Hardness, or the resistance to indentation or damage, is a very valuable trait in metals. Steelmakers can increase the strength of a material through different processes known as hardening. Steel hardening was common as early as the Greeks who used surface colors of the steel to determine quenching temperatures, a technique that was used up until the turn of the 20th century. Early smiths used a lot of “magic” to perfect their personal hardening techniques, often using quenching agents such as “dragons blood” or “dew of a May morning”. Luckily, today’s hardening techniques are based solely on science and not as much trial and error. There are two main types of hardening: case hardening, also known as surface, and through hardening.
Through hardening occurs via heat treatment of medium and high carbon steels (>0.25% carbon). First, ovens heat the steel above its critical temperature, which lies between 1500°F-1900°F depending on alloy. Second, the material is cooled rapidly by dunking the material in water or oil in a process known as quenching. Finally, following quenching, tempering or annealing by reheating eliminates excess brittleness in the material. This process results in increased hardness throughout the material. Consequently, the increased hardness also leads to increased brittleness. In fact, the relatively brittle metal can fracture under impact or shock loads.
Typical uses: seat frames, seat belt buckles, hand tools, springs, axles, blades, nuts and bolts, nails
Case (Surface) Hardening
Because through hardening has no effect on mild steel (<0.25% carbon), the material must be case or surface hardened. In a process known as carburizing, a chemically reactive source of carbon adheres to the mild steel during heating at a high temperature. Notably, the carbon source can be gas, liquid, or solid. Finally, after quenching, a hard surface layer, known as a “case”, will form. The most common depth is in the range 0.020”-0.030”. Importantly, the process results in increased hardness on the outside of the material; however, the low carbon core remains relatively soft. The outside is very hard but very brittle; however, the inside is soft and ductile. This combination is the best of both worlds. As a result, case hardened metals contain higher fracture hardness than through hardened steel.