Differential heat treatment (also called selective heat treatment or local heat treatment) is a technique used during heat treating of steel to harden or soften certain areas of an object, creating a difference in hardness between these areas. There are many techniques for creating a difference in properties, but most can be defined as either differential hardening or differential tempering. These were common heat treatment techniques used historically in Europe and Asia, with possibly the most widely known example being from Japanese swordsmithing. Some modern varieties were developed in the twentieth century as metallurgical knowledge and technology rapidly increased.
Differential hardening is done by either of two methods. One of them is heating the steel evenly to a red-hot temperature and then cooling part of it quickly, turning that part into very hard martensite while the rest cools more slowly and becomes softer pearlite. The other is heating only part of the steel very quickly to red-hot and then rapidly cooling it by quenching, again turning that part into martensite, but leaving the rest unchanged. Conversely, one may selectively harden steel by differential tempering, that is, by heating it evenly to red-hot and then quenching it, turning it into martensite, and then tempering part of it by heating it to a much lower temperature, softening only that part.
Introduction Differential heat treatment is a method used to alter the properties of various parts of a steel object differently, producing areas that are harder or softer than others. This creates greater toughness in the parts of the object where it is needed, such as the tang or spine of a sword, but produces greater hardness at the edge or other areas where greater impact resistance, wear resistance, and strength is needed. Differential heat treatment can often make certain areas harder than could be allowed if the steel was uniformly treated, or "through treated". There are several techniques used to differentially heat treat steel, but they can usually be divided into differential hardening and differential tempering methods.
During heat treating, when red-hot steel (usually between and ) is quenched, it becomes very hard. However, it will be too hard, becoming very brittle like glass. Quenched-steel is usually heated again, slowly and evenly (usually between and ) in a process called tempering, to soften the metal, thereby increasing the toughness. However, although this softening of the metal makes the blade less prone to breaking, it makes the edge more susceptible to deformation such as dulling, peening, or curling.Knife Talk II: The High Performance Blade By Ed Fowler – Krause Publications 2003 p. 115
Differential hardening is a method used in heat treating swords and knives to increase the hardness of the edge without making the whole blade brittle. To achieve this, the edge is cooled faster than the spine by adding a heat insulator to the spine before quenching. Clay or another material is used for insulation. To prevent cracking and loss of surface carbon, quenching is usually performed before beveling, shaping, and sharpening the edge. It can also be achieved by carefully pouring water (perhaps already heated) onto the edge of a blade as is the case with the manufacture of some kukri. Differential hardening technology originated in China and later spread to Korea and Japan. This technique is mainly used in later Chinese jian, Chinese dao, and the katana, the traditional Japanese sword, and the khukuri, the traditional Nepalese knife. Most blades made with this technique have visible temper lines. Earlier Chinese jian from the ancient era (eg. Warring States to Han dynasty) used tempering rather than differential heat treatment. This method is sometimes called differential tempering, but this term more accurately refers to a different technique, which originated with the broadswords of Europe.
Modern versions of differential hardening were developed when sources of rapidly heating the metal were devised, such as an oxy-acetylene torch or induction heating. With flame hardening and induction hardening techniques, the steel is quickly heated to red-hot in a localized area and then quenched. This hardens only part of the object, but leaves the rest unaltered.
Differential tempering was more commonly used to make cutting tools, although it was sometimes used on knives and swords as well. Differential tempering is obtained by quenching the sword uniformly, then tempering one part of it, such as the spine or the center portion of double edged blades. This is usually done with a torch or some other directed heat source. The heated portion of the metal is softened by this process, leaving the edge at the higher hardness.How to Make Your Own Steel Knifes (sic). Primitiveways.com. Retrieved on 2012-11-11.
Differential hardening
Bladesmithing
Differential hardening (also called differential quenching, selective quenching, selective hardening, or local hardening) is most commonly used in bladesmithing to increase the toughness of a blade while keeping very high hardness and strength at the edge. This helps to make the blade very resistant to breaking, by making the spine very soft and bendable, but allows greater hardness at the edge than would be possible if the blade was uniformly quenched and tempered. This helps to create a tough blade that will maintain a very sharp, wear-resistant edge, even during rough use such as found in combat.
