Cold Chisel heat treatment

[matt993fod]

I broke my dear father's favourite chisel whilst bashing apart a rusty old chimney setup for a forge I bought.

I have promised to replace it, and intend to make a replacement from a section of vehicle coil spring of around 2/3" thickness.

The problem is, all the how-tos I have read for chisel making suggest running the colours to temper. I have been told several times from various smiths that this is not an effective method of tempering. Can anyone suggest a suitable time and temperature for oven tempering a cold chisel?

Thanks in advance.

[Sam Salvati]

LOL for chisels it is fine. personally i would make it from H13, then heat the whole up to yellow and let it cool in still air, but that's just me:). Cool about a little more than 1/4 inch of the very edge in water/oil, then quickly remove it and file the surface shiny and let the colors run, for a cold chisel purple should be fine.

[Steve Sells]

With an unknown metal. heat to non-magnetic, quench in warm to the touch oil Test with file to see if it is hard, if not and sure it was non-mag, then redo the hardening process, using water rather than oil. We warm the quench liquid because thermal transfer if better than room temp would be.

after you are sure it hardened, place in oven at around 350F to 375F for 2 hours (you can add a frozen pizza so the wife isn't upset at the oven being on), ou can temper higher but I suggest a 2 stage temper, this is the stress refeif. dont wait to temper because it may crack while you are waiting.

IF you plan on heavy abuse to this, like hitting it with a hammer, I suggest using a torch heat the hammer contact side to high temp to soften this end a bit more, which should help to prevent chipping. Do not allow the cutting side to get hot. The easy way that I do this is to place the cutting edge into about 1 inch of water, while you heat the other exposed end. Heat the contact end to blule or purple.

[Glenn]

BP0352 Chisels Reconditioned

The photo is posted as an example of running the colors.

[matt993fod]

Ha! Reconditioning it would certainly save me a bit of trouble. He hasnt noticed yet!

So colour running is ok for chisels? Thats good. This should be no problem, once I get the forge finished.

[steveh]

if the colors run out ,nothing lost,just reheat the tip and start again.when i do jackhammer chisels for concrete i quench at blue,for granite at straw .just an example.

[R Funk]

Steve Sells,

I don't understand the dynamics of quench baths as well as I would like to. I wonder if you could clarify what you mean by the following...

"We warm the quench liquid because thermal transfer if better than room temp would be"

I thought in water the important factors were the agitation and the temperature with the higher temperature difference creating the higher heat transfer. Also could you further explain what you mean by "a 2 stage temper"?

GENERAL DISCUSSION

Steve is exactly correct that if you temper in an oven you need to further temper the body and head of the chisel.

The advantage with letting the colors run is that this leaves the body of the chisel and the strikening end in a softer condition than the cutting edge, which is desirable. The problem with letting the colors run is that it can occur very rapidly and the tempering temperature could be off by 25 or 50 deg F. I have missed the colors running and had to reharden and try again on more than one occasion. However this is a simple, almost one step hardening and tempering process and has been successfully used by blacksmiths for many years.

But this brings up another problem. If we let the colors run and then quench, the steel is at the correct tempering temperature for just a few seconds. We blacksmiths behave as though the steel reponds instantly to temperature changes.

In reality steel reacts rather slowly as the commercial heat treaters rule of thumb is 1 hr for every inch of thickness. Thus a chisel should be held at tempering temperature for approximately 1 hour to extract the optimal results from the heat treating processes.

This will make certain steel will spend ample time at the accurate tempering temperature. Using an oven allows allows the cutting edge of the chisel at an optimal state But as Steve says an additional opertion softening the body and striking end of the chisel needs to be completed before the tool is used.

[stretch]

If your colors run to fast then you left the piece out of the water to hot. After heating end of chisel to cherry red(canadian cherry) I usually dip in water from 1 to 1 1/2" until the piece above that loses all color, then polish the hardened piece til shiny and turn it hard side up in a vise to wait for the colors to run. It will take sometimes 5 minutes to run and you will have a wide color band, it is less likely to have a fracture line this way. Then drop in water after the color you want reaches the cutting end. I usually have about 1" of the end hard this way and have only had one chisel break because of to hard in 30 years. I have done 2" diameter concrete chisels to 3/8" chisels for fine work this way with great results. The key is to have slow color run and dip the chisel up and down so no fracture line. Packing the metal after forging and before tempering helps a lot too.

[Ice Czar]

jackhammer chisels for concrete i quench at blue,for granite at straw

we quench impact hammer chisels (for hot chiseling steel) at blue > purple in linseed oil

[Tom Lumpkins]

Boy the info is definitly here on this site.

[maddog]

R Funk.

"We warm the quench liquid because thermal transfer if better than room temp would be"

Temperature difference is a major factor in the rate at which heat flows from the steel into the quenchant. However there has to be good contact which means the coolant has to wet the surface of the steel and quickly. This is why a quench in soapy water is faster than plain water. Heating the oil does give up some of the temp diff but in exchange you get better contact. Its a trade off that balances out to a faster quench.

[williej]

Being a total novice I find that you guys are speaking a language that I don't understand. So, I went to Wikipedia and found this statement:" The cumulative effects of time and temperature can also be gauged by monitoring the color of the oxide film formed while tempering a well-polished blade." Is this what you mean when you refer to "the colors running"?
williej

[Charlotte]

Yes indeed! Also hard to do. Underline well polished! Given a choice I use my oven. However since I am the chief cook and bottle washer I don't have a problem with putting things in the oven.

