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Toni, retained austenite is not a desirable structure in a blade. Yes, it can facilitate more impact toughness, but this is my understanding....If you take a knife that has significant RA % and chop into "whatever", the damage may be a bit less with the RA present. However, on that spot that impacted the medium being struck, you now have a weak area that will continue to fail to a greater extent if the RA wasn't there to begin with. Sorta like, OK RA is fine to have for impact toughness...but it only works once (on that spot). That is WAY over simplified, and I have no data to support that claim. Just what I have heard.
No, Verhoeven does not say to use different tempering temperatures. If you have a target hardness of 61HRC, then you need to temper at a temperature that will give you 61HRC. The charts are a good place to find that data. Sometimes the charts aren't exact, tho, so you have to "dial in" the temperature to reach the desired HRC number. Example, if you are working with a new steel, and you don't want to overshoot and end up with lower HRC than planned, it is a good idea to "walk in" that temper. Start low. If goal is 61HRC and 61 comes at 400F, but you're not sure and haven't tried before and nailed it down, then it's a good idea to do a lower cycle first, say 375F or even 350F, and walk it up.
There are steels that need higher tempering temperatures, I say "need" but that's not entirely accurate. This is for the secondary hardening of particular alloys like D2 and stainless. However, the concept is still the same. Pick the temperature that gives you the desired HRC number, and stick with that temperature. Do 2 or 3 cycles AT that temperature....unless, again, you need to "dial it in".
Note the Japanese with their "low alloy" carbon steels. They temper once, often only a few minutes. But good to note most often their knives are kitchen knives, run very hard, and are strictly kitchen knives with the thin grinds. Not a knife you'd wack with.
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