Thermal aging embrittlement of W alloyed 9%Cr ferritic steels and its evaluation by electrochemical technique

Thermal aging embrittlement of newly developed W alloyed 9%Cr ferritic steel KA-STBA29/KA-STPA29 (ASME T92/P92) has been investigated. In order to clarify the controlling factor of the embrittlement, Charpy impact tests and determination of precipitates have been carried out using materials aged under various conditions. The decrease in the upper and lower shelf energies and the increase in the fracture appearance transition temperature (FATT) due to thermal aging were caused by the precipitation and coarsening of Laves phase. The decrease in the impact toughness was found to be uniquely correlated with the increase in the area fraction of Laves phase. In order to develop a non-destructive technique for detection of the thermal aging embrittlement, changes in electrochemical properties of KA-STBA29/KA STPA29 due to thermal aging have been investigated. The electrochemical polarization technique has been applied to thermally aged materials. Experimental results on electrochemical polarization measurements revealed that the peak current density `I<sub>p</sub>` which appeared at a specific potential during potentiodynamic polarization measurements in 1N KOH solution linearly increased with the degree of embrittlement evaluated by the impact energy at 0 °C. The increase in I_p corresponded to the increase in selective dissolution of Laves phase. As a consequence, the degree of thermal aging embrittlement can be estimated by the electrochemical technique.