The following is an examination of the process of heat treating edge tools (with some history mixed in).
A Brief History and Background
Heat treating of ferrous metal tools is an ancient practice, it is known as far back as the early Greeks, and may be a good thousand years older.
Homer who was writing around 800 B.C. knew about heating and quenching of iron
In fact he compared Odysseus jamming a spear point of heated wood into the Cyclops’ eye as “giving a sound like that of a blacksmith quenching his iron” at the forge, remarking of the quench: “For that is the strength of iron, specifically…”.
That last word shows that he was aware of the opposite effect of softening for the same process used with a copper-based alloy like bronze.
Iron which has a very small amount of carbon in its composition, can be hardened by heating to a given temperature and then cooling suddenly.
If the carbon content is larger than a critical amount, the materials melts and pours more easily, but becomes what we call “cast iron” which does not have heat treating properties.
It is strong on impact and compression, but brittle and poor in tension, so it cannot be used for edged tools, which is probably why bronze (which can be work-hardened by hammering a knife edge) continued as the basic material for tools for so long.
It is hard to know when the iron in the crucible has the proper amount of carbon for hardening, but it can be estimated by trial and error.
Perhaps the earliest iron was found in ferritic meteoric pieces which had burned out all the carbon in hot transit.
By adding bits of wood to the mix, if carefully done, people did learn finally to produce a”20/30 point” iron suitable for tools.
This must have seemed a magical process in the early days, and the process was guarded in great secrecy, which explains the slow development of an Iron Economy in antiquity.
When steel, which is just the name we give to iron which has the right carbon percentage, is heated to a bright yellow in daylight, it changes its molecular state.
We now know that it loses its ability to be magnetized, the molecules are actively randomized.
And it is the sudden cooling of the randomized state to a solid stage again which locks the structure into its hard, “molecule frozen” state.
At that point steel is very hard but it is also brittle, we would place it above R 60 on our standard Rockwell hardness test.
It can be used in that state but only with care, and it was soon discovered that reheating it cautiously softens the hardness bit by bit.
When a tool is reheated to red/yellow heat, and then allowed to cool slowly, even by sticking it into a pile of cool wood ashes, it will finally be completely softened, or “annealed” when cold.
At this point it can be bent without breaking, twisted and tweaked, but to form it into a new shape it must be heated again and hammered while red hot.
The old adage says: “The cardinal sin of the blacksmith it to hammer cold iron”, since that produces internal stresses which later cause breakage, even if the material is reheated to red hot.
The Golden Mean of Stiffness Without Brittleness
But by bit people have learned over the millennia how to harden and temper (soften to a desired state) steel.
There is an exact middle-point at which the material has “maximum strength”, which means reasonable stiffness without brittleness.
A spring is a good example of this state, it must not snap on bending, and it must return to its previous position repeatedly.
Only a hundred years ago, steel was hardened under what a modern engineer would consider primitive conditions.
The blacksmith or factory hand-worker heated the cutting end of a chisel to a bright red, plunged it into warmish water agitating it a few seconds, and then set it aside for “tempering”.
In order to temper it, he first ground or sanded the black surface scale off the tip a few inches back so as to see the colors appearing.
And then he applied heat behind that point, watching colors on the surface of the chisel progressively change as the heat traveled up toward the tip.
The order of the colors was the key to tempering, since each oxided color had a specific range of temperature.
First color to appear (the lowest temperature) was pale straw color, then a light straw, then light brown, brown, light gold, gold, dark gold, faint purple, and finally blue, giving a range of from about 400F for pale straw to 600F for true blue.
This was perfectly attuned to craftsmen’s needs.
Tempering Colors and Uses
The pale straw would be excellent for a chisel which was used in soft to medium density woods, carving with a pushing action.
If in harder wood or lightly tapped a medium straw to brown would be less likely to chip, while for real driving with a mallet, as in timber framing mortises, it would be better to go for a light blue.
