Differential heat treatment

Differential heat treatment (also called selective heat treatment or local heat treatment) is a technique used during heat treating to harden or soften certain areas of a steel 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 treating 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 heat treatment (also called selective heat treatment or local heat treatment) is a technique used during heat treating to harden or soften certain areas of a steel 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 treating 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 consists of either two methods. It can involve heating the metal evenly to a red-hot temperature and then cooling it at different rates, turning part of the object into very hard martensite while the rest cools slower and becomes softer pearlite. It may also consist of heating only a part of the object very quickly to red-hot and then rapidly cooling (quenching), turning only part of it into hard martensite but leaving the rest unchanged. Conversely, differential tempering methods consist of heating the object evenly to red-hot and then quenching the entire object, turning the whole thing into martensite. The object is then heated to a much lower temperature to soften it (tempering), but is only heated in a localized area, softening only a part of it. 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 1,500 °F (820 °C) and 1,600 °F (870 °C)) 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 400 °F (204 °C) and 650 °F (343 °C)) 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. 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 the Chinese jian and the katana, the traditional Japanese sword, and the khukuri, the traditional Nepalese knife. Most blades made with this technique have visible temper lines. 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. 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. 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. 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. 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 400 °F (204 °C) for a short time, to bring the hardness of the edge down to around HRc60 on the Rockwell hardness scale.