II - CSC de l’Alliage 600 dans le milieu primaire des réacteurs à eau préssurisée

Stress corrosion cracking of alloy 600 in primary water

Abstract

Some mechanical features concerning the mechanism of stress corrosion cracking of Alloy 600 in primary water have been studied focusing attention on three points: the plastic deformation behaviour of Alloy 600 in air, the effect of primary water exposure (in terms of intergranular embrittlement) and the interaction between mechanical behaviour and environmental effects.

Modelling of the plastic deformation behaviour of Alloy 600 requires mechanical test data without any environmental effect. Fatigue and fatigue relaxation tests have been conducted in air at room temperature and in the domain 300-400°C. The results are being analysed with an elastoviscoplastic law. The microstructures induced by cyclic straining of the material are being characterised by transmission electron microscopy.

The effect of a primary water exposure on the surface of Alloy 600 has been studied. Tensile samples and thin foils were subjected to a primary water exposure without any mechanical loading. Surface examinations performed after exposure tests showed that numerous grain boundaries could easily be fractured and open to a depth of several microns by pulling the specimens in tension at room temperature after the primary water exposure. This indicates an embrittling effect of the environment on the grain boundaries near the specimen surface.

The interactions between mechanical behaviour and environment will also be studied by crack propagation tests in primary water. Preliminary tests have shown that it is possible to produce intergranular cracking in Alloy 600 at 650°C in air.