If you harden a screwdriver at the bit and then grind it to shape, you get a pretty good indicator of wear after using it a while.
If “full blue” you will find it rounding over at the edges, but if pale straw there is a good chance that it will snap off on a hard twist.
So like many a l9th century craftsman, you will harden to a good healthy “brown” and the screwdriver will keep its edge for years.
(We won’t talk about the people who file a screwdriver tip to a V, they think that is sharp but we know it is a dull idea.)
Heat Treating a Plane Iron
A plane iron is a little harder to work with, since you have to get just the front edge hot, so clamping a piece of steel or aluminum about half an inch from the cutting edge would keep the heat down near the edge.
Since such a thin piece of steel is more liable to cracking, I would plunge it into a bath of a cup of motor oil, which has a slower rate of heat transfer, and is often used as an alternative to water.
(Remember oil quench is great, but oil can burn.)
In any case what you are trying to do is bring the heat down from about 1600F to under 1200F in a few measured seconds interval.
Water quenching gives more hardness, oil gives less if that it desirable.
But tempering to a middle range, even on the soft side, is necessary for a plane iron, since it can hit knots which will crack a hard edge. So for plane irons, go for a decent “brown”.
Most of the steel you will find in woodworkers tools is “carbon steel” so called, which you can identify by the characteristic spark against a grinding wheel.
Carbon steel sparks go out straight, then divide clearly into clusters of bright yellow sparks, whereas any of the “tool steels” throw out straight patterns of dull red spark without branching or sparkling.
Of course there are hundreds of specialized steels in common use, and many of the modern steels cannot be torch-hardened at all.
If you do have to make a new part, for example a moulding plane blade, look for “water hardening steel = W1” or “oil hardening = O1” which can be handled in the traditional way.
Highspeed steels, so called, need very special rates of cooling from a specific topheat, often in an inert atmosphere, with prolonged cooling rates, but I have hardened unknown steels from the junkyard successfully.
Although from sparking I know they were not suitable.
Test and try!
Basic Rules to Follow When Heat Treating Edge Tools
But there are a few basic rules to follow:
- Never heat metal to the point at which it starts to emit sparks, since these are the carbon molecules departing from the steel, and you need carbon or the steel will not harden.
- Never heat steel to be hardened in the forge, since even good blacksmith’s coal will have small amount of sulfur which is death to hardening steel.
In fact it is introduced into Free Machining Steel to make it cut more easily, of course losing strength.
- If you heat your steel with an oxyacetylene torch, go easy because you want a heat of 1600F, whereas the flame is almost 6000 F (sic).
And your torch must have a neutral flame, which means you adjust the acetylene until there is just a touch of a “beard” ahead of the blue cone, and then diminish it to “no beard”, which is the neutral flame.
Too much beard means too much carbon which will weaken the steel, too much roaring oxygen flame will burn out the carbon, so there is reason for caution here.
Best have some who knows the flames show you — or experiment and check the results!
- Never try to bend or hammer on hard steel, which can shatter and fly into your hand or eye.
But when heated red hot and softened in ashes, hard steel is ready for bending and forming.
Some people use files for making tools, which is great so far as the carbon steel is concerned.
But files snap badly, so beware.
In conclusion, heat treating certain woodworking tools as repair for overheated chisel blades or making a new tool from bought stock, is something which can be done by a person who:
- knows the procedures I have outlined above, and
- has access to an oxyacetylene or propane/compressed-air torch, or for small work even a propane flame directed against a carbon block (which retains heat and emits more as it burns).
It is more than a little magical to see a piece of steel which is totally soft turn so hard that it can carve into the bar from which it was made.
And when you heat treat steel you feel you are in the trail of a very ancient and very important tradition.
But always remember to touch iron lightly because it can be hot, to wear glasses through the whole process.
And if you learn the process well, teach someone else in the belief that this old tradition of the metalworking trades should be passed along.