More on nitriding
Nitriding is the process of adding nitrogen to the surface of an alloy steel to produce a hard, wear-resistant surface. Commercially, the introduction of nitrogen into the surface layers of alloy steel is brought about by subjecting the practically finished parts to an atmosphere of ammonia gas. The process requires special heat treating furnaces which are air tight and capable of holding the parts at a high temperature. At this heat level (975 degrees Fahrenheit), the ammonia gas flowing into the furnace is broken down into its elements of hydrogen and nitrogen, and this is the source of the nitrogen which penetrates the surfaces of the steel. In order to produce a satisfactory nitrided surface, the process is operated for an extended period of time, generally from 25 to 80 hours.
Through-hardening describes a process of heat treating to infuse a deep consistency to the integrity of the cylinder walls. This process takes effect to a depth of about 1/100th of an inch. Proponents of through-hardening include manufacturers of aftermarket cylinders. They see through-hardening as sufficient for the rigors of aviation piston powerplants to reach TBO. This may indeed be true.
The OEMs use a process called nitriding on their cylinders. After subjecting their cylinders to a through-hardening stage, nitriding is performed. This gives extra strength to the cylinder wall surfaces, to a depth of about .025 inch. Nitriding is accomplished by an ammonia gaseous infusion process and permeates, rather than coats, the surface.
Almost-finished parts are subjected to heat (975 degrees Fahrenheit) and ammonia in an airtight state for between 25 and 80 hours. The ammonia breaks down to its components — hydrogen and nitrogen — and the nitrogen then penetrates the steel. The result is reduced cylinder wall wear, improved piston ring life, the ability to use chrome-plated rings for longer life, and better compression, and an increased fatigue strength.
Rockwell "C" hardness
What Rockwell "C" hardness numbers does each process indicate? Through-hardened cylinder walls have a Rockwell "C" hardness of between 30 and 35. Nitriding adds to this, and factory cylinders test out at 53 to 55 on the scale, to a depth of .025 inch. (To put this depth in perspective, a typical first-oversize re-bore is just .0010 inch). Nitriding has been applied to Continental and Lycoming barrels since 1960. Aftermarket parts makers contend their through-hardened cylinders will achieve TBO, and according to the FAA standards, they should. Each process has its proponents and critics.
Confusion in terminology?
Perhaps some of the confusion lies in terminology. "Through-hardening" sounds — and is — tough. But the factory nitriding process is applied after through-hardening has been done, so naturally nitriding, while not a very positive descriptive, is a much harder surface, as verified in the Rockwell "C" tests.
Engine technology continues to evolve, develop, and improve. Both factory and aftermarket component suppliers produce FAA-approved parts which theoretically are equal, or at least equal to the FAA standards. In the two instances discussed here, the reader must make decisions. Is it acceptable to you to inherit reworked cylinders which may have gone through several TBO cycles? If so, then your choice is easy.
If you want assurances that no one has abused or fatigued your cylinders, then the only guarantee is factory new cylinder assemblies. Statistically, the long-term costs are about the same.
When it comes to choosing between normal through-hardening or hardening plus nitriding for your cylinder barrels, again, it is a matter of choice. It is fallacy to suggest, as some do, that aftermarket parts manufacturers are alone in R&D, while the OEMs twiddle their thumbs. Competition driven by customer demand has driven both OEMs and PMA manufacturers to improve their products.
The main beneficiary of this continued development is the owner of the engine. Over the past 10 years quality and strength have improved, and prices have gone down. Competition is the best prodder of product improvement and price containment, and the engine overhaul market is a perfect example of this theory in action. It's your money, and it's also your choice of overhaul and manufacturing procedures. The more questions you ask, the better educated you will be on this important subject.
There are many opinions when it comes to treatment of cylinders. With so much information on the pro's and con's of the different processes, you need to make the decision that best fits you. Below are companies that manufacture engine cylinders, both OEM and PMA. Contact them for more information on their products and services. As the author suggests, ask lots of questions in order to be better educated on this subject.
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