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Creating Heat for Hardening

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Martin Dilly 207/10/2019 23:16:04
32 forum posts
4 photos

I've bought a set of El-Cheapo woodcarving chisels and want to try hardening and re-tempering them. I need to get them to cherry red before quenching. I have a gas stove and a butane torch, but using both together doesn't quite get the metal up to cherry red. Any suggestions as to how to get things a bit hotter, short of building a furnace?

Grindstone Cowboy07/10/2019 23:24:04
300 forum posts
27 photos

Get some firebricks and make a bit of a hearth - no need for anything fancy, just a right-angled corner will do. They'll help contain and reflect the heat back into the work.

mechman4808/10/2019 09:01:39
2663 forum posts
410 photos
Posted by Grindstone Cowboy on 07/10/2019 23:24:04:

Get some firebricks and make a bit of a hearth - no need for anything fancy, just a right-angled corner will do. They'll help contain and reflect the heat back into the work.

Vermiculite blocks work superbly as a hearth as well, I have made a small one using these, I bought them off eBay reasonably cheap. If you have a supplier of wood burning fires/stoves local to you they will most likely have 'spare' blocks available, or may have sheets of it & would cut a couple of pieces off for you.


SillyOldDuffer08/10/2019 09:14:27
5762 forum posts
1226 photos

As Grindstone says, the answer is to build a hearth out of firebricks. The problem is most of the heat is wasted because it's free to radiate away from the job. Box it in.

Nothing fancy about the hearth - just a stack of bricks big enough to contain the heat while allowing access. It needn't be complicated or permanent, a sort of partial igloo arranged under and around the job to contain the heat better.

There's always a booby trap! Firebricks come in two varieties. One of them is designed to soak up heat like a sponge and release it slowly as in a night storage heater. This type is heavy and strong. Wrong sort!

The other is an insulating brick designed to retain heat in kilns etc.. They come in various temperature grades, are light-weight and soft (can be cut with a wood-saw) and are a little delicate. This type is what's wanted, and unless you're building a proper furnace, the grade is unlikely to matter. Try searching for 'Insulating Fire Brick' or 'Vermiculite' on ebay etc. Never tried them myself but 'Aerated Concrete Blocks' are a cheap common building material that might be good enough.


Dusty08/10/2019 09:22:14
471 forum posts
8 photos

Thermalite blocks also work well, they have the added advantage that you can drive nails into them to hold parts during soldering etc.

Emgee08/10/2019 09:28:03
1485 forum posts
217 photos


If the tools were very cheap perhaps the steel the chisels are made from is not suitable for through hardening by heating and quenching, only method would be to case harden if that is the case.


Andrew Tinsley08/10/2019 10:01:40
1110 forum posts

You may be already up to temperature. The quoted colours are for the metal glowing in reasonably dark conditions. The full light of day usually screws things up and gives a false colour temperature. It took me a while to realise this and my first efforts at hardening and tempering were not very good.


Hopper08/10/2019 10:36:52
4527 forum posts
94 photos

And note that cherry red is not a bright red. It is a dark red, the color of cherries in the 19th century, not today's bright cherries bred to look good for marketing purposes.

Kiwi Bloke08/10/2019 10:45:37
424 forum posts
1 photos

Well, it depends on what sort of cherry we're talking about. A common error when hardening is to not heat to a high enough temperature. The correct temperature (for silver steel) has been described as 'cooked carrot', considerably hotter than morello cherry. Food-based terminology is colourful but unreliable and ambiguous. Safer to heat to the Curie temperature - it's objectively testable.

Martin Dilly 208/10/2019 10:52:14
32 forum posts
4 photos

Many thanks, gents, for your rapid and helpful replies (and for figuring out what on earth 'hedat' meant). It may well be a low grade steel, but I thought it worth trying to see if it hardens. At present you can scratch it with the tip of an X-Acto blade.

ega08/10/2019 10:55:41
1709 forum posts
150 photos

Don't forget the magnet test.

Kiwi Bloke08/10/2019 11:16:27
424 forum posts
1 photos

X-Acto blades are pretty hard. My guess is that your cheapo chisels are made from chinesium and are not heat-hardenable. You could probably case-harden them, however. Or buy good'uns...

Just for info. - the Curie temperature is that temp at which magnetic properties are lost. Conveniently also the temp you need for hardening. The 'magnet test' (above) detects that temperature, far more reliably than misleadingly-named colours.

roy entwistle08/10/2019 11:53:50
1172 forum posts

If you case hardened them I doubt that you could sharpen them after

Roy ( I could be wrong )

mechman4808/10/2019 13:55:01
2663 forum posts
410 photos

Latest pic of my mini hearth, I have also lined it with glass fibre / ceramic wool (?) blanket from Cup alloys , usual disclaimer, the top brick can be laid down to form an enclosed 'oven'... exceedingly good cakes hearth Mr K*****g face 20

modified hearth (1).jpg

John Reese09/10/2019 22:28:25
842 forum posts

My uncle, an engraver hardened his gravers at the kitchen range. I believe he used an alcohol lamp when drawing back the temper.

