Rapid repair of thermal aging embrittlement in 17-4PH martensitic stainless steel using pulsed electric current

Abstract

The 17-4PH stainless steel serves as the main steam isolation valve in a light water reactor due to excellent comprehensive mechanical properties and corrosion resistance. Nonetheless, the thermal aging resulting from prolonged service poses a threat to the safe operation of nuclear power plants. The mechanism of thermal-aging embrittlement of 17-4PH stainless steel and its repairing behavior under the action of the pulsed electric current were studied using transmission electron microscopy and 3D atom probe tomography. The ductile to brittle transition occurred following the spinodal decomposition and G phase formation during the thermal aging of 5000 h at 673 K, resulting in a 100 HV increase in hardness of the 17-4PH stainless steel, of which 94.4 HV was attributed to the spinodal decomposition. Under the action of a pulsed electric current at 733 K, the hardness and impact energy can be restored to its original state, whereas heat treatment at the same temperature is unable to restore these properties. This is because the pulsed electric current treatment decreased the thermodynamic barrier to dissolution for the spinodal decomposition and G phases. The Kolmogorov-Johnson-Mehl-Avrami model was used to calculate the kinetics of spinodal decomposition dissolution. The results suggest that the pulsed electric current could increase the diffusion pre-exponential factor of Cr element and accelerate the elimination of spinodal decomposition. This process ultimately facilitates the quick regeneration of the 17-4PH stainless steel's performance.