Research On The Fatigue Behavior Of Nickel Based Alloy 690 And Its Welds For Nuclear Power Plant

Fatigue is an important form of nuclear material failures.The experiences of manufacturing and operation of nuclear power in the world show that the nickel based alloy welds used for connection between reactor pressure vessel nozzle and the safe-end of piping in nuclear power plants will usually produce some small sized defects,such as micro-cracks and so on during the welding process.These small sized defects are generally acceptable according to the existing NDT means.In this work,the differences of the fatigue properties of nickel base alloy 690 forgings and the same series of nickel base alloys welds were studied,and the effect of micro-cracks on the fatigue behavior and crack growth rate of the butt welds were studied.Based on the international experience and data,the main experiments and conclusions are as follows:1.The microstructures of nickel base alloy 690 forging and its ERNiCrFe-7 and ERNiCrFe-7A welds were examined.According to the design,the nickel based alloy ERNiCrFe-7 welds include C49-1 and C49-2.In the two kinds of specimens,C49-1 has no micro-cracks and C49-2 contains some micro-cracks.In the same,the nickel based alloy ERNiCrFe-7A welds---C50-1 has no micro-cracks and some micro-cracks are contained in the safe-end P specimens.The observation results showed that the coarse austenite dendrites appear in the four kinds of welds.Many micro-cracks were found in the butt weld of both C50-1 and C49-2,but no micro-cracks were found in both C49-1 and safe-end P specimens.Besides small amount of long cracks,most of micro cracks were observed to distribute unevenly along the dendrite grain boundaries with a micro-cracks size below 0.3mm.2.The strain fatigue tests for the nickel base alloy 690 forging and the four kinds of welds were carried out at room temperature.It was found that the fatigue life of the nickel base alloy 690 forging and its welds were significantly increased with the decrease of strain amplitude.Under 0.2%,0.3%,0.45%,0.6% strain amplitude,the strain fatigue lifeof Alloy 690 forging is higher than those of its welds.Comparing to the two kinds of welds made of ERNiCrFe-7,the fatigue life of C49-1 with no micro-cracks is much higher than that of C49-2 specimens containing many micro-cracks except under the 0.45% strain amplitude.Comparing to the two kinds of welds made of ERNiCrFe-7A,the fatigue life of safe-end P which has no micro cracks is much higher than that of C50-1 which contains many micro-cracks.The fatigue life of welds with micro-cracks is lower than that of the corresponding welds with no micro-cracks.The main reason is that fatigue crack growth on the grain boundaries will derive from the micro-cracks.The fatigue life of each weld specimen is significantly lower than that of the Alloy 690,and the experimental data are obviously dispersed due to the weak grain boundary strength,the coarse welding grains,the uneven microstructure and defects.3.The fatigue crack growth tests for nickel base alloy 690 forging and its four kinds of butt welds were carried out at room temperature,it was found that fatigue crack growth rate of nickel based alloy 690 and its two kinks of butt welds(C49-1,safe-end P)without micro-crack have little difference in the steady state crack growth region,the same results were found between the alloy 690 butt welds with or without micro-crack.These reasons may be explained by that the crack fatigue growth is mainly a transgranular fracture and the pre-existing micro-cracks locating in the dendrite grain boundaries plays a less role in promoting the fatigue crack growth.