Re- hardening chisels?

[garywayne]

Hi, everyone.

I realise that there has been a topic on making your own chisels, (hammer) but I was wondering if, and how, :-k I could re-harden used chisels that had been re-ground by there previous owner, :evil: which has resulted in heating up the blades and softening them :?:

Also, is there a way of telling wether a secondhand chisel has been softened or not, just by looking at it, at a car-boot sale forinstance :?:

ATB Gary.

 

[CHJ]

Hi, everyone.

I realise that there has been a topic on making your own chisels, (hammer) but I was wondering if, and how, :-k I could re-harden used chisels that had been re-ground by there previous owner, :evil: which has resulted in heating up the blades and softening them :?:

Basic:

1: Heat to Cherry Red (800 deg.C). quench in water at room temp or preferably oil. (not so brittle hard)

2: Polish surface

3: Gently reheat cutting edge area to a Straw Yellow (220-230degC.) and re quench. (Tempering to remove brittleness.)

The darker Yellow the colour the softer the material, pale Yellow (200-220 deg.C)will be harder but more brittle.

Also, is there a way of telling wether a secondhand chisel has been softened or not, just by looking at it, at a car-boot sale forinstance :?:
ATB Gary.

Take a small swiss file or ladies nail file with you, if chisel is hard then file should skid on surface.

EDIT: A Steel Surface Colour Temperature Chart that may be of interest.

 

[garywayne]

Thanks Chas.

That information will help my chisel quality tremendously.

ATB Gary.

 

[Chris Knight]

Garry ,
When tempering, it is awfully easy to overshoot the straw colour. I prefer to heat back from the edge and "float" the colours along the blade to the edge.

 

[CHJ]

Garry ,
When tempering, it is awfully easy to overshoot the straw colour. I prefer to heat back from the edge and "float" the colours along the blade to the edge.

I go along with Chris on that Gary but do not overdo the temp at any point near/behind the cutting edge. Any metal heated above the straw range will have to be rehardened if you reshape or sharpen back into it.

This is most likely the reason that your old chisels appear soft.

 

[garywayne]

Thanks for the tip Chris.

With the help of you both, I can now build my sets of chisels from the car boot at a price that I can afford.

Thank you both for your help, and knowledge. ccasion5:

ATB Gary.

 

[garywayne]

Hi all.

I have just received information saying that the first thing that I should do before re-hardening, is to anneal the metal first.

To do this, I should heat the metal to cherry red, then quickly insert it into a tin packed quite tightly with Lime or ash,(from a fire). This allows the metal to cool slowly which softens it, and changes the molecular structure or something. Then it can be shaped, hardened, and tempered.

I hope I got this right, I'm sure I have, thats what I was told.

ATB Gary.

 

[CHJ]

Gary, I personally would not think there would be any need to anneal the chisels before re-hardening.

In theory annealing first is the ideal to get the material back to a fine stress free grain structure, but I would not expect the chisels to have had that much heating or mechanical abuse as to have seriously affected the material structure.

If you wish to reform the chisels into a different shape then yes you will need to anneal to get them in a workable condition.

 

[garywayne]

Hi Chas.

OK, lets see if I have this right.

To, just re-harden, use your recipe. To re-shape, use the annealing recipe?

I do apologize if I am taking up your time, but I need to clarify things for my confused little grey cells. :?

ATB Gary.

 

[Chris Knight]

Gary,
I agree with Chas. Annealing gets the metal soft enough that you can reshape it by filing, sawing, bending etc. If I want to do any of these things - and I usually do because I modify tools - then annealing is a must. However, just to resore the temper to a tool, I would heat and quench it then temper it as previously described. I have no idea if this is the best possible way but it works fine on the chisels I am usually playing around with.

I suggest you have a play with a small, scrap chisel and see how you do.

 

[bugbear]

Hmm. Given that skilled tempering was one of the prime skills that was held in reverence in Sheffield, I'd be nervous of trying to improve any thing other than a junk chisel.

A badly ground (burnt) old chisel is normally easy to spot, since it will have a shiny and horrid bevel, and a great big blue mark next to it!

Since decent old chisels in premium brands (I. Sorby, Marples, Ward & Payne etc) are quite common at 3-4 quid at car boots, I'm not motivated to try and second guess (or beat!) the skilled temperers of old Sheffield.

BugBear

 

[garywayne]

Thanks Chris.

I intend to go to the tip today and see if I can get hold of an old B-B-Q cheap to use as a forge.

Once I have some charcoal I'll give it a go and let you know how I got on, probably at the weekend.

bugbear, my whole intention is to improve old soft chisels by re-hardening them. As they where dirt cheep and already soft, I can only improve them, or they will remain soft. Surely I have nothing to loose.

ATB Gary.

 

[CHJ]

Hmm. Given that skilled tempering was one of the prime skills that was held in reverence in Sheffield, I'd be nervous of trying to improve any thing other than a junk chisel.

A badly ground (burnt) old chisel is normally easy to spot, since it will have a shiny and horrid bevel, and a great big blue mark next to it!

Since decent old chisels in premium brands (I. Sorby, Marples, Ward & Payne etc) are quite common at 3-4 quid at car boots, I'm not motivated to try and second guess (or beat!) the skilled temperers of old Sheffield.

BugBear

As someone who did a 5 year apprenticeship as an instrument maker (complete with heat treatment of metals) and someone whos last employment involved control of some 3 megawatts of heat treatment equipment conditioning Aircraft materials, IMOHO there is nothing sacrosanct about the treatment of normal high carbon steels found in all but the most exotic tools.

It is only the treating of High Speed and specialist Tool Steels that is beyond the average DIY enthusiast.
Older methods for these were to use molten lead or dry air to treat these, (modern methods can involve vacuum furnaces and argon gas etc.) soaking at 800Deg.C then raising quickly to 1150-1300 Deg.C to prevent excessive grain coarsening and then temper at 550-600 Deg.C, this in contrast to high carbon steels increases the hardness by converting the retained austenite to martensite.

 

[CHJ]

...snip...I intend to go to the tip today and see if I can get hold of an old B-B-Q cheap to use as a forge.
.

If you do decide to play, then after you have eaten your stickle fish/steaks etc. if you heat an old chisel steel to cherry red and place it immediately in the charcoal as it burns itself out, this will anneal it for you.

 

[garywayne]

Thanks chas.

I'll give it a go. You seem like an honest type of person.

There weren't any old B-B-Q's down the tip, I'll wait till after the weekend.

ATB Gary.