There are two properties which distinguish a good tool from a bad one. The tool must have the correct shape for the work which it is to do, and the tool must be hard enough to last a long time. This class will teach you to make tools hard enough to last without being so hard that they are too brittle and break.
Tool steel has three different states: annealed, hardened and tempered. New steel comes annealed; the steel is as soft and workable as it gets. Hardened steel is just the opposite, as hard as it gets. Fully hardened steel is not useful as it is so hard that it will break if struck (it is brittle). Tempering is the process of removing some of the hardness. Tempering trades off toughness against hardness. More details will be found under Additional Information.
The process of making a hardened tool requires several steps:
- Shaping the tool to final form (while steel is still annealed)
- Hardening the tool by heating to an orange glow and then quickly cooling (quenching)
- Tempering the tool by gentle heating to increase toughness
- Final polishing of working surfaces
These steps will be described in more detail below.
Shaping and Hardening a Tool
First shape your tool (while still annealed) by whatever method works best for you — forging, grinding, belt sander or file. Put the finished surface on the tool; if it is to be polished, remove all scratches and polish the surface while still annealed. Scratches should be removed as they will focus stress and lead to short tool life. This is especially important in thinner tools such as blades and chisels.
When your tool shape is finished heat it to an orange heat and keep it there for a minute or two. Now take the tool out and let it air cool. This step is called normalizing, and reduces any stresses in the steel. Once cool, heat again to orange and keep it there for five minutes. The molecules in the steel must have time to rearrange themselves. Five minutes is long enough for 1/2″ and smaller tools; you may need to leave the tool in longer for larger pieces.
Once the tool is thoroughly heated, remove it from the heat, grasping it by the end you strike with the hammer, not the point. Immediately plunge the tool straight (point first) into a bucket of oil (cooking oil — see later information on quenching). It is important that the tool enter the liquid perpendicular to the surface, or the tool can bend. Plunge below the surface a couple of inches so that the oil doesn’t catch on fire. Hold the tool with a pair of tongs long enough to keep your hand out of any fire; if you are using vise grips, wear a heavy glove. Stir the tool in a figure 8 pattern until it is mostly cooled off (less than 400°F).
Be careful not to drop your tool — it is brittle enough that it might break.
Tempering and Finishing a Tool
Tempering is removing some of the hardness to improve the tool’s toughness (resistance to breaking). This is done by heating the tool to several hundred degrees — the higher the temperature, the more hardness is removed. You determine the temperature by watching for the oxide colors on the surface of the metal (you’ve probably had the experience of accidentally tempering a tool on a high speed grinder by turning the steel blue). What color you temper to is determined by the tool’s use. Here are some guidelines:
- Cutting tools – Straw (yellow)
- Stamping (chasing) tools – Bronze (brown-yellow)
- Leather tools, screwdrivers – Peacock (purple-blue)
First you need to clean the tool to remove oil from hardening (soap and water, or wipe with towel). Clean the surface scale off so that you have a shiny steel surface. Scale may be removed by a powered wire brush, but it is much easier to remove if you soak the tool in vinegar and salt for awhile first. Scale can also be removed by rubbing on an abrasive stone or on a file (this is not good for the file). You don’t need to get all of the scale off — just enough to allow you to see the tempering colors. Now heat the tool gently and slowly in a torch flame. Don’t use the main part of the flame; keep the tool out at the edges of the flame so that it doesn’t heat up too fast. It is quite easy to fall asleep at this point and go too far — be alert! Watch the steel carefully for the first sign of color change. Once you detect some color (a faint straw, light yellow), remove the metal from the flame and watch until you are sure the color is stable. Now resume heating even more cautiously, moving the tool into and out of the flame. When the final color is reached, cool the tool in water. You want the color change to go very slowly to be sure the core of the tool is also heated. If you should go too far, re-harden the tool and try again.
You can selectively temper different parts of the tool to different levels of hardness. For instance, a cold chisel must be very hard at the blade, but should be no harder than blue at the end you hammer on. If the hammer end is as hard as the blade, the hammer may chip when you use the chisel (steel chips are quite painful). Selective tempering is quite easy; first temper one end, then temper the other to a different hardness. This is something that modern oven tempering methods cannot do. The temper colors can be quite pretty.
Now polish the working surfaces to remove any scale the tool picked up during hardening, and you have a new tool!
