Subzero quenching

[Steve Sells]

From Knifemaking 2.0

Another thermal process to address is called subzero quenching. The purpose of a subzero quench is to force the conversion of retained austenite after the initial quench into martensite, allowing us to get the metal tempered, rather than allowing it to remain as the brittle untempered martensite if it were allowed to convert on its own. 

The sub zero quench is mainly applicable to high alloy steels. The home freezer is not cold enough for more than half conversion. Crushed dry ice and acetone can result in a temperature drop to about -170°F (-110°C). A quench with liquid Nitrogen will result in a drop to about -300°F (-185°C) and is used for many stainless steels.

The down and dirty way is to crush dry ice and place in an insulated container, along with the cleaned blade. Pour acetone into the container and allow to equalize. This process can be volatile, so take proper precautions. Never add dry ice to the acetone, it can explode. Liquid Nitrogen has less risk.

Remove the blade after 30 seconds, and allow to reach room temperature. Do not lay it down until reaching ambient temperature. A hanger for the blade is good for this. You must use gloves and tongs, and be sure to wear face protection and long sleeves. Do not allow the metal to contact any bare skin. 

Either method requires a follow up with a tempering cycle to address the newly formed martensite. Most high carbon blade quality stainless steels require a hardening process that is atmospherically controlled and done at rather exacting temps that are hard to reach and hold for the time necessary in a home forge. But it is not out of reach of the dedicated smith.

UPDATE it appears that there is a better way to do this, read  down the page for the link for details

[ThomasPowers]

CHECK THAT THE CONTAINER MATERIAL DOESN'T DISSOLVE IN ACETONE BEFORE TRYING THIS!

(yeah we all know this; but...)

[Frosty]

Dad spun some high alloy parts I think for rockets but it was too classified to even let me know the alloy beyond it being titanium. The parts were domes more shallow than a radar dish and about 24" dia. After being spun they needed stress relieved badly they wouldn't even make the short drive to the heat treater's, at about a 30% failure rate. 

Sooooo, it ties in here thusly.  We broke up about 10 lbs. of dry ice into a wash tub of denatured alcohol and dunked the bulkheads (?) till they quit screaming. The trick was to get them completely immersed as quickly as possible, it was like slipping a soup bowl into the dishwater. Dad or Mother lifted them out but I got to dunk them. And no fooling they literally screamed, making the alcohol dance, for IIRC 10-15 seconds, once quiet for maybe another 10 seconds the parts were stress relieved enough to take them to the heat treater. 

So, my question for the experienced is. Will denatured alcohol serve in place of acetone for a cryogenic quench?

Frosty The Lucky.

 

[JHCC]

A little google-fu informs me that a denatured alcohol/dry ice bath is -110F, not quite as cold as the dry ice/acetone bath. Not sure if that's cold enough for these purposes, though. Steve?

[ThomasPowers]

I wonder about that as the temperature of the dry ice should not change and if the alcohol is full strength it has a freezing point of -173.5  degF

so why would the bath  be *less*with alcohol than with acetone? (Acetone freezes at -139 degF; Dry  ice is at -109 degF)

[Steve Sells]

I have never used alcohol so never had the opportunity to measure the temperatures resulting from doing it, or what the resulting chemical reaction would be from doing it,.  Also I dont know what the actual temperature is for forcing conversion, I am just a blacksmith

[Frosty]

Any temperature difference between alcohol and acetone MIGHT be explained by evaporative cooling but I don't think it could be significant. 

I don't know what kind of chemistry occurs between a high alloy steel and a solvent at -109 f. One chemistry fact I DO know is alcohol fumes are less explosive than acetone, it's also a lot less likely to dissolve the container.

Do I sniff some IFI experimenting with this one? Hmmmm?

Frosty The Lucky.

[ThomasPowers]

At -100 degF I believe that evaporation will be a fairly low contributor 

[JHCC]

16 hours ago, Steve Sells said:

Also I dont know what the actual temperature is for forcing conversion, I am just a blacksmith

I was just reading Jay Fisher's (lengthy) article "Heat Treating and Cryogenic Processing of Knife Blade Steels", and this detail struck me:

Quote

Specifically, in cryogenic quenching, the sudden and drastic exposure of the steel to shallow or deep cryogenic temperatures can impose such stresses and shock to knife blades that they can crack. Even if the crack is not visible to the human eye, too fast of a drop in temperature can have a detrimental effect on the actual crystalline structure of the steel with microscopic fractures, and that failure will present itself as high wear of the steel, and a markedly less-than-optimum condition. In order to quench at an effective and continuous rate, quench staging can be employed. Depending on the steel, an initial quench based on the medium (oil, air, water), followed by freezing to below zero, and then slow cooling to shallow cryogenic temperatures, and finally deep cryogenic temperatures, if required. The rate must be controlled carefully, and each type of quenching has specific means, specially designed devices, and equipment to control this rate so the cooling is continual, even, steady, and uniform for the specific cycle and range.

What is the specific rate of cooling below room temperature for most of these steels? 4-5 degrees Fahrenheit per minute. That means in order to reach -100°F, it should take about 40 minutes (from room temperature), and to reach -325°F should take about an hour and a half (from room temperature). This is why simply dipping blades into cryogenic baths of dry ice and alcohol or liquid nitrogen is a huge and destructive error, yet knifemakers who are uneducated in this process frequently do this, and tell others that it's the proper way to quench! Sad, truly sad for the knife client. The cryogenic process cooling rate is absolute and critical.

There's a lot more there (most of which is waaaaay over my head), but I'd be interested in any analysis or counter-arguments.

 

(NB: In keeping with the IFI TOS, I'm not going to link to the article, which lives on Mr. Fisher's commercial site. However, it is easily found by doing a websearch for "Jay Fisher" and "Heat Treating and Cryogenic Processing of Knife Blade Steels".)

[Steve Sells]

thanks for posting that, I printed it out and have started reading it already, its great

 

[Frosty]

Thank you John! I'm not a bladesmith guy but I'm interested in all things metal and this is a good thing to know about.

I'm downloading the article now and bookmarking Jay's site.

I'm liking this article better with every paragraph and I'm only part way through the introduction. I'm going to save and rewrite his response to critics for my use here. Double thanks John!

Frosty The Lucky.

[stevomiller]

Kevin Cashen and Achim Wirtz, as well as Verhoeven are good gents to research as well if you want to learn the best methods of getting the most out of creating high performance tools, via all angles of heat treating. They have much work aimed at highest performance vs what’s best economically for large industry, or for large dies and molds vs something the size of a knife or hand plane blade. 

Additionally subzero using dry ice and cry do two different things to tool steels, subzero gives you greater transformation of retained austinite to martensite, cry temps can create additional carbide nucleation site.

 

[BartW]

Some very good articles about this :

cryogenic-part1

cryogenic-processing-of-steel-part-2

I've talked extensively to Achim about this; and I'm not a fan of dry ice. Not cold enough really.

 

mvg; bart

[stevomiller]

Bart, thanks for posting that detail rich information. The fact it’s written for non-metallurgists is a plus.

[jlpservicesinc]

Great info.. 

[Sanda220596]

Thank you for Posting, Bart