Quote:
Originally Posted by Bored&Stroked
We tend to mix the terms hardening and tempering - such that folks think they're the same. I used to heat treat tool and die steels --> first to harden them, then to draw them back to temper them. Tempering is the softening of the hardened steel . . .
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Good point. The metallurgy behind the tempering process is as much or more complicated than the blacksmith temperature chart shown above. Tempering to soften is only partly true in reality. To truly soften a metal you have to anneal it as a whole. The best way I can describe it is the difference between hardness and ductility. Hardness is a factor of all metals than can be changed either to increase or decrease it but ductility is also affected at the same time. A combination of specific hardness and ductility is what is important for the starter ring and it can be greatly affected by any major change in temperature subject to the part after it's initial tempering process. Most starter rings have the surface color in them when you get it.
Many tempering processes only affect part of the materials structure such as the surface used for abrasion or cutting. Springs also have a balancing of ductility and hardness that can only be affected by a specific tempering process. Carbon levels of the steel can also be a factor for both hardness and ductility. High carbon steels are hard but they aren't as ductile. The harder it gets the more brittle it is. You don't want brittle starter ring gear teeth for sure.