Research On Microstructure And Corrosion Resistance Of Zirconium Alloy Welding Joints

As the cladding material of light-water nuclear reactor, M5 zirconium alloy is widely used in industrial applications with its excellent mechanical properties and corrosion resistance,. However, with the development of the nuclear reactor technology of high fuel consumption and long period of changing the cladding material, there is a new challenge for the exsiting cladding material. In view of the good performance of the martensite in steel, The comprehensively study on the martensite in M5 alloy welding joints to meet the needs of the development of the nucear power plant.The microstrucuture, mechanical properties, corrosion dynamics, corrosion resistance and corrosion products of Zr-4 and M5 alloy were mainly investigated by using of OM, TEM, SEM, Vickers hardness test, dynamic corrosion test and energy spectrum analysis. The results were as follows.The metallograph and microhardness test of M5 alloy cladding tube showed that the structure of base metal was azr+βNb with fineβNb homogeneously distributed in azr matrix. The soldered area consists of grain boundary a-Zr (Nb) and acicular martensite with high density of dislocations under TEM; The microstructure of HAZ presents the coexisting structures of a and a'duplex phases system, or the three-phase coexisting strucuture of a',α'andβ. The width of martensite became narrower from WM to HAZ. The Grain size of Center of the weld was 6-7, the fusion zone is 8-9 and the size of base metal was only 13-14.Microhardness test indicated that the grain size of melted zone on the welding plug head was higher than that of other positions. For the different distributions of dislocations the highest microhardness value was on HAZ.The dynamic corrosion test at 380℃and 400℃revealed that the corrosion dynamics of Zr-4 and M5 alloys abided by parabolic law and the corrosion speed of M5 alloys was obviously lower than that of Zr-4 alloys. The surface of the two alloys changed from bright to gray. There were some tiny gray oxidation film on the surface of Zr-4 rather than on M5 alloys, which showed that corrosion resistance of M5 was superior to that of Zr-4. The spectrum analysis showed the oxide products of M5 were probably ZrO2 and ZrO.