Microstructural characterization and analysis of inclusions in C-Mn steel and weld metals
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I. INTRODUCTION
SIX of the Magnox nuclear power stations presently in service have reactor pressure vessels (RPVs) made of CMn steel plates and forgings joined together by either manual or machine-made weld metals of a similar basic composition.[1] The exposure of steel pressure vessels to a service environment of temperatures up to 360 8C and a range of neutron fluxes induces time-dependent changes to the microstructure and mechanical properties of these materials.[2,3] In order to monitor these changes, each reactor has a surveillance scheme in which specimens of the plates, welds, and forgings are irradiated in canisters placed at various locations within the pressure vessel since the date of its construction. Canisters from selected locations are then withdrawn at regular intervals, and the specimens are examined and tested for alterations in microstructure and mechanical properties. Shifts in the Charpy impact transition temperature and changes in the yield strength have been found to be influenced by a number of the service environment and material variables including fast neutron dose, irradiation dose rate, temperature, chemical composition, and microstructure. Trend curves illustrating mechanical property changes generated from the surveillance and accelerated irradiation test results indicate that, among the materials tested, submergedarc (SMA) weld metal exhibits the greatest sensitivity to neutron irradiation.[4] Studies focused on the neutron irradiation embrittlement of various types of RPV steels[3,5] have shown that the role of some alloying and trace impurity elements is particularly important, e.g., Cu, Ni, P, Sn, and N, but the degree of C. YOUNES, Research Fellow, P.J. HEARD, Research Associate, and R.K. WILD, Senior Research Fellow, are with the Interface Analysis Centre, Bristol University, Bristol, BS2 8BS United Kingdom. P.E.J. FLEWITT, Branch Manager, is with the Technology and Central Engineering Division, BNFL-Magnox Generation, Berkeley, Gloucestershire, GL13 9PB United Kingdom. Manuscript submitted March 16, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
embrittlement depends on the distribution and relative concentration of these elements in the as-manufactured material.[5,6] The RPV steels often contain traces of boron (,10 ppm), and the influence of this element may also be significant because it could contribute to any potential changes in material properties when exposed to a spectrum of high energy, fast, and low energy thermal neutrons.[7] However, the role of boron could be significantly different depending on whether it is present as free atomic species or incorporated into specific microstructural features such as inclusions or precipitates in the metal matrix. Earlier work of Abbott et al.[8] on unirradiated SMA weld metal indicates that boron is associated mainly with silicate inclusions as a result of partitioning to these particles during cooling from the molten phase. However, more thorough investigations are still required to further clarify this finding and to eluci
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