The isothermal segregation of alloying and impurity atoms to prior austenite and lath boundaries has been investigated in a 2·25Cr–1Mo low-alloy steel with an overaged microstructure. Scanning transmission electron microscopy (STEM) X-ray microanalysis and X-ray diffraction analysis have identified three carbide types on both of these boundaries within the initial microstructure. Embrittling heat treatments undertaken over the temperature range 793–873 K resulted in extensive regions of intergranular fracture in samples broken under liquid nitrogen. STEM X-ray microanalysis established that the prior austenite boundaries become progressively enriched in silicon and manganese and the M6C type boundary carbides become enriched in phosphorus. and silicon. De-embrittling heat treatments at 973 K removed both the low-temperature intergranular fracture and the segregation of both the prior austenite grain boundaries and carbides. The results are discussed in terms of existing theories of temper embrittlement.
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