a new approach to the micro-peening method with very small peening balls – between 50 and 150 microns in size – is.

We are currently conducting such a study with USF 100 steel balls from Winoa, and ceramic micro balls Zirshot HDC from Saint Gobain. Results will be presented on our blog somewhere in the middle of next year.

For now, there’s enough for thinking – maybe even too much for the format of this blog… The final Part 4 of this series of blog posts is coming up in two weeks, and I will present the latest results for nitrided and carbonitrided gears with comparative tables and a general conclusion.

And it’s a joy for me to wish you, as well as your families and loved ones a Merry Christmas and a healthy and happy New Year 2021!

Hope to meet you soon on my next blog and thank you for following us!

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