Quenching to harden

After reading some of Mr. Eastburn's reply below, I started thinking of back when I was going to welding school in the Marine Corps almost 20 years ago.
One of our lessons was to make a chisel. We started with mild steel, got it red hot, pounded it into shape, then quenched it in oil.
We were told that quenching the steel in oil was better than quenching in water. I'm sure there was an explanation with it, but I don't remember that part. I just wanted to light that fire! :)
So: Why? What different happens when steel is cooled in water verses oil? Also, what does cooling something in sand do? I've heard of this technique before, but it seems to me that this would make some very soft steel.
I understand that the water has a lower boiling point than the steel, but other than that, I'm at a loss.
Thanks! ~Joe
P.S. ...And I hope other readers are benefiting from answers posted to my lack of knowledge questions. I can see that I am the source of 4 of the last 14 questions asked on this newsgroup.
"Martin H. Eastburn" wrote:

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Probably it wasn't mild steel, as the definition of "mild steel" is low carbon i.e. not hardenable.
Oil has less thermal shock than water, and thus gives a slightly less violent quench, gentler if you will. There are water-quenching tool steels (e.g. W-1) and there are oil-quenching (e.g. O-1) just as there are air-quenching (A-1).
You probably just didn't notice at the time what kind of steel you made your chisel out of.
I don't know about cooling steel in sand.
Grant
jp2express wrote:

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Ok, so they probably had us use oil because they only had cheap mild steel.
Also, that explains why we used the sand on the cast iron blocks.
Thanks!
"Miller Nut" wrote:

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So, do most iron and steel sellers sell tool steel or does that have to be purchased from special shops?
"Grant Erwin" wrote:

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A couple quick basics:
It is difficult to quench harden mild steel. As Grant mentioned, what you used for making the chisel most likely was NOT mild steel.
As you increase the alloy content of steel (including carbon), the ability to quench harden increases. The general concept is that alloying elements such as Cr determine how easy it is to quench harden and the amount of carbon almost exclusively determines how hard it will be when it is quenched.
As Grant mentioned, there are different tool steels that are rated by what is needed as the quenching medium (water, oil, air) to achieve the "full" hardness. An increase in the alloying elements (including carbon) increases the ability to quench using "slower" quench media (going from water, which produces the fastest cooling rates, to oil to air, which is obviously the slowest cooling rate). In other words, a steel with a low alloy content and realtively low carbon content will require a full water quench in order to harden it. As the alloy content is increased, then a fully hardened structure can be obtained with a slower quench in oil and, if enough alloying elements are added, it may actually fully harden with just air cooling.
Some examples: Tool steel W-1 requires water quenching to fully harden. It has a carbon content of about 1.0% C, which is MUCH higher than mild steel (typically 0.30 % C or less) but almost no other alloying elements. Tool steel O-1 can use oil quenching to fully harden. It also has a carbon content of almost 1.0% C but also has about 0.5% Cr and some other alloys (e.g. 0.5% W). Tool steel A-2 will fully harden with just air cooling. It also has about 1.0% C but uses about 5.0% Cr and 1.0% Mo. (see the designation code? W-x for water quenching steels, O-x for oil-quenching steel, and A-x for air quenching steels)
Note that these three tool steels have about the same carbon content. Therefore, as a general approximation, the maximum attainable hardness will be the same for all three materials. It is just that the O-1 and A-2 can achieve this maximum hardness with much slower cooling rates.
Of course, there are some other reasons for having the different alloys (it costs a lot more for the higher alloy materials, higher alloy materials will retain their hardness at elevated temperatures, etc.)
It is desirable to use the slowest cooling media possible (i.e. to use oil rather than water) IF the desired hardness can be achieved because quenching introduces a lot of internal residual stresses and can lead to cracking. The more complex the shape and the more there are differences in sections thickensses, the more desireable it is to have a slower quench rate. Using the higher alloyed steels (e.g. O-1 or A-2) allow the fabricator to achieve the required hardness without resorting to excessive quench rates. So, it is preferable to use an oil quench rather than a water quench if you can get the hardness you want.
What about sand cooling or vermiculite or ashes? This for situations where you do NOT want the component to harden. You are trying to keep the cooling rate as slow as possible to make the piece as soft as possible or keep it from cracking (due to heat-generated internal residual stresses). Someone mentioned cast iron. A very brittle material that is very prone to cracking from heat-induced thernal stresses. Slow cooling is desirable for this type of material.
I am sure there are a lot of newsgroup participants that make knive blades or their own tools that can add a lot of details and practical information about this.
Regards
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Wow! Great info. Thanks!
So, what is with getting the steel red hot and pounding on it with a hammer? Does this harden anything or is it not necessary with the W-1, O-1 or A-2 metals?
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jp2express wrote:

That's just forging. Heating to red allows grain growth, and over time this is undesirable. You are pretty much mixed up about the difference between heating metal to forge it, heating it then quenching it to harden it, and heating it and letting it cool to temper it or anneal it. Time to get a book on the subject, or at least to read up a little on the Web.
Grant
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On Jul 13, 10:37 am, snipped-for-privacy@austin.rr.com wrote:

Another reason for using alloys that harden at lower quench rates is that the fast quench rate is at the surface of the part. If you have a 6 inch cube of high carbon steel, you can't harden the middle of the block. 4140 is used for parts that are not too thick. 4340 is used for thicker parts that need to be hardened all the way through.
lso not mentioned is that the strength of steel is proportional to its hardness.
Dan
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I buy tool steel from MSCdirect as they qualify the vendor via sales... And I can generally get what I want.
Martin
Martin H. Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net TSRA, Life; NRA LOH & Endowment Member, Golden Eagle, Patriot's Medal. NRA Second Amendment Task Force Charter Founder IHMSA and NRA Metallic Silhouette maker & member. http://lufkinced.com /
jp2express wrote:

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On Fri, 13 Jul 2007 13:03:45 -0230, "Miller Nut"

Vermiculite, available at most gardening shops, is the old standard for slow cooling media.

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"Gunner" wrote: Vermiculite, available at most gardening shops, is the old standard for slow cooling media. ^^^^^^^^^^^^^^^^^ I've always been told to use ashes. I haven's seen a pile of ashes in years. My suggestion would be to build a big bonfire, and then throw the metal piece in and rake the coals over it. But not on a "spare the air" day.
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Not enough carbon to mention.
Please get this book: http://www.feine-klingen.de/PDFs/verhoeven.pdf Which is a downloadable book on metallurgy for knifemakers, and free. Read pages 61 through 65. Note the rate of carbon migration in steel relative to temperature. Also note that carbon is absorbed into iron primarily through carbon monoxide gas. Given the slow rates of carbon transfer at quenching temperatures (1500F-1700F), even if you could get oil from the carbon, it wouldn't move in far enough in the less than 2 second window you have to get an appreciable amount into the steel.

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Great book, thanks for the link.
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Oil cools slower. Water absorbs more heat before boiling which cools what is left.
There are oil metals and water metals.
The AR material I use is water metal. The QA docs state that.
Martin Martin H. Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net TSRA, Life; NRA LOH & Endowment Member, Golden Eagle, Patriot's Medal. NRA Second Amendment Task Force Charter Founder IHMSA and NRA Metallic Silhouette maker & member. http://lufkinced.com /
jp2express wrote:

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