Harden Up (your chisels)

Got a new way of doing something? Or maybe an old method that needs some clarification.

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Trevor Gore
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Harden Up (your chisels)

Post by Trevor Gore » Thu May 19, 2016 11:00 pm

Many of you will have chisels of the Stanley/Marples/Irwin ilk. I’ve had a set of Marples for, idunno, 20 years or so, but was never particularly impressed with their edge holding when used in a guitar building context. My blue handled Irwins were added maybe 10 years ago and they were of similar capability, i.e. not really cutting it.
Irwin-Marples.jpg (127.17 KiB) Viewed 3377 times
You can see from the pic of the Irwins that I had had a go at hardening the brace carving chisel, with no real beneficial effect. I took to using a variety of old chisels bought at junk sales which performer better, but still not as well as I wanted. I even made a chisel from an old file, which turned out to be the best chisel I owned, holding an edge way sharper and longer than anything else I had.
File Chisel.jpg
File Chisel.jpg (162.46 KiB) Viewed 3377 times
I was just about getting to the stage of junking the Stanleys and Irwins when a correspondence with an old mate happened to mention a method of hardening some of the alloyed tool steels that somehow I had in mind couldn’t be heat hardened. I’d previously tried and failed with the Irwin brace carver and a couple of old Stanley plane blades, which reinforced that thinking. I thought “nothing to lose here”, so give it a go.

The process is to heat the last 50mm of the tool to ~750°C, hold at temperature for ~ 1 minute then plunge it into olive oil. That’s it! To get to 750°C I needed two butane torches flat out and matched the hot steel colour to the chart attached (cherry).
Colour Chart.jpg
Colour Chart.jpg (66.45 KiB) Viewed 3377 times
I was expecting some “fireworks” when the blade hit the oil, so tooled up with a stack of safety gear, but was disappointed. Just a bit of mild bubbling. If you try this, I’d suggest that you use all the safety gear, at least the first time around. When the chisel has cooled, clean off the scale (at least) and buff out the discolouration if it bothers you. A previously sharpened chisel will be rather brittle on the cutting edge due to it being so thin (and so cooling too fast) and it won't take an edge. Just grind it back a bit, but keep it cool, then sharpen it up.

Well, I was impressed! I used the brace carver to flush trim a figured gidgee heel cap, reckoning if it could do that well, it would be fine. (The gidgee was some of that “mad” figured stuff from Pete McCurly; hard as the hobs of hell).
Starting heel trim.jpg
Starting heel trim.jpg (67.97 KiB) Viewed 3377 times
Not only did it flush trim it fine, afterwards it would still shave hairs off my arm without a strop or re-hone. Not even my “file” chisel will do that. So I’d call that a pass. Previously, that chisel would have curled up its toes after one stroke.
Heel cap trimmed.jpg
Heel cap trimmed.jpg (85.29 KiB) Viewed 3377 times
Here's a detail of the brace carver after doing the heel cap.
Irwin brace carver.jpg
Irwin brace carver.jpg (119.67 KiB) Viewed 3377 times
I did the rest of the chisels with similar results, but left out the 38mm chisel because I only just had enough heat to get the 32mm one to temperature.
Chisel Set.jpg
Chisel Set.jpg (135.08 KiB) Viewed 3377 times
On the wider chisels, if you get the back of the blade into the oil first, you can get the back to dish slightly like some of the Japanese blades, which eases honing.
Dished back.jpg
Dished back.jpg (150.1 KiB) Viewed 3377 times

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Mark McLean
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Re: Harden Up (your chisels)

Post by Mark McLean » Fri May 20, 2016 6:20 am

Nice tute Trevor. My Irwins are not great at holding an edge and I wouldn't mind putting them at risk (and disfiguring them a bit) if it improved their performance. Thanks for the idea and the good instructions.

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Re: Harden Up (your chisels)

Post by Nick » Fri May 20, 2016 9:01 am

To anybody interested in what is going on in Trevor's process you are essentially annealing the chisel. Cherry red or close to it, is the colour most alloy steels anneal at and then the slower quenching in oil rather than water, avoids thermal shock to the steel thereby allowing the molecules in the steel to better 'align' and therefore making it stronger, (think of a piece of chipboard and you hit it on an edge, a piece will break off, now try hitting a board of 'real' wood on the end!). This would explain why they're loosing there edge in the first place, whilst they are 'hardened' at the factory they obviously haven't undergone the full heat treatment process and been annealed! While the steel is technically 'hard' it is still very brittle and prone to chipping out and the extremely thin cutting edge (if you could call a point an edge?) able to break off at a microscopic level. I'd never even considered that this (the heat treatment) process hadn't been followed fully!
A plot maybe by the makers? Don't anneal the blade which saves on production costs plus it has the added benefit of, you the consumer, will constantly be at the grinder & oilstone re-sharpening the chisel thereby wearing it down until you have to go buy a new chisel/set......Hmmmm, interesting. Not saying you still won't have to re-sharpen occasionally but it would be a lot less frequently if you follow Trevor's tute.
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Trevor Gore
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Re: Harden Up (your chisels)

Post by Trevor Gore » Fri May 20, 2016 9:54 am

Nick, before the recent heat treatment the steel was too soft and malleable (formed a "plastic" wire on honing) rather than too hard (forms "dust" on honing as the edge crumbles). I think the manufacturers probably go for a softer, less brittle heat treatment for general purpose chisels so that the edge doesn't shatter when the chisel is belted with a mallet. Mostly, we're looking for chisels to do paring type work, so we want a much harder edge so that it stays sharp for longer and there is much less risk of the edge shattering because there are no impact forces. Or maybe we credit the manufacturers with being smarter than they actually are and they don't have a proper heat treatment process, or if they do it is not under control!! I'm sure these edges could have been a lot harder and the chisel still be used with a mallet. Anyway, I'm pretty happy with how they are now for what I use them for. Much better them them sitting around as ornaments as they did for most of the last 20 years!

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Re: Harden Up (your chisels)

Post by kiwigeo » Fri May 20, 2016 11:42 am

Interesting looking chisel rack/junk box there Trevor :mrgreen:

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Re: Harden Up (your chisels)

Post by routout » Tue May 24, 2016 5:16 pm

Yep I don't think you can go too far wrong with the old chisel they are not that higher grade of steel, the file however I remember getting a slap around the head for doing that to a file machine shop practice :lol:
I was going to make some bent Chisels for Instrument making with a slight bend for pairing down the braces as I have access to the rite steel and a heat treatment place just up the road ,
I do make small inlay ones out of what they call silver steel you can quench harden it after you grind or file it then buff to a mirror finish. I have a selection of carving chisels that stay sharp but most don't want to pay $40 plus for just one they last a life time so the price is not a factor these are polished to a mirror finish they just slide through (fingers as well )all good ,cheers John.
John ,of way too many things to do.

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