Tip: get a small metal rectangular cake pan with a sliding lid and be sure all oil is removed for the item before putting in oven. It causes less freaking out all the way around.

Edited May 16, 2009 by Charlotte
add words left out.

[John B]

Hi Williej,
Sorry Charlotte, I have to disagree with you slightly on this one, I am afraid I don't understand the term " The colours running " by the Wiki description, although I agree with the statement as it stands, iethe colour of the oxide film produced by heating indicates the temperature of the material

The best way I can describe it to a newcomer to see this phenomenon is to obtain or make a piece of polished steel, maybe 1/8" thick by 1" by 6" plus long.

Then using a blowlamp, oxy/acetelyne, MAPP gas torch or some other controllable heating device Heat one end of the bar by oscillating the burner tip in a restricted area, and as you gently raise the temperature in this specific area, you should see a variegated band of colours from light yellow through to steely blue, start to pass down to the cooler end of the bar,(the slower the heating, the wider the colour bands) A bit like a rainbow effect. The colours correspond to a temperature of the material at that point somewher in the range of from 420degrees F (216C) to 650 F (343C)

The trick is to catch the required colour at the quench. That is why it easier to control in an oven environment where you can control the temperature.

In days gone by, there were ways of holding temperatures by the use of molten salts, or varying a lead and tin mix which where molten at a specific heat, (a practical example of this is when pit saws or hand saws required tempering to a pale blue colour, they were placed in a vat of boiling linseed oil as it was known that this was 600F, (more importantly it would give the right temper for the saws) and so tools could be left soaking in one of these molten baths, allowing it to reach the temperature throughout.

That is the reason for the time to size ratio allowing all the part to reach the same temperature.

They were then removed from the soak and rapidly quenched in a suitable medium (oil, water, or brine)

Google in tempering chart for some ideas http://www.westyorkssteel.com/Heat_Treatment/tmpchart.htm indicates colours, there are plenty of other sites with good information including anvilfire

Edited May 16, 2009 by John B
Link posted

[Frosty]

John:

The term "running colors" refers to the way they travel along the steel as it heats. Or the range of colors. For instance. Temper colors run from pale straw to dark blue.

Gee, you'd think a guy from England would understand simple English.

Frosty

[John B]

Then using a blowlamp, oxy/acetelyne, MAPP gas torch or some other controllable heating device Heat one end of the bar by oscillating the burner tip in a restricted area, and as you gently raise the temperature in this specific area, you should see a variegated band of colours from light yellow through to steely blue, start to pass down to the cooler end of the bar,(the slower the heating, the wider the colour bands)

Ho Frosty, They were running, just very slowly.

Thanks for clarifying that, I think I overloaded that reply with too much information (I should have stuck with the KISS)

A Wise old man once said "There are only three things you get in a hurry, babies, bent bumpers (fenders) and bad decisions." He may have been the same one that said "If you are doing it fast, Take your time"

Both apply when one step tempering

Edited May 17, 2009 by John B

[forgemaster]

After heat treating many chisels this is my 2 cents worth. If you use the colour method, be sure that you don't have the bit of the tool still out of the oil too hot, ie orange, as when you quench the end you will make that bit that was orange almost glass hard, unless as suggested you quench a fair distance back which gives a longer temper running time and allows the back to cool down more. This also as stated allows the colours to space themselves out more and it makes it easier to judge when to quench the colours.
Move the chisel around and up and down as you quench (only need 10 to 15mm of up and down) this keeps the chisel in cooler quench medium and helps to prevent a quench crack occuring at the liquid line.
After heat treating the cutting end we will heat the striking end to a dull red and then bury it in sand or lime or rake together a pile of dirt on the floor (another benefit to having a dirt floor) and bury the heated striking end in that to try to achive a annealled end.
Our commercial way of heat treating chisels when you have a number of chisels to make is to forge, grind, normalise then using the oxy heat approx 50mm of the blade to a cherry red and quench right out in oil, remove from the oil, then we burn off the remaining quenching oil still adhereing to the chisel blade with the oxy torch, just until it stops flaring, (oil will flash at about 380 deg) hence the burning off of the oil, it provides an indicator of the temperature reached with the torch. This method is I have found a nearly fool proof method of training apprentices (we have trained a fair few now) to heat treat chisels that are to be sold to customers (who want their tools to work and be heat treated properly).
Running the colours is fine with experience but I have found that my above points have to be taken into account.

Namely

Be careful with the heat remaining in the back of the tool, not to hot.
Be mindful of quenching long enough to allow the colours to space out,
Also the heat at which the tool is first quenched can vary with the amount of background light in the workshop, which is why traditionally forges have been not overly brightly lit workshops.
I always advise my blokes that I would prefer a chisel to dull over or wear out then snap.
Better a little to soft and you have to reheat treat then to have a breakage.
Cheers N good luck
Phil

Edited May 17, 2009 by forgemaster

[Frosty]

"Hurry slowly" is a common bit of advice for doing important things.

After I sent the post I thought of a better example, Chromatography. Then it occurred to me that temper colors are in fact a form of chromatography. I just never know what'll happen when I start thinking about something.

Frosty