Phil White Posted May 18, 2007 Report Share Posted May 18, 2007 Notes on Heat Treatment of Carbon Steel tools When I first started carving seriously, I was short of cash, and in need of tools. I began to make my own, and with the help of a local gunsmith, who I got to know, I began to learn a few things about tool making. This took me down the separate, but parallel path of metalworking. Most of following notes on heat treatment of carbon tool steel are a collection of musings that have been passed along to me by three older gunsmiths and blacksmiths that I have known, befriended, or otherwise worked with in the museum world. The most significant of which was a Norwegian master that I informally apprenticed with, who worked exclusively with the bare basics of tools. All of these master craftsmen were trained to work by hand, and by eye, in a very low-tech manner. Their milling machine was a hammer, a good file, and maybe a good cold chisel. The rest I have picked up on my own, from putting these practices to use. These are the traditional techniques that have been used by edged toolmakers and gunsmiths for hundreds of years, before any thought was given to crystal size, temperature gauges, or modern alloys. Although some readers may have other preferred techniques for making tools, and may prefer a more measured or technical approach, I know that these simple materials and methods work for me. Carbon Steels: Carbon steel is generally considered to be an alloy of iron and carbon, with other elements occasionally thrown in for various reasons, such as to improve tensile strength or wear resistance. The percentage of carbon can very between about 0.1 percent to about 1.2 percent The lower end of this range is considered to be mild steel, such as “hot rolled steel” or “cold rolled steel”, and can not be hardened by heating and quenching, and is therefore useless for making tools. There are a large range of steels available, but the basic carbon steels are identified by a four-digit code that indicated the percentage of carbon. For example 1070 steel has 0.7% carbon, 1080 steel has 0.8% carbon, 1095 steel has 0.95% carbon, etc. Generally, the more carbon in the steel, the higher the potential for hardness, wear resistance and edge-holding ability. However, with this comes the potential for brittleness. The lower the carbon, the tougher the steel becomes, with more ability to absorb shock and torque, but it is less likely to hold a fine edge over time. For example, High carbon steels can be found in files and engraving tools, where wear resistance is important. Medium carbon steels can be found on edge tools, where both wear resistance and flexibility are required. Low carbon steels can be found in tools where shock absorption is important, such as axes and hammers. Tools that have been made from medium to low carbon steels Tools that have been made from 1095 steel Steel “drill rod” or “silver steel” is a tough high quality tool steel that can be hardened and tempered easily, and will hold a very superior edge. It is used where a hard and tough steel are required, such as making taps or drills, or for small chisels and gouges. It is available in round stock, polished in various diameters from 1/16” on up. It comes annealed and can be filed, ground, or machined with conventional tools. It can be a bit hard to work though, but with patience it will make an excellent graver or fine chisel. Tools that have been made from drill rod Gunsmith’s suppliers are one of the best places to pick up small quantities of carbon tool steel, as gunsmiths are frequently required to make small tools and parts that are not easily available. Brownells at: www.brownells.com is one of the best, and sells carbon spring steel in small quantities of both flat bar stock and round rod. Another source is Metals Supermarket. They are online at: www.metalsupermarkets.com is also very good. If you happen to have one nearby, they also sell metals such as bronze sheet. Drill rod is available from Metals Supermarket, as well as many hardware stores. I believe I have seen it in the metals rack at Home Depot. Steel can also be obtained through recycling old files, scrapers, hay rake teeth, very old leaf springs, or any source where similar steel was once used. On of my favorite recycled steels comes from worn out files. Tools that have been made from old files Shaping Shaping of the steel should be done before hardening and tempering. It is beyond the scope of this tutorial to get into the methods of working steel, however, it will suffice to say that carbon steels can be sawn, ground, filed, machined, forged, or any combination thereof. Most tool steels are purchased soft, or annealed, unless otherwise noted. If using recycled steel such as a file, the steel will have to be softened before it can be machined of filed to shape. Annealing can be done by heating the steel to a red heat and allowing it to cool slowly in ashes. Putting the file into a fireplace with a good wood fire, and leaving it to cool with the ashes after the fire has gone out will usually do the trick. If hard spots are encountered while working the steel, heating to a very dull red and burying in ash for a couple of hours will usually soften them. Heat source: Any heat source that will bring the steel up to a bright red or “cherry red” will do. For small tools under ¼ in I use a propane brazing torch, just like the ones that you buy in a hardware store in the plumbing section. An oxy acetylene torch works well also, or a coal or propane forge. I have not specified a tempering oven, because if you have one, you know what you are doing anyway. In the following photos, a forge has been used as a heat source. The forge The forge fire Heat Treatment (Hardening and Tempering) There are two basic procedures for creating a lasting edge or surface on a steel tool: hardening and tempering. During the hardening process, a piece of steel is heated to temperature that allows the carbon (iron carbide) in the steel to come out and blend with the iron in a grainy crystalline structure. While at this heat (known as the eutectic) the steel is cooled very quickly by quenching, which freezes the crystalline structure in place, resulting in a very hard material. In the following photos, two pieces of steel, a round bar of mild steel and a hexagonal bar of medium carbon steel (1070) are heated to a “cherry red” heat and quenched in water. The two pieces are then tested for hardness with a file to make sure that they are hard. The file skates across the hexagonal piece without biting in, while the round piece is easily cut. The hexagonal bar has also turned grey, indicating physical changes have taken place. With the range of steels previously discussed, that heat can be judged by it’s color, which is a bright glowing red, like a maraschino cherry. This color was known to the old toolmakers as “cherry red” This is putting