Material used for bearing races?

gt-arrowhead

Just reading up on what different type of bicycle bearings are made of etc, and i managed to find somewhere that says that bearing races are case hardened, i.e. the steel in the middle retains its toughness, but the outside becomes hard.

But how is it possible to get just the outside of such a thin piece of steel hardened but not the inside. Wouldn't they just harden the whole thing? But then wouldn't hardening the whole race make it quite brittle and not great at surviving shocks/large forces.

The Rookie

Heat it and quench it, just like many items are surface hardened, sometimes if heated enough extra carbon will be alloyed into the surface. It's all well known techniques, Stanley knife blades for example are treated the same way so they don't snap too readily but keep their edge.

gt-arrowhead

So not fully heating the material to its core before quenching it will create the surface hardening?

I actually ended up on a knife forum eariler while googling, and read some stuff about blades being made out of old bearing races which was quite interesting.

The Rookie

No you heat the material throughout, you quench the surface, then as the core contracts as it cools it pulls in that quenched outer layer making it harder, similar principle is used for toughened glass.

The Rookie

https://en.m.wikipedia.org/wiki/Case-hardening

rockmonkeysc

I used to do it as a tool making apprentice.
Heat to cherry red and quench in oil. The steel takes some of the carbon from the oil which hardens the surface.
It can then be tempered if you don't want it too brittle by heating to a straw colour.
If bearings were fully hardened, they'd shatter when you try to fit them.

The Rookie

I've also made my own cutting edges by heating to cherry red and sprinkling on sugar, again it takes the carbon out, reheat cherry red and then quench in water.

gt-arrowhead

Thanks guys

I also found a great vid on how bearing races are produced, starting right from molten steel
https://youtu.be/VOysbd2ZMxE?t=3m41s