We temper a blade at say 350deg. for 2 hr. then let it cool to room temperature and repeat it two more two more times. What happens when we forget (CRS) and leave the blade in the oven for an extended amount of time say 4 hr or over night? Will this blade need to be re normalized and heat treated? How do we proceed in this malfunction. I don't think that I am the Lone Ranger when this happens. The steel in question is 10XX, 5160 and 01. Gib

I did that on a hawk head once Gib. Didnt seem to hurt it at all. In fact it was just as tough as a normally cycled one.

Tempering temperature is sort of logrithmic while time at temperature is linear. Especially at only 350 F I would doubt you encounter any trouble. I have never tried tempering for 12 hours though.

RL

Cactusforge: You can leave a blade at tempering temp indefinitely, and it will have no effect at all.

Cactusforge: I did some double checking on that answer, and, because of the steels in question, and the fact that you're not at the high end of the tempering range, the longer tempering times probably had a very minimal effect on the steel.
for more highly alloyed steels, tempered at the high end of the range, you really want to stay at the recommend times, or the hardness can fall off dramatically.

Yup. At 250F-350F your probably do most of what is going to get done in the first 1/2 hr. Tempering is a diffusion reaction that allows carbon trapped in the interstitial sites of the iron lattice to escape and diffuse to form carbides. At low temperatues, the carbon cannot diffuse very rapidly. When tempering more than once, you need to exceed the previous tempering temperature by, oh, 25 degrees or so, or you will probably not accomplish much. That may mean you start 50 degrees below the temperature that will get you the desired hardness. Conversely, since time and temperature are inversely relate in tempering, you can use the same temperature but for much longer periods of time. Eventually, the hardness may drop measureably.

I overdid an A2 blade for about 8 hrs at 450?F once and it came out quite a bit softer than the expected HRC ~60 by the time I got home and took it out (a file would bite into it readily). Instead of just pitching it, I did a second freezing treatment on dry ice and retempered it. It appeared to come back to the normal range of hardness(the file skated on it), but I didn't trust it and didn't finish it from there. I think it's still kicking around the shop somewhere.