AbstractFatigue-crack growth rates and fracture-toughness values were determined for a low-alloy ESR steel heated between i10 and 1400°C. The effect of α-MnS precipitation on the high-temperature austenite grain boundaries on crack growth rate in the IIb regime (ductile striation mode) and the onset of static overload was shown to be negligible. Only a few ‘scaly’ facets lying in the crack path were observed at low stress intensities. At high stress intensities and heating temperatures, well-defined facets were observed but these had little influence on crack growth rates. Moreover, fracture toughness values remained approximately constant over the entire heating range. Thus, overheating to 1400°C may be disregarded as a factor likely to reduce gun-barrel life significantly.