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Characterization of copper precipitates in Fe-Cu alloys using micromagnetic NDE techniques

: Pirlog, M.; Altpeter, I.; Dobmann, G.; Breme, J.

Dobmann, G. ; Japan Society of Applied Electromagnetics and Mechanics -JSAEM-:
Electromagnetic Nondestructive Evaluation VII : 12.-14. June 2002, Saarbrücken
Amsterdam: IOS Press, 2006 (Studies in Applied Electromagnetics and Mechanics 26)
ISBN: 1-58603-594-0
International Workshop on Electromagnetic Nondestructive Evaluation <8, 2002, Saarbrücken>
Conference Paper
Fraunhofer IZFP ()
magnetic Barkhausen noise; copper precipitation; hardness

The copper containing ferritic steels used in pressurized water reactors (PWR) and boiling water reactors (BWR) are know to be prone to hardening and embrittlement by thermal ageing. Preliminary investigations concluded that the changes in mechanical properties are caused by copper-precipitation. Small (2-3 mm diameter) and coherent copper precipitates cause an increase of hardness and brittleness.
For a better understanding of these phenomena, 10 Fe-Cu model alloys with Cu content between 0.65 and 2.1 wt.% were manufactured and investigated. The samples were prepared from pure (99.8) Fe-rods and pure (99.9) Cu-chips. Fe und proportional Cu were molten together in an electric arc furnace under protective gas (Ar, 700 mbar) in rods casting mould. After melting, these rods were annealed at 1000 °C for 4 hours, quenched in water and cold-rolled ( = 20%). In order to get Fe-Cu-alloys containing small and coherent copper precipitates, the specimens were solution treated at 1000°C for 2 hours, quenched in water and thermally aged at 550°C.
These 9 samples were characterized by Vickers hardness (HV10) and electromagnetic methods, and their dependence on the Cu content was investigated. It was observed that the Vickers hardness increases with rising Cu content. Hysteresis measurements showed that the values of coercivity increase with growing Cu content. The coercivity and the Vickers hardness exhibit a similar behaviour with respect to Cu content.
Eddy current measurements show that secondary induced voltage is dependent on the Cu content. Using a measurement system for Barkhausen noise and field upper harmonics analysis, the hardness was determined non-destructively. The Vickers hardness and the non-destructively determined hardness correlate well and increase with rising Cu content. Barkhausen noise measurements show that the mean Barkhausen noise amplitude and the width of the Barkhausen noise profile curve at the 25% and 50% of the maximum amplitude decrease with Cu content.