So your chisel is too soft or too hard, now what?

[ED65]

Following on from the Chisel quality ? thread, what if your chisel is too hard (brittle) or too soft? Do you live with it, shop around for alternatives or go with door number three?

The third option is the purpose of the thread and that is doing some heat treating of the steel yourself. This is not widely known about or the thought fills people with trepidation but it can be quite simple and straightforward as you'll see below, it can be done in the workshop or the kitchen using the most basic setup, e.g. a gas torch (butane, propane or MAPP) and a container of water.

Too brittle is the easiest to deal with. This can be simply tempered out of the steel by heating to lower, 'non-critical', temperatures – usually you're aiming for a surface oxidation of a yellowish or light straw colour – and even a cooker's gas burner is more than capable of providing enough heat for this. Almost any tool steel should respond to this treatment.

For too soft you have to re-do the heat treating from scratch, how-to on this is shown in the link and explained by the pictures below. All basic high-carbon tool steels can be hardened in this way and a few of the modern steels as well.

A demonstration of how easy this sort of thing can be is seen in this video, Small Woodworking Shop Tips & ideas from Jim Cummins, editor of Fine Woodworking, where he takes only a few minutes to re-do the heat treating on a chisel and a key point is he doesn't even need take the plastic handle off to do it. The video is an hour long so if you want to go directly to the relevant part here's a direct link.

Here are a couple of other references from publications for more details. Since both are American publications they give temperatures in Fahrenheit, 1450°F is 788°C.

From "Best Tips From 25 Years Of Fine Woodworking":

From "Making Wood Tools" by John Wilson:

 

[Trigs]

Cracking idea, would never have thought you could do it at home. A project for next week me thinks

 

[Sheffield Tony]

Ignore the instruction to quench in water. Try first with oil - cooking oil is good - and only if it will not harden sufficiently to pass the file test after an oil quench, try again with water.

O1 is meant to be oil quenched and will likely crack in water. W1 or silver steel needs the quicker cooling of a water quench.

Leave any firm of HSS well alone, you may anneal it but will not likely get it hardened ever again !

 

[D_W]

Ditto Sheffield Tony's comments. I don't think there is much in thin chisels around that can tolerate a water or brine quench.

 

[Corneel]

Before resorting to heat treating, I would first test if a higher bevel angle isn't all the chisel really needs.

 

[Sheffield Tony]

Indeed. And worth grinding back by a millimetre or so. Modern chisels are often ground before heat treatment, which might leave the thin edge liable to overheat or decarburise. Narex specifically recommend this.

 

[ED65]

I agree that if you want to err on the side of caution you should try an oil quench first. Nothing wrong with being cautious and it won't cost much in time if it doesn't work, just a few extra minutes.

Ignore the instruction to quench in water. Try first with oil - cooking oil is good - and only if it will not harden sufficiently to pass the file test after an oil quench, try again with water.

Any thoughts or experience with oil floated over water? I've read conflicting things about why you'd do it in the first place but I don't see that there's much harm in trying it.

Leave any firm of HSS well alone, you may anneal it but will not likely get it hardened ever again !

I was going to say something HSS being an exception to being re-hardened at home, but as almost no bench chisels are made from it I decided to keep it shorter and not refer to it.

One of the other things I left out to keep it simpler is deliberately quenching in oil with a water-quenched steel, apparently you can get a good hard/tough result in one go that way. Probably something in the worth trying category though but best not to assume you'll get good results.

 

[Corneel]

You can read everything you want to know about quenching a certain steel in the TTT diagram, like this one for O1 tool steel.

You start heating up the steel to 740 degrees. That will form austenite crystals in the steel. The nose shape in the diagram is the perlite nose. Perlite is a kind of quenched steel that is not as desirable as martensite. Martensite is what we really want, it is the quenched variation of Austenite, so to speak. Perlite isn't nearly as hard as martensite. Cooling very slowly gets you bainite, but you need special salt baths to get into that area.

Now, you start with the hot steel at the upper left corner of the diagram. If you manage to cool it to less then 600 degrees within 10 seconds, you have avoided the Perlite nose. Cool further, and around 350 degrees or so the austenite is starting to transform to martensite.

So, for O1 you have 10 seconds. For W1 you only have about 1 second to avoid perlite. That is why O1 is an oil hardening steel and W1 a water hardening steel. But, things like chisels or knifes are very thin. It is very possible that you get enough cooling rate in oil for W1 steel. The recommended hardening procedures are always for engineering stuff and they talk about much thicker tools.

After quenching in the right manner you get martensite which is way too brittle for a tool. So you temper it to create tempered martensite. The suggestion you made about experimenting with oil quenching a water hardening steel to get the right hardness without tempering, leads you into the perlite area. It gets all very finicky, and you get a type of steel that just doesn't have the same ideal properties as tempered martensite. Perlite is just not hard enough. It is just much better practice to create as much martensite as you can and then temper it.

 

[Jacob]

In case there are any beginners reading this- this sort of interesting metal work is no doubt effective but it's not what your typical woodworker would ever bother with. More likely to dump an ineffective chisel and pick up another one instead.
You don't have to become your own blacksmith, though I can see the advantages.

