rehardening a chisel

[Eric Dippold]

I recently purchased a 1/4 pigsticker mortising chisel online. The only mark on it is an owners name on the handle. Unfortunately the tip is soft, since I'm not sure of the carbon content I plan on oil quenching and draw temper to a blue to start. My worry is the handle, I will be heating the tip to cherry red in a small forge. Do I need to worry about the handle charring on the inside? If I do, how do I remove the handle?

Thanks for your help, Eric

[Jim Koepke]

Howdy Eric and welcome to the Creek. If you have been reading this forum since 2013 you may have seen my question about where folks live. You may live near someone who has done this and is willing to offer their insight and possibly even help.

Most often with my pigsticker chisels the handle was easy to remove by wrapping the blade for safety and then hitting the handle with angled blows from a mallet.

Many older mortising chisels are made by laminating a piece of carbon steel onto softer steel. Is it possible yours was beveled on the wrong side?

A little abrasion on the side with a fine stone should reveal the lamination. Newer mortising chisels may not have this lamination.

jtk

[Patrick Chase]

I recently purchased a 1/4 pigsticker mortising chisel online. The only mark on it is an owners name on the handle. Unfortunately the tip is soft, since I'm not sure of the carbon content I plan on oil quenching and draw temper to a blue to start. My worry is the handle, I will be heating the tip to cherry red in a small forge. Do I need to worry about the handle charring on the inside? If I do, how do I remove the handle?

Thanks for your help, Eric

Usually when the tip of a chisel is soft it means that that the tip has been detempered, usually by overheating. If that's the case then you might want to grind it back a millimeter or two and see if you have better luck - that's going to take a lot less of your time than "amateur heat-treating", and may very well obtain a better edge than you can via re-treatment.

[Steve Voigt]

First off, I agree with Patrick, try grinding a bit off first and see if that works.

If it doesn't, I have rehardened a number of pigstickers just fine. Definitely pull the handle. Grab the chisel in a metal vise, if you have one, and hit angled blows like Jim said. Try pulling too; I have just pulled some off.

I normally quench vintage tools in brine (about 4-5 gallons of water to a box of morton canning or ice cream salt), but oil should work for a 1/4" chisel. The smaller the chisel, the less it matters.

I wouldn't go to blue right away. I'd try for a golden brown, with maybe just a hint of purple. Then reflatten the back, sharpen, and test. If it chips, try purple. I have never had to go to blue on a mortise chisel.

It's pretty hard to avoid losing a bit of thickness on the sides, because you will need to abrade off the decarb from HT. One thing that can dramatically mitigate your loss is to use some kind of decarb-prevention coating. There are commercial variants, like Brownell's, but I have always used borax. Heat the steel past blue, into grey, and sprinkle the borax on before there's any red glow.

Good luck. I've had very good luck with rehardening laminated steel; it's not that difficult.

[Eric Dippold]

Thanks for all the advise. My thought on drawing temper to blue was to reduce the chance of chips and if still to soft re-harden and re-temper to a cooler temp, I think maybe I'll start at purple. It doesn't appear to be laminated, the finish on it is pretty rough, I'm not sure that it's not home made, but it's still got some age to it. My fear is that if is home made the they might have used low carbon steel to make it. luckily, even if it doesn't harden I don't chop to many mortises that it bothers me to resharpen more often. I live in a small town in southeastern Alaska, not to many people into hand tools around here, even though I try to preach the gospel.

[Patrick Chase]

Thanks for all the advise. My thought on drawing temper to blue was to reduce the chance of chips and if still to soft re-harden and re-temper to a cooler temp, I think maybe I'll start at purple. It doesn't appear to be laminated, the finish on it is pretty rough, I'm not sure that it's not home made, but it's still got some age to it. My fear is that if is home made the they might have used low carbon steel to make it. luckily, even if it doesn't harden I don't chop to many mortises that it bothers me to resharpen more often. I live in a small town in southeastern Alaska, not to many people into hand tools around here, even though I try to preach the gospel.

OK, let's use some numbers here: Assuming this is HCS/O1 and that you achieve optimal hardening temperature and do so without decarbing the surface (see Steve's comment about Borax), blue will give you ~600F and Rc57, which is pretty soft for a chisel, even a pigsticker. I wouldn't do that even if I knew that my processes were perfect and that I would indeed achieve Rc57.

Steve's recommendation of golden brown is about 450F and Rc61. That's potentially on the hard side for a pigsticker, but I'm going to go out on a limb here and guess that Steve thinks you won't quite perfectly nail the heat-treatment for one reason or another, so he's recommending a mild starting temper to allow for a point or two of hardness loss. IMO he's right to do that, and you'd be wise to heed his advice. You can always retemper if the metal is too hard, but you have to start over if it's too soft.

[Kees Heiden]

Also, you shouldn't reharden steel too often. At each hardening you loose some carbon and burn some steel. So it is better to strive for a chisel that is a bit too hard and retemper it later on.

I also wonder if you could make a coating with sand. Sand is often used instead of borax in black smithing. When the steel is just red, dip it in the sand, the sand will melt and form a glassy coating. But all that is more for later experiments. I would first just heat up the steel and quench it in oil. It is quite a challenge the first time anyway, without any further tricks.

[Patrick Chase]

Also, you shouldn't reharden steel too often. At each hardening you loose some carbon and burn some steel.

