tempering a knife blade?

[airborne2037]

Im in the process of makeing a knife blade...I have no forge and im almost to the point where im ready to temper the blade...any ideas as to how to heat the blade without a forge..

 

[ohio ramrod]

I find a lead melting pot to be ideal for tempering blades if you want tempering temperature in the 700 degeee range, electic stove ovens to temper at the 500 degree range. :idunno: For hardening before tempering a wood stove will seve as a forge. Just let a good bed of red hot coals form , then heat the blade in the coals. :idunno:

 

[LRB]

Temper or harden? Those are two different things. You can temper in your kitchen range. To harden you need a source of heat that will at least heat the edge zone to a bright red, or red-orange. Evenly, within reason. Quench in canola oil heated to about 125/130°. Temper at between 425° to 475°, using a separate oven thermometer. 2 tempers for 2 hours each, quench in water when you remove it from the oven each temper cycle, or you can just let it air cool in between. Charcoal fed with air from a hair dryer will give enough heat for hardening. Don't let it over heat beyond red-orange.

 

[Rickey Brown]

Use an old BBQ pit, for hardening. Build the Charcoal fire on TOP of the cooking grate and use a hair dryer to blow air into the coals. With the blade inserted into the coals heat it till orange and will not stick to a magnet. This is called critical temperature. Quench in motor oil or cooking oil. When the blade is cooled heat it to 450 degrees, in an oven, to temper for 1 hour, let air cool in the oven do this 2 to 4 times. The more cycles the more the metal molecules align

 

[LRB]

Acually, critical temperature is the target temperature at which to quench a given steel. This temp can vary widely depending on the type of steel you are working with. A magnet loses attraction to iron/steel at 1414°. Critical temperatures generally run from 1450° on up as high as 1950° for some of the stainless types. 1450° would appear as about one shade of red past non-magnetic. The temper process has nothing to do with molecular alignment. The reason being that there are no molecules to align. Steel is a structure of atomic based crystals. When you heat steel above 1333° the iron atom groups begin to allow carbon atoms to join with them. When quenched at critical heat, the carbon atoms are locked in place. If slow cooled, they begin leaving. When you temper, you allow some of the carbon atoms to leave, relieving some of the high stress they caused by crowding in with the atomic iron structures.

 

[crockett]

Do you know what kind of steel it is or what was the source of the steel? There are some stainless air quenchable steels and I think w2 can be quenched in water (brine) rather than oil. On the oil- do outside and obviously have a flash cover in case of a flame up.
Do whatever Wick tells you as he is an expert.

 

[LRB]

Yes, W-2 can be quenched in brine, but it is a gamble. A safer quench would be warmed canola oil. You may sacrifice a point or two of hardness, but you can be relatively comfortable in keeping the blade in one piece. One of the worst choices of quench would be motor oil. It will not cool fast enough give the blade an optimum hardness, and the vapors are pretty toxic, as well as they really stink.

 

[LRB]

To add more to the above. 2, 2 hour temper cycles is plenty, although a third can be done, though it will make no measurable difference. A small gain in the quality of the temper can be had by quenching the blade in water after each temper. The hardening of the blade does not happen until the blades temp drops under 400° to 450°. There is most always some of the Austenite, the structure of the steel that forms when heated, that does not convert to the hard Martensite condition. During the cool down from each temper cycle, some of this left over Austenite converts to untempered Martensite, and requires a second temper cycle to prevent any brittleness this may cause. If cooled too slowly though, some of this new Martensite will revert back to Austenite. This can be prevented by quenching in water after each temper. It is really only a small gain to the steel in most cases, but does no harm, and allows you to start the second temper sooner.

 

[crockett]

Wick- do you get a flare/fire very often when quenching in oil? How much oil do you use in the container? Thanks.

 

[LRB]

I get a small flare up almost every time. That's because I don't submerge the tangs. There is little to no flare up if the entire blade is submerged. The flames confine themselves to the immediate area around the tang and only last seconds with a stick tang. A little longer with a full tang. The flames go out when the steel temp drops below the flash point. I have a vertical tank that will hold at least 6 gallons, but keep 5 gallons of Parks AAA in it. For a 4" to 7" blade you need at least 2 gallons of oil to keep it from over heating. My tank sits on a propane fish frier unit and is bolted to a heavy work bench, and is about 2 feet away from my oven door. It is about time I change out my oil with a fresh batch.

 

[crockett]

Thanks Wick- I thought if an oil quench tank caught on fire it would be very hard to put it out.

 

[LRB]

Some oils may be. Most commercial quench oils go out as soon as the blade cools below the flash point. Just keep a cover handy.

 

[gmarc]

Wick...if it is really quenching oil, filtering the solids out will make it fine for re-use.

Heated oil baths have a shorter life, but this is usually because the user don't clean 'em out.

I strain mine through cheese cloth

 

[Troy Allen Christianson]

Great points Wick. I like your advice about quenching after each tempering cycle. I have always let mine cool to room temperature after each cycle. I have a bevy of blades to do tomorrow and I am going to try your sage advice. Thanks -Troy

 

[LRB]

Austenite transforms to martensite when the steel temperature drops below 400°/450°. There is always a little austenite that does not convert. With the first temper you are not only allowing some of the carbon atoms to release a little from their atomic stacking, but also promoting some of the retained austenite to form to more martensite when the steel cools. This new extra martensite is untempered, and that is why a second, or even third temper is desirable. Air cooling between tempers is sufficient to make a good blade, but allows a slight bit of the newly formed martensite to slip back into the austenite condition. By cooling quickly, this is prevented, and you have a slightly stronger blade. If the blade were cooled too slowly, all the newly formed martensite would go back to austenite, and there is no gain. Bear in mind, this gain is only minor, but quenching after the temper does no harm and speeds up the temper process.