advice on direction to take

[J adams]

Hey all, newcomer here. I've searched threads as a visitor many times and figured I might as well get on with it and jump in.
I'm about to differentially harden a file knife of unknown steel (other than its not case hardened) and need some direction to take. I have a piece of the file I used to experiment with if that makes sense to do. I want to brine quench it in hoping for best performance results but am a touch fearful of what you all know can happen.
Does anyone one have a stepped process to advise for best luck?
The blade is 1/8" thick, 4" long, tapers in width 5/8" at ricasso to 7/8" near tip. Profiled after a surgeons scalpel.
I realize I may be asking an unanswerable question, but I'll take anything that may help. Thanks!

 

[Cactus Kid]

Hey J Adams,
You kind of have a tough question going on here. Do you mind me asking a couple of questions first? Why do you want to use a brine instead of oil? Is there a way you can determine the hardness of the file as it stands now?

 

[Stacy E. Apelt - Bladesmith]

Go with the assumption that it is W2 or 1095. A water/brine quench on a thin knife in those steels will be iffy. A differential quench would be another problem you don't want.
You will do much better to quench in 130°F canola oil. Use at least 1/2 gallon for that knife. A gallon would be even better.

Heat the knife as evenly as possible, and check against a magnet as it gets to a bright red. At the point it stops being attracted to the magnet, it is about 1415°F. Heat about 75-85° hotter, or about one full shade of red brighter. Once at that temp and evenly heated along the edge ( the only part that really matters), quench straight into the oil point down...all the way in. Hold in the oil for 60 seconds. Wash off gently and place in a pre-heated 400°F oven for an hour. Take out and cool off in running tap water. Dry of and put back in the oven for another hour. Take out, cool off in water again, and you are ready to finish the blade.

 

[J adams]

Thank you much, was afraid the oil quench would be to slow and the water quench too dangerous, just needed to be nudged forward, the blade is annealed at this point so pretty soft. Is it safe to think if I dont achieve a "good" hardness in oil, I can restart with brine and just know I may end up with pieces?😌

 

[Cactus Kid]

I'm a bit sketch on the brine, but that's because I've never used it myself (I've always used oil). I have done files before and have always used oil to great success. The advice Stacy gave you is really good and if you follow it you should have no problems in the hardness category. I guess for what it's worth, you can always use the brine for a practice run if you don't mind taking the chance. Who knows, you might learn something the rest of us could use in the future???

 

[J adams]

This is true, all I've got to lose is a free file and a few hours of grinding. I know variables are endless with this sort of thing. I think I'll quench my scrap in brine and inspect it....then likely follow staceys instruction. But for the sake of us all I sure wouldn't mind sacrificing for the knowledge of us all. Thanks Stacey and cactus for a good experience on first post!

 

[Stacy E. Apelt - Bladesmith]

All a quench needs to do to harden a blade is bring the austenite (steel at 1450-1500°F) down to below the pearlite nose fast enough ( below 1000°F). For 1095/W2, that is pretty fast. In thin sections, the speed of a moderately fast oil is fast enough. Once below that nose the steel will cool down to the martensite range ( 400°F) and convert into hard martensite.

Using a more aggressive or faster quench will not harden the blade any harder than using one that just reaches the above requirements. Brine has its uses for thick and large blades, and in yaki-ire, but for the normal knife blade fast oil is more than sufficient. I use Parks#50 (the most popular fast quench oil) for all my high carbon/shallow hardening blades with complete success. Canola oil isn't Parks #50, but it works quite well in its place.

Quenchant volume is a big part of why many new makers have poor results. Many are quenching in a quart jar of oil. That is just not enough volume to extract the heat fast enough. I usually state that a gallon is the minimum. On a very small blade, like yours, a half-gallon would work, but a gallon would be safer. After the oil cools, pour it back in the bottles and use it again many times.