It’s sometimes useful to know how to harden steel. Although it’s possible to do woodturning with very few tools, I can never resist trying something new. Sometimes I buy tools, but often I make them myself. Back in the days before factory-made tools were available, turners knew how to harden steel and temper it. They made their own tools out of high carbon steel, or got them from the local blacksmith. Of course, the best way to harden steel and temper it is to do it properly, using modern methods based on good science and metallurgy. This requires the right steel for the job, with careful control of temperatures and timing and the rates of heating and cooling. Acceptable results for many purposes can usually be obtained using the cruder methods described here. But modern high speed steel cutting tools are best, particularly if they are to get a lot of use.
Turning tools are now rarely manufactured from carbon steel, but it is still used for many other purposes. Even for turning tools, it still works as well as it did when the old-time turners made their living with it. You can buy high carbon steel in various grades from steel stockholders, some suitable for hardening in water and others in oil.
Scrap carbon steel is easy to find. If you want to harden steel for tool making, springs, files, saw blades, masonry nails, crow bars, cold chisels, woodworking chisels, old screwdrivers, plane irons and many other things are made of steel that is suitable for reworking. I have used the tines of an old garden fork and the rings from old ball races. Keep in mind that if you put the tool under significant stress, steel that has or may develop cracks is unsuitable. For example, the valleys between the teeth of a file are weak points where cracks can start.
If you are using scrap, old steel is often the best choice because some modern steel alloys may not respond well to these basic heat treatment procedures. One way to tell what you’ve got is to touch the steel on a grinder. Lots of bright bursting sparks like a child’s ‘sparkler’ firework mean it is probably high carbon steel. Try a bit of mild steel such as an ordinary nail for comparison. It will also make a lot of sparks, but there will be fewer bursters. The use of the metal before it became scrap is a clue – if it was subject to a lot of stress, the metal is probably high carbon steel. But there is a continuum of carbon content – to be sure, test-harden a piece before making the tool.
Low carbon mild steel
Ordinary low-carbon mild steel is not suitable for most cutting tools. You cannot harden steel of that kind, and it will not keep a sharp edge. To harden steel, the metal must have a high enough carbon content. The metal is harder than wood, and a cutting edge on mild steel may last long enough for a one-off job. But if the edge is thin it will just give way under pressure.
You can case-harden mild steel. This gives it a very thin layer of higher carbon content on its surface. Then you can use it for some light-duty cutting tools provided you don’t grind away the hardened skin when sharpening. To case-harden, you have to coat the metal with a special compound before heating it. You can use mild steel as a holder for inserted or brazed-on cutters.
To harden steel, you must first make it hot. You can use a magnet to check that it is hot enough for hardening – when the magnet stops attracting it, the steel is ready. More simply, just get it red hot, which is also the temperature needed for hot forging. The bigger the piece, the more heat you will need. If the item is small, you can harden steel using a burner on a gas cooker. You can use a charcoal fire with a blower to supply air. You can do a lot with a reasonably powerful blowtorch. Larger pieces of steel may dissipate the heat as fast as it is applied, never getting hot enough. If using a blowtorch, you can stack a few dry bricks to make an enclosure to retain the heat.
A simple forge burning solid fuel or propane is not hard to improvise. There is video on YouTube about making a simple but very effective propane forge. I made a coal forge that burned anthracite and worked 7/8 inch bar without too much trouble. More recently I made a propane forge using just two insulating fire bricks and a blowtorch, and was able to bend steel strip of about 2 inches width and 3/8 inches thick.
If you make two pieces of steel white hot, you can weld them together by hammering. Hotter still, the metal will burn and spoil. But these temperatures are harder to reach with the sort of equipment described.
If you want to do any hot forging, you will need something to use as an anvil, a hammer, vise and heavy pliers. You may also need a hacksaw, angle grinder, file, bench grinder and a drill press. To work thick steel, you will need heavier tools and a forge. If it’s hot enough, it’s surprisingly easy to bend steel using a vise or wrenches, or to hammer it into shape on an anvil. You can forge high carbon steel and ordinary mild steel in the same way.
You may find that some scrap carbon steel is too hard for sawing or filing into shape. You will have to anneal it to make it softer and workable. To do this, the first step is to make the metal red hot. While it is hot, you can forge it, but don’t try to work the metal when it has lost its red heat or it may crack. You have to ‘strike while the iron is hot’. After any forging work, get the steel red hot again and then anneal it by cooling it slowly. The slower it cools, the better – traditionally the metal was left to cool buried in hot ashes. If using a propane forge, you can just leave the metal to cool in the forge so the residual heat will slow the cooling. You want the whole piece uniformly softened, so try to heat and cool it evenly.
When cooled, the annealed steel should be soft enough to file. Check that all parts are soft, then carry out whatever further operations you need. Shape the tool, including rough grinding the cutting edge.
How to harden steel
To harden steel, heat the part to be hardened bright red hot again, if possible ‘soak’ it in the heat for a bit, then quench it. It’s the rapid change from red hot to cold that will harden the steel. You can use various quenching liquids, but a bucket of water will usually do the trick. Plunge the red hot metal straight in, and swirl it about to cool it as rapidly as possible. If the steel warps or cracks when quenched, try using oil instead of water, or use different steel. If using oil, fire precautions are necessary. Use a metal container, not a plastic bucket. The hot steel will heat and ignite the oil, so have a metal cover handy to extinguish the flames.
You don’t always have to harden all parts of a tool to the same degree. If it must withstand stress in use, you can leave the bulk of it soft, for toughness and strength, with just the cutting tip hardened.
After quenching, the steel should be glass-hard and a file will just slide off it without cutting. Don’t try too hard with the file, the hard metal will soon make it blunt.
The freshly hardened tool will be brittle. If you were to use it in this state, the edge could chip or it could shatter. For most purposes, you must heat it one more time to temper it before use. Tempering takes away the brittleness. It makes the metal tougher, but softer. The higher the temperature reached during tempering, the softer and less brittle the steel will become. Each tool has its own optimum compromise between hardness and toughness.
Before tempering, clean up the metal using abrasives. You want the steel bright and shiny for this stage. Warm the tool very carefully above, not in, a clean flame. Watch the bright metal surface carefully as it heats up. Let the heat start away from the edge and creep towards it, aiming for a uniform temperature over the surface. The edge and any other thin parts will heat up too fast if exposed to the heat directly.
Watch the colour change
As it gets hotter, you will see the bright steel change colour. It will go from silvery to a pale yellow, through brown to blue and then to grey. You have to catch the moment when the oxidation colour you want reaches the edge. Normally a pale yellow-brown colour at the edge of a cutting tool is about right for cutting wood, but different tools may need other colours. A blue colour will leave the steel tough and strong, but not hard enough to keep a cutting edge. (Springs are tempered to blue, and some steel components such as screws are blued for decorative purposes.)
Don’t let it get too hot
If you let the metal get too hot you will have to repeat the hardening stage. You can repeat the annealing, hardening and tempering without harm to the metal. But too long at high temperature can tend to burn out the carbon from the surface layers. When the colour is right, quench the tool again. It is then ready for final clean up and sharpening before being put to use.
Sometimes you need more accurate tempering, for example if you need a larger piece evenly tempered. You can do this by heating the item in an oven at a set temperature, or in oil. You will need a thermometer for these methods. They allow for soaking at the proper temperature, which may give better results.