309 is an austenitic heat resistant alloy with useful oxidation resistance to 2000 degrees F under constant temperature conditions. When frequent heating and cooling is involved the alloy is resistant to about 1850 degrees F.

309 is particularly suited for oxidizing environments involving constant temperature or mild cycling with slow rates of heating and cooling. Because of its relatively high rate of thermal expansion the alloy is not suggested for applications involving severe thermal cycling, such as liquid quenching.

The high chromium and relatively low nickel contents of 309 make it the preferred choice among the austenitic grades for high temperature sulfur bearing atmospheres. Under the most severe conditions, however, alloys completely free of nickel may be required.

309 is one of the most commonly used heat resisting alloys in the range of 1500-2000 degrees F under oxidizing conditions.

Carburization: 309 has a degree of resistance to carbon absorption in some environments. Low cost, good sulfidation and moderate carburization resistance combine to make 309 the most widely used alloy for carbon saggers.

Sigma: Prolonged exposure to the 1100-1600 degrees F (590-870 degrees C) temperature range results in the formation of sigma phase in 309. Sigma formation reduces ductility and impact strength at room temperature, although the material remains ductile at elevated temperature. Care should be taken to avoid heavy mechanical impact or thermal shock to fabrications which have seen extensive service in this temperature range.

Sigma may be removed by annealing above 1900 degrees F (1040 degrees C). However, it will again form should the metal subsequently be exposed for long times in the 100-1600 degrees F (590-870 degrees C) range.

The annealed structure of 309 is primarily austenitic. Small amounts of ferrite may occasionally be present.

Applications include burner parts, heat exchangers and combustion chambers, kilns, annealing covers and boxes, incinerators, muffles, retorts, radiant tubes, power boiler tube hangers, anchor bolts, brazing fixtures, glass forming equipment, chemical plant equipment, furnace fans, shafts, and housings, thermowells, paper mill equipment, neutral salt pots.