Influence of Ultrafine-Grained Layer on Gaseous Nitriding of Large-Sized Titanium Plate

In this paper, mechanical shot blasting on a large sized titanium plate is conducted to induce severe plastic deformation, which generates an ultrafine-grained surface layer. The effect of an ultrafine-grained layer on nitriding is evaluated at nitriding temperatures from 600 to 850 °C. The structural phases and mechanical property improvements are investigated and compared to those of a coarse-grained specimen by using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and mechanical property measurements. The results indicate that an ultrafine-grained layer enhances the nitriding kinetics and produces a thicker nitrided layer than that of a coarse-grained plate at the same gaseous nitriding temperatures. The improved kinetics are attributed to a greater number of grain boundaries and defects introduced into the titanium plate surface by the mechanical shot blasting treatment. Meanwhile, the surface and cross-sectional hardness values improve compared to the coarse-grained plate due to the thicker nitrided layer resulting from deeper nitrogen diffusion.