Connection between cyclic thermal shocks and reliable operation of boiler steels

In the pressure vessels of nuclear power plants macros. copic cracks can occure owing to the combined effect of creep and fatigue. According to the experiments the specific fracture energy in specimens after low-cycle thermal fatigue may decrease to a crytical value which is dangerous to working on. The scanning and transmission electron microscopy of the material revealed that during fatigue loading cracks initiate along the ferrite-pearlite grain boundaries. The crack initiation is preceded by micro voids and wedge-type microcracks on the grain boundary. Also plastic deformation and an increased dislocation density can be met with in the grains. A stress peak around the carbide particles hinders the dislocation in movement, thus promotes the woid formation. During heat cycles the smaller carbide particles dissolve, the size of greater carbide particles increases. It seems to be important to check permanent by the damage in the nuclear pressure vessels by the acoustic emission method. (K.A.)