The Charpy ν-notch impact properties at -196°C of casts of Type 316 austenitic stainless steel have been compared, after ageing at 650°C for time periods up to 90 minutes. The impact energy of all steels depends on ageing time; the degree of response to ageing depends on original delta ferrite content, which varied from −0.5% to 5.0% in the steels studied. For steels with delta ferrite content in the lower half of this range the impact energy, during ageing, initially decreases to a minimum, subsequently rises to a maximum and finally decreases. Steels with delta ferrite contents near 5% suffer a continuous decrease in impact energy as ageing time increases.

Microstructural investigation reveals, during early stages of ageing, the rapid formation of sigma phase, at the ferrite-austenite interphase boundaries. Some delta concurrently transforms to new austenite. Microfractographs indicate a change in the nature of the fracture as ageing progresses. Initially, fractures have large dimples, but as ageing progresses, a finer dimpling appears and extends in area. These dimples contain fine particles, identified mainly as MnS. The variations in impact behaviour are explained on the basis of the structural changes observed.