What hardness?

[ALW]

I'm new to knife making and had a question about blade hardness.  What kind of hardness do you get/expect to get from say high carbon steel when you finish tempering?  What's a good hardness to shoot for?  I have some industrial planer blades I'm wanting to give a try sometime.  I know they are very hard.  When I get some kind of forge set up I was planning on annealing them, shaping them, then hardening them back up and trying to temper them.  Where I work I have access to a steel hardness tester (Rockwell hardness) so I can check the hardness before and after.  Just wondered what I should be shooting for to get a good durable blade.

Thanks.  Aaron

[Jeremy]

My opinions, FWIW.
The answer to most of those questions is "depends"  

"High carbon" covers a lot of different steels.  Some can only get to 57-58 as quenched.  Others are high 60's and need to be drawn back quite a bit.

Hardness in the blade depends on the size of the blade, intended use and hardening method.

A small caper can be left fairly hard - 60 Rc or so and not have any issues.  Leave a big camp knife that hard and you'll have some problems though. 54-57 is more like it for big knives...

Unless you've edge quenched it or plan on differentially drawing the temper back.  Then you can get away with an edge that's 58-60, as long as your spine is in the low 50's.  I like to have less than 1/3 of the blade hard when doing this.

A knife that's only going to be used for skinning can be left a little harder, but if you're going to be splitting wood, digging broadheads out of trees or using it as a prybar go softer or you're likely to snap a blade.  Edge geometry plays into the decision as well.  If you have a real delicate, thin edge and leave it harder it's prone to chipping or cracking with lateral force.  Leave it a bit beefier though and you're fine.

Like I said, "depends"  :D   Choose a design, make it, temper it, test it.  If the temper doesn't work for you retemper.

I'd think the industrial planer blades are HSS, aren't they?

[ALW]

Thanks Jeremy.  The planer blades are single beveled right now so I will have to grind it back some to be able to bevel both sides.  They aren't real wide but I think I can get a blade probably 3/4" to 1" wide, edge to spine.  I'll probably just make a drop point hunting knife from it with something like a 3 1/2" to 4" blade.  The thickness of it is about 1/8", maybe a hair thicker.

I'll check the hardness before I do anything to it.  I was hoping to harden it back up after shaping by quenching it in old trans. fluid.  If I can get it back to a good hardness I was planing to try and temper it in an oven/toaster oven.  See how that works.  Can I soften the spine up by submerging the edge in water then heating up the spine with a propane torch?  I think I read about doing it that way somewhere.

Thanks.  Aaron

[kbaknife]

Your biggest obstacle to over come will be to get the blade back up to full hardness after you've shaped your knife. What heat source are you using to do this? Seems as if you are focused on "tempering", which will only be successful after a full "hardening". Tempering a blade that was not successfully hardened will accomplish little.

[ALW]

I don't have anything built yet to do the heating for the hardening.  I'm planning on building a washtub forge or a kiln brick forge.  Something just big enough to heat up knife blades to critical temp. to do the quenching.  I'm actually not overly concerned about the exact hardness as long as I can get a decent knife.  I just figured since I had the resources at hand to check the hardness it would allow me to attempt to be more precise and see how good of a hardening job I can do.

I'm still a ways off from doing any of this so right now I'm just trying to soak up as much info. as I can.  Thanks.

Aaron