The void swelling rate in Type 316 steel is dominated by the instantaneous composition of the austenite matrix rather than by the concurrent nature of the dislocation and void microstructure alone. This is the conclusion derived from a series of neutron-irradiation experiments designed to determine which microstructural components account for the variability of void development observed in this alloy in response to changes in composition, preirradiation treatment, and irradiation history. The attainment of steady-state swelling is thought to be related to the development of a saturation microstructure of precipitates and dislocations. The existence of a saturation state for dislocation microstructure has been confirmed, but this condition is reached at fluences substantially below the onset of steady-state swelling. Relative to a solution-annealed treatment, 20 percent cold working accelerates the approach to saturation of the dislocation microstructure; thus the delay of swelling by cold work cannot be rationalized simply in terms of the time or fluence dependence of dislocation density.

In this alloy the accelerated swelling state is always associated with extensive second-phase precipitation. These precipitates are rich in nickel, molybdenum, silicon, and carbon and continue to form to very high neutron fluence levels. They deplete the austenite matrix of most of the carbon and silicon and about one third of the nickel and molybdenum. The acceleration of the swelling rate with fluence can be correlated with the instantaneous nickel content of the matrix. The primary influence of cold work on void development in Type 316 stainless steel is the disruption of the normal sequence of second-phase formation and the prolongation of the depletion sequences. Preirradiation aging at temperatures substantially above the irradiation temperature also leads to altered phase stability and altered precipitation sequences.