it very simply, of course, but for the home toolmaker, that is all we really need to know. Mild steel and carbon steel heated to cherry red Quenching the two steels for hardness comparison Testing the two steels with file Results of test Ideally, the steel should just be heated enough that the carbon goes into solution, and no more. This will produce a fine in the steel, which is very hard. To illustrate this, the steel is hit with a hammer, breaking off a small piece, and revealing a fine grain structure. Brittleness test with hammer Close up of break It should be noted that the higher the steel is heated, the larger the grains in the crystal structure get, and the more brittle the steel will become when quenched. The following photos show the same piece of hexagonal steel heated beyond the ideal temperature to a bright yellow heat, nearly melting the surface. The steel is quenched in water, and scored every ½ inch, then broken off in sections to show the grain structure. At the tip, the steel is so weak that only a light tap is necessary to break off a piece. Towards the left, where it was relatively cool when quenched, it will not break at all, indicating that it has not been heated enough to harden. Returning rod to fire to heat higher Results on grain at yellow heat quench Close-up of grain where toughest Understanding how grain size affects the process is important, in that if a finished tool breaks, and the grain is coarse, as in the previous photos, you can be relatively sure that the steel was quenched too hot and adjust your quenching temperature accordingly. After quenching, the steel will be very hard, often referred to as “glass hard” but very brittle. Some of the brittleness must be taken out before the tool before it can be used for its intended purpose. This is done by re-heating, or “tempering” the piece, just a little, to break down the crystals enough to restore some strength. The steel is usually cleaned of any scale or soot, to get down to the bare metal, and slowly heated until a color appears on the surface. The colors will appear in the order of: pale yellow (pale straw), yellow (straw), brown, purple, dark purple, bright blue, pale blue. The metal is then quenched again to stop the heating process. The more the tool is heated, the more flexible it will become. The less it is heated, the more hardness is retained. Edged tools that are not tempered enough may chip at the edges too easily or even break. If heated too much, they mat bend or not hold an edge. Balance of hardness and flexibility are a personal preference. I prefer the tool to be on the hard side, so it will retain an edge, but I have to be careful not to pry. The following photo shows a piece of polished drill rod that has been heated from the left to allow the temper colors to creep up the rod. The hotter end shows a pale blue, while the cooler end next to the polish shows a pale yellow. Temper colors on polished steel The following chart was taken from Appleton’s Cyclopedia of Applied Mechanics, and accurately shows the temper colors as they apply to various types of tools. Quenching medium: The role of a quenching medium is to cool the metal quickly, freezing the crystal structure that is formed at high heats. There are two basic quenching mediums: water and oil. Both have different effects. Water cools steel very fast, and is ideal for maximum hardness, such as small hammer faces, engraving chisels. Oil cools more slowly, and is ideal for toughness, such as knives, carving tools, and springs. Some tool steels are sold as water hardening or oil hardening, which implies the medium that they should be quenched in. For example, an oil-hardening steel may become too brittle, or even crack from the shock of being quenched in water. Conversely, a water-hardening steel will not become completely hard if quenched in oil. One note of caution when quenching is that thinly shaped edges may deform or even crack from the shock of being quenched. Therefore, it is ideal to leave a bit of thickness at the edge when shaping an edged tool, to avoid problems when hardening it. Keep in mind Joseph Moxon’s saying from the 17th century “If you will a good edge win, temper thick and then grind thin” There are other options that can be used as well, such as wax or salt water, but I will stick to water for now. Case Hardening I have included this section for the purposes of explaining the term “case hardening”, rather than for any real practical purposes, although I have used the technique from time to time. The technique is not used on carbon steel. Case hardening is a method of hardening a mild steel or iron surface to improve wear resistance. During this process, the iron is heated to the same temperatures as you would with a carbon steel, but in close contact with a carbon source. During the heating, the iron absorbs some of the carbon into the surface, effectively turning it into steel. Traditionally, the iron parts that were to be hardened were placed in a crucible, and then covered with a carbon-rich material. The composition of this material varied, but usually consisted of ground charcoal and ground bone. Burying iron parts in a crucible The top of the crucible was then covered and sealed to keep oxygen out. This is an essential step. Without oxygen, the carbon will absorb into the surface of the iron, forming a layer of iron carbide on the surface. Covering the lid of thee crucible and heating The crucible was then heated for a period of time to allow the iron to migrate into the iron surface. The amount of time depended on the size of the piece, but ½ to 1 hour was common. The pieces were then dumped into water, straight out of the crucible. This created a hardened steel surface on the piece that is very resistant to wear. This technique was commonly used where moving parts were bearing against each other, such as in flintlock or percussion lock mechanisms on firearms. Similar results can be produced by heating the iron part red hao, sprinkling with 'Casenite" powder, re-heating to red, then quenching in water. This flintlock was made by me using mild steel, which was case hardened, 1095 steel for the springs, and drill rod for the internal working parts. The following photos show the mottled colors of case hardening on the larger pieces, the pale blue on the springs where flexibility is necessary, and the purple or brown on the internal parts, where wear resistance is important. Flintlock showing case hardening colors and temper colors of various carbon steels With a little experimentation on a couple of pieces of steel, a feeling for these techniques will be achieved very quickly. By following these methods tools can be custom-made for special purposes, and they will hold an edge that is superior to any tools purchased off the shelf. Phil Link to comment Share on other sites More sharing options...
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