Differential Heat Treatment - Differential Tempering

Differential tempering (also called graded tempering, selective tempering or local tempering) is the inverse of differential hardening, to ultimately produce similar results. Differential tempering begins by taking steel that has been uniformly quenched and hardened, and then heating it in localized areas to reduce the hardness. The process is often used for cutting instruments, softening the back, shaft, or spine, but simultaneously tempering the edge to a very high hardness. The process was very common in ancient Europe, for making tools, but soon was applied to knives and swords as well.

The most common use for differential tempering was for heat treating cutting tools, such as axes and chisels, where an extremely hard edge is desired, but some malleability and springiness is needed in the rest of the tool. A chisel with a very hard edge can maintain that edge longer and cut harder material, but, if the entire chisel was too hard, it would shatter under the hammer blows. Differential tempering was often used to provide a very hard cutting edge, but to soften parts of the tool that are subject to impact and shock loading.

Before a tool is differentially tempered, it is first heated to red-hot and then quenched, hardening the entire tool. This makes the tool much too hard for normal use, so the tool is tempered to reduce the hardness to a more suitable point. However, unlike normal tempering, the tool is not heated evenly. Instead, the heat is applied to only a part of the tool, allowing the heat to thermally conduct toward the cooler cutting edge. The quenched-steel is first sanded or polished, to remove any residual oxidation, revealing the bare metal underneath. The steel is then heated in a localized area, such as the hammering-end of a chisel or the handle-end of an axe. The smith then carefully gauges the temperature by watching the tempering colors of the steel. As the steel is heated, these colors will form, ranging from yellow to brown, purple, and blue, and many shades in between, and will indicate the temperature of the steel. As heat is applied, the colors will form near the heat source, and then slowly move across the tool, following the heat as it conducts toward the edge.

Before the yellow or "light-straw" color reaches the edge, the smith removes the heat. The heat will continue to conduct, moving the colors toward the edge for a short time after the heat is removed. When the light-straw color reaches the edge, the smith will usually dip the steel in water, to stop the process. This will generally produce a very hard edge, around HRc58-60 on the Rockwell scale, but will leave the opposite end of the tool much softer. The exact hardness of the soft end depends on many factors, but the main one is the speed at which the steel was heated, or how far the colors spread out. The light-straw color is very hard, brittle steel, but the light-blue is softer and very springy. Beyond the blue color, when the steel turns grey, it is more likely to be very malleable, which is usually undesirable in a chisel. If the steel is too soft it can bend or mushroom, plastically deforming under the force of the hammer.

Unlike with differential hardening, there is no distinct boundary between the harder and softer metals, but the change form hard to soft is very gradual, forming a continuum, or "grade" (gradient), of hardness. However, higher heating temperatures cause the colors to spread less, creating a much steeper grade, while lower temperatures can make the change more gradual, using a smaller portion of the entire continuum. The tempering colors only represent a fraction of the entire grade, because the metal turns grey above 650 °F (343 °C), making it difficult to judge the temperature, but the hardness will continue to decrease as the temperature rises.

Heating in just one area, like the flat end of a center punch, will cause the grade to spread evenly down the length of the tool. Because having a continuous grade along the length of the entire tool is not always desired, methods of concentrating the change have been devised. A tool like a chisel may be heated quickly but evenly along the entire shaft, tempering it to a purple or blue color, but allowing the residual heat to quickly conduct a short distance to the edge. Another method is to hold the edge in water, keeping it cool as the rest of the tool is tempered. When the proper color is reached, the edge is removed from the water and allowed to temper from the residual heat, and the entire tool is plunged in the water when the edge turns the proper color. However, heating in localized areas with such low temperatures may be difficult with larger items, like an axe or a splitting maul, because the steel may lose too much heat before it can conduct to the edge. Sometimes the steel is heated evenly to just below the desired temperature, and then differentially tempered, making it easier to control the temperature change. Another way is to partially embed the steel in an insulator, like sand or lime, preventing too much heat loss during tempering.

Eventually, this process was applied to swords and knives, to produce mechanical effects that were similar to differential hardening, but with some important differences. To differentially temper a blade, it is first quenched to harden the entire blade evenly. The blade is then heated in a localized area, allowing the heat to flow toward the edge. With single-edged blades, the blade may be tempered with fire or a torch. The blade is heated along the spine and tang only, allowing the heat to conduct to the edge. The heat will need to be applied evenly, allowing the colors to spread evenly across the blade. However, with double-edged blades, the heat source will usually need to be more precisely localized because the heat must be applied evenly along the center of the blade, allowing it to conduct to both edges. Often, a red or yellow-hot bar is used to supply the heat, placing it along the center of the blade; perhaps fitted into a blood gutter. To prevent too much heat loss in the blade, it may be preheated, partially insulated, or sandwiched between two red-hot bars. When the proper color reaches the edge, it is immersed in water to stop the process.

Differential tempering can be made more difficult by the shape of the blade. When tempering a double-edged sword with a taper along its length, the tip may reach the proper temperature before the shank does. The smith may meed to contol the temperature by using methods like pouring water along certain parts of the edge, or cooling it with ice, causing the proper temperature to reach the entire edge at the same time. In this way, although it is less time-consuming than differential hardening with clay, once the process starts the smith must be vigilant, carefully guiding the heat. This leaves little room for error, and mistakes in shaping the hardened zone cannot easily be corrected. This is made even more difficult if the knife or sword has a curve, an odd shape, or a sharply tapered tip. Swords tempered in this manner, especially double-edged swords, will generally need to be rather wide, like broadswords, allowing room for a gradient to form. However differential tempering does not alter the blade's shape.

When a sword, knife or tool is evenly quenched, the entire object turns into martensite, which is extremely hard, without the formation of soft pearlite. Tempering reduces the hardness in the steel by gradually changing the martensite into a microstructure of various carbides, such as cementite, and softer ferrite (iron), forming a microstructure called "tempered martensite." When tempering high-carbon steel in the blacksmith method, the color provides a general indicaton of the final hardness, although some trial-and-error is usually required to match the right color to the type of steel to achieve the exact hardness, because the carbon content, the heating speed, and even the type of heat source will affect the outcome. Without the formation of pearlite, the steel can be incrementally tempered to achieve the proper hardness in each area, assuring that no area is too soft. In broadwords, for instance, because the blade is typically rather wide, thin, and heavy, a blade can be prone to bending during combat. If the center of the blade is too soft, this bending may likely be permanent. However, if the sword is tempered to a springy hardness, it will be more likely to return to its original shape.

A sword tempered this way cannot usually have an edge as hard as a differentially-hardened sword, like a katana, because there is no softer metal directly underneath the edge to back-up the harder metal. This makes the edge more likely the chip away in larger pieces. However, in combat with hacking-type swords, like broadswords, such an extremely hard edge is not always desirable, as blocking and parrying is often done with the edge. With these swords, it is generally desirable to have a more impact-resistant edge, even at the expense of the ability to hold that edge. The edge may need to be tempered to dark-straw or more to achieve this, and the center tempered to a blue or purple color. This leaves very little difference between the edge and the center, and the benefits of this method, over tempering the sword evenly (through tempering), may not be very substantial. However, a sword tempered in this way can usually be resharpened many times, and, although the hardness will decrease with each sharpening, the reduction in hardness will usually not be noticeable until a large amount of steel has been removed.