understanding the tempering process

[bigin]

I understand the need to raise and lower the temp during tempering to bring the hardness back and lock it in where you want it. Where I am confused is the correlation between time at temp and hardness. Crucible recommends 2ea 2hr soaks at the given temp as a minimum for temper cycles. I know that blade materials are much thinner than average things that Crucible talks about. A lot of people are doing 2ea 1hr soaks. Some are doing 2ea 2hr and 1ea 1hr all at the same temp.
My question is what is the difference to the RC in say, 2ea 1hr @400f-vs-2ea 2hr @400f-vs- 2ea 3hr @400f?
Just wondering if I am wasting my time with 2hr temper cycles.
Thanks guys.

 

[samuraistuart]

Tempering can be a strange science to understand. But I think I can at least answer the question. There would be no difference in Rockwell readings between those times at 400F. If the RC readings were identical out of quench on all three knives, and all were held at 400 for at least 30 minutes, regardless of how long, the RC readings will not be "less" the longer you leave them in to temper. Now let me add a disclaimer......maybe many hours upon hours might start to reduce the hardness level, but it's of course much more temp dependent than time.

I would recommend a minimum one hour per cycle. If two is what you want to go with, or three hours, nothing wrong there. There is a lot of things going on when tempering that require the time. Stress relieving, retained austenite conversion, precipitation of tempering carbides. To name three anyway.

My tempering schedule for carbon steels is usually something like this....
(if target HRC is going to be at 400F, then)
1 hour at 350F
1 hour at 375F
1 hour at 400F
1 hour at 400F

(if target HRC is going to be at 420F, then)
1 hour at 350F
1 hour at 400F
1 hour at 420F
1 hour at 420F

All get water quenches in between. In all reality, my schedule is probably overkill to some extent. But it gives me piece of mind for whatever reason.

 

[bigin]

Thanks. Appreciate the info.

 

[David Stifle]

Tempering can be a strange science to understand. But I think I can at least answer the question. There would be no difference in Rockwell readings between those times at 400F. If the RC readings were identical out of quench on all three knives, and all were held at 400 for at least 30 minutes, regardless of how long, the RC readings will not be "less" the longer you leave them in to temper. Now let me add a disclaimer......maybe many hours upon hours might start to reduce the hardness level, but it's of course much more temp dependent than time.

I would recommend a minimum one hour per cycle. If two is what you want to go with, or three hours, nothing wrong there. There is a lot of things going on when tempering that require the time. Stress relieving, retained austenite conversion, precipitation of tempering carbides. To name three anyway.

My tempering schedule for carbon steels is usually something like this....
(if target HRC is going to be at 400F, then)
1 hour at 350F
1 hour at 375F
1 hour at 400F
1 hour at 400F

(if target HRC is going to be at 420F, then)
1 hour at 350F
1 hour at 400F
1 hour at 420F
1 hour at 420F

All get water quenches in between. In all reality, my schedule is probably overkill to some extent. But it gives me piece of mind for whatever reason.

I've never heard of ramping up tempering heats like that. Of course, that doesn't mean a thing, as I'm sure there's tons of things I've not heard nor completely understand. But I've had good results simply going with a one hour heat at 400, water cool, and a second hour at the same temp (which I suspect isn't even needed). There's probably a thousand ways to skin the cat, and it used to drive me crazy, as I was always looking for the "right" way.