Microstructure And Property Of Austenitic Stainless Steel For PWR Primary Loop In Simulated Working Condition

Casting austenitic stainless steel used in primary loop of PWR always occurs thermal aging embrittlement during long time servicing in the pressure of 15.5 MPa and the temperature of 288 ℃～327 ℃, which poses a great threat to the safe operation and t life of PWR. Therefore, exploring the microstructure and mechanical property change of austenitic stainless steel after long term service in this condition has an important practical significance.In this paper, thermal accelerated aging test for domestic centrifugal casting Z3CN20-09M steel are respectively carried out in air and water vapor condition at 400 ℃. After different time of thermal ageing, metallographic microstructure and substructure of the steel have been observed by OM and TEM, Vickers hardness, nano-indentation hardness, SPT property, and charpy impact energy of the steel have been tested, and fracture morphology of SPT and charpy impact are observed by SEM. The thermal aging behavior of the steel under simulated working condition could be explored by means of analysis and comparision with the influence of thermal aging on microstructure and mechanical property in air condition.Metallographic microstructure of Z3CN20-09M is consisted of austenite phase and ferrite phase, which is distributed with the acicular, island and stripped type in austenitic matrix. With the aging time increased, the shape and size of ferrite phase have no obvious change. But the density of dislocation and stacking fault in austenite decreases, and a small amount of Cr2N precipitates in matrix and dislocation line after aging for 300 hours. When it was aged for 3000 hours, spinodal decomposition occurred in the ferrite phase and generated a black and white spots. With the aging time extended to 5000 hours, the product of spinodal decomposition in ferrite phase appears a homogeneous distribution, and it indicates that the spinodal decomposition is gradually close to the state of equilibrium. Compared the product of spinodal decomposition in different conditions, it could be found that the degree of thermal aging for 5000 hours in air condition is similar to 3000 hours in simulated working condition, and in simulated working condition the boundary of the product which was aged for 5000 hours is distinct than aged for 3000 hours. It can be concluded that aging for same long time, the degree of spinodal decomposition in simulated working condition is greater than in air condition.Vickers hardness and nano hardness of ferrite phase are both slowly increased with the extension of the thermal aging time, and the hardness of austenite phase in both two conditions has no significant change. The hardness of ferrite phase in simulated working condition and air condition has a significant difference, and statistical inspection shows the difference is obvious. With the aging time extended, the change of charpy impact energy has no significant difference in both two cnditions. Analysis of the impact fracture surface shows that indifferent condition, with the aging time prolonged, fracture mechanism gradually changes from micropore aggregation fracture which is charactered by dimples to quasi-cleavage fracture which is charactered by tear ridges and cleavage step. The fracture mode was diverted from ductile fracture to brittle fracture.Small punch test indicates that, with the aging time extended, the yield strength and the ultimate strength of SPT are both increased, while the SPT specific fracture energy declines. Statistical inspection indicates that the difference in both two conditions is unobvious. Analysis of the SEM fracture surface shows that fracture characteristics changes from a large number of dimples to a large number of tear ridges with the aging time prolonged in air condition, while the fracture mechanism in simulated working condition changes from micropore aggregation fracture charactered by a large number of dimples to quasi-cleavage fracture charactered by some tear ridges and some cleavage steps. The comparision of fracture property in different condition reveals that the thermal aging in simulated working condition has more significant change to fracture property than it in air condition.