Creating Heat for Hardening

[Martin Dilly 2Participant @martindilly2]

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?

[Martin Dilly 2Participant @martindilly2]

[Grindstone CowboyParticipant @grindstonecowboy]

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.

[mechman48Participant @mechman48]

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.

George.

[SillyOldDufferModerator @sillyoldduffer]

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.

Dave

[DustyParticipant @dusty]

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

[EmgeeParticipant @emgee]

Martin

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.

Emgee

[Andrew TinsleyParticipant @andrewtinsley63637]

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.

Andrew.

[HopperParticipant @hopper]

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 BlokeParticipant @kiwibloke62605]

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 2Participant @martindilly2]

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.

[egaParticipant @ega]

Don't forget the magnet test.

[Kiwi BlokeParticipant @kiwibloke62605]

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 entwistleParticipant @royentwistle24699]

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

Roy ( I could be wrong )

[mechman48Participant @mechman48]

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

[John ReeseParticipant @johnreese12848]

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 2Participant @martindilly2]

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 yetParticipant @notdoneityet]

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!).

[JohnFParticipant @johnf59703]

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

John

**LINK**

[CirclipParticipant @circlip]

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.

[SillyOldDufferModerator @sillyoldduffer]

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.

Dave

[Tim StevensParticipant @timstevens64731]

The link from John F includes an excellent colour chart of the various colours involved in the hardening and tempering processes. But, what is does not explain is that the lower half of the chart (up to 800F) is what you see by reflected light, and the steel surface needs to be cleaned and bright to show the colours properly. In reality, the coulrs are due to the thin layer of oxide on the steel surface (thicker as the temperature rises) – rather like the reflected colurs of a soap bubble. Above that temperature (top half of the chart) shows the colours of the radiant light (glowing) from the steel. These colours do not rely on a shniy surface, and can be seen in pitch darkness because the heat itself is producing the light. If this is not explained it is difficult to see how or why the colours change from red-ish at 500F, through blue, and back to red-ish again at 1200F.

The top half of the temperature chart is for hardening colours, and the lower half for tempering colours.

Cheers, Tim

[Martin Dilly 2Participant @martindilly2]

A bit of a pause there while other projects came along. However, I've made a fairly crude version of the two-bricks- and-a-torch hearth shown here: https://www.youtube.com/watch?v=ifTIuNt3aNY

However, I have a problem. When I fire up the propane torch and introduce it into the hole the flame goes out.The torch's air intake holes are outside and clear of the brick. The main internal width is around 2 inches, but it's not actually circular in section. I suspect it may be connected to the flame point 'bouncing back' from the wall it faces; if I leave the top half of the hearth off all's well till I put it back, at which out goes the flame. Suggestions welcome.

[not done it yetParticipant @notdoneityet]

Unfortunately it is a fact of life that the flame requires oxygen to burn the fuel. Disrupting the flow will cause the flame to extinguish. Either simply because products of combustion are being drawn in – instead of air – or that increased pressure is slowing the combustion air being drawn in by the ’venturi’ effect of the fuel flow. Improved design of the combustion chamber – shape, size – and improved exhaust exit from the combustion area are required. Slower heating is another option, utilising more heat from the hot gases generated before they exit the chamber.

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