Bendy Gumby D2

E

elementfe

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May 3, 2008
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OK, once in a great while something so odd and wondrous happens in the course of knifemaking that I just have to talk about it...

Bending during heat treating has been a bit of a bane in my knifemaking- I've never had any success in straightening during tempering using clamps and such, though occasionally I can get a thin blade straight while at tempering temp. The most annoying problem has been distortion of stainless during HT/subzero.
Carbon steels aren't a big issue, since I just pull em out of the quench and straighten on the anvil, but the SS HT is so much more involved that I'm still trying to get good at catching all the times when things can go wonky.

So today, when I pulled some D2 hunters out of their foil packs and sighted them, I was really annoyed to see bending (not terribly surprising, since they were tapered tang and distal, just lacking some finish grinding on the bevels.)
I was holding it in my bare hands, deciding whether to straighten in the vice or on the anvil, idly thinking, "Jeez, I wish I could just bend it like this...."

To my immense surprise, it bent...EASILY....WITH MY FINGERS....about like 15 ga mild steel sheet metal "knife" would have. The parts that were full .150 didn't bend, but the distortion was in the thinner parts.

I didn't want to write this until I'd done a quick file test between temper cycles, sure enough it was plenty hard.

Blade steel is such fascinating stuff....
 
Um hmmm.
I think of D2 as the Mister T of steels, it was freaky to be bending it like it was mild.
 
Nothing spooky going on.

Until the super-saturated austenite drops to 400F and starts the conversion to martensite, it is as soft as rubber (figuratively). You can take a freshly quenched carbon steel blade and in the temps between 900 and around 300F bend it as you wish with gloved hands. This is how many makers straighten their blades during quench. Once you feel the blade stiffen up, quit straightening.

On a high alloy blade, like stainless steels and D-2, the conversion will not be completed until it reaches -100F. This would allow hand straightening at bare hand temps during the first ten minutes or so after quench.
Even without a sub-zero treatment or cryo, after the blade sits for many hours/days, the retained austenite will continue to convert to some degree. This is called age hardening.
 
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Thanks for the clarification, Stacey!
It's easy to think of hardening as something that happens "because you dunked it in the quench." Fascinating that there's so much more to the transformation.

Now to test my Google mojo and see if I can get some understanding of the physics of what makes the transformation happen and how it propagates even as the steel is at cooler temps.

It's just so cool when my assumptions about materials are blown up- D2 is so bloody tough that having it tweak so easily at room temp almost gives me vertigo :)
 
Matthew Lomas said:
Stacy has a very elegant way of explaining these processes in an easy to understand format.
Click to expand...

Well, here is the Cliff Notes version:
Austenite is soft and pliable...Martensite is hard and brittle. As long as the steel is predominantly austenite, it will bend easily. Once it is 50% martensite, stop all bending and stress.

When steel rises above the critical point ( Ar1) it converts into an austenite structure.
Stainless steel needs to go to 1850-1950F to get the carbide formers ( alloy) into solution.
Upon cooling, as it passes Ac1 ( around 1320F) it can do one of two things ( this area between 900F and 1200F is called the pearlite nose)-
a) if the cooling rate is slow enough, it will convert into a pearlite structure. The carbides will form rods and balls.
b) if the cooling rate is fast enough, as it passes the pearlite nose the structure will remain austenite with the carbides in solution. This is called super-saturated austenite.
Once safely past the pearlite nose, the cooling rate can ( and should) be slowed down. This short window allows straightening of any warps and twists in the soft austenite. Don't hold it here too long though, or bainite may start to form ( don't ask about bainite, just avoid it).
Around 400F ( Ms) the super-cooled austenite starts converting into martensite. By 200F carbon steel has finished its conversion (Mf). Stainless steel has to drop to -100F to reach Mf. This is called Sub-zero, and is the Mf for stainless steel. If dropped to -300F or lower, it is called cryo. At this point, the carbides transform into eta carbides ( you really don't want to ask why or how).
 
Thanks, Stacy! As always, great information in a small package.
The mechanics or chemistry of how that reaction propagates and why it does so at certain temps would likely be like trying to read heiroglyphics for me. It's pretty cool just experiencing and working with it in the shop.
And, since warping during HT has been such a problem for me with stainless, such a relief to know that it can still be straightened as it cools, similar to Carbon steels.
I'm so used to having to do that hot that I made an erroneous assumption that by the time it was cool enough to get that foil packet off, it was too late to straighten.....BUT NO! :)
 
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