Sensitization of Austenitic Stainless Steel
Austenitic stainless steels in service at temperatures above approximately 800°F can
become sensitized to intergranular corrosion. This is similar to the knife-line-attack
referred to in Section 731, but it is not directly related to welding. Intergranular
corrosion attributable to sensitization is not considered a serious problem in most
process environments. However, the use of stabilized grades of austenitic stainless
steel (i.e., Types 321 and Type 347) for service at temperatures above 750°F in
process environments that contain sulfur compounds significantly reduces the risk
of intergranular corrosion.
Dye-penetrant examination (PT) of austenitic stainless steel surfaces exposed to the process environment will usually reveal intergranular corrosion and is an especially useful testing tool for cladding and weld overlays. Ultrasonic examination (UT) cannot detect intergranular cracks in cladding or weld overlays and can be very difficult to use for solid stainless steel vessels. The fine, multiple, and branched morphology of the intergranular cracks makes them poor reflectors of ultrasonic pulses. Furthermore, the coarse grain structure of austenitic stainless steels results in considerable “noise” that additionally complicates interpretation of the ultrasonic reflections. Specialized ultrasonic examination (UT) procedures using very low frequency ultrasonic transducers can sometimes be successfully applied to austenitic stainless steel vessel components.
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