INORYS®

Increased surface hardness

Cross-section of hardened steel

The main family of stainless steels used in the manufacturing of watch exterior components belongs to the austenitic group, generally classified under the two primary grades: AISI 316L and AISI 904L. Depending on their manufacturing methods and chemical composition, austenitic stainless steels typically achieve a hardness ranging from 200 to 450 HV (Vickers hardness).

Only a thermochemical treatment can enhance the surface mechanical properties of austenitic stainless steels while preserving their corrosion resistance, regardless of the geometry of the parts being treated.

The treatment involves the diffusion of carbon into the austenite, carried out at low temperature (< 500°C) and in a gaseous phase to prevent the formation of chromium carbides.

As a result, the treated layer consists solely of a carbon interstitial solution within the austenite (S-phase treatment), with the carbon atoms occupying the octahedral sites of the face-centered cubic metallic lattice.

After treatment, a strong carbon concentration gradient is observed, which leads to significant solid solution strengthening. This results in a substantially enhanced surface hardness profile on AISI 316L stainless steels.

Furthermore, the high number of slip systems in the face-centered cubic structure of austenite provides remarkable ductility, allowing the base steel to accommodate plastic deformation without cracking or failure.

The hardened layer exhibits excellent wear resistance. Another key feature of the treatment is the significant reduction in galling tendency, with the coefficient of friction halved between hardened material (0.55) and non-hardened material (1.3), as demonstrated by pin-on-disk testing.

Inorys®

The drawback of the initial structural hardening phase is a slight increase in surface roughness [Ra], which is not acceptable from an aesthetic standpoint for high-end watchmaking standards. To overcome this issue, MTS SA has developed the INORYS process.Phynox® alloy qui permet, grâce un traitement unique de remettre en conformité l’aspect de surface et de revenir le plus proche possible de l’état initial de la rugosité de la pièce tout en conservant la plus grande profondeur de la zone durcie.

Furthermore, thePhynox® alloy process also enhances corrosion resistance, fully meeting the stringent requirements of the NIHS 96-50 standard. After four days of salt spray exposure followed by six days of artificial perspiration testing on 1.4435 stainless steel watch case components treated withPhynox® alloy, no visible corrosion was observed.

This exceptional corrosion resistance also ensures that components treated withPhynox® alloy comply with nickel release limits as defined by the EN 1811 standard.

Alteration of stainless steels

Swiss watchmaking is renowned for its uncompromising standards and has built its success on the exceptional quality of its products, down to the surface finish of every watch component. However, it is well known that during handling and daily use, the surface finishes of watch exterior components can progressively lose their original brilliance (surface deterioration). This is particularly problematic for a new product and remains a concern throughout its lifetime.

The formation of surface scratches, often visible to the naked eye, affects the perceived quality of finished products prior to delivery, potentially leading to rejection or refinishing. Ultimately, this also impacts financial costs, which are often hidden.

Several solutions have been developed to address this issue. In most cases, great care is taken during the handling of components, or protective measures—of varying effectiveness—are applied to shield exposed surfaces. However, these approaches involve the use of protective materials that can be costly and generate waste. Enhancing the surface properties of stainless steel now offers a new solution fully compatible with our aesthetic standards.

The introduction of new materials with wear resistance properties far superior to stainless steel—such as ceramics and composite materials like carbon—has opened new possibilities. However, when combining these materials with stainless steel components to create a watch case or bracelet, noticeable differences in surface ageing can appear within just a few weeks of wear, becoming easily visible to the naked eye.