Non-Destructive Examination NDE
The simplest and most common manual NDE technique for detecting CLSCC in austenitc stainless steel is dye penetrant testing (PT). This method requires that a surface under inspection is clean, accessible and visible to the inspector. Disadvantages of PT in detecting CLSCC
are:
(i) A high standard of surface cleanliness is required and it is relatively
slow.
(ii) Cracks must break the surface being inspected.
(iii) It can be difficult to differentiate cracks from volumetric defects such
as corrosion pits or welding pores.
Where both surfaces can be inspected, the effectiveness of PT in finding through-wall CLSCC can be increased by applying dye penetrant to one surface and developer to the opposite surface. PT can be effective in locating CLSCC but sizing through thickness cracks will not be possible unless both surfaces can be inspected.
Eddy current testing (ECT) is also a common NDE technique which can be applied manually and automatically for detecting CLSCC. It can be particularly effective when automated with
purpose-designed probes that have been calibrated on test samples containing known defects. ECT is used for heat exchanger tubing and pipework where the surface condition and geometry
are fairly consistent over long distances. The disadvantage of ECT is that it will only find surface breaking cracks and cracks just below the surface. In other words the penetration is
limited in steel and it is most effective when testing surfaces from which cracks have initiated rather than the opposing surface. Crack sizing by ECT will be limited in the through
thickness direction.
Ultrasonic flaw detection (UT) of fabricated and welded austenitic stainless steel requires a
specialised technique because weld metals are coarse grained and this causes high sound attenuation and false signals. Normal methods of UT, for example as covered by EN 1714:1998 “Non-destructive testing of welds —Ultrasonic testing of welded joints”, are normally based on ferritic materials. An appropriate UT procedure for austenitic stainless steel can be applied as a manual or an automated NDE technique for detecting CLSCC. Providing access is available, UT can be applied to a surface from which cracks have initiated and/or fromthe opposing surface. If accurate drawings are available and an appropriate technique is used, UT may be capable of obtaining information on the crack size and orientation.
The reactor that was studied as part of this work was fabricated from 5mm to 7mm thick material and CLSCC was found in a range of welded details. As described in Section 4 of this
report, Mitsui-Babcock compared the effectiveness of PT, ECT and UT on samples cut from the reactor. The conclusions of the report were:
1 ECT using standard and purpose designed probes had limited penetration and significant sensitivity to surface imperfections that could not be distinguished from cracking.
2 Manual UT was more promising but the procedure required qualified personnel, a range of probes, and several complimentary scans. The procedure was considered to be very time consuming and would not cover all design details and possible crack position orientations found on the reactor. Sizing of defects was found to be difficult due to access limitations.
3 PT was the recommended method of NDE
Related References:
1. austenitic stainless steel
2. Stress Corrosion Cracking SCC
3. Chloride Stress Corrosion Cracking (CLSCC)
4. Stress Corrosin Cracking SCC of Duplex Stainless Steel
5. Chloride Stress Corrosion Cracking in Austenitic Stainless Steel
6. Recommendations for Assessing Susceptibility to CLSCC
7. Main Findings on CLSCC in the Reactors
8. Literature Review to Chloride Stress Corrosion Cracking
9. CLSCC Chloride Stress Corrosion Cracking Mechanism
10. Factors Affecting CLSCC Chloride Stress Corrosion Cracking
11. Controlling Chloride Stress Corrosion Cracking
12. Structural Integrity Assessment
13. Non-Destructive Examination NDE
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