Martin Dilly 209/10/2019 22:33:17
32 forum posts
4 photos

Good idea, John, but sadly our Ideal boiler, which would have done the trick I'm sure, got the heave-ho in 1987 when we installed central heating.

Thanks to the rest of you, too. I suspect it's a case of chinesium, which may defeat my best efforts.

not done it yet10/10/2019 10:12:36
4637 forum posts
16 photos
Posted by roy entwistle on 08/10/2019 11:53:50:

If you case hardened them I doubt that you could sharpen them after

Roy ( I could be wrong )

Quite! About like hard-point saws. Particularly cheap hard-point saws! Use and throw away. At least with chisels one should be able to re-sharpen them.

In the case of wood chisels cheapness reflects on how often the cutting edge needs a ‘tickle’ on the sharpening stone (probably forget honing, for the worst of them!).

JohnF10/10/2019 11:48:03
981 forum posts
142 photos

Martin, Much has already been said but why not do a test on one chisel, just make a simple hearth, heat say 1/3 to 1/2 the blade to a cherry red in subdued daylight and quench in water -- test with a file, if it hard your on a winner if not dump them and buy better !

Assuming it goes hard then clean up to bright metal and temper by heating the rear of the blade allowing the colours to traverse up to the tip, temper to a dark straw and quench. Tempering MUST be done in daylight, not artificial light and not in bright sunlight, on a cloudy day its fine or move to a shaded area.

Case hardening for this purpose is a none starter unless it was a single use job, resharpening would remove the case and your back to soft metal.

Here is a link with far more info than you need but you may find it interesting



Circlip10/10/2019 12:45:41
1101 forum posts

And to get a better grade of cheapo wood chisels go round car boot sales and look for wood handled versions NOT the plastic handled variety. Wood handles will usually mean Carbon steel blades, not the modern HSS non keeping a cutting edge rubbish.

Cheap furnaces can be quickly put together using the Radiants from gas fires. Cannon Gasmisers were A source but have now been superseded by Flavel. Local tip/recycling centre is a good start.

Regards Ian.

SillyOldDuffer10/10/2019 12:50:26
5762 forum posts
1226 photos
Posted by Martin Dilly 2 on 09/10/2019 22:33:17:


Thanks to the rest of you, too. I suspect it's a case of chinesium, which may defeat my best efforts.

Could be! I'd love to put those blades in front of an Alloy Analyser.

I suspect there are a number of explanations. In the good old days Chisels were made from Tool Steel. This is a simple carbon-steel, straightforwardly mass produced cheaply at high quality since about 1880. Although easy to harden by heat treatment and good at holding an edge, it's fallen into disfavour, for example cutlery and working knives are rarely made of it, and the same may be true of chisels. Reasons for falling out of favour include staining, prone to rust, excessive brittleness, poor heat performance, and high maintenance. (Frequent resharpening needed.)

Nowadays most small kitchen knives are made from stainless steel. Eating knives are rolled from a soft stainless alloy and won't take an edge. Sharp knives are made from a very hard stainless alloy that isn't as easy to sharpen as carbon-steel. Neither of the stainless alloys used respond to heat treatment. They're semi-disposable - when sharpening is needed it may easier to buy a new knife.

Martin's chisels are likely made from an Alloy Steel. The manufacturer might have valued rapid factory sharpening and not snapping in customer hands more than making the best possible wood-chisels. If it's an alloy steel, it may not behave like an ordinary engineering steel during heat-treatment. If you don't know what a steel is, all bets are off.

Alloy steels with non-standard properties have been squeezing the more ordinary steels out of manufacture for at least a century. A 1938 Austin is mostly dead-mild steel, mild steel, cast-iron, and a sprinkling of specialist steels for springs, gears, and shafts etc. Nice and simple. A modern car contains a much wider mix of specialist metals, each chosen carefully to maximise performance and safety at minimum cost. Boron steels and other fancy manufacturing techniques may be marvels in the right place, but they' can be downright unfriendly in a home workshop! Those Chinesium chisels may be from the same camp: cheap replaceable tools cost-cut for light amateur use, not intended to make professionals happy!

By all means experiment but don't be surprised if the metal doesn't behave with textbook simplicity. Please let us know what happens, good or bad.


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