Tempering steel: heating, cooling & timing
Tempering the metal to hold an edge. Recorder/Paul Franz
Yellow will hold an excellent edge but may break before bending. Recorder/Paul Franz
Purple has excellent impact resistance, good for hammers, cold chisels.
Blue is generally is the best compromise between hardness & resilience.
Gray: Bendable, the right choice if you’re making a paper clip.
Cooling hot metal. Recorder/Paul Franz
In the good old days, tinkers traveled the land repairing small metal things. They do no longer, leaving this vital function up to you. Let’s say you snap a chisel or the rivet in your favorite pair of scissors. Or your favorite paper clip needs a tune-up. You have two options: discard these items and buy new ones, cheaply and easily, or spend precious hours of your life repairing them.
Time to break out the hammers and scrap metal.
Steel is easy enough to shape with a little effort and no idea what you’re doing. Making the steel act as a spring or hold up to repeated impacts requires an understanding of tempering through heat.
Heat-treating steel strikes a balance between hard but brittle and soft but tough and resilient.
Once you know how to harden or soften steel, and strike a balance between these two, you can make your own tools, repair any number of things, and do all sorts of other fun and potentially dangerous stuff.
Ancient smiths are supposed to have believed that iron was hardened into steel by the urine of a red-haired child. Fortunately for the modern steel industry, as well as do-it-yourselfers and red-haired children everywhere, that is not the case.
Steel is a versatile alloy of iron and carbon, the carbon at first accidentally introduced when carbon-rich coal replaced wood as a fuel in the fires used to melt raw iron ore into a workable form, according to historian P.E. Cleator in his excellent 1967 book “Weapons of War.”
Today, high-tech equipment allows professionals to achieve scientific accuracy in the pursuit of specific tolerances. On the other hand, a propane torch and some informed guesswork is fine for most home applications, from hardening a cheap chisel edge to replacing a broken leaf spring.
Relatively reliable results can be achieved with little or no investment in new tools. A propane or MAPP gas torch, both cheap and available at most hardware stores, a pair of tongs or sturdy pliers — vice grips are great — and a bucket of water should suffice for most small projects.
The first stage of heat treating is also the simplest. Grasp the steel article with your tongs or pliers. Point the lighted torch at the steel and wait. Note: the section of steel touching the pliers will not heat up as much because the tongs will leach away the heat, so plan accordingly. Design the object with a throw-away tab that you can use to grasp the object, if necessary.
Keep in mind that pliers, tongs, etc. are metal and therefore conduct heat: you risk burning your hand and dropping a piece of red hot metal on your feet or similar flammable surfaces. Gloves are good, safety goggles are always advisable, but the heavier the tongs, the slower they will be to transfer heat and the safer your hands will be.
When the metal begins to glow red, you have two options. If it needs bending or hammering, do so at this stage, being careful to keep the metal glowing or risk cracking the steel. If not, or once you have reached the desired shape, quench the steel by dunking it quickly into the waiting bucket of water. The steel should be red hot right up to the moment you quench it or the desired effect will not be reached. Various oils and commercial solutions can be used to provide a faster or slower quench than water, but water won’t catch fire or poison you. If you are unsure when the steel is ready for quenching, test it with a magnet; at the correct temperature, the steel will be non-magnetic.
Remember, steel is an alloy of carbon and iron. By bringing the metal to red hot, you allow the carbon molecules to move with relative freedom in the iron. Rapidly cooling the metal locks these molecules into a rigid crystalline structure. At this point, the steel should be at its hardest.
Unfortunately, at its hardest, steel is also at its most brittle. A knife, for instance, is of limited value if it holds a perfect edge but shatters while chopping celery.
In order to achieve the appropriate balance between hardness and toughness, go back to the torch.
First, polish the surface of the steel with fine sandpaper or steel wool, then clean it with rubbing alcohol to remove any grease, including fingerprints. Heat the steel as slowly and evenly as possible, paying careful attention to the steel’s surface. As the temperature of the steel rises, the surface will change color, from a light blond through deepening shades of purple to dark blue and finally a muddled slate gray. This corresponds to a temperature range of approximately 400 to 800 degrees Fahrenheit. The more you heat the steel within this range, the more the crystalline structure will relax and the softer the steel will become. Detailed color charts can be found online. Simply enter the search term “steel tempering color chart.”
Each successive color corresponds to a softening of the steel.
At yellow, most steel will hold an excellent edge but will hardly bend at all and may break before bending.
Purple will provide excellent impact resistance, appropriate for hammers, cold chisels and the like.
At blue, it is flexible and resilient, bending and returning to its original form.
At gray, it will bend and stay bent, and can be bent back without breaking, the right choice if you’re making a paper clip.
Tempering is a compromise, selecting the proper balance between hard but brittle and tough but soft. Blue generally is the best compromise between hardness and resilience, hence the characteristic color of band-saw blades and gun barrels. Stopping at yellow is appropriate for files, wood chisels, and dental tools, should you feel inclined to dabble in amateur dentistry or impress and terrify your dentist with a thoughtful gift.
The color shift may only be apparent if you take the steel away from the heat source. To halt the heating process at the desired color, simply quench in water.
It is important to note that this is all relative. If you are working with a low-carbon steel, the hardest it gets may not be hard enough for any useful application.
Sources of good-quality steel include worn-out files, broken saw blades and bits of broken automotive suspension springs.
With any of these objects, the steel is already tempered and will be too hard to cut or file easily into the necessary shape. So, to begin with, you will want to remove the existing temper. Do this by heating the steel to less than red hot, maintaining that temperature for a few seconds or a few minutes depending on the steel, then allowing it to cool slowly without quenching. Repeat as necessary. The slow cooling allows the internal structure of the steel to relax as far as possible. Unfortunately, as you heat and cool the steel, you lose carbon and will eventually end up with soft, pliable, utterly un-temperable metal. This is known as annealing.
Annealing can also be accidentally achieved while working the metal, so when doing anything involving smithing, try to keep the working time short.
As in all things, the secret is not to burn yourself or others too badly in the process.
Staff reporter Chris Curtis started at The Recorder in 2011. He covers Montague, Gill, Erving and Wendell. He can be reached at firstname.lastname@example.org or 413-772-0261, ext. 257
Staff photographer Paul Franz has worked for The Recorder since 1988. He can be reached at email@example.com or 413-772-0261 ext. 266. His website is www.franzphoto.com.