Warming Oil

[Cody]

I have been reading here and I have seen that if you warm the oil it will give a faster quench. Did I read this right? I have warmed the oil before, but I did it because I was told to and I thought that it must slow the quench. Being an "armchair engineer" I would think that warm oil would slow and ice cold would speed up. Is it because the oil gets thicker when cold and thiner when warm? Is this true with quenching in water? Thanks for your patience for the newbie ????

[son_of_bluegrass]

It is my understanding that the lower viscosity of warm oil allows for faster convection which takes heat away faster. I believe there is a point where warming the oil further doesn't add significantly to the speed of cooling.
Water is already very thin and doesn't become faster when warmed. Water tends to be such a harsh quenchant that it is sometimes warmed to reduce the speed of cooling with the idea it will also make it less harsh.
Interestingly adding salt to the water will speed the quench and supposedly is less harsh than plain water.

We all didn't know sometime.

ron

[macbruce]

I remember quenching in below freezing oil once and my piece cracked.......Warmed the oil and no problem.......................

[Cody]

Bluegrass.... that's very interesting that if you add salt that it speeds up the quench and makes it less harsh. That seems contradictive to a dumb country boy like myself. I think I'll have to look into that further!!! Thanks for that tidd bitt!

[ThomasPowers]

adding salt raises the boiling temperature of the water and as it's the steam jacket that plays a major point in making a water quench uneven and a bit slower; lessening that steam jacket makes it a "stronger but more even" quench.

Makes perfect sense to me. And explains some of the old smith's tales about special quenching liquids being better like Theophilus' suggestions in "Divers Arts" from 1120 A.D. "urine from a a small red headed boy or from a goat fed ferns for 3 days" or a whole list of ones from the renaissance in "Sources for the History of the Science of Steel" that include radish juice and "worm water".

BTW the urine one works but has such an amusing smell when red hot steel hits the stale urine...

[Cody]

Thomas, most of that make sence now. Not sure what difference the color of the boys hair makes, but the salt raising the boiling point, and thus changing the steamming point makes sence. Thanks for taking the time to explain it!!

[brian.pierson]

the hair color makes no difference but the "magic" of red heads in those days does. just like rubbing the head of a red headed boy for luck. Okay not relevant in a science world but interesting anthropology.

Brian Pierson

[MattBower]

The salt doesn't just raise the boiling point. As water flashes to steam it actually forms tiny salt deposits on the hot blade, which then basically explode from the heat and disrupt the vapor jacket. See page 13 of Houghton on Quenching. The vapor jacket slows down the quench and also tends to make it extremely uneven, which is hard on steel. So by eliminating the vapor jacket you make the quench both faster and more uniform -- and the lack of uniform cooling is one of the things that makes water quenching so dangerous.

And here's an interesting aside. If you look at page 12 of Hougton on Quenching, you see this quote:

The stability of the vapor phase is dependent upon the surface finish of the component. The vapor film is very persistent on flat smooth surfaces, but breaks up readily with the onset of the boiling stage at sharp corners,

rough surfaces

, defects or other stress risers. This variation in stability can produce markedly different cooling rates across the component, resulting in distortion and cracking.

I always used to wonder why the Japanese put a thin clay wash on the edge of the blade, which is supposed to get full hard. It seemed like the clay would only insulate the edge, which would be counterproductive. (When you're differentially hardening with clay you want to insulate the spine, but the edge...?) But I think this explains it pretty well. The thin clay wash a little roughness, and roughness breaks up the vapor film and gives a faster quench. (That's assuming that the clay is quite thin, of course. At a certain point the insulating effect outweighs the extra quench speed.)

I think it probably also helps explain why Japanese smiths are able to get away with quenching in water as much as they do. The clay likely makes the quench speed more uniform.

Clay quenching. It ain't just for pretty. :)

[son_of_bluegrass]

You may find it useful to read a basic metallurgy text (or the relevant parts). I think "Metallurgy Theory and Practice" by Dell Allen (I think that is right) is a good start and can often be found in a used bookstore for a few dollars. If you can find it, "Metallurgy of Steel for Bladesmiths & Others who Heat Treat and Forge Steel" by John D. Verhoeven is available as a free pdf for non-commercial applications.

ron

[MattBower]

Actually, I have heard that that the free version of Verhoeven's book that's currently available on the web is in violation of his copyright. Verhoeven allowed it to be freely distributed for a while, but once he turned the revised version into a full scale book published by ASM (a rather pricey one, at that), he withdrew the license for free distribution of the older version. However, since the website that's hosting the PDF is German, apparently it's not that easy to resolve the problem.

For folks with no formal knowledge of the metallurgy of heat treating, I think this is a pretty decent intro to the basics:

http://www.asminternational.org/content/ASM/StoreFiles/ACF180B.pdf

It's not a how-to, but it introduces you to the important concepts and terminology.

[Steve Sells]

Or read the FREE heat treat sticky here. its not that bad as a reference, and may even explain a few things to you.

[MattBower]

Good point, Steve! Sorry about that!

[Nakedanvil - Grant Sarver]

I always used to wonder why the Japanese put a thin clay wash on the edge of the blade, which is supposed to get full hard. It seemed like the clay would only insulate the edge, which would be counterproductive. (When you're differentially hardening with clay you want to insulate the spine, but the edge...?) But I think this explains it pretty well. The thin clay wash a little roughness, and roughness breaks up the vapor film and gives a faster quench. (That's assuming that the clay is quite thin, of course. At a certain point the insulating effect outweighs the extra quench speed.)

I think it probably also helps explain why Japanese smiths are able to get away with quenching in water as much as they do. The clay likely makes the quench speed more uniform.

Clay quenching. It ain't just for pretty.

I believe they apply the clay to the spine and not the edge. The quench transformation to martensite of the edge is also what puts the curve into the blade.

[ThomasPowers]

He was pointing out that there usually is a thin "clay wash" on the edge as well as the thick layer on the spine and the reasons why that might help.

[Nakedanvil - Grant Sarver]

Thanks Thomas!

[thingmaker3]

I've occasionally wondered if the "red hair" requirement was to allow the urine to age while being collected. Something in the urine breaks down into ammonia over time.

Regarding the thin layer of clay over the edge, I remember reading an engineer's analysis of it. It speeds the cooling of the edge due to increased surface area. I'll try to find the article.