| 압력 용기 재료 SA508에서 열적 처리에 따른 입계 파괴의 원인 |
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| The Cause of Intergranular (IG) Fracture by Thermal Embrittlement
in SA508 of Reactor Pressure Vessel (RPV) Steel |
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Published online: 30 November 1999. |
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| ABSTRACT |
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Intergranular(IG) fracture due to thermal treatment has been reported in a reactor pressure
vessel(RPV) steel of Russian light water reactor in last decade. This is attributed to grain boundary
segregation of phosphorus (P) or precipitation of carbide, etc.. This is a finding a difference in microstructure
before and after IG cracking; this cannot explain the cause of the IG embrittlement. This old paradigm follows
only correlation. Recently, a mechanism in which IG embrittlement occurs due to a decrease in entropy of
a material has been reported at a temperature where atomic diffusion is possible. It is anticipated that new
paradigm can explain the IG embrittlement of RPV based on a causal relationship. Thus, the thermal
treatment at 350-420 oC was applied to RPV steel of SA508 and IG cracking was confirmed. DSC analysis
was applied to confirm whether a decrease in entropy due to a short range ordering reaction occurs in SA508.
It was possible to quantify the entropy change( S= Q/T) through DSC measurement. A lattice changes due
to thermal treatment were confirmed using XRD analysis in aged specimens. The results showed that lattice
contraction by aging causes a reduction of fracture toughness. The internal stress formed inside the material
due to entropy reduction can be calculated by multiplying the exothermic energy per unit mass by the density.
This relationship is expressed by a equation of stress(σ) = exothermic heat( Q) x density(ρ). |
| Keywords:
reactor pressure vessel (RPV), SA508 (RPV steel), DSC(differential scanning calorimeter), short range
ordering (SRO), exothermic reaction, entropy |
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