page_title: Differential heat treatment
section_title: Metallography
hierarchical_section_title: Differential heat treatment / Differential hardening / Bladesmithing / Metallography
caption_attribution_description: English: A katana of the kobuse type, made in the traditional Japanese style. The hardened edge is separated from the softer back by a bright, wavy line, called the nioi. The edge of this bright line, called the hamon, marks the boundary between the martensitic portion (the yakiba) from pearlitic center area (the hira). The wavy shape of the hamon is from the manner in which the insulating clay was appllied before quenching. The inset shows a close-up of the nioi, which is made up of niye. The niye are single, bright grains of martensite surrounded by pearlite, which are seen as the speckled zone between the bright yakiba and the darker hira. The wood-grain appearance comes from alternating layers of steel that have different hardenability.
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: 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.
  Although both the pearlite and martensite can be polished to a mirror-like shine, only the back and spine are usually polished to such an extent. The hardened portion of the blade (yakiba) and the center portion (hira) are often given a matte finish instead, to make the differences in the hardness stand out. This causes the various microstructures to reflect light differently when viewed from different angles. The pearlite takes on longer, deeper scratches, and either appears shiny and bright, or sometimes dark depending on the viewing angle. The martensite is harder to scratch, so the microscopic abrasions are smaller. The martensite usually appears brighter yet flatter than the pearlite, and this is less dependent on the viewing angle. When polished or etched with acid to reveal these features, a distinct boundary is observed between the martensite portion of the blade and the pearlite. This boundary is often called the "temper line", or the commonly used Japanese term, the "hamon". Between the hardened edge and the hamon lies an intermediate zone, called the '"nioi" in Japanese, which is usually only visible at long angles. The nioi is about a millimeter or two wide, following the hamon, which is made up of individual martensite grains (niye) surrounded by pearlite. The nioi provides a very tough boundary between the yakiba and the hira.
caption: A katana, shown at a long angle to reveal the nioi, which is the bright line following the hamon. The inset shows a close up of the nioi, which appears as the speckled area between the bright hardened edge and the darker soft zone. The wood-grain appearance is from the folding techniques used during forging.
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