Just because you do not see any difference does not mean that there is no improvement in immediate tempering. There IS. Read about it. It makes your tools last longer.
And,it is not about fully hardened tools left overnight to crack or explode,though that can be a problem. It has to do with improving the micro structure of the tool steel.
I use an ordinary toaster oven,but with a long shanked hi temp thermometer stuck inside it through a slot in the back side of the oven. Don't trust the oven's own temperature control. They can be way off,and there is a 25º window in both hardening and tempering in order to get the most out of your tool steel. Doubt this and you are not doing the best job.
Using mystery metal is very poor practice. We had a hard headed gunsmith in the museum who thought he was more macho if he could make something out of nothing. He made some mainsprings out of some car springs he found. After hours of work,they broke when he tried to cock the locks. Turned out the springs were actually made of some kind of air hardening steel. He wasted a lot of labor trying to save 50 cents worth of known alloy spring steel. And,he didn't look too smart in the end after all.
This guy was a great craftsman,but he never did learn diddly about steels. He had an anvil cast from steel,but it turned out to be too soft. When he went back to the shipyard that cast it,the foundry man asked him how hard he wanted the anvil to be. He could not answer the question. He came back and messed around for several days. Finally found a knife blade and took that to the shipyard. The foundry man said "Oh,you want it as hard as a knife?(Which didn't sound too swift either!). They ended up hard facing the anvil,and to this day the face of the anvil is a little bumpy,and you can see the swirls that the hard facing sticks left in it. Needs to be ground smoother,at least. Fortunately,the relatively light pounding from gunsmith work has not caused the thin hard facing to get pounded into the softer steel beneath it.
Fortunately,when I made anvil patterns for the blacksmith shop(18th. C. patterns,of course),they sent the pattern off to Texas,where they cast anvils from 4140 and hardened them. These anvils look good,and in 30 years+ have not gotten sway backed from hard daily pounding.