Gas nitriding of inner surfaces of deep holes

In industrial applications of thermo-chemical treatments, gas nitriding is a widespread method of nitriding. Machine parts and tools of various shapes and sizes are nitrided, including drills, cutters, dies, shafts, rings, bearing balls, etc. often characterized by a complicated shape. The nitrided layers should have a controlled thickness not only of the diffusion zone, but also the surface layer of iron nitrides to have good exploitation properties. With a properly prepared surface on low alloy steels, we obtain a uniform nitrided layer after nitriding. It may happen that the structure of the nitrided layer is heterogeneous or even completely lacking in the nitrided parts having a complicated shape, and especially thin inner openings, where it is difficult to properly prepare the surface. Therefore, the nitriding problem is not the external surface of the workpieces but the internal one. The aim of the work was to assess the possibility of nitriding deep holes with small diameter. The tests were carried out on cylindrical specimens of unalloyed and low-alloy steel with electro-bored and mechanically hollow through holes and blind holes. The quality of the obtained nitrided layers was evaluated by metallographic and hardness examinations.