D2 grains, carbides and cryo

[JTknives]

As some of you might remember I annealed a small trial batch of D2 planer blades. I have started my testing of this material the other day and wanted to share my findings as I went along. Plus my mind is full of questions as to how I should go about logically testing this stuff. And by testing I don't mean make a knife and see how it cuts. I mean how to go about settling on optimum heat treating temps as well as cryo times and temper temps and time. As I start adding it all up in my head it seams like an allmost limitless set of possibilities. How does one lay out a proper game plan on how to atack such a mountain of work/data.

I'm getting to a point in my life/knife making where "good" is not enough. If I'm going to spend the time I want the best I can get and that includes ringing out every last drop of performance out of my steel. Because of this I have said good bye to 5160. It has been my go to steel for a very long time but it's just not providing me with the performance I'm after. Most of the blades I do are hunting/skinning knives with a dash of fillet knives mixed in. Because of this I have chosen D2 as a steel worth the effort going after and investing the time/money into doing it right.

So with all that being said any advise you can provide would be most helpful. I'm hoping Nathan will pop in and set me stright. But here is what I got so far.
The blades where annealed from 1850° to room temp at a rate of 40°/hr. After that I cut 2 identical chunks except I pre quench one at 1750° in oil and then both where plate quenched from 1850°. I then clamped in the vise and beat them till the snapped off. I then inspected the grain on both and to my eye the grain size was the same. I don't know how fine a grain D2 can get but I was impressed with the results. I was dreading that my annealing would have messed up the grain as I had read that re doing the heat treat on D2 can be problematic. This is why I was hoping my annealing would clean the slate so to speak. The grain was so fine you could not see the grains with the nacked eye. I then got out my 30x magnifier and inspected the grain and it looks to me rather good.

Here is a picture of the grain at 30x magnification.

I measured the hardness after grinding away all the decarb and I'm sitting right at 87shore (63rc). I then did a test temper at 950° as I don't have my dewar yet and after the temper it still measured 62-63rc.
I'm not planning on using 950° temper I was just wanting to see how it would affect this D2. I feel the more data I can collect the better off I will be.

So that's it for now. I hope the dewar arrives soon so I can really get after this. But in the mean time I will use this time to plan out a proper "plan of atack" so any advise would be great.
Thanks guys

 

[nickandersonart]

I just got a length of Böhler D2 myself so I'll be watching this thread too, as I haven't used D2 previously.

I did read a great thread by Nathan from a while back about how doing a snap temper on D2 before cryo treatment can cause fatal issues in the steel, specifically the chippy edge that some folks seem to associate with D2. Best is straight from quench to cryo. I'll let him chime in but here's a link to that thread:

http://www.bladeforums.com/forums/showthread.php/839148-D2-heat-treat-edge-retention-testing

Also seems that people have had issues using the upper tier of tempering ranges for D2 and had much better results in the lower range in the 400's or so. Again, just what I've read but yet to try myself. Excited to try out some of this steel.

 

[jdm61]

Well, we examined the picture, and your knife appears to have eczema.

 

[mete]

Unfortunately the photo doesn't tell me much ! D2 has large carbides and that limits the sharpness [definition -sharpness is the smallest radius capable with that type of material.] the carbides wear a bit but then break out of the matrix exposing new carbides. So D2 has wear resistance but not the sharpest edge. You want small carbides but you don't have too much ability to change that .Higher austenitizing temps help there but that means multiple Hardening. After dealing with the carbides then deal with the grain size . Cryo comes last and it's not necessary to snap temper That would be done ,if you want ,at no higher than 300F. Suspend the blade in LN for 4-6 hours ,then temper at 400 F.

 

[JTknives]

So are you saying do a multi quench but run the first one hotter at say 1900°+ then the second one at a lower temp like 1800°. I had always thought or at least read that D2 has enough aloying elaments in it to pin the grain size down. I guess By using a higher austenitizing temp you desolve more carbides into solution but at the risk of larger grain size. but then re quench at a lower temp to deal with grain size but not hot enough to desolve the carbides?

 

[Nathan the Machinist]

So are you saying do a multi quench but run the first one hotter at say 1900°+ then the second one at a lower temp like 1800°. I had always thought or at least read that D2 has enough aloying elaments in it to pin the grain size down. I guess By using a higher austenitizing temp you desolve more carbides into solution but at the risk of larger grain size. but then re quench at a lower temp to deal with grain size but not hot enough to desolve the carbides?

It doesn't exactly work that way. Some complex steels, M2 and D2 for example, can experience extreme grain growth from a second heat. That doesn't mean they always do (quite the opposite is often true) but it is a common issue with these steels and is the reason accepted industry practice is to anneal before attempting a second heat.

This is what happens: The considerable carbides pin the boundaries and allow these materials to soak a long time at high aust temps without much grain growth. But on the second heat the steel has smaller carbides (carbon in solution rather than carbide) and the smaller carbides pinning the boundaries can completely dissolve on a second heat before hitting equilibrium allowing grains to merge and grow. Thus, two heats is usually considered bad practice.

That said, two heats is the basis of prequenching these materials and is part of grain refinement. Very fine grain D2 is possible by utilizing prequenching, but the first prequench temperature would be in the range of 1600-1800. That's quite a range, and that's a real problem because if the particular piece you have needs to be at the top of the range and you use the bottom - nothing happens. And if the piece you have needs to be at the bottom and you go to the top - bad things happen. And, when bad things happen, the line is so thin that you can have one piece bad and the one next to it good. Or even good and bad areas in the same piece. This is because relatively minor differences in temp in a work piece can cause different reactions. And relatively minor differences in heat condition going into HT. And probably even random variations in alloy distribution in a piece.

So, with D2 you can reliably refine the grain some with prequenching, but you must have an even homogeneous structure going into it or you'll run into problems. And you have to experiment with your particular material and find what it wants and back down from that a little. Given your material and your setup, I'm not sure I'd be comfortable playing with multiple heats. For sure, 1900 for the first heat is probably too high. But without knowing the particulars of your material even that is just a guess. The amount of vanadium plays a big role in this, and that varies a lot from one makers of D2 to another.

 

[Nebulae]

Have you looked at the planer blades as received? I would imagine that as they come from the manufacturer they are probably heat treated pretty well, maybe as good as they will ever be. Good planer blades may be run a little harder than we would like for some knives, but they should be excellent quality, and many are salt bath heat treated in a very controlled and well refined processes.
Looking at what the as received blades look like may give you a very good idea of what you should be working towards, or at least give you a reference point for comparison.

 

[mete]

Nathan , that will surely confuse JT but true for sure !
He should deal with carbides first as that is the easier way.Then deal with grain size.