Microstructural Investigation of the 15H2MFA Steel Used in Nuclear Pressure Vessels

Anna Csordás -Pintér¹, Márta Horváth¹, László Gillemot² and Gábor Uri¹

¹KFKI Atomic Energy Research Institute, Budapest POB 49, Hungary

²ABB Power Generation Ltd., Budapest POB 156, Hungary

In order to determine and manage the lifetime of structural materials in nuclear reactors, it is very important to study the ageing phenomena of nuclear pressure vessel steels. Ageing phenomena are very complex, because thermal and radiation induced processes are taking place in the same time and there are interactions between them.

Embrittlement of the reactor vessel steels are influenced by several factors including some non metallic impurities especially the sulphur and phosphorus.

The aim of our investigation was to detect, follow and compare inclusions of the as received and aged 15H2MFA steels. For comparison mostly microstructural methods were selected, however some mechanical properties were also measured.

Raw material of 15H2MFA and weldments were manufactured in Izhora Plant in Russia, meeting the demands of the Russian type of reactor vessel (WWER-440 RPV) steel. Thermal ageing was done at 350°C for 2500 hours. Microstructure was studied by an optical microscope supplied with a PC. The novel of our SEM and EDS studies was the application of the Charpy test probes for revealing some inclusions containing non metallic impurities. Fractured sampIes were taken from Charpy test probes produced above and under DBTT.

Both in the as received and in the aged steel samples mostly globular inclusions were found in the microvoids of the ductile fractures. As to the elemental composition of these inclusions Mn- sulphide, further S, Cl, small amounts of K, Ca and sometimes P were detected. In brittle fractures most of the inclusions have flaky shape, however there are some globular and irregularly shaped inclusions, too. In the elemental composition of these inclusions the same impurity elements were detected as in ductile fractures. Non metallic impurity elements (S, Cl, Si and small amount of P) were found at the transition between ductile and brittle fractures, too.

Both in the as received and in the aged weldments the most frequently found inclusion has globular shape and in the elemental composition Mn, Si and Ti has been found. It seems that the number of the inclusions was slightly decreased by the effect of thermal ageing.

It was found that the tensile strength was slightly increased after ageing at 350°C for
2500 hours, while the elongation was decreased.

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