Mechanical design using low-temperature carburization

Fluid systems in a variety of industries use tube fittings to readily install leak-free connections. Robust tube-fitting performance depends on adequate differential hardness, the gripping element on the tube, and a guided plastic deformation of the tube-gripping components. Low-temperature carburization technology hardens the surface of austenitic stainless steels. Since 1999, this treatment has been applied to the ferrules of the Swagelok tube fitting, ensuring tube grip, gas-tight seal, and vibration resistance in hundreds of millions of fluid system connections around the world. The process involves activation of the surface followed by a gas-phase treatment, performed at temperatures low enough to avoid the formation of carbides, for a sufficient time to allow carbon diffusion to occur. The result is a hardened conformal case on the treated parts without distortion or change to dimension. The treated case remains austenite (with verified carbon concentrations over 12 atomic percent at the surface) and retains its ductility. This paper will discuss the design features and performance characteristics of tube fittings enabled by low-temperature carburization.