Quenching 440c air or oil?

[Brian Dougherty]

I will need to heat treat some non-knife related parts in a couple of weeks that are machined from 440C.  (I agreed to help out a student team at the college I work for so this is a shoe-string budget job)

 

I've never worked with 440C, but according to the datasheets I found it can be air hardened with forced air, or interrupted quenched in oil.

 

I had planned to wrap the parts up in foil to keep the scale minimized.  This makes me a little concerned about air hardening since the foil will slow things down a bit.  However, I think what the TTT curve is saying is that I have over 6 minutes to get below 1200F.  Am I reading that correctly? (PDF attached)

 

These parts are all lathe turned from 1/2" diameter stock, and only a couple inches long.  It would seem to me that even wrapped in foil, such small parts will easily cool from 1900F to 1200F in less than 6 minutes without the stress of oil quenching.

 

Their target hardness is around 54C, so I don't need to eek out every bit of performance with this.

 

Would anyone care to weigh in on air as opposed to oil in this case?

 

Thanks in advance...

 

Data Sheet 440c.pdf

[Jerrod Miller]

Delicate parts should be heated carefully.  Heat to about 1425 and hold until fully evenly at temp.  Or if they are extremely delicate, do a hold at 1000, too.  This alloy has a lower thermal conductivity than most blade steels, so it takes a little longer to come up to temp throughout the part.  Should still only be a couple minutes of soaking, you just don't want to blow right through it.  These little guys probably will want to be cooled more from 1850-1875; unless corrosion resistance is necessary, then you can go all the way to 1950.  Soak at temp should be in the range of about 45 minutes (or 90 minutes if the steel was fully annealed first).  Oil quenching will get you not just maximum hardness, but maximum corrosion resistance and ductility.  Martempering would be great.  For maximum dimensional stability, cool the parts all the way down to -100F from the quench (before temper).  Double temper at 550F.  

 

When I don't need to cool overly quick but quicker than air, I use room temp canola.  I think if you go on the hot end of things, and make your SS packet easy to open, then you should be able to remove your packet, open it, and get the parts in oil without too much problems and be in pretty good shape.  And yes, you were reading the TTT correctly.  

[Brian Dougherty]

Thanks Jerrod,

 

I was going to heat to 1100F and equalize, then bump up to 1450f and equalize, then go up to the quench temp.  Most of the part looks pretty robust in their drawings, but there is one feature I am a little worried about.

 

I've got some McMaster 11 sec. oil at that I could use.  That's a bit slower than warm canola as I understand it, so it might approximate the cold canola.  I might just quench in the foil.  I've done some work with stainless where I have used a vacuum press to make sure the foil was tightly pressed against the part to minimize thermal impedance, but those were pocket knife blades that were very simple shapes.