When austenitic stainless steel tubes are heated or cooled through the temperature range 425-900C (800-1650F), chromium tends to combine with carbon to form chromium carbides. The carbides precipitate preferen- tially at grain boundaries depleting chromium from the adjacent areas. This reduces the corrosion resistance of the chromium depleted areas, sensitizing the alloy to Intergranular Attack (IGA).
The extent of carbide formation is dependent upon time at temperature and the carbon content of the alloy. Thus, exposure in the temperature range stated does not automatically mean that sensitization, or IGA will occur.
Sensitization may also result from slow cooling from solution annealing temperatures, or stress relieving – after welding – in the 425 to 900C (800 to 1650F) temperature range. In welded fabrications, sensitization and IGA may occur in corrosive environments in a rather narrow band on either side of or on the side opposite the weld, known as the heat affected zone (HAZ).
It is important to note that even if sensitization does occur, it is not of significant consequence unless the alloy is exposed to a corrosive environment. Sensitized stainless steel performs in a normal manner and safe manner in non-corrosive applications.
Order of Resistance
| 904L stainless steel | Highest | This table lists some of the common stainless steels as to general resistance to pitting or crevice corrosion in aqueous environments where corrosive conditions may exist. |
| 317L stainless steel | ||
| 329 stainless steel | ||
| 316/316L stainless steel | ||
| 304/304L stainless steel | ||
| 430 stainless steel | ||
| 410 stainless steel | ||
| 420 stainless steel | Lowest |
Methods of Minimizing Intergranular Attack – IGA
1) Solution anneal above 1040C (1900F) followed by a rapid quench.
2) Use type 347 stainless steel, a Cb stabilized grade, or 321 stainless steel, a Ti stabilized grade.
3) Use a low carbon, 0.03% max. carbon steel grade such as 304L stainless steel, 316L stainless steel, 317L stainless steel or 904L stainless steel.
With today’s technology, carbon steel is economically reduced to very low residuals. The low carbon grades are the standard forwelded fabrication.
ASTM A262 practice A to E are standard tests to determine susceptibility to IGA. Practice E, the Huey test, is widely used.
Source: wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)
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