A modified middle scaled creviced bent beam (CBB) test was conducted in order to investigate stress corrosion cracking (SCC) behavior around fusion boundary between nickel base alloy (Alloy82) and low-alloy steel (LAS) under high stress conditions in light water reactor (LWR) coolant environment.

An Alloy182/LAS clad plate specimen was placed between the CBB jigs so that the tensile stress occurs in Alloy182 and the SCC cracks propagate from the Alloy182 surface to the fusion boundary. In pure water, only oxides and cracks propagating along the fusion boundary were observed. Meanwhile in Na2SO4 injected water with electrical conductivity 0.3μS/cm, deep cracks propagated into the LAS. The higher conductivity enhanced the driving force of the SCC cracks.

A notched specimen was also tested to investigate SCC propagating behavior in more severe stress conditions than those in a smooth surface specimen. When a notched specimen was used, even in pure water condition, some cracks penetrated the fusion boundary and oxides were observed. In the notched specimen, crack nucleation points are limited in the notch root and the stress relaxation hardly occurs due to multiple crack nucleations. Such severe stress conditions also contribute the cracks to propagate into the LAS.

To discuss the crack propagation behavior in the vicinity of the fusion boundary, the cross section was observed with an optical microscope. The observation indicates that the crack propagating depth into the LAS and the diameter of oxides increase with increasing the Alloy182 thickness. Although an effect of applied strain and welding direction of specimen to SCC behavior was investigated, there was no significant difference in the morphology by them.

Since 1% strain was applied to CBB specimen surface and much greater strain was observed at a notched specimen, elasto-plasticity should be considered to stress condition of a SCC crack tip. An ‘equivalent stress intensity factor, KJ’ was, therefore, proposed to describe a stress condition in elasto-plastic area. A good relationship was observed between depth of SCC crack in LAS and KJ value. It was found that the proposed parameter KJ was effective to describe the stress state at the crack tip from the elastic region to the plastic region up to approximately 1% strain.

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