Abstract

In recent times, iron-base materials have been developed to replace Stellite hardfacings often used in valves for nuclear power plants. This evolution allows to reduce the radiation exposure to plant maintenance personnel since the activation of cobalt coming from Stellite material that goes into the fluid circuit is drastically reduced. However, NOREM02 which is one of the iron-base materials implemented in the newer valves presents a higher cracking susceptibility than Stellite deposits. This paper describes the work conducted on actual nuclear power plant valves to characterise the main features of microcavity clusters leading to cracking in NOREM02 hardfacing deposited by Plasma Transferred Arc (PTA) welding. SEM nano tomography and other techniques such as neutron diffraction were used to find the cracking origin mechanism of these valves.

Specmens from a nuclear power plant valve were mounted for serial sectioning and SEM imaging and EDX data collection to analyse interconnected cavities and crack propagation through the dendritic microstructure. These characterisations allowed a more precise quantification of the cavities density, as well as evidence of the cavity-interconnections leading to crack initiation which was assisted by welding residual stresses.