Research and Development Project - Shot peening of gears treated with the three most common methods of Improvement (cementing, nitriding, carbonitriding)
Hello, dear readers!
I am glad to see you back again on the FerroECOBlast Blog!
In my previous blog, we learned some basic facts about the shot peening process. That’s fine, but how well do you really know the shot peening process itself?
In addition to the general belief that this process significantly improves durability and corrosion resistance, would you like to know the physics of what happens to the material during shot peening, how we measure it and what we can expect? Someone once wrote a great comparison that simply hits the point: Shot peening is the same as walking on freshly fallen snow: The surface shrinks, becomes compact and smooth. Compressed snow is stronger and lasts longer. So, keep on reading…
It all starts with choosing the right steel to make gears. It is usually a low carbon steel with the addition of Chromium, Nickel and Molybdenum. Such a steel composition is perfect because it allows slight flexibility of the tooth at high loads and absorbs external energy without fracture. However, for the gear to simultaneously be strong, flexible, and resistant to wear, it must be heat-treated, a process called improvement. With improvement, carbon is added to the top layer of the gear (a few microns thick) by heat treatment and become hard, while the core of the tooth remains flexible to absorb large forces and shock loads. Such gears are quenched and get a hard surface and a tough interior.
As is well known, there are mainly three so-called “schools” for gear improvements in the world, namely – simply put – Japanese, French and German. They differ in the improvement procedures that have a decisive influence on the correct shot peening procedure. And this is where the basic problem arises: when the person who decides on the most suitable surface treatment with shot peening does not know the differences between them, which happened to us decades ago as well. How to choose the right technology and design of the most optimal equipment, then? In addition to the fact that the process and equipment for shot peening are known today and a lot of scientific literature is available, it is far from easy for engineers in companies to choose the appropriate process and key parameters that will be optimal for their “Gears and Pinions”, or – God forbid – harmful.
Three years ago, we started our own, self-funded research project on the shot peening of gears and involved leading external Institutes, Universities, and measurement laboratories. We carried it out precisely because of the frequent questions and doubts of our clients. The research was carried out on a series of common test gears, which were precisely manufactured, measured and treated with a variety of heat and surface treatments, because the known methods of shot peening and results from the professional literature of the last thirty years do not give an unambiguous answer to our users about when and in which case and most importantly which shot peening process is appropriate and justified for a particular situation. We have successfully completed our study this year and our scientifically proven results are right here – in front of you.
Gear manufacturing technologies as well as the pre-heat treatment precedeing shot peening vary from manufacturer to manufacturer. The input parameters differ, the knowledge and beliefs of the manufacturers are different, and the data obtained often contradict the expectations and results published in the professional literature. A review of the available professional literature has shown that in some cases they even contradict each other. Simply explained: methods that apply to the “Japanese” type of gear making, cannot be applied to the “German” or “French” type.
Our R&D Project in this study addressed one standard: very common gears made of low-carbon DIN/ISO 18CrNiMo7-6 steel, which was heat-treated and surface-engineered in three ways before shot peening: Cemented (C) ), Nitrated (N) and Carbonated (CN). In this phase, we measured the hardness of all gears before and after carburizing and heat treatment, as well as before and after shot peening.
Shot peening was performed with three different media and with the prescribed parameters. Additionally, the same shot peening process at room temperature and at elevated temperature was included in the study.
Multiple intermediate measurements of hardness and depth of changes were performed for each heat treatment separately, as well as measurements after shot peening for each individual process. At least three replicates were performed on each sample to ensure an appropriate average of the measurement results.
Measurements of material fatigue up to tooth fracture were performed at different frequency ranges, we simulated the real load of the gear in the gearbox during acceleration, braking and load running.
For each heat treatment procedure, we performed precise hardness measurements in the entire cross section of the tooth, so we measured the hardness at different depths below the surface. The microhardness profiles on the tooth of the test gears after different heat and surface treatment are shown in the diagrams in Figures 2a-2c. As can be seen from the diagrams below, the effects of heat treatment on the hardness and depth of the modified layer are different, depending on the selected improvement/heat treatment technology.
The hardness in the core of DIN 18CrNiMo7-6 steel gears is 700 HV10, which is the basic starting point for further comparisons.
Our research showed that the mechanical and technological characteristics of heat-treated gears are significantly improved with appropriate procedures and properly selected shot peening materials, but the line between success and failure is thin. We should not treat nitrided or carbonitrided gears with the same parameters as, for example, cemented gears! With incorrectly selected process parameters or just improperly selected granulation of the shot peening media, we can even worsen the characteristics of the machined part!
In the next post, I will present the concrete procedures and measurement results for each type of heat-treated gear and the associated shot peening effect, while the fourth and last part, will discuss the effect of elevated temperatures on shot peening and the final report. Also, there will be most likely a few words about the future dilemmas of shot peening processes and technologies.
I wish you a pleasant continued reading!
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