page_title: Differential heat treatment
section_title: Bladesmithing
hierarchical_section_title: Differential heat treatment / Differential tempering / Bladesmithing
caption_attribution_description: English: A sword, made from 5160 carbon steel, that has been differentially tempered by first quenching it and then heating it in a localized area to reduce the hardness. The center is tempered to a spring hardness, while the edges are tempered to a very high hardness (slightly higher than a hammer). The sword measures 42 inches overall length, with an unfinished weight of 6.8 lbs. (Estimated weight upon finishing is 4.5 lbs.
context_page_description: Differential 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, turning only part of it into hard martensite but leaving the rest unchanged.
context_section_description: 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, typically fitted into a fuller. Modern gas torches often have the ability to produce very precise flames. 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.
caption: A differentially tempered sword. Made of 5160 carbon steel and balanced right at the end of the fullers, the edge has been tempered slightly harder than a hammer, whereas the center, hilt and handle are tempered to a spring hardness.
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