Standard 316L stainless steel, the alloy used in roughly 90% of Swiss and German watch cases, has a Vickers hardness around 220 HV. That number is high enough for normal wear but low enough that everyday key-and-coin contact in a pocket scratches the polished surfaces visibly within weeks. Soft-corner brushing and re-polishing have therefore been a routine part of luxury-watch service since the 1950s.
Sinn, the German tool-watch maker founded by Helmut Sinn in 1961 and acquired by engineer Lothar Schmidt in 1994, took a different path. Schmidt's background in industrial process engineering led to the development of Tegimentation, a proprietary surface-hardening process patented in the late 1990s and refined continuously since.
"Tegimented surfaces have a slightly grey, satin look, almost ceramic in feel. After two years of daily wear, my U50 has not a single visible scratch."- Frequent enthusiast review of Sinn U50 ownership
The process works by diffusing carbon into the outer surface layer of an untreated 316L case at high temperature under low-pressure conditions. Carbon atoms penetrate roughly 0.5–1.0mm into the surface; the resulting carbide structure raises surface hardness to approximately 1500 HV, very close to the hardness of zirconia ceramic and roughly seven times the hardness of the underlying steel. Crucially, the case core remains untreated 316L, meaning it retains the impact-absorbing toughness of steel rather than the brittleness of ceramic.
The trade-offs are real. Tegimented surfaces are essentially uniformly black-grey rather than the cold-white of polished 316L; aggressive polishing during service can in principle remove the hardened layer, so Sinn recommends only light bead-blast restoration. But for the genuine tool-watch use case, where impact-resistance and scratch-resistance both matter, Tegimentation represents one of the few genuine technical differentiators in modern stainless-steel case construction. References include the U-series (U50, U1, U2, U212), the EZM-line mission-timers, and the Frankfurt-issued Bundeswehr pieces.