Stress relief cracking behaviour has been studied in controlled purity and (540 ppm) phosphorus-doped casts of 2 1 4 Cr 1 Mo steel, using a test developed to simulate conditions in the coarse-grained region of a weld heat-affected-zone (HAZ) during post-weld heat-treatment (PWHT). Two distinct cracking regimes were observed in the phosphorus-doped alloy during the PWHT cycle. At low temperatures (700–750 K), a smooth, intergranular failure mode was produced, whereas at higher temperatures (800–875 K) the cracking mode changed to a ductile intergranular fracture, with voids centred on MnS inclusions at the grain boundaries. The controlled purity alloy exhibited a similar type of ductile intergranular cracking at the higher temperatures but no cracking was observed at lower temperatures. Auger electron spectroscopy (AES) revealed a maximum in the grain boundary concentration of phosphorus during the stress-relief heating period of the phosphorus-doped alloy, confirming a prediction based on segregation thermodynamics and kinetic theory. The temperature at which this peak occurred was coincident with that of the low temperature cracking regime.