Tempering a Swiss sword
One of the most magnificent images of a tank in production – a glowing hot Swiss Army Panzer 68 hull being heat treated in an oil bath, at an Eidgenoessische Konstruktionswerkstaette facility in Thun, Switzerland, 1977. The hull would have previously been cast, fettled (cleaned of any sprues/risers/runners/ etc.), and descaled (cleaned of oxide scales on the surface).
The cleaned hull would then be reheated, and kept at high temperature for many hours, to homogenize the metal. Homogenization allows additives and impurities dissolved in the steel alloy to diffuse more uniformly into the grains within the resultant component – since during the cooling of the initial cast, a high fraction of additives is segregated out to the grain boundaries, which weakens the metal. Once the alloy is sufficiently homogenized, the hull is tempered in an oil bath, to decrease its hardness (resistance to permanent deformation under compressive force), but increase its toughness (ability to absorb energy before fracturing).
Finally, following the oil bath tempering, the hull is face-hardened by quenching (rapid cooling). Face hardening produces a metal component with a hard surface, but a tough interior, so that the resultant armor has a higher probability of preventing an incoming projectile from penetrating the hard face (either by deflecting or shattering the projectile), but also will be more difficult to fracture entirely in case the projectile does penetrate the hardened surface. (Text credit Oleg Sapunkov)
The finished product:
Some 390 Panzer 68s were made in the 1970s and remained in service with the Swiss until they were replaced by the new Panzer 87 (license-built Leopard 2) in 2003.