Heat treating high alloy blades.

I had to heat treat a couple blades yesterday, and since I haven’t talked much on this blog about heat treating, I thought I’d make a post of it.

One blade is made of D2 tool steel, the other one (the small one) is D5.  They’re heat treat schedule is similar, so I can do them at the same time.

Here they are mostly ground and finished to 240 grit.  It’s always a good idea to finish blades to at least 240 grit before hardening.  If left rough, you might get some cracks….


Since these require a higher temperature than the simple carbon steels, the blades are put in stainless steel foil envelopes which protect them from oxygen.


The oven is set to 1850 degrees F:

and the blades are put in to “soak” for about 40 minutes.

While that’s happening, it’s time to work on the quenchant.  For carbon steels, I use different types of oil.  For air hardening steels like D2 and D5, I first quench them in a plate quenching setup, and then they go into a mixture of dry ice and acetone, for what’s called a “cryogenic quench”.  They reason for the cryo, is that many high alloy steels don’t fully convert to martensite (the hard stuff) at room temperature. They need to be taken much lower.


So, here’s some dry ice crushed up (I think I used 2 or 3 pounds total):


And here’s a tank made of PVC with almost a gallon of acetone, with dry ice added.



So, when the 40 minutes is up, the blades are taken out (one at a time) and squeezed in my plate quencher (which is a carpentry vise with a couple of heavy aluminum plates attached):



When the blade is well below 1000 degrees, the envelope is removed and cut open:


Then the blade is checked for straightness (the steel is still soft until it gets below 200 or so), and left to cool to room temperature.

The D5 blade needs to be tempered a little bit at 350 F before going into the cryo, so it goes in the oven (otherwise it might crack), and the D2 blade goes into the acetone:


The D2 is left in the cryo for a half an hour, then it switches places with the D5 blade.  Here’s how it looks at about -109 F:



After both have been cryo quenched, then they are tempered at successively higher temps until they reach the hardness I want. (actually the softness I want, since tempering reduces hardness).  Looks like the D2 is going to finish up at 450 F, but I think the D5 is gonna have to go higher.




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