Influence of helium on impact properties of reduced-activation ferritic martensitic Cr-steels

Instrumented Charpy impact tests of the reduced activation type 8Cr2WVTa steel F82H have been performed after homogeneous implantation of 300 appm helium at 250 degrees C. The results are compared with investigations oil mixed spectrum neutron irradiated (HFR Petten) specimens. After neutron irradiation at 250 degrees C to the same low damage dose of 0.2 dpa, the ductile-brittle transition temperature shift (Delta DBTT) amounts to 18 degrees C, whereas a much higher Delta DBTT of 42 degrees C has been measured after helium implantation. These results are compared with other neutron irradiated ferritic/ martensitic steels having different boron levels and thus different helium contents. A model is proposed which describes the dynamic brittle fracture of martensitic/ferritic steels by a stress-induced propagation of micro-cracks, taking into account radiation induced hardening as well as helium bubble formation. (C) 1999 Elsevier Science B.V. All rights reserved.