Effect Of Simulated Working Condition Thermal Aging On Dynamic Mechanical Properties Of Nuclear Stainless Steel

Z3CN20-09M stainless steel has been widely used in primary coolant loop of PWR due to its good anti-hot cracks,anti-stress corrosion and good mechanical properties,but the pipes will be embrittled after long-time service in the medium of high temperature,pressure and high flow rate to threaten the safety of the nuclear plant operation.Therefore,it is very important to explore the properties of the stainless steel after long term service.In this paper,Z3CN20-09M stainless steels are treated with 400?and 350?simulated working medium for 1 000 h,3 000 h,5 000 h,1 0000 h and 1 5000 h,the microstructure,the instrumented impact and impact-tensile properties of the aged specimens have been investigated.Moreover,the fracture morphologies of the instrumented impact and impact-tensile specimens have been observed by use of SEM.The results are as follows:There is no obvious influence of long time aging on the shape,size and volume fraction of ferrite phase.When the thermal aging time is up to 3 000 h at 400?,the spinodal decomposition occurred in the ferrite phase.With the aging time extending to 10000 h,the spinodal decomposition in ferrite took place with a homogeneous distribution,and it indicated that the spinodal decomposition was gradually close to the state of equilibrium.When the thermal aging time is up to 1 5000 h,the spinodal structure fully decomposed.Spinodal decomposition occurred in the ferrite phase after aged 350?for 1 0000 h.With the aging time increasing,the spinodal decomposition further developed.There is a similar variation rule in austenitic substructure under 400?and 350?working conditions that the density of dislocation and stacking fault decreased with the prolong of thermal aging time.Instrumented impact test indicated that with the aged time increasing the impact energy of the specimens at 400?and 350?reduced greatly,due to the spinodal decomposition in ferrite.Moreover,the quantitative analysis of impact energy revealed that the decrease of W_iuu is the main reason of the decrease of the impact energy.SEM observation indicated that with the aged time increasing the fracture mechanism gradually changed from microvoid aggregation fracture with elongated dimples to quasi-cleavage fracture with tear ridges and cleavage steps.Impact-tensile test results showed that the thermal aging caused the decrease of both yield strength and tensile strength and reduction of both elongation and reduction of area.For the thermal aging time is too short,the fracture of specimen is still characterized by ductile fracture under high speed tension load.