MATERIALS SCIENCE AND ENGINEERING

ALLOY SURFACE HARDENING BY COLOSSAL SUPERSATURATION OF INTERSTITIAL SOLUTES

Using 316-type austenitic stainless steels as an example, we study a new concept for significantly improving the performance of structural alloys used for medical devices: paraequilibrium surface alloying with interstitial solutes. The basic idea of this approach is to expose alloy parts in their final shape to a gas phase providing atomic species that dissolve interstitially (carbon, nitrogen, or oxygen) and rapidly diffuse into the alloy surface, while all other atomic species in the alloys are basically immobile. Under such, so-called "paraequilibrium" conditions, homogeneous solid solutions can be obtained with interstitial solute concentrations that significantly exceed equilibrium solubility limit.

For 316-type austenitic stainless steels, in particular, a novel, low-temperature gas-phase carburization process has been developed by Swagelok Company to generate homogeneous, carbide-free solid solutions with more than 12 at% interstitially dissolved carbon [1]. This is more than 600 times the equilibrium solubility at the processing temperature (470 °C) – and about 10⁵ times the equilibrium solubility at room temperature. Such a "colossal" supersaturation (CSS) dramatically improves the surface hardness (from ≅ 300 to ≅ 1200 HV25), the fatigue strength, and – rather unexpectedly –the corrosion resistance [1-4]. At the same time, the carburized layer retains ductility and the properties of the bulk material remain basically unaltered. Figure 1 shows the carbon concentration profile below the surface of a low-temperature-carburized 316 austenitic stainless steel and how it affects the profile of hardness and residual compressive stress (induced by lattice expansion).

According to our present understanding and the results of initial experiments with other alloy systems, the new concept can be applied to a broad variety of structural alloys including iron-, cobalt-, and titanium-based alloys.

Fig. 1. Carbon mole fraction, X_C, surface hardness H, , and surface residual compressive stress, σ¹¹, as a function of depth, z, below the surface of a 316L austenitic stainless steel after low-temperature carburization.