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138 THE DISTRIBUTOR’S LINK ROB LaPOINTE CASE-HARDENING – CREATING FASTENERS WITH A DURABLE SKIN & FLEXIBLE CORE from page 90 TABLE 1 CARBON CONTENT OF LOW-ALLOY STEEL Most case-hardened products are made from lowalloy carbon steel. Low-alloy carbon steel is an alloy of iron and carbon along with many other elements, but principally manganese, sulfur, and phosphorus. Carbon is the principal element that allows the metal to achieve high hardness. The ability to be hardenable is due to the geometry created by the bonding structure of carbon and iron in the crystal lattice. Low-alloy carbon steel can be subdivided into low, medium, and high-carbon steel. See Table 1 for the carbon content and classification of lowalloy carbon steel. Most case-hardened fasteners are produced from low carbon steel with a carbon percentage by weight of 0.15-0.25 which falls into the low-carbon steel category. Medium-carbon steel can also be used, especially if the core hardness of the fastener needs to be a bit higher than the annealed hardness of steel. If the core hardness needs to be higher, the material can be through-hardened first and then case-hardened. Types Of Case-Hardening The term “case” in case-hardening comes from the process itself. Case-hardening requires the diffusion of carbon or nitrogen into the surface of the steel to raise the carbon or nitride content of the outside of the material to a required depth, usually a few hundred microns (a few thousandths of an inch). This is done by creating a carbon or nitrogen-rich atmosphere to surround the steel while being heated. Early case-hardening used organic material to produce carbon monoxide (CO) when burned and would be kept close to the steel by means of a “case” which isolated the steel and carbon-rich atmosphere from the atmosphere of the furnace itself. The organic material was usually comprised of horns or hoofs of animals. So, the term “case-hardened” literally means hardened in a case. The term “case” has also come to be known as the layer or shell of harder material around a softer core, such as a case of harder material surrounding softer material as illustrated in Figure 5. FIGURE 5. A CROSS-SECTION OF A SERRATED FLANGE BOLT THAT IS CASE-HARDENED SO THE SERRATIONS WILL CUT INTO THE BEARING SURFACE TO ACHIEVE RESISTANCE TO LOOSENING. Carburizing is done by placing steel (usually lowcarbon) in an atmosphere rich in carbon monoxide. Carbon monoxide could be pumped into the furnace, or a case filled with materials to produce carbon monoxide could be used. The furnace is brought to a temperature of about 1700 °F (927 °C) and held there for the amount of time needed for carbon to diffuse into the material to the required depth. A general rule is 0.1 mm per hour for carbon penetration. The material is then quenched to achieve its maximum hardness. If a lower hardness is required, it is then heated to a tempering temperature and held until the desired hardness is reached. Nitriding is done by introducing nitrogen to the surface and allowing it to diffuse into the material. This is usually done by placing the material in contact with ammonia (NH 3 ) which will thermally decompose into nitrogen and hydrogen, introducing nitrogen gas to the material (Figure 7). In the steel, the diffusing nitrogen reacts with the nitride forming elements (aluminum, chromium, and molybdenum) which harden the steel. CONTINUED ON PAGE 166
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