Since I may have to reproduce a hardened valve rotator cap, not knowing the degree of hardness they were originally tempered to, I did a search and came up with this list from Wikipedia:
Faint-yellow – 176 °C (349 °F) – engravers, razors, scrapers
Light-straw – 205 °C (401 °F) – rock drills, reamers, metal-cutting saws
Dark-straw – 226 °C (439 °F) – scribers, planer blades
Brown – 260 °C (500 °F) – taps, dies, drill bits, hammers, cold chisels
Purple – 282 °C (540 °F) – surgical tools, punches, stone carving tools
Dark blue – 310 °C (590 °F) – screwdrivers, wrenches
Light blue – 337 °C (639 °F) – springs, wood-cutting saws
Grey-blue – 371 °C (700 °F) and higher – structural steel
It seems to me that maybe I can use this info to get an approximate hardness measure of an existing cap, by scratching it with some of the above to see where the hardness falls. Kind of like doing a hardness scratch test to identify minerals.
Handy that the translation to a tempering color can be got by using this list as well.