Changes in electromagnetic properties during thermal aging of duplex stainless steel

To maintain the integrity of applications of duplex stainless steels currently in service, a study using two types of electromagnetic testing, i.e. electric resistivity testing and coercivity measurement testing, was conducted to develop a method to nondestructively estimate their Charpy-impact energy at room temperature. Materials tested included four grades of duplex stainless steel with differing ferrite content that had been subjected to long-term heating. Changes in Charpy-impact energy and electromagnetic properties due to aging were observed. It was found that electric resistivity and coercivity are reliable indicators of the process of embrittlement, but their relationship to impact energy is dependent on the material. Further, the dependency on the material differed between the two testing methods employed, though the main cause of embrittlement for duplex stainless steels can be assumed to be ferrite phase separation. Coercivity testing seems to show the property change in the ferrite phase more sensitively than it shows the difference in material, whereas electric resistivity testing distinguishes the difference in material more clearly than the property change in the ferrite phase. Change in Charpy-impact energy due to embrittlement is mainly caused by ferrite phase separation, but Charpy-impact energy is also influenced by the austenite phase. As the conclusion of this study, an equation involving electric resistivity, coercivity and ferrite content was presented as a method applicable to the nondestructive estimation of Charpy-impact energy at room temperature.