Tool shapes (and mechanisms in an articulated tool like pliers) are crucial to their proper function. If you have a tool which works very well, study it for fine details of its construction. Try to figure out why it works as well as it does. A tool which works poorly can also offer instruction. You are your own best source to turn to in improving a tool shape — you have your own methods of work, slightly different from everyone else’s. See the attached taxonomy of chasing tool shapes.
Tool steel comes in a large variety of types and shapes. Unless you plan on buying a lot of it, a shape called “drill rod” is quite convenient (it costs a bit more). Drill rod is sold in many precision diameters, sized by 1/64″. It also comes in several types of steel, depending on your need. The type we are using in this class is “oil hardening” tool steel, commonly called O1. Tool steel is sold in the annealed state — when you buy it, it is soft enough to cut with a file or a machine tool like a lathe.
There are two other common varieties of tool steel: water hardening and air hardening. Water hardening steel can be quenched in water for larger pieces. Smaller pieces like 1/2″ size must be quenched in oil to avoid cracking. Water hardening steel is more prone to distortion during hardening than oil or air hardening steels.
Air hardening steel is quite nice to work with, as it hardens by simply air cooling. It has two disadvantages — it is expensive, and it is terribly difficult to anneal once it is hardened. An excellent choice for thin, hammer struck tools is S7 air hardening steel. You will be amazed at how well a thin tool of S7 will hold up against hammer blows.
The correct quench liquid is described by the type of steel — oil hardening steel is quenched in oil, etc. Small pieces of steel will sometimes need the next milder quench, especially with water hardening steel. Quenching liquids from severe (very rapid cooling) to mild:
Motor oil is a bad choice for quenching, as it gives off fumes and smoke that are bad for you. Cooking oil works, although you may not get quite as hard a tool. You can also buy “quenching oil” which is specially designed for a high flash point and quick cooling to ensure thorough hardening.
The amount of oil needed depends on the tool size. For 4-6″ long tools, I recommend 2-4 gallons. You need enough oil so that it does not get too hot in use — if it gets too hot, your tools will not harden sufficiently.
The oil should be in a container skinny enough to give you sufficient depth to get the tool at least one inch below the surface. Your container needs to have a tight fitting lid in case of fire. Ideal is an all metal container; ammo boxes work pretty well. A heavy plastic container works, but could be a problem in case of fire. Do not use a glass container.
Steel is annealed by cooling it very slowly. Heat the steel to an orange glow and bury it overnight in ashes or perlite (from a garden supply store). This will remove all hardness from the steel and allow you to cut and shape the steel. You will need to anneal used steel, such as car springs, before working and shaping it. It is not practical to anneal air hardening steel at home; you need an incredibly slow cooling rate (done in a controlled oven).
Steel is hardened by heating it above the “upper transformation temperature” and then cooling it rapidly. The faster you cool it, the harder it gets (up to the maximum for that type of steel). Cooling too rapidly can cause the steel to crack — quite upsetting after putting in a lot of work on a tool. Not cooling fast enough will cause the steel to not become as hard.
The upper transformation temperature for most tool steels is between a red and an orange glow (1400°F to 1600°F). Air hardening steels are generally a bit higher (bright orange, 1800°F). You should not heat a tool steel to yellow as it will allow crystal growth and weaken the steel (this can be reversed by forging, which breaks apart the crystals).
Tempering is the only part of tool making that is tricky. If a tool breaks, either it was too hard or the tempering was too uneven, leaving a stressed zone. If a tool wears too rapidly, or needs sharpening too often, try leaving it a bit harder the next time you make one (or re-harden the one you have).
Tempering does not need to be even the entire length of the tool. A cutting edge needs to be left pretty hard; the other end of the tool might be struck with a hammer, and want to be a bit soft (blue). You can achieve this by careful work with the tempering flame (and a bit of practice).
Sources and References
Steel can be bought new from:
MSC (Manhattan Supply Company)
Call and ask for a catalog; they will be happy to send you one for free. It weighs about 12 pounds, and has all manner of machine tools and hardware in it. Their service is excellent. Minimum order $25.
You can also get decent steel at a junkyard. Automobile springs make good oil hardening tool steel; garage door springs are good for smaller diameters. You will need to anneal springs before filing.
I strongly recommend the three books by Alexander Weygers:
These three have recently been republished as one volume. They are an excellent way to learn how to make tools.
For a good introduction to blacksmithing including detail on heat treating:
The New Edge of the Anvil by Jack Andrews