As detailed in the patent documentation on Google Patents, “Stop-off composition,” a novel method and composition for preventing the diffusion of nitrogen, carbon, or oxygen into the surface of a metal at elevated temperatures has been introduced. This is achieved using an air-setting aqueous mixture of refractory oxides combined with a modifier and silicate binders. The mixture is selectively coated onto the metal workpiece surface where diffusion is not desired, cured at room temperature, and then subjected to elevated temperatures for treatment. After the process, any residual coating is removed by conventional methods.

The process addresses a significant challenge in metal surface treatment, particularly for steel and other ferrous metals undergoing nitriding, carburizing, or carbonitriding. These treatments, which occur at temperatures ranging from 950°F to over 1700°F, aim to create a hard, wear-resistant surface layer by diffusing nitrogen and/or carbon into the metal. Traditionally, preventing diffusion in unwanted areas has involved either treating the entire piece and removing the hardened layer where not needed or applying various masking materials. Both methods have limitations and drawbacks.

This innovation provides an efficient, effective alternative. The air-setting mixture acts as a barrier to diffusion, preventing the hardening of selected areas while allowing other parts to achieve the desired hardness. This solution is advantageous because it is easy to apply, economical, and does not introduce adverse effects on the treated surface. It ensures uniform, repeatable results, making it a superior choice for selective surface hardening.

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Article with all rights reserved, courtesy of patents.google.com