Before we leave our study of materials, I want to share a few thoughts on heat treating, lab tests, and hardness testing. When you start working with gears they throw lots of terminology at you and leave it to you to figure out what they said. It is a situation that can lead to serious misunderstandings that cost time, money, and even careers.
Do not fall into the trap of nodding your head as if you understand when someone rattles off a bunch of metallurgical jargon at you. Find someone who will take the time to explain the terms and processes to you, even if you have to admit that you are clueless. Better an informed newbie than a career handicapped by ignorance. It took me many years to finally get my questions fully answered, but I kept plugging away at it.
Another suggestion is to find some entry level heat treat text books and read them. One of the best explanations of metallurgy and thermal processing is found in Engineer to Win, by Carrol Smith — a book on automobile race car preparation. The late Mr. Smith is not kind to the know-it-alls who give the sort of advice that results in broken parts because failures in racing can injure or kill people.
Take something as simple as hardness testing. Even if your drawing calls for HB [Hardness Brinell] or HRC [Hardness Rockwell C Scale], the actual lab measurements may be made in a completely different system [such as Vickers] and then converted into the desired units mathematically. Calibration frequency, indenter design, test location, and even the hardness comparison blocks used for calibration, can influence the results. This seems like “inside baseball” trivia — until your parts are rejected and your delivery promise is endangered.
Hardness test blocks, for example, have a tolerance on them. Your heat treater’s blocks, your inspection department’s blocks, and your customer’s blocks may not be exactly in synch. The more you learn about these “insider details” the better prepared you will be to develop robust processes that insure good parts made on schedule.