Abstract
A 0·4C–Cr–Mo–Ni steel has been studied to determine the major microstructural factor controlling the plane strain fracture toughness K Ic of desulphurised ultrahigh strength steel and to develop a method to improve the fracture toughness. For desulphurised steel containing 0·002 wt-%S, intergranular failure occurred during the fracture toughness test, so that there was little difference between the values of K Ic for the desulphurised steel and those for steel containing sulphur at the commercial level (0·016 wt-%). This is attributed to the ‘overheating phenomenon’, in which manganese and sulphur, which are taken into solution in the hot rolling temperature range, segregate to the grain boundaries and reprecipitate there asfine MnS. Reheating at high austenitising temperature followed by rapid quenching significantly improved the fracture toughness, but it had a detrimental effect on strength and Charpy impact energy. The addition of 0·0061 wt-% Ca (Ca/S=3) in the desulphurised melt just after vacuum degassing modified the morphology of the inclusions from fine MnS precipitated along grain boundaries to spheroidised CaS and consequently significantly improved the fracture toughness. This beneficial effect is briefly discussed in terms of metallography, fractography, and the crack extension model in the fibrous mode.
MST1212
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