Tempered too long?

J

John L

Joined
Oct 28, 2004
Messages
1,330
I fitted up a small folder in D2 last night and put spring back in kiln at 1200 and put blade in my kitchen oven at 500. Set the kiln for two hrs and forgot to pull the blade out of the oven until 4:30 this AM. Any harm to the blade which stayed @ 500F for about 9 hrs?
 
I have read the diffusion process that occurs during tempering is time dependant and if left for way way way to long it will become too soft. I don't think 9 hours is a problem. 9 days, that would be a problem.
 
Hey John,
9 hours may or may not be too long a time at 500 degrees. The only way that you'll know for sure is to really test it. And, if worse comes to worse, just HT it all over again. I know that sucks to hear, but, if your blade won't hold an edge, what else can you do? I've had to do it.:grumpy:
 
mete said:
9 hours is not a problem.
Click to expand...

Robert, you sound very confident. How can you be so sure? He's talking about a blade for a folder, right? So, we're probably talking a thin blade. I know I'm only an Apprentice, and I know you have more knowledge that I do, .....but still. Nine hours at 500 would concern me. Please let me know how you know that 500 at nine hours is O.K. :confused:
 
As a complex steel we would temper D2 for 2+2 hours minimum . That's about half of nine right there ! There was a discussion of long term tempering and possible problems on one of the forums but that was much longer than 9 hours......Long tempering times is what you use for making hardtack. It is baked thoroughly until all moisture is removed .Zwieback, and biscotti are similar .Hardtack will last for many years as long as it's kept dry !
 
Relax Troop, 500F is on the low end of temper for D-2. The thickness of the section has little to do with tempering. Nine hours is long if you are waiting for your daughter to come home from a date, but not long for a piece of steel in the oven. I would calculate that it would take somewhere in the realm of 100 to 500 hours before you would have a noticeable effect on the blade. There should be no measurable difference in a two hour vs a nine hour temper for a complex steel like D-2.
Stacy
 
Would you guys say the same for A2? It's a complex steel, but I don't think the tempering temps are as high. I'd be scared that A2 at 425 for nine hours would soften it to butter!
 
A-2 and D-2 are very different steels.
A2 is not near as high in chromium as D-2 (4.5-5.5% vs 11-13%). It tempers from 345F to 1000F,depending on what you want to achieve in toughness vs hardness. With A-2,for knife use ,350-575 is the normal range. With D-2, the range is from 400F to 1000F , and the knife use range is from 450F to 800F. With A-2 the hardness drops off steadily, to Rc58 at around 550F, but D-2 is flat at Rc58 from 550F until nearly 900F.
As you can see, it takes much more time and temperature for the complex carbides in D-2 to start to convert into softer structures.
Stacy
 
Tempering is a diffusional process, all diffusion is both time and temperature dependant, but temperature always trumps time. I would expect mete to be sure about this because he is familiar with concepts such as Ficks law and diffusion rates in things like carburizing where the distance carbon can travel in a given time at temperature can be determined with a formula without ever lighting an oven or a forge. And 500F doesn't even register on most of those charts.

Most of the rapid effects that one sees in 2 hours at 400-500F is a result of the relaxation from BCT (body centered tetragonal) to BCC (body centered cubic). Now that displaced carbon has to go somewhere to get out of play, which will require much greater distances to travel and at 500F the traffic is grid-locked; in something as complex as D2 there will be some serious chemistry to overcome even if you bump the temperature up a bit.

I would still test the knife, simply because I am neurotic about such things and have enough confidence in my own incompetence to believe I could overcome the laws of physics and screw it up anyhow. Heck I have thrown blades away simply because I took longer than expected to forge them, and developed a bad feeling about them.
 
Wow! The only thing I'm qualified to say after that is...."Mommy, where's my crayons?":D
 
At 500F things are slow .First the carbon comes out of the martensite crystal to form carbides then there will be a tendency to lose coherency with the matrix then agglomerate into larger carbides .But again this is all very slow at 500F. A jump to 600F would make a much bigger difference than long times at 500 F.
 
mete said:
At 500F things are slow .First the carbon comes out of the martensite crystal to form carbides then there will be a tendency to lose coherency with the matrix then agglomerate into larger carbides .But again this is all very slow at 500F. A jump to 600F would make a much bigger difference than long times at 500 F.
Click to expand...

Thanks, Robert and Kevin.:thumbup: Got it.
 
You must log in or register to reply here.
Back
Top