Liquid Nitrogen vs Dry Ice

[Larrin]

I've tried to keep up with this interesting topic and forgive if I've missed it during such BUT........

How long does a cyro or sub zero treament need to be? I've read 10 minutes, 1 hour and 24 hour?

I've actually read one knife makers web page articles that stated cyro was definitely needed but ALSO BETWEEN TEMPERS?

The transformation of austenite to martensite is near instantaneous and is based on temperature not time. So to convert austenite it only needs as long as to ensure the whole piece has reached temperature. However, some have found that enhanced eta carbide formation results from longer times such as 24-36 hours. I would personally do an hour. In the past Roman has recommended cryo between tempers. I’m not sure what he bases that recommendation on.

 

[Jrmysell]

The transformation of austenite to martensite is near instantaneous and is based on temperature not time. So to convert austenite it only needs as long as to ensure the whole piece has reached temperature. However, some have found that enhanced eta carbide formation results from longer times such as 24-36 hours. I would personally do an hour. In the past Roman has recommended cryo between tempers. I’m not sure what he bases that recommendation on.

Would you also go straight from quench to cryo or do the snap temper before? I've read both ways. I don't see how cryo works after tempering though since (from what I have read, it should be a single drop in temp from aust. temp (is this also correct)?

 

[Larrin]

Would you also go straight from quench to cryo or do the snap temper before? I've read both ways. I don't see how cryo works after tempering though since (from what I have read, it should be a single drop in temp from aust. temp (is this also correct)?

Cryo works best direct from the quench so that would be my recommendation unless you are getting cracking.

 

[mete]

From what I have learned is that cryo tweaks the matrix which makes room for the formation of eta carbides . But when it comes to timing, evidence is that it's dependant on the alloy. So tests would have to be made for each alloy you want to use !! I have been mentioning at least 6 hours .I don't have any data for different alloys.
My background in industry for bainite is that times can be very long for bainite transformation making some alloys foolish to use .Maybe cryo is the same thing ??

 

[LCoop]

Ok this question just came to my mind tonight! Can I with appropriate equipment (oven heat verified with temp sticks and new RH hardness tester) heat treat with DI afterwards and only due one temper cycle and then a day later do the second temper cycle?

I guess my question is how long can you do between the tempering cycles?

 

[mete]

The stresses in the martensite transformation are great ! The temper should be done quickly otherwise your blade may crack. A second or third temper can wait but I wouldn't wait more than about 24 hours.

 

[Nathan the Machinist]

That’s what I was gathering from Nathan! Makes sense. My tests won’t be as intricate as yours, but when I get my equipment, I plan on doing plenty of tests, with various steels. It’ll be years before I know anywhere near what you guys do, but I’m trying to learn! Thanks Larrin.

People seem to have different ideas on what is meant by "toughness". One way to define it would be the amount of energy absorbed in a fracture impact which can be done by swinging a pendulum through a sample and measuring how far it continues to swing after the break. Unhardened steel is quite tough by this test. But folks using knives have a different idea of toughness and one very important aspect is the ability of an edge to shrug off damage in rough use ("tough" knives see rough use). Obviously this is very geometry dependent, but if you control all your variables such as alloy, hardness and geometry, in controlled cut testing you'll find that a heat treat that is designed to minimize RA will often have improved edge stability and will be "more tough" in use while possibly having lower measured impact toughness values. It's a little counter intuitive but strength and toughness often have an inverse correlation. So, when the literature says that RA increases toughness (and possibly durability in something like a stamping tool) it doesn't necessarily mean it will make a more durable knife.

It's just my opinion but I don't think there this is much utility to RA in a knife, it plays hell with edge stability and that's very important. I think of zones of soft material in a knife edge (RA, ferrite, nickel, undersoaked or over tempered martensite, etc) as being like the perforations in a postage stamp, allowing an otherwise hard strong structure to roll or break free when those soft spots go across the edge.

 

[shqxk]

“Tool Steels” 4th edition page 346 has a little info on 52100 regarding quench speed and retained austenite.

Quenched from 1550*; water quench = 5.8% ra, oil quench = 7% ra, salt-quench to 450* then water quench = 6.1% ra, salt quench to 450* then oil quench = 6.2% ra, salt quench to 450* then air cool = 10.6% ra.

I would think you could get very low or no ra with a sub-zero or cryo quench, if that’s what you want.

Hoss

I don't really understand why 52100 salt quench then air cool has highest amount of RA? Isn't quenching fast enough to past the pearlite nose then lets it slowly cool just behind the MS point is the ideal to get the highest percentage of martensite? I could be totally wrong but from my experience 52100 salt quench to 400F 2 min equalize does yield much better hardness than veg oil quench... like 62HRC vs 59.8-60HRC from my test (Both tempered at 400F)