Questions on Heat Treating O2 Steel

I'm currently working on a couple home-made blades and I have some questions for anybody who has heat treated O2 steel at home.

I found the following on the Hock Tools web site:

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When it's hit critical temp, remove it from the heat and quickly dunk it into a sufficient quantity of oil (preheated to about 150F.) Swish it around a bit until it's cooled then let it cool to ambient in the air.

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So, here are my questions:

[1] What type of oil is best to use? I've read everything from automatic transmission fluid to peanut oil.

[2] My understanding is that one wants to use a lot of oil so as to reduce the chance of flash fire when the hot metal hits the oil. How do I heat up a couple gallons of oil without risking fire?

The thinner the oil, the faster the cooling, and the harder the steel.

But not by much.

I use a gallon of linseed heated in a double boiler rigged from a kerosene space heater and a steel bucket. The linseed gets used later in boat soup. Keep a wet towel handy to drape over it should the can catch fire.

If you have the proper formula for heat treating your steel, you don't have a furnace and you haven't done a lot of this, then you might wanna use Templac temperature crayons from brownells.com for accuracy.

Otherwise, place a steel bucket on it's side nest to your torch to stick the workpiece in....the deep shade provides more accurate color than natural or artificial lighting, and if you do it outdoors, the bucket makes a good winf break.

Get a big roasting pan and use a camp stove or for that matter any outside grill and heat it up using a cooking thermometer. When it gets to 150-160 turn the flames off and soak your steel. Should work. :eek: ;)

Oh ya if I remember right, it should be a heavy grade of oil.

Hi Dick-

Are you asking about heat treating A-2 or O-1 ? A-2 can be done by the hobbiest, but the results are iffy at best and most folks leave it to the pros. There is a very particular schedule for ramping up the temp, a dwell time, and then ramping down. O-1 on the other hand is easily done by the home hobbiest with nothing more than a propane torch and maybe some fire bricks if the piece is large like a plane iron. For oil you can use almost anything. I personally use either walnut oil or peanut oil because they have high flashpoints and generate less smoke than motor oil. Besides, the smell isn't as bad. If you seach the old posts, I did a description of how to treat O-1 back a few months ago.

I followed Dave Anderson's method of heat treating, with a magnet for the temp point, and walnut oil (couldn't find cherry oil). But, I did not preheat the oil to 150. How important is this? What does it do? Does it eliminate the next step of backing off the brittleness in the kitchen oven at 300 degress for 30 minutes or so?
Dave's method worked fine on two marking knives, but I love to learn, like most of the guys here at SMC.

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Originally Posted by Alan Turner

But, I did not preheat the oil to 150. How important is this? What does it do?

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I don't know either.

My WAG would be that it helps prevent splattering?

Dave?

Somebody?

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Does it eliminate the next step of backing off the brittleness in the kitchen oven at 300 degress for 30 minutes or so?

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I don't think so. BWDIK?

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Here's another question though:

How quickly must one go from quenching to tempering? Is it okay to stop and give the blade a good washing first to get the oil off?

Well, I can't answer the first ones, and I haven't preheated the quenching oil with the old carbon steel I've retempered. Maybe it is an added safeguard against cooling down so quickly as to cause the steel to shatter or crack. While I did have a chisel crack when quenching in water one time, I've not had that happen with oil quenching.

As to tempering after quenching, shouldn't be a time limit involved. As to cleaning, that is a neccesity along with using an abrasive to clean back to shiny metal if you are gonna use the color of the heated metal to judge your tempering.

From what I understand with O1, once quenched in oil it needs to be straw tempered. A good way to do it is stick it in a 350 degree home oven or toaster oven.

The reason for preheating the oil is to lessen the shock on the steel. It also decreases the viscosity of the oil, which allows it to come in contact with the steel better.

Here are the critical steps to get simple steels (01, 1040-1090,5160) hardened.

Bring it to critical temperature. This is also non-magnetic temperature.
Quench it. How quickly the temperature drops determines how hard the steel gets. The faster it drops the harder the steel, but this also puts a ton of stress into the steel. So if you quench it in something too agressive for the steel then you end up with cracking.

Below is the list of quenches from slowest to fastest. If you heat it or cool it then it affects just how fast it quenches.
Oil
Water
Brine

At this point you have a full hard piece of steel. Drop it on concrete and it might just shatter like glass. Good luck sharpening it. A common test to see if you got it hard is to try and file it. The file will just skate across the steel.

Now you want to temper it. This reduces the hardness but makes the steel less likely to chip. This can be done in one of two ways. By colour or by temperature.

By colour you clean the crap off the steel and then heat the piece until the desired colour is seen. You then quench it to prevent going past where you want to.

By temperature you just put it in an oven set for a specific temperature and leave it for 1 hour per inch of thickness. Get an oven thermometer since most home ovens are notoriously bad for being inaccurate. (This is also a good peace offering for the SO, and helps prevent burned pizzas, A pizza stone would also help things).

Check out this link for more info on steel hardening.
http://www.key-to-steel.com/Articles/Art12.htm
Machinery's Handbook also has way more info than you ever want to know about hardening steel and other fun topics.

Scott,

Very quick simple expose on hardening/tempering. I'd only add that if going by color, for chisels and plane irons and the like, you want to get a straw color. If it goes to blue, you blew it and have to start over.

Now, about that pizza stone, will that help the tempering process, or just the pizza cooking? And why would one want to sharpen a pizza, anyway?:D

Thanks Jerry,

A straw colour is probably best for chisels and plane blades. The key with tempering is you can always temper more if you go too low the first time. If you go too high, it will become too soft and then you have to anneal it and reharden it.

As for the pizza stone, it will just help the oven hold a steady temperature. This helps both pizza and tempering. The more consistent the temperature for the steel the more accurate results you will get.

As for the 'za, it also helps make sure you don't end up with a burnt crust and uncooked toppings.

Treat O2 like O1, for our imprecise methods there isn't much difference. I use olive oil with good success on O1. I preheat the oil either by setting it's container double boiler style into a vat of very hot water. If I'm in a hurry I just stick a hot piece of bar stock in the oil once or twice until it warms up nicely.

Since I'm only doing one or two small tools at a time I just bought the olive oil in one of those tins and cut out the top. Looks like about a gallon tin. It's deep enough for a knife and wide enough for a BIG plane blade. I keep the oil tin inside a small galvanized trash can like receptable with a lid i can drop on if it ever catches fire (which it hasn't).

Watch out for splatter - hot oil is much worse than hot water.