The component shown in the above picture, the tripod joint, is a typical example for hardening. In this case, the surface becomes hard and the inside remains tough. When the first front wheel drive constant-velocity joints (homokinetic or CV joints) appeared in the 1960s, the demands became clear. Full torque transfer, often also at a somewhat greater flexion angle, demanded too much from the material. Nowadays, these problems with substantially higher torques, have quite likely, been overcome.
Fine-grained structure increases the strength of steel.
Ferrous material is hardened either by changing it's internal structure or by the specific application of hardening substances. A crystal-clear hardness is achieved, e.g., when tool-steel, at approx. 800°, is doused with water, oil or air. The excessive brittleness is reduced by renewed heating (annealing) at less than half the temperature and then slowly cooling down, the hardness however, is also somewhat reduced. Ferrous material, when cooling down, has the characteristic, of arranging it's atoms in the form of crystals. This process is only ideally possible, if attention is paid to the time which the material remains in certain temperature ranges. In this case, iron becomes tough. If it is too rapidly cooled down, it becomes more brittle.
Nitrogen gives the steel a glass-hard surface.
Should the surface and the edges be made durable and at the same time the inside remain tough, e.g., gearwheels, they must either be case-hardened from the outside, be fortified with nitrogen (nitration) or the structure must be changed by specific heating. The above picture shows clearly the paths where the rollers run and the colouring which occurs through flame-hardening. 11/11
Nitrided materials maintain their qualities up to 500°C.