Interactions with the macroscopic specimen surface can profoundly modify phase-separation processes. This has previously been observed in liquids and polymer films and is theoretically described by the theory of surface-directed spinodal decomposition (SDSD). Here we report first observations of SDSD in a metallic alloy on a macroscopic scale. The influence of the surface leads to the development of concentric domains extending over the whole 10 mm thick cylindrical steel specimen, due to long-range interactions via elastic stresses and long-range diffusion of the interstitial elements nitrogen and carbon.