Influence Of Thermal Aging On The Microstructure And Mechanical Properties Of Cast Duplex Stainless Steel In The Primary Circuit Piping Of Nuclear Power Plants

Duplex stainless steels (DSSs) have high strength, corrosion resistance,and good weldability as their duplex structures (austenite and ferrite). They are widely used in major components such as the primary coolant piping of pressurized water nuclear reactors (PWRs). However, DSS are susceptible to thermal aging embrittlement after long-term service at temperatures ranging from 280 to 450 ?.This embrittlement may cause the brittle fracture of the primary circuit pipes and even the serious nuclear leakage accidents. For these reasons, thermal aging embrittlement in DSS has been concerned for serveral decades.In the present work, the DSS materials from the domestic PWR nuclear power plants were analyzed after thermal aging at 400 ? for as long as 10,000 h in order to understand the deformation behavior of these steels. Microstructural evolution in ferrite and austenite in different deformation regions were observed by the electron backscattered diffraction (EBSD) technique and a transmission electron microscope (TEM). The effect of localized strain incompatibility and high stress concentration on the crack initiation were investigated in order to understand the plastic deformation behaviors and fracture mechanisms. These research results are useful to understand the failure mechanism of DSS, and also important to the localization and safe operation of the domestic nuclear power plants.In situ tensile tests at room temperature were conducted on the unaged and aged DSS to investigate both the plastic deformation mechanisms and the effect of long-term thermal aging on the crack initiation. After thermal aging, the ultimate tensile strength of DSS increase and the plasticity has a significant decrease. The fracture morphologies change from ductile fracture with shallow dimples to the mixture of cleavages in ferrite and tearing in austenite. The EBSD technique was used to determine the crystallographic orientations of austenite and ferrite grains on the three different deformation degree areas. The EBSD analysis results indicate that multiple high strain areas exist near the austenite grain boundaries and the ferrite/austenite phase boundaries. This localized strain incompatibility is considered to be related with high stress concentration and crack initiation.Long-term thermal aging affects the crack initiation mechanism and cleavage cracks initiate in the ferrite grains of the aged DSS.The plastic deformation behaviors and fracture mechanisms of the unaged and aged DSS at high temperature were also investigated. Comparing with that at room temperature, the tensile strength at high temperature has an obvious improvement, but the yield strength has a slightly increase after long-term thermal aging. Microcracks initiate in the ferrite and extend to the phase boundaries,leading to the fracture of ferrite before the tensile failure of the specimen. The nano-indentation test results indicate that hardness in ferrite continuously increases with aging time, but not strongly affected by the deformation degree.High stress concentration on the phase boundaries causes the phase boundary separating. The dislocations pile-up at spinodal decomposition precipitates and G-phase in ferrite phases results in the reduction of the micromechanical property of the ferrite.The effects of thermal aging and the associated phase transformations on the impact toughness of DSS were also investigated at room temperature. After long-term thermal aging, the impact energy decreases significantly and the cracks initiate and propagate more easily. The plastic deformation of ferrite decreases and the wavy profiles of impact fracture become flatter. The effect of thermal aging on deformation ability of ferrite leads to the degeneration of the impact property.High stress concentration areas are observed near the phase boundaries and the austenite grain boundaries in the aged materials. The cracks initiation at the phase boundaries have more opportunities to expand to the ferrite grains due to the hardening in the aged ferrite grains.The nanoindenter and the EBSD technique have been conducted to investigate both the plastic deformation behavior and the influence of the crystal orientation on the nanohardness H and the indentation modulus E. Both nanohardness H and indentation modulus E are correlated with the orientation factor averaged over three normal directions of the contact surface. After thermal aging, the dependence of hardness and indentation modulus on the crystallographic orientation obviously changes for the ferrite phase. For the ferrite and austenite phases, the maximum and minimum values of hardness H and indentation modulus E are observed in the near-<111> and near-<001>-oriented grains, respectively. The TEM results indicate that the area of plastic deformation decreases in the ferrite grain. The interactions of a' phases and G-phases with the dislocations are considered to be responsible for the degradation of plastic deformation ability in ferrite.