Trying to understand D2 heat treat and temper

R

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I have read way too much about D2 heat treat methods, and I'm frankly confused. I believe some of what I read was from Nathanthemachinist, but I'm having trouble finding everything I've read, so I'm hoping you all can lay out the facts.

The actual annealing, stress relief and quench methods don't seem to be part of the great D2 debate, but please correct me if I'm wrong about that.

The parts I believe are confusing me have to do with discussion of which tempering temperature range is preferred and what seemed like a related discussion of retained austenite. From what I could gather, Nathan felt that the supposed tougher 950ish tempering range left more RA - but that doesn't make any sense to me, so I think I'm completely missing the point.


Anyway, hopefully the experts will weigh in to explain the different schools of thought on quench, temper and cold treatments for D2. It seems that this steel has excellent potential, because several people make absolutely excellent knives from it, but many heat treats are lacking, giving the steel a bad name.

Thanks!
 
Look at any D2 data with the thickness and hardness desired for the final product in mind.

Knives are thin and you want them hard.

Austenitize to the lower end of the range. Use cryo. Temper at the lower range ( the high range is for tough, but less hard tools).

For suggested numbers:
Pre-heat slowly to 1100-1200F and equalize for 5 minutes
Heat slowly to 1400F and hold for 10 minutes
Heat to 1800-1850F and hold for 30-45min ( I use 1825F)
Plate quench ( You can oil quench. The difference is that the plates will be about 1 Rc point lower...but the blade will be straighter.)
Do cryo or sub-zero finish to the quench
Temper twice at 400F. One to two hours each temper. (The 1000F range is for impact tools that need high toughness.)
 
D2 can be pushed hard by converting as much over as possible... come straight from the plates to a sub-zero cycle (plates, hand warm, water cool, sub zero/cryo) and do a low temperature temper.

I prefer the above method but a machinist customer of mine told me he would like to try one with his heat treat... he would air cool to hand warm and do a higher temp temper cycle for a blade that has a toothy edge.
 
I never used a snap temper..Stacys formula there is pretty much in line with what I have found to work well..Nathan the Machinest has an excellent post of D2 and its heat treat..Its what we use now and am totally happy with it..
 
Chemically, what happens if you temper at 400, then do a second temper at 950?

Where does the extra toughness come from?
 
I generally temper between 425 and 450 for ~HRC 61-62

It's unlikely that Bob Dozier tempers at 600. Who told you that?

Tempering at 950 is the industry standard for tooling and it makes a mediocre blade. Soft carbon lean martensite and an excessive carbide volume fraction that is not particularly cohesive with the matrix is fine in a stamping tool but crummy in a thin knife edge. It's like the perforations in a postage stamp. The edge folds or flakes off easily in use.

950 decomposes most retained austenite (into various things, not just martensite) and makes a good tough abrasion resistant tool with minimal dimensional changes and low risk of cracking.

A relatively rapid quench to Mf (-100F works pretty well) largely eliminates the need for the secondary hardening hump so you can temper in the 400's. This gives you a much more durable, stronger structure that holds an edge much better. If you're cutting hide and meat it may make no difference, but as soon as you hit bone, a cutting board or cut harder materials the difference in fine edge stability become extremely significant.

Think about what a Rockwell hardness test is actually measuring and you'll see that two steels with the same measured hardness can be very different.

D2 can be a brilliant steel, but not with the industry standard heat treat developed for thick stamping tools.
 
Nathan the Machinist said:
I generally temper between 425 and 450 for ~HRC 61-62

It's unlikely that Bob Dozier tempers at 600. Who told you that?

Tempering at 950 is the industry standard for tooling and it makes a mediocre blade. Soft carbon lean martensite and an excessive carbide volume fraction that is not particularly cohesive with the matrix is fine in a stamping tool but crummy in a thin knife edge. It's like the perforations in a postage stamp. The edge folds or flakes off easily in use.

950 decomposes most retained austenite (into various things, not just martensite) and makes a good tough abrasion resistant tool with minimal dimensional changes and low risk of cracking.

A relatively rapid quench to Mf (-100F works pretty well) largely eliminates the need for the secondary hardening hump so you can temper in the 400's. This gives you a much more durable, stronger structure that holds an edge much better. If you're cutting hide and meat it may make no difference, but as soon as you hit bone, a cutting board or cut harder materials the difference in fine edge stability become extremely significant.

Think about what a Rockwell hardness test is actually measuring and you'll see that two steels with the same measured hardness can be very different.

D2 can be a brilliant steel, but not with the industry standard heat treat developed for thick stamping tools.
Click to expand...

