Max toughness from D2

J

jlmetcalf

Joined
Jul 5, 2005
Messages
75
I'm a hobbyist maker and I've used D2 almost exclusively for the last 20+ years. Over the years I've settled on a heat treat process that seems to work best for me: foil wrap and air cool from 1875, temper twice at 450 with a dry ice treatment between tempers. I get rc 59-60 with good toughness (for D2).

I've often read in various places that D2 yields max toughness if tempered near the secondary hardness peak (somewhere around 975). I can get the advertised hardness at 975 (about 60) but I get much less toughness than I get at the same hardness with the 475 temper. I've tried austenizing temps from 1825 to 1925 and draw temps from 900 to 1000. I'm using a commercial hardness tester and a home-made impact tester.

Anyone else done any testing along these lines? I'm sure I must be missing something.

Thanks,

Jim
 
It's generally not recommended to heat treat using that higher temperature, it actually reduces toughness and corrosion resistance, it does increase wear resistance, though. It's a myth perpetuated by some that it actually makes it tougher. Peak toughness for D2 is somewhere around 475-525F temper like you're already doing. Lowering the austenitizing temperature to 1850F and raising your temper to 500F might increase the toughness slightly, for a tradeoff of a slight decrease in hardness. That's the peak toughness heat treatment as listed by Crucible steel. You probably have a pretty good heat treat already.

Edit: Carpenter lists the toughness peak as 450F.
 
Thanks Larrin, I guess I'll stick with what works. I had just seen so many comments over the last few years from "experts" about retained austenite in D2 and how the proper way to deal with it was via high temperature temper cycles that hit the secondary hardness peak. Looks good on paper; just does not give good results.

Jim
 
Getting rid of retained austenite won't raise the toughness, austenite is dead soft, and many toughness tests show that retained austenite will raise toughness, but getting rid of it does raise the hardness reading. We don't really want soft spots in our edges either, which is why many want to get rid of retained austenite. If you want to convert some of that austenite, you should add cryo to your heat treatment, either dry ice and acetone, or liquid nitrogen. Even using your freezer will convert some. The thing is, you're trying to get down to what's called the martensite finish temperature, which basically means that that is the temperature at which martensite has a complete transformation, the closer you get, the more that transforms. With very simple carbon steels, that temperature is above room temperature.
 
Unacceptable amounts of retained austenite are more often caused by excessive hardening temperature than anything else.There isn't all that much reduction in retained austenite after tempering to 475 F as that stabilizes RA.Better to go right into cryo after quenching or a flash temper of 350F. Dry ice gives you -100 F and LN gives -300 F. You ought to give plate quenching a try.
 
mete said:
Unacceptable amounts of retained austenite are more often caused by excessive hardening temperature than anything else.There isn't all that much reduction in retained austenite after tempering to 475 F as that stabilizes RA.Better to go right into cryo after quenching or a flash temper of 350F. Dry ice gives you -100 F and LN gives -300 F. You ought to give plate quenching a try.
Click to expand...
That's good advice, mete.
 
Thanks all for the good info. I've been using dry ice in acetone between tempers. I'll look at the dry ice as soon as the quench is down to RT.

Mete,
One thing I've never understood though, most spec sheets suggest that you temper the part while it is still warm before it gets to RT. Based on this I was always afraid of a sub-zero right after the quench. It just seemed strange that we are supposed to not let the part get to RT yet we can let it sit in dry ice all night. I know that you would not suggest the sub-zero right after the quench if it was not the way to go but, well ... I'm goint to try it and I bet it will work and I'll just remain a little confused.

Thanks,

Jim
 
Mete,

I got up today and treated a few pieces using the changes you suggested.
1. dropped the high heat to 1850
2. 3 hours in dry ice and acetone right after quench
3. draw twice @ 450

All good results! hardness went from 60 to 61 and I'm getting about 5-10% more impact strength.

I guess the suggestions on the Crucible and Timken spec sheets about sub-zero after the first draw are to allow for issues that might arise with more complex shapes.

Thanks for being a resource here.

Jim
 
Larrin,

I use a home-made contraption that I clamp a 3/8" x 3/8" cantilevered izod type sample in. The sample is 3" long with a 3/32 v notch. Above this I have a 5' vertical rod that I can slide a 5# or 10# weight down. Next to the rod is a scale marked every few inches with the potential energy associated with that height. I just keep increasing the height the weight is dropped from until the sample breaks. I know it does not correlate to true izod values but it works well for comparative testing which is all I want anyway. Plus, the price was right.

Thanks for your advice.
 
You must log in or register to reply here.
Back
Top