Thermal Aging Mechanism Of Casting Duplex Stainless Steels For Primary Coolant Loop In PWR

Casting duplex stainless steel with good corrosion resistance and mechanical properties is used in the PWR primary coolant loop. However, with the extension of service time, it will become embrittlement at its working tempercture, thus affecting the safety of nuclear power. Therefore, it is very important to study its thermal aging mechanism.Microstructure and mechanical properties of casting duplex stainless steel thermally aged at 400℃for 100,300,1000,3000,10000,12000 and 15000 hrs respectively are investigated by using of OM, TEM, SEM, tensile test, impact test and micro-hardness test. And based on the relationship between the microstructure changes, such as spinodal decomposition, dislocation configuration and precipitation phase, and the mechanical properties with the thermal aging time, the thermal aging mechanism has been deeply investigated. The results are as follows.The morphology of spinodal decomposition from the beginning to the full stage has been observed by TEM. Spinodal decomposition takes in ferrite phase thermally aged at 400℃for 3000h, to form the black Cr-rich a'phase and the bright Fe-rich a" phase. The wavelengths of spinodally decomposed structure decreases with the increase of aging time. When the thermal aging time is up to 15000h, the spinodal structure has decomposed fully. It is the spinodal decomposition and the precipitation of G-phase resulted from the spinodal decomposition that induces the embrittlement of casting duplex stainless steels.TEM investigations have shown the dislocation tangles and coalescences in ao-received specimens. The dislocation density reduces and the dislocation network forms with the increase of aging time. The dislocations are of low density and straight line by long-term aging. The precipitation of Cr2N, x-phase and M23C6 inα/δinterface with the ageing time also indireetly affect the embrittlement of casting duplex stainless steels.The results of tensile test, impact test and micro-hardness test show that the impact toughness and the elongation as well as the percentage of area reduetion decreases, and the tensile strength and the yield strength increase with the increase of aging time respeetively. The micro-hardness in ferrite phase increases gradually with the increment of the long thermal ageing time, while the effect of the long thermal aging time on the micro-hardness in austenite is negligible.SEM observations indicate that the long thermal ageing time only decrease the size of dimples when the thermal ageing time is lower than 300h.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 fracture mechanism casting duplex stainless steel is of the microvoid coalescence fracture when the thermal ageing time is lower than 300h; the fracture mechanism converts gradually from the microvoid coalescence fracture into the cleavage between 1000h and 3000h. The fracture mechanism of the material is the cleavage after the thermal ageing time is up to 10000h. The results are consistent with the test of mechanical property of casting duplex stainless steel.Thermal aging parameters method has been used to forecast the service life of casting duplex stainless. The results show that the high temperatures impact toughness and the room temperatures impact toughness of the material thermally aged for 15000h reduce about 70.01% and 75.41%. Casting duplex stainless steels for primary coolant loop in PWR is safe during the service life of 40 years.