309 is an austenitic heat resistant alloy with oxidation resistance to 1900°F under constant temperature conditions. When frequent thermal cycling is involved, the alloy is resistant to about 1850°F. The high chromium and relatively low nickel content of 309 provide good resistance to high temperature sulfur bearing atmospheres. 309 has only moderate resistance to carbon absorption and is not suggested for use in highly carburizing atmospheres.

Often in many product forms, the grain size and carbon content can meet both the 309S and 309H requirements. Prolonged exposure to the 1100-1600°F temperature range will cause some sigma to form, lowering room temperature ductility and impact resistance. Should sigma formation be anticipated, care should be taken to avoid heavy mechanical impact at room temperature. 309 has a machinability rating of 43% relative to AISI B1112 steel. Room temperature forming is suggested. Weld with AWS E309-16 covered electrodes or ER309 bare wire.