Abstract: A steel blade for turbomachinery, for example, a vane or blade of stainless steel for a steam turbine, has its working surface smoothed, for example, by tumble polishing, to a smoothness of about 0.25 microns Ra or less. Then the smoothed surface is subjected to plasma nitriding to harden the surface to a value of, for example, about 1000 on the Vickers hardness scale. The hardened nitrided layer in the steel can be 25 to 100 microns thick. The plasma nitriding process does not affect the smoothness of the area of surface being nitrided, and the resultant surface hardness preserves smoothness of the surface while the blade is in use.