| Summary: | The present study investigates the toughness degradation mechanism(s) present in a series of 12% CrMoVNbN steel turbine studs using both virgin and ex-service material. It is demonstrated that the embrittlement mechanism is reversible using various re-heat treatments and also that following re-heat treatment and subsequent ageing, this material begins to re-embrittle. Early indications in the present study suggest that the phenomenon of reverse temper embrittlement (RTE) may be occurring in this material. Detailed microstructure examinations have been fully characterised by a combination of electron microscopy, X-ray diffraction and conventional chemical analysis of the ex-service and re-heat treated materials and of extracted carbides. However, the important finding of the analysis of the extracted particles from both the ex-service and re-heat treated samples was that they were Cr-rich M<SUB>7</SUB>C<SUB>3</SUB>-type carbides, contrary to data in the literature which indicates, that for this composition steel. Cr-rich M<SUB>23</SUB>C<SUB>6</SUB>-type carbides would be present. The presence of Cr-rich M<SUB>7</SUB>C<SUB>3</SUB> carbides, and not M<SUB>23</SUB>C6-type carbides, in both material conditions is demonstrated to be due to the low Cr content (10.7%) of this material. Thus, the kinetics of formation of the carbides may be the important factor in the observed embrittlement of this steel. It was observed in the present work, using Auger electron spectroscopy (AES), that P and G cosegregated to the prior austenite grain boundaries of various heats of material. The observations in the present work of significant reverse temper embrittlement (attributed to the higher carbon content of these steels) indicates that RTE is potentially a serious problem in these steels and is critically dependent on alloy chemistry.
|
|---|