Hi Nathan. Glad you found this thread.
In your above mentioned D2 thread Matthew Gregory quoted Bob Dozier from here:
http://forum.dozierknives.com/forum...-heat-treatment-on-d2-treatment-for-toughness

But now reading it in context, Bob was explaining how to get more toughness at a lower hardness than he prefers. The fact that he isn't suggesting going up to 950 suggests that he also doesn't think the secondary hardening temp is worthwhile for blades, either.

In contrast, Jay Fisher seems to be a proponent of 950 tempering:
http://www.jayfisher.com/D2_Wear_Resistance_King.htm
However, if D2 is tempered correctly (and at the correct time), the heat treating processer (the knifemaker, I hope!) can achieve the supreme balance of hardness and toughness that will yield incredible blade performance in both cutting, wear resistance, toughness, and even, to a degree, corrosion resistance, at the same 60HRC! What this means to the knife owner is that just because a knifemaker (or manufacturer) offers a D2 blade at 60HRC, it doesn't mean he has processed it right. This is why it's best to query the knifemaker about his heat treating practice, and true professional knifemakers should be able to answer every question you may have about your blade.

...If correctly processed in the higher range, D2 has a temperature resistance in use and exposures on the knife blade to 950°F (510°C).
Click to expand...

It sounds to me like the "secret" to D2 is to follow the heating directions, go with a rapid plate or oil quench, not snap temper, freeze, and temper 2 or more times at 400 to 450. Is that roughly accurate?
 
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Steels with that much chromium will tend to loose toughness in the secondary hardening range. This is primarily true for stainless steels, though D2 likely isn't too different considering it's chromium content. They will tend to loose corrosion resistance as well. The reason is part of the increase in hardness at secondary hardening temperatures comes from the precipitation of chromium carbides, some of which will be at the grain boundaries, providing a path for cracks with very little in the way to stop them, and also requiring very little chromium carbide to form.
 
Could Nathan or someone else explain where RA comes into all of this? Something Nathan mentions in his thread, but doesn't really get into where the variable RA comes from in D2. Is tempering a factor, or just austenizing temps?

And if hardness is increased with oil quenching, why plate quench?
 
The retained austenite increases with high austenitize temps.
In high alloy steels you can't get rid of it, but you can minimize it some ways. Nathan method is to stay low with the aus temperature and then quench to Mf; this way he puts the minimum carbon into solution necessary to have the stronger martensite, which he tempers to the lower range, aiming for strenght and finely dispersed little carbides.
Another way is to austenitize high, quench and cryo, and mess with the inevitable retained austenite (due to the supersaturation of the austenite), trying to destabilize it someway with high tempers.
The thing is that with high aus temps the martensite will be filled with RA, loosing it's strenght potential and, as Nathan pointed out, with the high temper you only convert part of that RA into carbide precipitates (at the grain boundaries), thus destabilizing a portion of RA wich will then converts part in fresh martensite, part into bainite etc..
In the end you will get super high tempered martensite (not so strong), a bunch of carbides in bad places, and still the same RA as in the alternative (and better for knives) method.
 
stezann said:
The retained austenite increases with high austenitize temps.
In high alloy steels you can't get rid of it, but you can minimize it some ways. Nathan method is to stay low with the aus temperature and then quench to Mf; this way he puts the minimum carbon into solution necessary to have the stronger martensite, which he tempers to the lower range, aiming for strenght and finely dispersed little carbides.
Another way is to austenitize high, quench and cryo, and mess with the inevitable retained austenite (due to the supersaturation of the austenite), trying to destabilize it someway with high tempers.
The thing is that with high aus temps the martensite will be filled with RA, loosing it's strenght potential and, as Nathan pointed out, with the high temper you only convert part of that RA into carbide precipitates (at the grain boundaries), thus destabilizing a portion of RA wich will then converts part in fresh martensite, part into bainite etc..
In the end you will get super high tempered martensite (not so strong), a bunch of carbides in bad places, and still the same RA as in the alternative (and better for knives) method.
Click to expand...

All the HT best practices I run across recommend lower austenizing temps to limit things like grain growth. Is there a metallurgical reason a higher temp would be preferred for D2, aside from limitations on temp control or time?
 
Anyone have an opinion about when higher austenizing temps would be used? Nathan?
 
lower austenitizing temperature are better for knives...maybe i wasn't clear in my post.
Higher temp austenitizing is used, as Nathan pointed out, when you are making dies and the like. In that case you will actually BENEFIT of a less strong matrix, filled with lot of wear resistent carbides, because toughness and wear resistance are the good features for the specific application.
In any case heat treating D2 is something for the people with good temperature control....with that much carbon and chrome there is little room for any guess.
 
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