Performance Of Advanced Dissimilar Metal Weld SA508-52M-316L In Simulated PWR Primary Water Environment

Long-term operation experience of worldwide nuclear power plants shows that environmentally assisted cracking, mainly stress corrosion cracking (SCC), of structural materials in high temperature high pressure water environments in the nuclear island has been one of main problems threatening the integrity of the whole system. High quality tests and evaluations for the cracking behavior of these key materials in the related environment are of great significance. In this work, an advanced dissimilar metal weld SA508-52M-316L for the key part between the reactor pressure vessel nozzle and the safe-end of main piping, which is used in the third generation pressurized water reactor (PWR) nuclear power plants API000, was investigated for this purpose. Following research works were carried out:1) The microstructure of all parts of the weld was examined. The results showed that, the microstructures of low alloy steel SA508 base metal, nickel-based alloy 52M weld metal and stainless steel 316L base metal were characterized as upper bainite, austenitic solidification dendrite structure and forged austenite structure, respectively. In heat affected zone (HAZ) of SA508, three specific regions were observed, that is, thin area with less carbides precipitation, coarse grain area and matrix-like area. In alloy 52M weld metal, some small defects, probably weld hot cracking, were observed in some straight grain boundaries. Dramatic change of chemical compositions and complicated microstructure were found in the narrow transition region near the fusion lines (also interfaces) SA508-52M and 52M-316L.2) The SCC behavior of whole dissimilar metal weld SA508-52M-316L in simulated PWR primary water environments at 290℃was investigated by means of slow strain rate testing (SSRT). The tests were performed at various applied electrode potentials which correspond to the electrochemical conditions of the weld in various water environments, from low potentials with ideal water chemistry to high potentials with oxygen-contaminated water chemistry. On the tensile specimens in SSRT, sharp notches were machined at important and typical places, i.e. at the two interfaces and in the typical bulk parts of the low alloy steel, Ni-based weld metal and stainless steel of the weld. Results showed that the specimens always failed in bulk zone of the Ni-based weld metal with ductile appearances when tested in the potential range from-780 mV to-300 mV (vs. SHE). When electrode potential was raised into the range from-200 mV to+200 mV which corresponded to oxygen-contaminated water chemistry, the weld exhibited significant SCC. The area around the SA508-52M interface was the weakest place where transgranular SCC (TGSCC) happened both along the interface and in SA508 HAZ, intergranular SCC (IGSCC) occurred in the Ni-based weld metal close to the interface.3) Using the similar method of SSRT with electrode potential controlling, the SCC behaviors of the joint SA508-52M and the joint 52M-316L in the same high temperature environments at 290℃were further studied independently with smooth specimens. Results showed that, the SCC characteristic of SA508-52M specimens was similar to that of SA508-52M-316L notched specimens. The SCC susceptibility increased with the rising of electrode potential. A critical cracking potential for SCC (Escc) was in the range of+200-+300 mV (vs. SHE). When electrode potential was higher than Escc, remarkable SCC occurred in the area around the welding interface, which probably initiated in the low alloy steel HAZ. The SCC behavior of 52M-316L specimens was relatively complicated. The 316Lpart exhibited SCC sometimes at low potentials and high strain conditions. When the potential was raised to+400 mV, some SCC sign was found in the zone around the 52M-316L interface. In addition, the serrated yielding was exhibited in the SSRT curves of welded specimen, indicating that the plastic deformation of the weld had dynamic strain aging (DSA) characteristics at the temperature.4) The SCC behavior of the whole weld SA508-52M-316L in simulated PWR primary water at 54℃was also investigated by similar means as the tests at 290℃. Results showed that no clear SCC existed in all parts of the specimen under different stain rates and electrode potentials, smooth or notched specimens. Comparing to the tests at 290℃, there was no significant evidence of DSA of the weld materials at 54℃5) Since the low alloy steel part of the weld exhibited relatively higher SCC susceptibility, further studies on this part was important. A domestic low alloy steel SA-508Ⅲ, which had similar chemical compositions and heat treatment procedure as low alloy steel in the weld, was used because of the limited size of the weld. Results turned out that the SCC behaviors of the SA-508Ⅲand the low alloy steel in the weld were similar.The engineering practical significance for predicting the service performance was discussed in this thesis.