Thermal aging embrittlement of tungsten-alloyed 9%Cr ferritic steels and electrochemical evaluation

Abstract

Thermal aging embrittlement of newly developed W-alloyed 9%Cr ferritic steel KA-STBA29/KA-STPA29 (ASME T92/P92) was investigated. In order to clarify the controlling factor of embrittlement, Charpy impact tests were carried out, and characterization of precipitates was determined through use of materials that had been aged under various conditions. Drops in the upper and lower shelf energies and an increase in the fracture appearance transition temperature (FATT) induced by pre-aging were found to be closely associated with an increase in the amount of precipitates, such as the M23C6 type carbide and the Laves phase. As the result, the drop in impact toughness was found to be uniquely correlated with an increase in the area fraction of the Laves phase. In order to develop a nondestructive procedure for detecting thermal aging embrittlement, the changes in electrochemical properties of KA-STBA29/KA STPA29 induced by pre-aging have been investigated, and the electrochemical polarization technique has been applied to the pre-aged materials. Experimental results on electrochemical polarization measurements revealed that the peak current density "Ip" which appears at a specific potential during potentiodynamic polarization curve measurements in 1N-KOH solution increases linearly with the degree of embrittlement as evaluated by impact absorbed energy at 0°C. This increase in Ip was correlated with an increase in selective dissolution volume of the Laves phase. Consequently, the Charpy impact absorbed energy, along with the degree of thermal aging embrittlement, can be nondestructively estimated by the electrochemical technique.