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Research of metal after long time exploitation in critical conditionsAna Lypolt
Croatian Society of Non-destructive Testing
Key words: reactor, NDT findings, specimen, laboratory testing (NDT and metallurgic), material degradation, creep.
Even before the Yugoslav army attacked Croatia and prior to the beginning of the war in Croatia, the condition of the reactor was very critical. Material fatigue was established, even complete degradation (of the surface), but owing to team-work and joint efforts by the equipment users, authorised inspectors and testers, the critical war period was overcome successfully, regarding fuel requirements for civilian and military purposes. The plants have performed their designated purpose, and several times the equipment was successfully protected from damages that might have ended in fatal catastrophes.
Due to diagnosed material fatigue and complete degradation (of the surface), both the external and internal surfaces of the reactor were tested in detail once a year using non-destructive methods, and every half a year during the last two years.
In the period between two testing, extreme worsening of the material condition was established, macro-damages - cracks at critical points, noticeable changes in structure (micro-damages) - occurrence of micropores and cavities, their coalescence and combining into chains (micro-cracks), changes in hardness (softening and hardening).
These characteristics do not guarantee a steady growth of cracks since their propagation may be sudden. Therefore, it is very important to determine the condition at critical points by testing, and to insure such a testing plan that the interval between single controls is shorter than the time necessary for the cracks to form.
Microstructure findings of the reactor surface over the last four years (1:300)
Fig 1: external surface, HB 117
Fig 2: internal surface, HB 107
Fig 3: external surface, HB 113
Fig 4: internal surface, HB 102
Fig 5: external surface, HB 112
Fig 6: internal surface, HB 102
By comparing microstructure findings and the measured hardness, there is no doubt that the material (with findings and micro-damage analysis) is in the area of 3rd level of creep and that the changes in structure were becoming more obvious with time.
The reactor was replaced by a new one.
The plan was made in such a way as to achieve the best possible comparability of test results, i.e. so that the obtained findings of the non-destructive method testing would be indicators of changes in properties and material structure, determined by metallurgical tests of specimens taken at the base cross-section.
The analysis of test results showed that the material was completely degraded by high-temperature hydrogen corrosion, in fact by material fatigue caused by long-term operation in the creep range in hydrogen atmosphere.
Testing of the reactor by non-destructive methods and material condition assessment have led already earlier to such a conclusion. However, this opinion was now fully confirmed, in fact, results of mechanical testing (destructive testing) indicated without any doubt complete degradation of the material and unreliability in object exploitation.
All the findings obtained by non-destructive testing were confirmed by destructive testing results and can be traced to the obtained material properties, i.e. changes in material microstructure:
Fig 7: Lamination, 1:300
Fig 8: Inter-crystalline crack perpendicular to surface, 1:300
|Fig 9: Scheme of taking specimens for destructive testing|
|Test piece mark||Yield point load||Tensile load||Yield stress||Tensile strength||Elongation||Contraction||Impact value|
|Required values||min. 316||527-668||min. 20|
|Table 1: Results of testing mechanical properties|
The only deviation was found in measuring the ultrasonic attenuation through the whole wall thickness in relation to partial measurements (90 and 20mm), which is in accordance with the given procedure (the procedure defines the thickness of up to 50mm).
If all responsibility is rejected, efforts and dilemmas regarding maintaining the plant in function, since material was almost completely degraded, and safety was just a function of time, from the technical point of view then, this is a great advantage regarding behaviour of metal in critical conditions (high pressure, creep temperature range, extremely corrosive medium). In fact it is a contribution to science and research regarding extended life-times of materials without guaranteed properties in the border conditions of utilisation.
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