I have been reading Primitive Skills by Stephen Coote (members can borrow it by joining the BushcraftNZ Library) and was especially interested in the primitive blacksmithing chapter.

From what I understand the jist of it would be:
  • Start off with a medium to high carbon content steel (ie an old file).
  • Heat it to orange yellow and shape it, over and over
  • Harden by heating cherry red then cooling rapidly (in water or oil)
  • Tempering then helps lose some of the hardness to become less brittle and break resistant. I get that there are different colors that appear on the steel surface indicate a scale of tempering, from pale yellow to blue/grey.

I had a question that i hope Stephen or Warren can answer for me. How is this tempering physically achieved? Just by heating more for blue/grey than for yellow?

So a tang and back edge of a knife that must be well tempered and springy (to avoid breakage) must be tempered for longer and achieve the grey.black stage? I assume if at this stage it was overheated and got to a glowing red stage the process would have to start again.

Thanks in advance!

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one way to temper your knife so that you have a hard durable edge with a blade that is not going to break or shatter you would clean the blade with emery till bright then slowly & carefully heat the back of the blade & tang with blow-torch until it is blue & allow this color to travel evenly towards the edge until edge turns a straw color. remove heat & allow the blade to air cool.
I would probably temper a knife blade just like Warren has described. I'd add that if you get too much heat into the back of the blade you might find that the edge will reach the color you want..... but then get even hotter and go to another color further down the scale and the blade would end up softer than you might want. This could happen even if you have removed the blade from the heat source. So like Warren says... slowly and carefully is the way to go. You can keep on separating the blade from the heat source and carefully watch the color traveling across the blade.

Or you could immediately quench the blade once the desired temper color is reached. This should be ok provided that you haven't taken the back of the blade to above the 'critical' temperature which would cause it to become brittle when quenched.

A gas torch is great for tempering, but it can be done in a charcoal fire or even by rubbing the blade on a big piece of very hot steel. I remember that my Dad once tempered a special reamer he'd made in the kitchen oven. It worked well.

So how is tempering physically achieved? Hmmm. The practical side of it basically covered in what Warren wrote.

When steel with a certain amount of carbon in it is heated to a bright red and then quickly quenched it generally becomes very hard. I understand that this is because the structure of the steel is suddenly 'frozen' in a state that is relatively stressed. The desired 'temper' is obtained by heating the steel to the point where some of the stress is relieved. (My explanation may not be 100% correct, but the concept helps me to understand and explain the process).

Besides hardening and tempering steel, there are other forms of treatment that can be done with heat:

Annealing: This is done to soften hard or stressed steel. To soften a file, for instance, I'd heat it to bright red then bury it in a mound of soft, dry wood ash to allow it to cool as slowly as possible. There are probably better or more sophisticated ways to anneal, but in practical terms all you need is a method that works well enough for what you need to do.

Normalizing: The difference between annealing and normalizing doesn't seem to be clearly defined. But normalizing might be done to a bit of steel that has been subjected to a lot of stress through 'cold working'.... or it might be applied to a bit of steel that has been heated very hot for a long time to reduce the grain size of the steel thus making it less likely to break. To normalize the steel is heated to above critical temperature (where it loses its magnetism and is red hot) and then allowed to cool normally in the air.

I've forged a few blades etc and I've never conciously attempted to normalize anything, but I probably should have. Mind you, I don't recall anything breaking. When I follow what the real enthusiasts and experts are doing, it seems that some might normalize their work maybe a couple of times before hardening and tempering.

When hardening, some steels might crack if quenched in water. I've noticed this most, I think, with steel taken from car springs. So I'd use oil. Mostly I've used old engine oil, but I think a nicer thing to use would be cooking oil. Oil often bursts into flame when the quenching is done, so you have to do it in a safe place and take precautions. Use tongs and have a lid ready to put on the oil to extinguish it. And never pour water onto an oil fire to extinguish it because it can have an explosive effect and make things worse.

People have made some good tools out of files. I'm a bit cautious about files if the finished product is going to be subject to a lot of stress - especially levering. I might be quite wrong, but I get the idea that at least some files always seem to retain some brittleness even when heat treated.

My understanding is that up to a certain point, the more carbon steel has in it, the harder it can be when quenched. But generally harder steel will be more brittle. The less carbon steel has in it, the less likely it is to break when bent. So somewhere you have to decide what qualities you want in a tool.

Other elements are added to iron as well as carbon to try to achieve different characteristics.... like toughness while still able to retain a hard, durable edge. If you read what is written about knives, for instance, you might come across a bewildering range of steels and opinions.... and possibly some steel 'snobbery'. I tend to shrug off a lot of this hype because I've found that a simple bit of medium to high carbon steel will make a knife blade that is more than adequate for what I do, and it performs a helluva lot better than tools I might make from stone or bone. I doubt that many of the people who debate the steel types could show you a significant difference in performance in practical, everyday use.

If I had to name just one type of steel to use for a knife blade I'd say try old leaf springs from cars.
You can temper your blades in an Oven or even a bench top type oven as well. I have made several blades and only edge quenched them in oil, if you have to use a lower carbon content use used motor oil as apparently the metal will absorb some of the carbon in the oil, heat steel in oven to a straw colour and hold for an hour or so then allow to cool naturally. On the edge quenching, I have never tempered after this and have had no breakage. Edge quenching only means, hard edge and softer spine. Also when heating blades for quenching, use a magnet. When the metal is non-magnetic, e.g. the magnet is not attracted to the metal, heat slightly longer then quench in oil. Also you can use spring steel, fresh from a spring makers, no need to anneal, the blade can be made razor sharp, you can forge the metal thinner as I can only buy 6mm thick. And if you stuff up you have not lost $50 worth of 01 tool steel. I think I bought 60mmx6mmx2metres approx of spring steel for like $90.
if spring steel or any steel for that matter is heated to forging temp it will need to be hardened & tempered again...I think making a knife with a grinder is typically consumerist & gives an inferior quality blade over a hand forged blade. Forging forces the grain structure of the metal to flow with the contours of the blade which gives it a greater strength as opposed to grinding which cuts thru the grain therefore defeating the purpose of the structure ie: holding it all together.

Scott, i'd spend the $90 on coffee & coal for mah forge & use the leaf springs found at pritty well any junk yard... :)

I know this is an old thread but since I've only just joined and am a bit of a "knife knut"  the forum I'd like to add to it.

One of the best easily available tutorials on making a knife from an old file is published in the August/September 2006 edition of The Shed magazine. It was by Auckland cutler Pete Caulfield. Check it out your local library may still have it or you can back order it from the publishers.   

Cheers for that! Good to know

Here is a link to an excellent web site on making a bushcraft knife. It has been around for a while and you may have already seen it. It has a template for a Ray Mears knife and detailed instructions on all aspects included annealing ,hardening  tempering and creating a single bevel edge. In addition to written instructions there are 4 comprehensive videos on youtube which demonstrate the processes.

greenpetes bushcraft knife

Cheers for that, I'll have a go

HI, all there has been some good stuff here about hardening and tempering most of it close to the mark some a little confused and even confusing but good stuff all together the mention of files , a lot of files now on the market  are not suitable for making files because the body of the file is not hardenable  only the teeth have been case hardened   that is the reason they don't last very long doing real work beware cheep files  you can make an oven for  tempering a hardened blade by using a radiant stove element  and a simmer stat from an old stove ...It is probably better to use oil for a hardening quench bath and if you have to use  water get it up to body temp before putting the blade in if the blade will not harden id the oil quench then is the time ti use water  ... Rob

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