FRACTOGRAPHIC OBSERVATIONS ON A HIGHLY IRRADIATED AISI 304 STEEL AFTER CONSTANT LOAD TESTS IN SIMULATED PWR WATER AND ARGON AND AFTER SUPPLEMENTARY TENSILE AND IMPACT TESTS A. Toivonen 1 , U. Ehrnstén 1 , W. Karlsen 1 , P. Aaltonen 1 , J-P. Massoud 2 , J-M Boursier 2 1 VTT Technical Research Centre of Finland, Industrial Systems, Kemistintie 3, P.O. Box 1704, FIN-02044 VTT, Finland 2 EDF Electricite de France, Site de Renardieres, Route de Sens-Ecuelles Moret Sur Loing 77818 France Keywords: Irradiated stainless steel, fractography Abstract Intergranular cracking has been observed in highly irradiated AISI 304 following constant load tests at 340 C in both simulated PWR water and, to a lesser extent, argon. Complementary investigations to clarify the mechanical behavior of the irradiated steel were performed. Those investigations consisted of tensile tests in air at room temperature and at 300 C, and impact testing in air at room and liquid nitrogen temperatures. The prevailing fracture mode for the irradiated material was ductile-dimple at 300 °C (using a strain rate of 1·10 -3 s -1 ) and intergranular at room temperature and below. The results of the investigations show that the irradiated stainless steel studied is susceptible to intergranular fracture without environmental assistance. The results are discussed based on the possible influences of strain rate, temperature and environment on the fracture mode in highly irradiated stainless steels. Introduction The properties of austenitic stainless steels are known to degrade in light-water reactor environments as a consequence of exposure to fast neutrons. Neutron-induced microstructural changes, e.g., the formation of dislocation loops and radiation induced segregation (RIS) of elements at the grain boundaries, are associated with an increase in the material’s bulk hardness and strength and decrease in its ductility, as well as reduced corrosion resistance. At pressurized water reactor (PWR) operating temperatures, fracture surfaces of tensile specimens of irradiated stainless steels ruptured in air or in argon generally display dimpled ductile (transgranular) features. However, intergranular fractures in an inert environment have also been reported by several researchers for irradiated austenitic stainless steels tested by slow strain rate methods [1, 2]. Intergranular fracture is also known to take place under impact loading conditions for specimens charged with hydrogen [3]. In the tests performed in argon or air, the intergranular fracture has typically been located along the periphery of the specimen, around an otherwise ductile-dimple type main fracture surface. This paper describes the results of an investigation into the different conditions that can cause intergranular cracking in a highly irradiated austenitic stainless steel of the type AISI 304. Experimental Methods The test material was a solution annealed type AISI 304 stainless steel removed from the core internals of a decommissioned French commercial PWR. It had been irradiated during use to a fluence of about 30 dpa, at a temperature of 300 °C. The chemical composition of the steel is given in Table I, and the tensile properties measured earlier for the bulk irradiated materials are shown in Table II. For the current study, three different kinds of specimens were prepared and mechanically tested as outlined in Table III, generating the fracture surfaces that form the basis of the current investigation. The particular methods for each case are further described next. Table I. Chemical composition of the stainless steel (wt-%) C S P Si Mn 0.060 0.003 0.011 0.78 0.96 Ni Cr Cu Co 9.30 18.60 0.09 0.034 Table II. Tensile properties after irradiation Test Temperature (°C) YS (MPa) UTS (MPa) Total Elongation (%) Uniform Elongation (%) 20 1075 1077 17 0.4 300 892 896 > 2 0.4 340 891 896 > 3 0.3 Proceedings of the 12th International Conference on Environmental Degradation of Materials in Nuclear Power System – Water Reactors – Edited by T.R. Allen, P.J. King, and L. Nelson TMS (The Minerals, Metals & Materials Society), 2005 327