The 309S steel was examined in the solution-treated and quenched condition, and after heat-treatment at 975K for 15 or 300min. The material was carbide-free in the water-quenched condition, but 15min at 975K produced a sensitized microstructure of small discontinuous grain-boundary carbides. The 300min treatment produced larger and essentially continuous carbides. This structure was not sensitized. Depletion of Cr near to boundaries in the sensitized material was demonstrated by a stacking-fault energy of about 22mJ/m2; the matrix value was about 35mJ/m2. This change implied a reduction in Cr content from 23% to about 12%. Smooth and notched bars were tested in all three conditions. The tests were performed in air, in high-pressure hydrogen or after hydrogen-charging to supersaturation. The ductility was decreased only by hydrogen charging; regardless of the microstructure. That decrease was slight.

The Behavior of Sensitized 309S Stainless Steel in Hydrogen. A.W.Thompson: Materials Science and Engineering, 1974, 14[3], 253-64