Differential Heat Treatment

Differential Heat Treatment

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.

Differential hardening most often consists of coating the back or spine of a blade with an insulating layer. This 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.

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.

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, 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 more slowly than the edge.

When the edge cools fast, a diffusionless transformation occurs, turning the edge into very hard martensite. Because the rest of the blade cools slowly, the carbon in the steel has time to precipitate, becoming soft pearlite. The diffusionless transformation causes the edge to "freeze" suddenly in a thermally expanded state, but allows the back to contract as it cools more slowly. 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. This helps to facilitate cutting, with slashing-type swords, but increases the chances of cracking during the procedure. The sword may need further shaping after quenching and tempering, to achieve the desired curvature. Straight swords, such as ninjato, are often curved the wrong way before heating so that they straighten when quenched.

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 more quickly 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, they 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 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 line about a millimeter or two wide, following the hamon, which is made up of individual martensite grains (niye) surrounded by pearlite.

In Japan, from the legendary time of the famous smith Amakuni, hamons were originally straight and parallel to the edge, but by the twelfth century AD, smiths such as Shintogo Kunimitsu began producing hamons with very irregular shapes. By the sixteenth century AD, the Japanese smiths often overheated their swords slightly before quenching, to produce rather large niye for aesthetic purposes, even though a larger grain size tended to weaken the sword a bit. During this time, great attention began to be paid in Japan to making decorative hamons, by carefully shaping the clay. It became very common during this era to find swords with wavy hamons, flowers or clovers depicted in the temper line, rat's feet, trees, or other shapes. By the eighteenth century, decorative hamons were often being combined with decorative folding techniques to produce entire landscapes, complete with specific islands, crashing waves, hills, mountains, rivers, and sometimes low spots were cut in the clay to produce niye far away from the hamon, creating effects such as birds in the sky.

Although differential hardening produces a very hard edge, it also leaves the rest of the sword rather soft, which can make it prone to bending under heavy loads, such as parrying a hard blow. It can also make the edge more susceptible to chipping or cracking. Samurai warriors of Japan were trained to block and parry with the sides of their swords, to protect the cutting edge. Swords of this type can usually only be resharpened a few times before reaching the softer metal underneath the edge. However, if properly protected and maintained, these blades can usually hold an edge for long periods of time, even after slicing through bone and flesh, or heavily matted bamboo to simulate cutting through body parts, as is in iaido.