Structural Integrity Assessment
In general, wrought austenitic stainless steel have high fracture toughness with a very low susceptibility of brittle fracture down to cryogenic temperatures. Fracture toughness is also usually high in austenitic weld metal and weld heat affected zones, as was found in the laboratory examination of the reactor described in Appendix 1 of this report.
Austenitic
components and structures can therefore remain stable when they contain large through-wall defects generated by CLSCC (>100mm in the case of the reactor). Clearly, for pipework and
vessels Leak-Before-Break (LBB) is the most likely consequence of CLSCC. Note that a plausible but less likely failure mode would arise if pressures/stresses were high and cracks
reached a critical size for rapid ductile failure before LBB. It is recommended, therefore, that through-wall leaks developing in service with austenitic stainless pipework or vessels should always be investigated for possible CLSCC by NDE or by in-situ metallography.
It is also important to recognise that leakage is not a reliable indicator of either the presence or the size of through-wall cracks because cracks formed by CLSCC tend to be narrow and highly branched. The quantity of the fluid that escapes from a crack will also depend on its viscosity and the
pressure difference. Furthermore, leakage into the process stream (e.g. coolant leaking into
reactants) could be more difficult to detect than a visible escape of liquid or audible escape of
gas.
When CLSCC is a recognised mode of deterioration for a vessel or pipework, the absence of
leakage does not guarantee that cracks are not present; and in particular, it does not obviate a
requirement for periodic NDE. The reactor investigated in this work contained numerous very
long, through thickness cracks as a result of CLSCC. Its structural integrity had been seriously
compromised to the extent that some nozzles and attachments appeared to be close to full
separation under static loading from the reactor body.
This demonstrates that very large cracks
can form by CLSCC in structures whereas in the case of the reactors, the dominant stress was
residual stress from welding and operational stresses would have been low by comparison. If
undetected by NDE, the large cracks can introduce failure modes which were not anticipated by
the original design, for example complete separation of attachments, toppling of tall columns
under wind loading and collapse of long pipe runs due to self-weight.
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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