I’m enjoying the show “Forged in Fire”. It is a bit repetitive, but somehow it manages to be fun anyway. So I understand that the process of hardening a blade requires quenching it in oil.
What exactly does this do?
Is the fact that it is oil particularly important?
Are there other liquids which are better?
Is there any hypothetical exotic materials which could be better (but may not exist)?
The purpose of quenching is to cool down the workpiece as quickly as possible. The reason is because there are certain undesirable chemical reactions that occur at higher temperatures, and once you’ve heated the metal enough to harden it, you want to cool it down quickly to prevent those undesirable effects.
You can use any liquid, and plain old water is a popular choice. However, water may cool the workpiece too quickly, causing cracking. Oil conducts heat a little slower, but at the expense of slightly less hardness.
It all depends on the specific metallurgical properties of the material and what the desired results are. All kinds of different mineral oils are used, as well as custom-made water-based quenching fluids.
There are always new discoveries to be made in materials science. But plain old hardened steel is pretty great and has been around for a long time.
Quenching is about subjecting the blade (or whatever) to a fast temperature change. How fast is critical. Oil is faster than water (it doesn’t boil), which is faster than air. But it isn’t a matter that any is better than the other - it is that the rate is correct for purpose. In general the faster the quench, the harder the steel. But, and this is critical, you don’t want the hardest steel for many purposes, as with hardness comes brittleness. So you need to tune the quench to get exactly the balance you want (or at least can achieve.) The starting temperature and rate of quench are both critical.
One are where there is real art in the quenching and general heat treatment is to get a gradient of quench through the material. So that (for instance) the core of the blade is softer, stronger steel, and there is a controlled thickness of hard steel on the sharp edge.
Another step is tempering - which is the reverse. A hardened object may be reheated and cooled at a slower rate to reduce the hardness. This can yield just the right hardness profile.
The hardness changes (especially in steel) are all about the mixture between Austentic and Martenestic steel. These are two different forms of steel that result from where the carbon atoms reside in the matrix of iron atoms. The temperature profile the steel undergoes affects the location of the carbon atoms and thus the mix of the two.
Thanks for the replies. I know I could Google this but I find that the answers I get on The Straight Dope are often more insightful.
You each say the opposite with respect to water and oil. Soooo is oil faster or slower?
With respect to question #4, I mean is there a hypothetical liquid which would be best for quenching. Based on Francis’ reply it seems like maybe depending on what you’re trying to do, but maybe not because existing materials are just fine.
Can we get a decision from the judges on this?
ETA: yup, beep beep, I just saw.
Well, I could help myself I had to look it up on Wikipedia.
So it seems that oil cools more slowly than water.
Also, on wikipedia it talks about quenching with inert gases! Very interesting.
I’m still very interested in insights on the quench so please feel free to post anything you might have to add. As I mentioned above, that’s why I like having questions answered here.
Ah, faulty memory on oil. Water certainly has a much higher specific heat, so if it doesn’t boil should be faster.
Inert gasses will be needed if the metal is reactive. You don’t want it corroding or bursting into flame when it hits the air. A reducing atmosphere is used for some heat treatments as well. Hydrogen for instance. It wil actively remove oxygen from the surface of the tempered material.
Some legends suggest a sword cold be quenched in a passing peasant. :eek:
The finest Japanese blades had varying thicknesses of clay painted on them - from none at the edge, to the most at the back of the blade. When the blade was quenched, the edge cooled the fastest, and was therefore the hardest, while the rest of the sword was less hard and therefore less brittle.
You can purchase ‘tool steels’ designed to harden using different quenching media.
O1- oil quenched
W1- water quenched
A1 - air quenched.
To reach the proper temperature before quenching, a common trick is to keep trying a magnet on the hot steel. At the transition temperature, steel stops being magnetic. It has to be held at the correct temperature for a certain time depending on the thickness of the piece. A machinist would use a temperature controlled oven, but in the forge, it is all in the experience of the smith.
Dennis
That hardly seems sporting.
For some interesting demonstrations of forging techniques, check out Man at Arms. They frequently show oil quenching, but in the most recent episode, Kennosuke’s Sword: Kuromukuro, they demonstrate what happens with water quenching as well as traditional Japanese smithing techniques.
During the water quenching, you can see why a sword might crack, given just how huge a flex can occur.
Thanks for sharing that link! Amazing! I love that they explain what they are doing.
Wasn’t there some legend about Damascus Steel blades being quenched in the blood of a slave? (without extracting the blood into a container first). The idea was that blood also contained iron and carbon, thus assisting the metallurgy.
A bit of googling (I always heard it as “quenched in the belly of a slave”) brings up the claim that that idea is actually from a 19th century German joker.
Boiling is a phase change and takes a lot of energy. Speaking of phase changes, quenching inhibits phase change of steel. Check out a phase diagram.
I think the objection to allowing the water to boil is that the steam provides an insulating layer, reducing the rate of heat transfer.
I suspect the final answer can be covered under the umbrella “it depends”.
Yeah, I had assumed such. Just depends upon how long ago the joker thought up the idea. Some more modern ones are attributable to Prof F Lirpa.
That seems far fetched because Damascus was made in India and there were no slaves in India at that time.
The source I read suggested that virgin girls were the optimum, but I imagine that they were always in short supply.
It may be naive of me, but I would have thought Damascus steel blades were made in … Damascus! Which is not in India. And definitely had slavery in parts of its history.