What is Sensitization in Austenitic Stainless Steel & how it happens

What is Sensitization

In this article, we are discussing What is Sensitization, its consequence on austenitic stainless steel & austenitic stainless steel welding, and how we can prevent the sensitization.

As we know, Carbon (C) is an unwanted impurity in austenitic stainless steels due to its excessive thermodynamic affinity for chromium (Cr) i.e. carbon easily combines with chromium at a higher temperature. When austenitic stainless steel is exposed to the temperature range of 370 °C to 815 °C (700 °F to 1500 °F, API 571 reference) during manufacturing, welding, or high-temperature applications, chromium-rich carbides precipitate along the austenite grain boundaries. During carbide precipitation, interstitial carbon (C) diffuses quickly to the chromium available in the material along the grain boundaries and form chromium carbides. As the diffusion of chromium (Cr) is slow, it cannot diffuse from the body of the grains to replace the Cr which has gone into the carbides. A high concentration of chromium in the carbide particles decreases the chromium content immediately adjacent to the grain boundaries.

Resulting, the grain boundary has lower corrosion resistance and can be attacked in an environment the stainless steel would normally resist. The steel is said to be sensitized and is susceptible to intergranular corrosion, IGC (also called grain boundary attack). A complete steel section may be affected after service or heat treatment in the critical temperature range, or part of the heat affected zone of a weld may suffer the sensitization problem.

In these areas, the concentration of chromium falls below 12 wt% (the level at which stainless steels acquire their ‘stainless’ characteristic) and thus the depleted areas become anodic in the presence of an electrolyte and are susceptible to corrosion.

Figure 1. Chromium carbide precipitation at austenitic grain boundaries

Chromium carbide precipitation can occur during exposure to high operating temperatures. This is not deemed to be an issue if the operating temperatures remains above the dew point temperature. However, in the presence of moisture and air, intergranular attack can occur due to the localized depletion of chromium adjacent to the grain boundaries. For materials with a metal sulphide scale from exposure to Sulphur (S) containing species during operation, the scale may react with air and moisture to form polythionic acid. This in combination with a stress, residual or applied, can lead to a phenomenon known as Polythionic stress corrosion cracking as per API 571.

Figure 2 – Polythionic SCC crack

Chromium carbide precipitation can also occur by slow cooling from elevated temperature such as solution annealing during manufacture or welding. The phenomenon associated with welding is commonly termed weld decay where the heat affected zone is locally sensitized and thus susceptible to intergranular attack.

Prevention of Sensitization in Austenitic Stainless Steel

  1. Susceptibility to sensitization can be minimized by using a low carbon variant (i.e. 304L, 316L). These grades are manufacturer with very low carbon content thus naturally limited carbon atoms to form the chromium carbides. Regular and high carbon grades of stainless steels such as Types 304/304H and 316/316H are particularly susceptible to sensitizing in the weld HAZ. Low carbon “L” grades (<0.03 % C) are less susceptible and usually can be welded without sensitizing. The L grades will not sensitize provided long-term operating temperatures do not exceed about 750 °F (400 °C).
  2. Improved resistance to sensitization can be achieved with chemically stabilized versions of these alloys containing small amounts of titanium (Ti) or niobium (Nb) and tantalum (Ta) such as Types 321 & 347. [Note that Niobium was also called Columbium (Cb), but Niobium is now the generally accepted name.] Alloy 20Cb-3, as well as Alloys 825 and 625, are also chemically stabilized. These additions are stronger carbide formers than chromium (they will form their carbides so no carbon will be left to react with chromium) and tie up the carbon so that chromium carbides are not formed. Titanium or niobium additions also reduce the solubility of carbon in austenite resulting in MC-type carbides, where M represents titanium or niobium. Although these alternative materials reduce susceptibility to sensitization at short exposure times to elevated temperatures, chromium-rich carbides still precipitate after long-term aging.
  3. Supplemental requirements in ASTM specifications provide for mill products to be delivered in a thermally stabilized condition rather than simply solution annealed. This heat treatment will minimize potential sensitization problems at higher temperatures.
  4. A thermal stabilization heat treatment at 1650 °F (900 °C) may be applied to chemically stabilized austenitic stainless steel welds after all welding is complete to reduce sensitization at the welds. This heat treatment is also applied after welding material that was thermally stabilized in the mill in order to restore the thermal stabilization destroyed by the heat of welding.
  5. Solution heat treatment, commonly called solution-annealing, is used for material that got sensitized. The material is heated to a temperature of about 1950° to 2050°F to dissolve the carbides and then water quenched, thus not allowing the carbides to precipitates. This is useful in the case of welded components that got carbides precipitation.

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