Insulation coatings A differentially hardened blade will usually be coated with an insulating layer, like clay, but leaving the edge exposed. When it is heated to red-hot and quenched, the edge cools quickly, becoming very hard, but the rest cools slowly, becoming much softer.Knife Talk II: The High Performance Blade By Ed Fowler – Krause Publications 2003 pp. 114–115NOVA | Secrets of the Samurai Sword. Pbs.org. Retrieved on 2012-11-11. The insulation layer is quite often a mixture of clays, ashes, polishing stone powder, and salts, which protects the back of the blade from cooling very quickly when quenched.Japanse Swordmaking Process ~ www.samuraisword.com . Samuraisword.com. Retrieved on 2012-11-11.Smith, p. 49 The clay is often applied by painting it on, coating the blade very thickly around the center and spine, but leaving the edge exposed. This allows the edge to cool very quickly, turning it into a very hard microstructure called martensite, but causes the rest of the blade to cool slowly, turning it into a soft microstructure called pearlite. This produces an edge that is exceptionally hard and brittle, but is backed-up by softer, tougher metal. The edge, however, will usually be too hard, so after quenching the entire blade is usually tempered to around for a short time, to bring the hardness of the edge down to around HRc60 on the Rockwell hardness scale.
The exact composition of the clay mixture, the thickness of the coating, and even the temperature of the water were often closely guarded secrets of the various bladesmithing schools. With the clay mixture, the main goal was to find a mixture that would withstand high temperatures and adhere to the blade without shrinking, cracking, or peeling as it dried. Sometimes the back of the blade was coated with clay, leaving the edge exposed. Other times the entire blade was coated and then the clay was cut away from the edge. Another method was to apply the clay thickly at the back but thinly at the edge, providing a lesser amount of insulation. By controlling the thickness of the edge-coating along with the temperature of the water, the cooling rate of each part of the blade can be controlled to produce the proper hardness upon quenching without the need for further tempering.
Quenching
Once the coating has dried, the blade is heated slowly and evenly, to prevent the coating from cracking or falling off. After the blade is heated to the proper temperature, which is usually judged by the cherry-red glow (blackbody radiation) of the blade, it will change into a phase called austenite. Both to help prevent cracking and to produce uniformity in the hardness of each area, the smith will need to ensure that the temperature is even, lacking any hot spots from sitting next to the coals. To prevent this, the blade is usually kept in motion while heating, to distribute the heat more evenly. Quenching is often done in low-light conditions, to help accurately judge the color of the glow. Typically, the smith will also try to avoid overheating the blade to prevent the metallic crystals from growing too large. At this time the blade will usually be plunged into a vat of water or oil, to quickly remove the heat from the edge. The clay, in turn, insulates the back of the blade, causing it to cool slower than the edge.
When the edge cools fast a diffusionless transformation occurs, turning the austenite into very hard martensite. This requires a temperature drop from around 750 °C (cherry-red) to 450 °C (at which point the transformation is complete) in less than a second to prevent the formation of soft pearlite. Because the rest of the blade cools slowly, the carbon in the austenite has time to precipitate, becoming pearlite. The diffusionless transformation causes the edge to "freeze" suddenly in a thermally expanded state, but allows the back to contract as it cools slower. This typically causes the blade to bend or curve during quenching, as the back contracts more than the edge. This gives swords like katana and wakizashi their characteristic curved shapes. The blade is usually straight when heated but then bows as it cools; first curving toward the edge as it contracts, and then away from the edge as the spine contracts more. With slashing-type swords, this curvature helps to facilitate cutting, but increases the chances of cracking during the procedure. Up to one third of all swords are ruined during the quenching process. However, when the sword does not crack, the internal stresses created help increase the toughness of the blade, similar to the increased toughness in tempered glass. The sword may need further shaping after quenching and tempering, to achieve the desired curvature.
Care must be taken to plunge the sword quickly and vertically (edge first), for if one side enters the quenching fluid before the other the cooling may be asymmetric and cause the blade to bend sideways (warp). Because quenching in water tends to cause a sudden loss of surface carbon, the sword will usually be quenched before the edge is beveled and sharpened. After quenching and tempering, the blade was traditionally given a rough shape with a metal-cutting draw knife (sen) before sending to a polisher for sharpening,Smith, pp. 49–52 although in modern times an electric belt sander is often used instead.
Metallography Differential hardening will produce two different zones of hardness, which respond differently to grinding, sharpening, and polishing. The back and center of the blade will grind away much quicker than the edge, so the polisher will need to carefully control the angle of the edge, which will affect the geometry of the blade. An inexperienced polisher can quickly ruin a blade by applying too much pressure to the softened areas, rapidly altering the blade's shape without much change to the hardened zone.Smith