USE OF A BIOCOMPATIBLE MATERIAL OF STAINLESS STEEL HAVING A MARTENSITIC SURFACE LAYER FOR WATCHES, PARTS FOR WATCHES AND JEWELLERY

摘要

The jewellery is made from a ferritic chromium steel. Its martensitic surface layer is formed by heat treatment with nitriding, followed by cooling. The hardness of the surface layer, expressed on the Vickers HV 3 scale and determined in accordance with (DIN ISO EN 6507), is 30-300% greater than the minimum HV 3 hardness of the ferritic core. The difference is preferably 100-250%. The surface hardness is in the range 320-650 HV 3. The minimum core hardness is 160-260 HV 3. The internal boundary of the harder surface layer, is located where the metal becomes 30% harder than the inner core. The layer becomes harder as the surface is approached, when moving away from the core. The depth of hardening is 0.005 - 1.00 mm, preferably 0.01-0.3 mm. The modulus of elasticity of the martensitic surface layer exceeds that of the core by 1-50%. Hardness and/or modulus of elasticity decrease progressively from the surface towards the core, within the martensitic layer. The surface of the martensitic layer is polished, roughened or matt-finished. It may remain untreated. Surface roughness, measured in accordance with DIN ISO EN 4287is in the range of 1.5-40 mu m. The steel type is selected from 1.4016, 1.4000 and 1.4024. The surface of the martensitic surface layer exhibits no chromium carbide precipitation at the grain boundaries. The grain size is greater, in comparison with a non-hardened surface. The surface layer is obtained by heat treatment. In the surface layer, starting from the surface and moving towards the inner ferritic core, the microstructure varies continuously, accompanied by hardness variation. The treatment is used for stainless steel parts of watches, e.g. the cases, bezels and wrist strap catches. It is also used for jewellery e.g. bracelets, arm bands, chains and ear rings.