The segregation of alloying and impurity elements to prior austenite grain boundaries (PAGBs) in low-alloy steels controls temper-embrittlement although the precise microchemical and microstructural interactions are, as yet, unclear because of the many variables involved. Competing segregation and de-segregation phenomena are observed. For example, Auger analyses of fracture surfaces indicate that brittle fracture is caused by the segregation of P to the PAGB. The addition of small amounts ((approx) 0.5 wt%) of Mo appears to regard, but not stop, temper-embrittlement, possibly due to Mo(sub 2)C precipitates that form at elevated temperatures causing de-segregation of Mo from the PAGB. The relationship between segregation and temper embrittlement is further complicated in commercial alloys by both the number of segregating elements and the complex, multi-stage heat treatments. Auger analysis pre-selects the most embrittled boundaries and so the complete distribution of segregants across all PAGBs cannot be determined by this technique. Previous work has shown how X-ray mapping (XRM) in a field-emission gun scanning transmission electron microscope (FEG-STEM) offers a more complete view of the distribution of segregants on both non-embrittled and embrittled PAGBs. XRM was used to observe the evolution of the segregation and desegregation of five elements during four successive heat-treatment stages of commercial low-alloy steel forging. In the last and crucial temper-embrittlement stage, increases in the degree and frequency of Ni segregation occur while other elements either segregate, remain constant or desegregate from the PAGBs.
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