Mossbauer Spectrometry Study of Aging Deterioration in Duplex Stainless Steel

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Cast duplex stainless steels which are used as the pump casings and valve casings in the cooling system of atomic power plants are degraded in the mechanical properties after avery long term use. The life time of the steel may be evaluated by the activation energy obtained by the accelerated test performed at temperatures higher than the operation temperature. The present study aims to investigate the origin of the degradation and to measure the activation energy for the commercial CF3M cast duplex stainless steel aged at 623-723K.The average intemal magnetic filed of Fe, <H>, in the ferrite measured by Mossbauer spectroscopy increases with agin time. The change in <H> corresponds to a decrease in the mean Cr content that in dicates the phase decompositon of ferrite. The activation energy for the <H> change is obtained to be about 180kJ/mol.The ferrite hardness increases with aging time corresponding with the <H> change. The activation energy obtaine from the hardness change are 160-17 … More 0kJ/mol which is consistent with one mentioned above. These results suggest that the hardness change is ascribed to the phase decomposition which is controlled by the volume diffusion of Cr or Fe atom in ferrite.V-notch Charpy impact energy, CE,measured at 273K decreases with aging time. The decrease is mainly ascribed to the phase decomposition of ferrite. However the activation energy of 63kJ/mol obtained for the CE Change is unconsistent with those mentioned above. The CE change is assumed to be expressed by a tanh function : CE (t, T) =A-B {1+tanh [(logt-C- (Q loge/RT)) /D]}, in which unknown parameters are detemined experimentally by a least square fit. Charpy energy curves at the operation temperature of PWR and BWR are calculated from the above result.if a 30J value is assumed to guarantee areasonable ductility, 50years of life time will be expected for the PWR and BWR application.There is a good corelation between the average intemal magnetic field change and the Charpy impact decrease. Mossbauer spectroscopy may be useful for anondestructive evaluation of the aging embrittlement. Less