Nit-picking, and being brief because we've been down this road before. "Burning" is irrelevant if you're heat-treating, since the entire point is to get the metal over its hardening temperature and thereby bring it to fully-hardened state. You can't do so without passing through all of the shades of the "burnt steel rainbow (https://en.wikipedia.org/wiki/Tempering_(metallurgy)#/media/File:Tempering_standards_used_in_blacksmithing.JPG )" along the way, but that's of no consequence at all provided that you actually do achieve hardening temperature.

Decarburization ("loosing some carbon") is a real issue, particularly if as here you're retreating an existing tool and hoping to maintain its existing dimensions. Assuming you don't have a vacuum or inert-gas furnace, you'll be wanting to coat the part such that oxygen can't reach the surface once it's above critical temp (where the carbon becomes mobile) as Steve suggested.

Note that in threads about initially hardening plane blades Steve argued against coating when I brought it up. His rationale was (correctly) that you just have to grind away a few mils (and maybe 10-20 mils at most for treatment with a very long soak at hardening temp) of surface material to get past the decarb, so there's not much to be gained by taking aggressive measures to prevent it. It's probably worth noting that the coatings make it harder to tell when you've reached critical and hardening temps unless you leave a "window" somewhere.

That's fine for initial treatment of a plane blade, but not so good for a pigsticker that's supposed to make mortises of a specific width. You really don't want to have to sacrifice those sidewalls, hence Steve's suggestion of Borax.

[george wilson]

Steel starts going "crazy" if you re heat it to hardening temp. too many times. Rehardening it once or twice will be o.k.. You should NORMALIZE the steel if you have to repeatedly re heat tool steel. Google normalizing for your self.

[Larry Frank]

In addition to what has been said, the repeated heat treatment can result in grain size problems that can not be cured by normalizing. While not technical, George's comment is pretty good advice. I would only try re-heating treating once or twice.

[Patrick Chase]

In addition to what has been said, the repeated heat treatment can result in grain size problems that can not be cured by normalizing. While not technical, George's comment is pretty good advice. I would only try re-heating treating once or twice.

Agreed w.r.t. bottom line advice. Re-treating is a bad idea, particularly if it's potentially incomplete as when you use a MAP torch and sort of eyeball it.

I do have a question, though. Let's assume that you have a vacuum or inert-gas furnace and can basically treat at any temperature you want for as long as you want. Shouldn't prolonged treatment in the pure-austenite regime above A_3 or A_cm (depending on Carbon content) followed by normalization "reset" the grain structure? Or is there something more subtle at play that isn't captured in the phase transitions?

[Larry Frank]

The answer to Patrick's question is maybe. It is difficult/impossible to say without knowing the steel grade. There are some where the answer is yes and others you risk doing things like blowing the austenitic grain size. Another issue with tool steel grades is the carbide size and distribution with prolonged heat treatment.

The knife or blade forums have some good discussions on heat treatment.

[george wilson]

I most usually give plain advice that most people can understand. Making it too technical can result in people not reading it or in not understanding it. So,I leave out terms like austenite,martensite,liquidus(in soldering),etc., unless someone instigates a technical argument that they are wrong about. I wrote a LOOOONG reply in an issue about early iron and steel production years ago.

After all,most people here are woodworkers,not metal workers. Even fewer have done hardening and tempering of tool steels.

[Ron Hock]

http://www.hocktools.com/tech-info/diyht.html

I agree with the response that advised shooting for too-hard and re-tempering if needed. For the high-temp hardening part, use a magnet to determine when it's ready to quench. Simple high carbon steels become non-magnetic at the critical temp. Quench is oil (room temperature is fine) assuming the alloy is O1. If not, you may get sufficient hardening but it's a crap shoot. For tempering, apply heat a little away from the edge and let it run down. Quench to stop the heat flow just before the tip turns the very first hint of color. (If you have a temperature controlled deep fryer, you can dip the tip into 325F oil for a few minutes.) It should be Rc62 at that point. You'll need to aggressively flatten the back to remove any de-carbed layer (the back IS the cutting edge). Finish honing and try it out. Too brittle, do the tempering step again and let the tip get a smidge hotter. You shouldn't need to re-flatten, etc, this time. Repeat as necessary. Good luck!

[Jim Koepke]

I most usually give plain advice that most people can understand. Making it too technical can result in people not reading it or in not understanding it.
EDIT


I think most folks appreciate that George. The acronym KISS is about keeping things simple.

It can be annoying to ask a simple question expecting a yes or no answer and instead having to listen to a long boring dialog that leaves one to wonder, what was the question?

jtk

[george wilson]

I seldom have the time or patience to read real long posts. Or diagrams, or graphs where someone is trying to prove that he has a saw that is hard as CARBIDE ( 70) R.C.. Or was that 72? And claiming that he can STILL file it!!:)

[Patrick Chase]

I seldom have the time or patience to read real long posts. Or diagrams, or graphs where someone is trying to prove that he has a saw that is hard as CARBIDE ( 70) R.C.. Or was that 72? And claiming that he can STILL file it!!:)

Who claimed that they had a saw that's as hard as carbide?

[Jim Koepke]

Who claimed that they had a saw that's as hard as carbide?

I think George is referring to a post from before you came along.

jtk