Hardness of Roman Armour

[Dan Howard]

Quenching only has an effect if the carbon content is above a certain level. Every alloy has a different recipe for quench-hardening and it is difficult to get it right even today with modern technology.

I know brass and bronze can be work hardened bot sure about the medium carbon thing if this is heat treated. Best graph I know of is the one in Knight and the Blast Furnace.

I should have said "Bronze can be as hard as medium carbon steel BEFORE heat treating." After heat treatment, medium carbon steel becomes harder than bronze. Tylecote has the best data.

Were the Celts and Romans aware of the advantages of quenching and tempering?

Some of their surviving swords certainly seem to indicate that they were well aware of quenching and tempering. But knowing about it and applying the techniques correctly are two different things. Only the very best smiths would have been able to do it reliably.