The graph shows a Time-Temperature-Sensitization diagram (TTS) for 304 austenitic stainless steel with various carbon contents. Decreasing carbon content significantly delays the formation of chromium carbides and the onset of intergranular corrosion. In low carbon grades, defined as those with carbon contents below 0.030%, an exposure of about eight hours is necessary before sensitization occurs.
Low-carbon austenitics are intermediate in tendency to precipitate chromium carbides to the stabilized and unstabilized grades. The lower carbon content in an austenitic stainless steel will slow the kinetics of carbide precipitation, which allows longer time at temperature before the onset of sensitization, as shown in the graph.
Carbon contents of 0.03% and lower allow short excursions of several hours into the sensitizing temperature range without loss of corrosion resistance. Carbon in austenite can be retained in supersaturation for extended periods of time. This is why austenitic L grades do not sensitize even though they are slightly supersaturated. Sensitization occurs at higher levels of carbon by prolonged heating at 600 to 850°C.
Volume 4D, Heat Treating of Irons and Steels ⇒ Heat Treated High-Alloy Steels ⇒ Heat Treating of Austenitic and Duplex Stainless Steels ⇒ Annealing
