# Effect of Corrosion Characteristics on Long-Term Aging of Austenitic 304 Steel
## 3 Results and Discussion
### Microstructure Analysis
Figure 6 shows the result of the analysis of \(\delta\)-ferrite through EPMA. Figure 6a is a photograph showing the EPMA analysis result of the austenitized sample. The iron element content is higher in the austenitic matrix than in the \(\delta\)-ferrite. However, the chromium element content is higher in \(\delta\)-ferrite than in austenitic matrix due to the difference of Cr element content at each phase. Figure 6b shows the EPMA analysis result of the 100-h aging specimen. As previously explained, the \(\delta\)-ferrite may be decomposed by heat treatment. In this stage, the \(\delta\)-ferrite already started the transformation into \(\sigma\)-phase, M\({}_{23}\)C\({}_{6}\), and new \(\gamma\)-austenite. The chromium element content increased compared with \(\delta\)-ferrite and the iron element content increased within the center of \(\delta\)-ferrite interior due to the generation of new \(\gamma\)-austenite. This transformation is more progressive after 10,000 aging as shown in Figure 6c. During the sensitization of austenitic AISI 304 steel, the intermetallic M\({}_{23}\)C\({}_{6}\) carbides precipitated along grain boundaries and twin boundaries. Therefore, the Cr element depletion happens in the vicinity near these boundaries. In sensitized austenite steel, the grain boundaries and interfaces are thus more susceptible to martensitic transformation, sensitization induced martensite transformation. Figure 3: SEM micrographs of grain boundary precipitation; (a) 0 h, (b) 100 h, (c) 1000 h, (d) 5000 h, (e) 10,000 h.  
To analyze the sensitization induced martensite with long-term aging, X-ray diffraction, ferrite scope, and magnetic force microscopy (MFM) were analyzed. Figure 4: SEM micrographs of delta ferrite; (**a**) 0 h, (**b**) 100 h, (**c**) 1000 h, (**d**) 5000 h, (**e**) 10,000 h.  
Figure 5: Schematic illustration of the transformation mechanism for delta-ferrite to sigma plus austenite reaction [13]. Figure 7a shows the results of X-ray diffraction analysis. As already observed by the optical microscope and the secondary electron microscope, X-ray diffraction analysis showed that the peak of the X-ray diffraction is due to the presence of a strong optical microscope. The X-ray diffraction analysis shows that the peak of the X-ray diffraction is due to the presence of a strong optical microscope. Figure 6: The EPMA elemental mapping of AISI 304 at each long-term aging time; (**a**) 0 h, (**b**) 100 h, (**c**) 10,000 h.  
\(\delta\)-\(\alpha\)' increased as the heat treatment time increased due to the effects of \(\delta\)-ferrite and \(\alpha\) '-martensite transformation. Figure 7b shows the result of ferrite scope measurement. From these results, the ferrite fraction seemed to increase with increasing heat treatment time. However, as already observed through optical microscopy and scanning electron microscopy, the \(\delta\)-ferrite is decomposed into \(\sigma\)-phase, M\({}_{23}\)C\({}_{6}\), and new \(\gamma\)-austenite with increasing heat treatment time. Since only \(\delta\)-ferrite is usually a magnetic phase of the austenitized sample, it can be analyzed by magnetic force microscopy to evaluate magnetic properties. [PERSON] et al. observed the formation of martensite around grain boundaries through magnetic force microscopy [14]. Figure 8 shows the MFM micrographs of test specimens of 0, 5000, and 10,000 h. In this experiment, it is hard to observe the magnetic domain at 0 h, but the magnetic domain was developed at 1000 h. This magnetic domain can be observed near the grain boundary after 5000 h and 10,000 h heat treatment due to the generation of sensitization induced martensite transformation. In this study, the peak increases in the X-ray diffraction analysis and the increase of Ferrites cope reading mean the sensitization induced martensite had indeed been induced. Figure 2.0 Variation in delta-ferrite with long-term aging of AISI 304; (**a**) XRD patterns and (**b**) ferrite fraction.