The Research Of Plasma Nitriding Process On Control Rod Cladding Tube

In this paper, plasma nitriding of the AISI 304 L stainless steel is studied under the condition of different temperatures, times and gas compositions by X-ray diffraction(XRD), knoop microhardness meter, optical microscopy and transmission electron microscopy(TEM). The envolution of the precipitated phase, microhardness, thickness and microstructure on nitrided surface layer is investigated. The results reveal that the single expansion phase(γN) is detected on the surface under 400 °C nitriding. Besides, not only the γN phase but also Cr-nitrides(CrN and Cr2N) are found on the nitrided layer. The surface microhardness of nitrided layer increased with the extension of nitriding time, the value of surface microhardness reached 1600 HK when nitriding at 36 h. In addition, the amorphous and nanocrystalline regions are observed on the nitrded top layer by TEM.The fretting-wear in autoclave and the erosion-wear under normal impact resistance behaviors of the samples have been investigated in detail. The wear and erosion mechanisms of nitrided sampes are discussed. The fretting-wear resistance property has greatly improved after plasma nitriding, and the values of wear rate in autoclave accord with regularity. The erosion-wear resistance of all the nitrided samlples is degenerated, except for the specimen nitriding at 400 °C- 6 h. Notably, the erosion-wear resistance of the substrate after nitriding is improved, which is because that the N atoms are implanted in the substrate and improve the erosion-wear resistance. The material brittleness increased when the microhardness is too high, which reduce the fatigue life.The corrosion of the simulated spent fuel pool and the degree of sensitization(DOS) of intergranular corrosion(IGC) resistance behaviors of the samples have also been analyzed in detail. Besides, the corrosive mechanisms of non-nitried and nitrided samples are discussed. The change of average corrosive rate value in the three days is very big, which indicating the corrosion resistance degenerated after plasma nitriding. The average corrosion rate of the sample nitriding at 400 °C- 24 h is the smallest of all the nitrided samples, which means the best corrosion resistance. The corrosion rate has regularity to follow when continue to extend the corrosion time. When nitriding temperature is lower and nitriding time is shorter, the corrosion resistance in the spent fuel pool is better. The corrosion resistant behavior degradated with the increase of N potential during nitriding at 400 °C- 24 h. The DOS of IGC after plasma nitriding has a different degree of increase, which illustrate the intergranular corrosion resistance degradated of samples after the nitriding treatment sensitive performance. The Cr-nitrides(CrN and Cr2N) are precipitated on the grain boundry and the nanocrystals are detected on the surface layer, which lead to the chromium depletion area adjacent the chromium nitrides and the density increase of the grain boundary after the grain refinement. As a consequence, the microstructures of the samples after grain refinement have a greater length and network of grain boundary, which lead them to be highly attacked...