Research On Structure And Properties Of Duplex Stainless Steel For Nuclear Power Plant Pipes

Cast duplex stainless steels (CSS) used for PWR pipes have the merits of both austenitic steels and ferritic steels. But under the reactor coolant operating temperature (288℃to 327℃), CSS will become thermal ageing embrittlement during long-term service. With the increasing of thermal ageing embrittlement, the critical crack size of pressure parts will decrease, the wholeness of the pressure boundary in the primary loop will be weaken. So it is necessary to conduct an intensive study on its thermal ageing mechanism to prolong the lifespan and improve the economic benefit of nuclear power plants.The microstructures, mechanical properties at room-temperature and 350℃as well as ductile-brittle transition temperatures of Z3CN20-09M CSS both made in China and France treated with accelerated thermal ageing at 400℃for 0 up to 10000h were investigated by OM, TEM, SEM, tensile and impact test, the results are as follows:The microstructures of these two CSSs consist of austenite and a little stripped and island ferrite distributed on the austenite matrix. There is no significant change in the microstructures of ferrit with the extension of aging time. In the beginning of thermal aging, there are a lot of dislocations and high stacking faults and dislocation density on the austenite matrix. With the increment of aging time, the density of dislocations, stacking faults and dislocation density decreases obviously and there were precipitations in the dislocations, phase interfaces and grain boundaries. Thermal aged for 10000 hours, the spinodal decomposition occurred in the ferrite.The sampling position has no obvious influence on room-temperature and 350℃tensile properties of domestic steel, but it has significant effect on room-temperature and 350℃tensile strength of French steel and so is the thermal ageing time on both strength and plasticity at room-temperature and 350℃of domestic steel and on tensile strength at room-temperature and both strength and plasticity at 350℃of French steel. The mechanical hardening of steels at both room temperature and 350℃can be divided to yield-mechanical hardening, fore-uniform mechanical hardening, and rear uniform mechanical hardening designated by the ascending sequence of the mechanical hardening index, n1,n2 and n3.It is found that under the same thermal ageing condition, the impact toughness of French steel is better than that of domestic steel and the impact energy parameters reduce with the prolong of the thermal ageing time.The sampling position has no remarkable effect on impact toughness of both domestic and French steels, but the thermal ageing time has prominent effect on the impact toughness index, Wm, Wiu, Wa and Wt, of domestic and French steels. In additional, the ductile-brittle transition temperatures of French steel is less than domestic steel, and they both increase with the prolong of the thermal ageing time.For both domestic and French CSS loaded by tensile test, the fracture mechanism are the microvoid coalescence fracture. The long thermal ageing time only decrease the size of dimples. When the thermal ageing time is lower than 300h, the impact fracture mechanism is microvoid coalescence fracture. The tearing ridge as well as the interface fracture between austenite and ferrite and the river pattern of cleavage fracture can be seen in partial areas after 1000h and 3000h, respectively. After 10000h the characteristics of cleavage fracture can be more clearly observed. The results are consistent with the test of mechanical property and ductile-brittle transition temperatures of two CSSs.After comperhensive comparing the two kinds of materials in tensile properties and impact properties and ductile-brittle transition temperatures, there are still differences between domestic and French CSSs, but every performance index of domestic CSS is meet the challenge. All in all, domestic CSS can instead French CSS.