I sold an anvil, leg vice and other bits of kit some time ago - I sometimes wish I'd kept them

 

[lurker]

Before resorting to heat treating, I would first test if a higher bevel angle isn't all the chisel really needs.

Absolutely correct in most cases it's a question of bad workmen blaming their tools.
Any damage done to mine has been my fault.
Mind you other than aldi I don't think I have a chisel younger than 60 years old

 

[Trafalgar]

Seems a waste of time to (re) heat treat inferior steel, or expect good results on decent steel from a fellow who ruined the temper at the grinding wheel in the first place (wasn't this the case?)

Not sure what is being recommended here but it doesn't seem to make sense to me.

I would simply replace a bona fide clunker of a chisel with something slightly better. They're not hard to find -- new or used.

 

[ED65]

The suggestion you made about experimenting with oil quenching a water hardening steel to get the right hardness without tempering, leads you into the perlite area. It gets all very finicky, and you get a type of steel that just doesn't have the same ideal properties as tempered martensite. Perlite is just not hard enough. It is just much better practice to create as much martensite as you can and then temper it.

Fair enough, easy enough to do it the conventional way.

Thanks for posting the graph by the way!

 

[ED65]

Seems a waste of time to (re) heat treat inferior steel, or expect good results on decent steel from a fellow who ruined the temper at the grinding wheel in the first place (wasn't this the case?)

The "inferior steel" thing is a bit of a red herring. One man's version of that won't match another's: I know of a good few guys who look down their noses as CrV even though it's shown itself to be very capable.

And I think the point is that even a really top-class steel can be let down by the heat treating. Assuming steel quality in any set or class of chisels is uniform some are hardened less well than others, as shown by the occasional atypical example.

Not sure what is being recommended here but it doesn't seem to make sense to me.

With respect it seems pretty clear to me.

Obviously not everyone will want to try this sort of thing, I was just trying to spread the word that it's possible when many people are unaware of because it's seldom, if ever, mentioned.

And what's best of all that it can be relatively easily, and cheaply, done is a big bonus for the person who would prefer to try to fix a tool than buy a replacement.

 

[ED65]

Sorry forgot to address this point before.

Before resorting to heat treating, I would first test if a higher bevel angle isn't all the chisel really needs.

How steep would you go before you wrote off the chisel's steel as not being up to snuff?

Chisel type might be important here, so let's say for general bench chisels as well as purpose-made paring chisels and mortise chisels.

 

[mind_the_goat]

Ignore the instruction to quench in water. Try first with oil - cooking oil is good - and only if it will not harden sufficiently to pass the file test after an oil quench, try again with water.

O1 is meant to be oil quenched and will likely crack in water. W1 or silver steel needs the quicker cooling of a water quench.

Leave any firm of HSS well alone, you may anneal it but will not likely get it hardened ever again !

Well, I learnt something, I figure out on my own that oil quenching was quicker than using water, so thanks for the correction.
Have a slight concern though about using cooking oil, wouldn't the flash point of some oils be lower than the temperature of the steel you plunge into it?

 

[swb58]

Don't be surprised by some localised flame until it cools down a bit, it's the opposite end of the scale from a chip pan fire.

 

[Racers]

Yes you do get flames, lots of lovely flames.

Pete

 

[Sheffield Tony]

Have a slight concern though about using cooking oil, wouldn't the flash point of some oils be lower than the temperature of the steel you plunge into it?

I learned through experience that it is not wise to use a plastic pot to hold your quenching oil. Not sure why it wasn't obvious from the outset . Also worth having a damp cloth to hand.

 

[Trafalgar]

Seems a waste of time to (re) heat treat inferior steel, or expect good results on decent steel from a fellow who ruined the temper at the grinding wheel in the first place (wasn't this the case?)

The "inferior steel" thing is a bit of a red herring. One man's version of that won't match another's: I know of a good few guys who look down their noses as CrV even though it's shown itself to be very capable.

And I think the point is that even a really top-class steel can be let down by the heat treating. Assuming steel quality in any set or class of chisels is uniform some are hardened less well than others, as shown by the occasional atypical example.

Not sure what is being recommended here but it doesn't seem to make sense to me.

With respect it seems pretty clear to me.

Obviously not everyone will want to try this sort of thing, I was just trying to spread the word that it's possible when many people are unaware of because it's seldom, if ever, mentioned.

And what's best of all that it can be relatively easily, and cheaply, done is a big bonus for the person who would prefer to try to fix a tool than buy a replacement.

One always runs a risk when recommending a course of action that might result in not expanding one's own horizons. I've done that here. I didn't articulate my reasons very well and in retrospect it's really not a point worth making, certainly not one worth arguing.

 

[Bedrock]

...and if you or a previous owner has cocked up a grind, it's one way of resurrecting a decent tool.

Not for everyone, but with appropriate safety precautions, not that difficult either.
Do not try this in the kitchen, particularly if your other half is at home. Nor inside the workshop for that matter.