HT on a long thin 1080/15n20 blade? Help?

Bob Urban

Active Member
I am working on one of the longer blades I have made in an attempt to create a sushi knife similar to the one I have in the kitchen. Total blade with full tang handle is 14" and my HT set up is rather primative. I heat to non magnetic and dunk in heated vegi oil.

My qustion is to those in the know - how do you keep a long thin blade staight when HTing and if it does have a little bow or bend how do you straighten them post HT.

Thanks for any help - Bob Urban
 
Bob
IMHO thin blades such as yours should be groud after Heat Treat to avoid the warpadge issues. I know of a few makers that straighten at the tempering heat. this is done by several means, but can be done by eye. I also know a Smith who uses a small welding tip on an oxy/acet rig to localize heat a small spot or spots opposite the warp to bring them back. Ive seen it done, but never tried it.

Im sure others will have soud advise on how to help.

Good Luck and God Bless,
Mike
 
Prevention:
1. Normalize three times. This will relieve stress in the steel as well as refine the grain.
2. You could try something like marquenching, also know as martempering even though it has nothing to do with tempering. For that combination of steel you will need a tank of oil at about 450 degrees to quench the steel to just above the Ms point. This allows the thicker and thinner parts of the blade to come to the same temperature before being removed from the oil and air quenched. It also brings the temperature of the blade down into the bay on the IT curve and gives you the time to air quench the steel which produces less stress in the steel.
3. As Mike said, grind your bevels after heat treating, though this is not guarantee against warping.

Correction:
1. Straighten right out of the quench. This requires retained heat in the steel so it would be easier to do with marquenching. It can be done with a regular quench but you have to pull the blade out while there is still good heat in it. Either way, you only have seconds to do what needs to be done.
2. Straighten after a tempering cycle while the steel is still hot. This can be done with a mallet or a hammer or counter bending with a vice. Again you have to work quick.
3. Clamp the blade with steel C clamps to a ridged metal backing and then temper the blade. Do at least one tempering cycle beforehand just to be on the safe side. You also might need to shim the blade with something like ma small coin to counter bend the blade.
4. Cold straightening. Minor warping only. It can be done with a mallet or hammer but you can apply more force if you apply the force slowly, as in a vice. Put a rod at the peak of the warp and two other rods to the side of it on the opposite side of the of the blade in a vice. Slowly tighten the vice to counter bend. Remove to check and repeat the process as necessary. Yes, I have broken blades doing this but it has also worked for me.
5. Repeat normalization, straighten, re-heat treat.

Doug
 
Prevention:
1. Normalize three times. This will relieve stress in the steel as well as refine the grain.
2. You could try something like marquenching, also know as martempering even though it has nothing to do with tempering. For that combination of steel you will need a tank of oil at about 450 degrees to quench the steel to just above the Ms point. This allows the thicker and thinner parts of the blade to come to the same temperature before being removed from the oil and air quenched. It also brings the temperature of the blade down into the bay on the IT curve and gives you the time to air quench the steel which produces less stress in the steel.
3. As Mike said, grind your bevels after heat treating, though this is not guarantee against warping.

Correction:
1. Straighten right out of the quench. This requires retained heat in the steel so it would be easier to do with marquenching. It can be done with a regular quench but you have to pull the blade out while there is still good heat in it. Either way, you only have seconds to do what needs to be done.
2. Straighten after a tempering cycle while the steel is still hot. This can be done with a mallet or a hammer or counter bending with a vice. Again you have to work quick.
3. Clamp the blade with steel C clamps to a ridged metal backing and then temper the blade. Do at least one tempering cycle beforehand just to be on the safe side. You also might need to shim the blade with something like ma small coin to counter bend the blade.
4. Cold straightening. Minor warping only. It can be done with a mallet or hammer but you can apply more force if you apply the force slowly, as in a vice. Put a rod at the peak of the warp and two other rods to the side of it on the opposite side of the of the blade in a vice. Slowly tighten the vice to counter bend. Remove to check and repeat the process as necessary. Yes, I have broken blades doing this but it has also worked for me.
5. Repeat normalization, straighten, re-heat treat.

Doug

Good advice from Doug here, but as a moderator I feel I should give some input from my experience based on safety concerns:

I would strongly advise against heating oil for martempering/marquenching unless you can obtain martempering oil which is made specifically to go to these temperatures. Most oils have a flash point at or slightly below the temperatures necessary to interrupt at Ms, introducing a 1475F-1500F blade into this will result in an inferno much more threatening than a normal quench flame up. I know smiths that have been severely injured by attempting this. Even if you can obtain the martempering oil, 1080 or 15n20 will not respond well to the cooling curve of an actual martempering/marquenching method and you will produce much more pearlite than you would want in a kitchen knife. I do realize that a sushi knife should be thin enough to harden easily but this will not remove the fire hazards. And in the end the thermal extraction rate of most oils not designed for martempering drops like a stone above the 225-250F range, and the high heat will only shorten the life of the oil. I have personally used blades by folks who boast of how well this method works and have had them bend in my hands, prompting closer examination which reveled a 1/4" or less mildly martensitic zone at the edge; tough, yes, completely hardened- no. All of this put together with the serious safety concern always causes me to advise against heating oil above 150F.

A safer and much more effective method of approximating a marquench is to use interrupted quenching in a quality, fast oil heated to no more than 130F. Take a few practice runs with a similar piece of steel and count off from seven to twelve seconds of agitation in the oil before removing to allow it to air cool. If the temp is right you should have the blade entirely wet with oil but some light wispy vapors coming off from it, a dry blade that is heavily smoking is far too hot. If done correctly you will gain the benefits of the fast cooling of a normal quench to eliminate the pearlite and then still get all the benefits of the martempering effects, and it will all be much safer.
 
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Your caution duly noted, Kevin. Flash point is one of the reasons that I chose peanut oil. Even at that, I'm aware that I'm pushing it to within about 100-150 degrees of it's flash point when using it with something like 52100 which needs a higher temperature than the above steels. Didn't think about the rate of heat transfer. Something that I'll have to figure out how to test. I may end up having to grit my teeth and purchase a hardness tester but I need a new roof first. Also, thanks for the reminder that there is a limit on the thickness you can use marquenching to produce martensetic steel when using a shallow hardening alloy. Do you think that etching with ferric chloride will give me some idea if I'm producing pearletic steel towards the spine?

BTW, love the new logo. First time that I've been aware of seeing it.

Doug
 
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