Help with HT of cpm d2

[PMLKnives]

I recently decided to try out some new steels, just to try something different. I'm wondering how some of the other makers are HTing CPM D2? The info I found online was kind of bare bones. Any advice is welcome thanks alot.
Paul

 

[mete]

It's the same as regular D2 .You should do it the easy way with plate quenching.

 

[PMLKnives]

thanks, but I've never used regular D2, should have mentioned that:foot:. I've got some info
1850F 15-20 minutes
double temper @ 950
but I dont know if this is reliable or not.

 

[jlmetcalf]

After quite a bit of testing I've settled on these steps:

-air quench from 1875. (leave in foil until below appx 1000F)
-dry ice for two hours
-temper twice at 475 (two hours each)

This gives me RC61

Jim

Edit to correct temp

 

[Nathan the Machinist]

In my experience, 15-20 minutes may not be enough time at temp for D2. Actually, it might be for PM because the carbides are fine so they may dissolve faster. But generally, D2 needs to soak a long time. 30 minutes at temp. Grain growth is mostly a function of temp, not time, so don't sweat it.

Notice how Jim puts it in dry ice before tempering. That is the right way to do it for a blade IMO. He is also tempering at 475. I like that temp too. The higher temp is for converting retained austenite. I believe dry ice is a better way to do that. The high temp may yield a good HRC, but some of the hardness is from carbide precipitation. That means weaker martensite, larger carbides, and less free chromium (lower corrosion resistance). It is a good HT and that is how I HT tools, but not blades.

I soak at 1850.

I plate quench in the foil and end up around 62.

You can get five pounds of dry ice at the grocery store for five bucks. I mix it with acetone for better conduction and leave it in there for about two hours.

 

[jlmetcalf]

I think Nathan is exactly right about the high temp tempers. For gauges and tools it make sense to go the easy way and use the high temperature tempering to convert RA and provide dimensional stability. For those applications, a little loss of corrosion resistance and toughness are more than offset by the ease of not having to cryo to get the dimensional stability.

However, for blades the added toughness and corrosion resistance are easily worth the cost of a couple of pounds of dry ice.

Note: I've noticed that CPM D2 seems to be more corrosion resistant than standard D2.

Jim

 

[PMLKnives]

Thanks! Alot of help here and it is much appreciated! I like all the info I can get. Nothing like knowing to much!! My worst fear is having just enough knowledge to get me into trouble. Which I seem to do just fine BTW.
Like I said new to D2, so it does well at those high RC's 61-62? Seems kind of high, not that I'm doubting your knowledge. Just would like to know.
I've found a good suppier of dryice, around here it was a little hard to find. not to bad about 1.50 a pound and they are pretty generous about portions.
So speaking of dry ice, do you guys mix w/acetone? I put my prob into the stuff and after at least 24 hrs(possibly 30) the solution came in at -99 the small golf ball sized piece of dry ice was -100. Just so you know, I had heard you loss something like 30%.
Paul

 

[grease-man]

I don't want to ruffle anyones feathers with this, but I'm just asking for clarification purposes.

Wouldn't the sequence be to harden, then temper, then cryo, then temper again? Instead of cryo before the first temper? Isn't it the function of the cryo to convert the retained austenite, rather than the tempering temperature?

Also, the dry ice and acetone combo, may not be cold enough to to convert all the retained austenite to untempered martensite. My local cryo treater told me a few months ago that temps around -300f are what's needed for ideal results.

Again, I've been led to believe this, I'm only asking to clarify or get better information. Thanks. -Matt-

 

[butcher_block]

crucible calls for -100 but i go with if a little is good then more is betterer (ok i had a LN bottle thats why i use it )

and yes convert of RA is the goal i go right from quench to LN then temper in the morning
the LN is finishing the quench on high alloy steels

 

[jlmetcalf]

Some spec sheets for D2 might mention a flash temper before the cryo but that is normally a precaution when heat treating complex shapes. For a simple shape like a blade, I think it is best to go in the cold as soon as you reach room temp.

 

[grease-man]

Thanks Butch and Jlmetcalf, that's what I was looking for was some clarification and correction! :thumbup: -Matt-

 

[Nathan the Machinist]

Different steels have different sweet spots. The sweet spot for D2 is fairly hard. Softer tougher applications call for a different steel. IMO.

It will still be tougher and chip out less than stainless steel at this hardness and will have better edge stability than it would have at lower hardness, which means a finer stronger edge. It is still plenty tough. Softer (ductile/tougher) and the martensite won't support the carbides as well, which defeats the purpose of having them. IMO.

And as already said, the dry ice is to complete the quench. So it prevents retained austenite rather than convert it. Dry ice used on a tempered piece will convert what will convert, but there is plenty of stabilized austenite left at that point. In a tool that means dimensional stability at the expense of hardness and strength, which is a good tradeoff. In a blade we want all the martensite we can get (I do anyway) so adding dry ice as a continuation of the quench maximizes martensite with practically zero risk of cracking (would be a problem in a tool).

I used to have a chart for D2 that showed the % martensite at different temperatures. While -300 will convert more, -100 gets the majority. The degree of diminishing returns is such that the general consensus here is they are equivalent.

 

[Larrin]

More retained austenite is converted by using a "continuous cooldown", or going into cryo directly after the quench. Tempering can stabilize retained austenite (as well as convert it), so if you're not having problems with warping or cracking, cryo after quenching is best. However, even if you are having trouble with warping or cracking, you want to use a snap temper, a short low temperature temper, to stabilize as little retained austenite as possible.

As far as liquid nitrogen being better than dry ice, that is true, but not to the extent you might think. According to my information, if using a true continuous cooldown, the conversion of retained austenite is almost identical between the two. If you wait just ten minutes between the quench and cryo, then the liquid nitrogen will convert more. Even with liquid nitrogen it is important to get into cryo as quickly as possible, assuming maximum conversion of retained austenite is your goal.