Effect of applied stress on the ductility of RAFM steel during long-term exposure in flowing Pb-17Li

Abstract

The effect of applied stress on the ductility of reduced activation ferritic martensitic (RAFM) steel during long-term exposure in flowing Pb-17Li in the temperature interval 300–400 ℃ was investigated using tensile tests. The ductility of RAFM steels at a stress of 250 MPa decreases appreciably after exposure in following Pb-17Li at 400 ℃ for 1500 h. And a slight ductility loss occurred for the samples tested at 300 ℃, 350 ℃ and 400 ℃ for 500 h. Coarsening of martensitic laths occurred in RAFM steels owing to the thermal aging with a 250 MPa stress for 1500 h in liquid Pb-17Li. Due to the poor ductility of coarse laths, micro-cracks originated on the surface of necking zone during tensile test, which facilitates the penetration of liquid metal. With sufficient liquid Pb-17Li penetrated into cracks continuously, the surface tension of the newly formed fracture surface is declined by liquid metal in the crack tip. The fracture mode was altered from uniform to unstable crack propagation thus leading to a reduction of ductility. There was no depletion of Fe and Cr or Pb infiltration into the substrate for RAFM steels after exposure. The fracture mechanisms were proposed in terms of supply of liquid metal and micro-cracks caused by coarse laths. Accordingly, the reduction of ductility for RAFM steels after exposure in liquid Pb-17Li was the result of combined effect of applied stress and liquid metal.