Magnetic imaging of chromium depletion in thermally sensitized ni-cr-fe alloys

Magnetic imaging of chromium depletion in thermally sensitized Ni-Cr-Fe alloys, which is one of the origins for the stress corrosion cracking, was achieved. Magnetic force microscopy (MFM) observation was performed on Alloy600 as a function of heating time at 873K. After thermal treatment, precipitation of chromium carbides was confirmed at the grain boundary by SEM image and ferromagnetic contrast was observed along the grain boundary in MFM images. Magnetic properties of ternary model alloys suggested that the chromium depletion occurred in the vicinity of carbides leads to raise the Curie temperature of Alloy600 near the grain boundary. This study shows a possibility for the application of magnetic techniques to the evaluation of thermal sensitization of Ni-Cr-Fe alloys. Introduction: Alloy600, austenitic Ni-Cr-Fe alloy, has a high corrosion resistance and has been used in steam generators in nuclear power plants. However, thermal treatment including welding during construction causes chromium carbide precipitations and chromium depletion at the grain boundary [1-3]. This phenomenon is called thermal sensitization, and it is a serious problem because it leads to lower the corrosion resistance. Alloy600 is normally non-ferromagnetic at room temperature. However, a change of chemical composition during the sensitization may cause a ferromagnetic transition of Alloy600. It means that we can non-destructively evaluate the sensitization by magnetic method. This paper presents the magnetic properties of thermally sensitized Alloy600 and shows a mechanism of the magnetic imaging of chromium depletion. Experiments: Two types of specimens were prepared as presented in Table 1: Alloy600, and NiCr-Fe ternary model alloy. Specimens of Alloy600 were heated at 823K from 1 hour to 100 hours following 1 hour solution anneal at 1273K. After the thermal treatment, all specimens were electrically polished. Surface microstructures were observed by using field emission SEM system equipped with EDS apparatus (JEOL, JSM-6500F), and surface magnetic images were obtained by using MFM (SII, SPA400) at room temperature. MFM is a powerful technique for magnetic imaging with high spatial resolution because it uses a magnetic interaction between a sharp ferromagnetic tip and a ferromagnetic specimen. As for ternary alloy, specimens with different Cr concentrations below 16wt.% were prepared for simulating the chromium depletion of Alloy 600. Magnetic properties of ternary alloys were investigated by using SQUID magnetometer (Quantum Design, MPMS-XL5) from 4.5K to room temperature, and the Curie temperature was determined. Combined with the results of ternary model alloys, the effects of thermal sensitization on the surface magnetic images of Alloy600 were discussed. Table 1 Chemical composition of the specimens Wt.% Ni Cr Fe Mn Si C Alloy600 76.0 15.5 7.8 0.5 0.2 0.08 NiCrFe alloy (X = 9-12,16) 92-X X 8 Results: Fig.1 shows a typical surface SEM image of Alloy600 after 1 hour heating and its chemical composition profiles across the grain boundary obtained by FE-SEM-EDS system. Fig. 1(a) shows an augmentation of concentration in both carbon and chromium composition, which implies the chromium carbide formation. Contrary to the chemical analysis previously obtained by destructive TEM-EDS experiments [2-3], the chromium depletion could not be confirmed by the present FE-SEM-EDS system as shown in Fig.1 (b). Although the field emission SEM has an excellent beam focusing, the dispersion of an electron beam inside the specimen lower the spatial resolution of chemical analysis. The chromium depletion region of several tens nm width might be too narrow to be detected by the FE-SEM-EDS system.