Remove Scale after Heat Treat...quick question?

G

Gundisalv

Joined
Mar 17, 2020
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Greetings Everyone....new thread as I have read a post here but was from 2018 and rules i just read stated start a new thread if the thread is older than 12 months.

New to knife making AND the forums....plan is the eternal hobbyist as I don't have enough time in a week to work on this for more than that...only get 3 to 4 hours per week. (new family and am 46. Time is a luxury at this point....

What would the ratio of PH Down be to water to remove scale on my knife post heat treat. I plan on knives following the one i am working on to leave less finished so I can grind off the scale. This one I finished way too much and have some scale i need to remove without removing too much of the steel?
 
I'm not sure what you are using for an etching solution. PH Down ? Any way, type of steel, thickness of fire scale, type of fire scale, type of etchant, temperature, surface roughness of blade and orientation of blade all make a difference.

I usually use full strength white vinegar. Place blade(s) in a long narrow plastic tray. Support them so the vinegar can flow freely around them. I use plastic swizzle sticks or chunks of plastic cable ties for spacers. Cover everything with vinegar and let it soak for an hour. Ocassionally brush off any bubbles and agitate the vinegar. Repeat every hour until the fire scale has softened to your satisfaction. Too short of a soak, and the fire scale will be hard to grind off. Too long of a soak, and you can get pitting in steel. It is nice to set the tray outside so you don't have to smell the vinegar. Just beware, the squirrels may knock over the tray, leaves a nasty stain on concrete LOL. Make note of which steel, heat treat times and how long you have to soak the blade. This will help you estimate the soak time next time around.
Jim A.
 
Follow the directions on the can. Keep an eye on the progress though. If you forget about it and come back two days later you may be sorry.
 
Angle grinder with hard disks, less than a minute per side...
 
The approximate ratio for mixing pickle is 1 quart water to 1 cup pickle. Plain old sodium bisulphate (NaHSO4) works fine and comes in a tub from the pool supply as Ph Down.

Store in a plastic or glass container. A piece of 4" PVC pipe with a closed flange cemented on one end and a cap slipped on the other works perfect for both pickle and for your FeCl etchant.
 
Sanding sponges, scotchbrite belts, or leave your blade slightly thicker before heat treating and you can sand the blade down while removing the scale. If you go with the latter, keep a bucket of water or a squirt bottle close by to keep your blade cool so you won’t overheat it and ruin your temper.
 
As an additional comment, almost all blades should be sanded/ground after HT. You can't just remove the scale/oxides. There is a layer of of decarb that needs to be removed. I guess I remove about .005" or so on each side in post HT finish work. This exposes clean steel.

The only exceptions are knives with the decarb deliberately left on the upper bevel as in Brute-de-forge and kurouchi (blacksmith finish) on Japanese blades.
 
Thank you all for the info. My plan is for my next knife to be much thicker or virgin after getting a slight bevel put in.

I am going to try that PVC method. I also really really need to hit the scratch off lotto for a real belt sander lol.

Thanks again.....
 
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