Effect of oxidation and nitriding on the properties of zirconium alloys

In contrast to a large number of publications about the influence of interstitial elements (O, N) on the physical-mechanical properties of zirconium alloys, insufficient attention at present is paid to examining their influence on the characteristics of near-surface layers of the shells of heat generating element (HGE). Therefore, it is expedient to widen the knowledge about the influence of interstitial elements on the properties of zirconium HGE tubes. Authors experimentally established the influence of treatment in the controlled oxygen- and nitrogen-containing gas media on the mass increment and properties of the near-surface layer of samples-rings, cut out of the shells of heat generating elements. Differences in the saturation of internal and external surfaces of zirconium pipes were described. It was shown that roughness of the internal surface is less compared to that of the external surface. Results of examining the hardness of external and internal surfaces of the samples-rings after oxidizing and nitriding are presented here. For example, treatment of the samples-rings in the oxygen-containing medium (T=650 °C, t=20 h) leads to the formation of hardness at the external surface HV 0.49 =1190±90, and at the internal surface HV 0.49 =1190±90. However, after treatment in the nitrogen-containing medium (T=650 °C, t=20 h), the hardness on external surface is HV 0.49 =615±35, while on the internal surface it is HV 0.49 =445±35. For example, after treatment in the oxygen-containing medium (T=650 °C, t=20 h), depth of the strengthened layer at the external surface is l=70…75 μm and at the internal surface, it reaches l=60…65 μm. Treatment in the nitrogen-containing medium (T=650 °С, t=20 h) causes the formation of a strengthened layer on the external surface l=60…65 µm and on the internal surface – l=55…65 µm. The duration of isothermal holding in the oxygen mixture, which can lead to the crack initiation at the internal surface of zirconium HGE pipes, was experimentally discovered. Results of present work may be taken into account for the development of modes of treatment of zirconium alloys.