JANDOVÁ Dagmar Research and Testing Institute Plzen

Correlation of Microstructure Development of Cast CB2 Steel after Long-term Creep Tests with Electrochemical Characteristics

Co-authors STREJCIUS Josef, KASL Josef

Specimens for long-term creep tests were taken from two different positions in the pilot valve made of cast CB2 steel, which was developed within the COST programmes. Creep testing was carried out at 650°C and a stress range from 50 MPa to 120 MPa. The time to rupture is up to 70.000 kours, however some tests are still running. Ruptured samples underwent fractographic and microstructural analyses and electrochemical measurements. Quantitative evaluation of substructure was performed using SEM and TEM of extraction carbon replicas and thin foils. Microstructure of the virgin material and crept specimens were compared and correlated with electrochemical characteristics measured on the base of the potentiokinetic polarization curves with aim to test a possibility of access of material degradation using electrochemical measurement. Precipitation processes taking place during creep exposures resulted in changes of polarization curves. Secondary peak of the current density was detected at a specific potential applied on specimens after creep tests at 650°C. This peak increased with increasing time to rupture and is related to precipitation and significant growth of molybdenum rich Laves phase particles, which were demonstrated by means of detailed microstructural study.