Influence of delta-Ferrite Content on Thermal Aging Induced Mechanical Property Degradation in Cast Stainless Steels

Abstract

Thermal degradation of cast stainless steels was studied to provide an extensive knowledgebase for the assessment of structural integrity during extended operations of reactor coolant systems. The CF3 and CF8 series cast stainless steels with relatively low (5-12%) delta-ferrite contents were thermally aged at 290-400 degrees C for up to 10,000 h and tested to measure changes in tensile and impact properties. The aging treatments caused significant reduction of tensile ductility, but only slight softening or negligible strength change. The thermal aging also caused significant reduction of upper shelf energy and large shift of ductile-brittle transition temperature (DDBTT). The most influential factor in thermal degradation was ferrite content because of the major degradation mechanism occurring in the phase, while the nitrogen and carbon contents caused only weak effects. An integrated model is being developed to correlate the mechanical property changes with microstructural and compositional parameters.