Etching Technique for Assessing Toughness Degradation of In-Service Components

The toughness of low alloy steel components can degrade in service due to temper embrittlement. This embrittlement is caused by segregation of impurity elements to the grain boundaries of the steel. Nondestructive techniques are needed to assess the extent of toughness degradation, since this information is vital in making run-retire decisions. EPRI has successfully completed evaluation of a chemical etching technique for determining the change in the fracture appearance transition temperature (ΔFATT) of rotor steels. Laboratory steels and core samples removed from actual rotors, with varying degrees of embrittlement, have been evaluated. Depths of grain boundary grooves, produced by chemical etching, correlate well with ΔFATT and with the grain boundary coverage of impurity elements as determined by Auger spectroscopy. To render the technique even more nondestructive, a 3-stage replica technique has been developed. Groove depths measured from replicas of etched samples also yield an excellent correlation with the ΔFATT of the samples.