The microstructural constitution of stainless steels is quite complex, and exposure to high operating temperatures adds to the complexity as a variety of phases can occur. In addition to the matrix phases of ferrite, austenite and martensite, as well as duplex austenite-ferrite and (less commonly) ferrite-martensite, there are numerous possible minor constituents. In the carbide family, M23C6(face-centered cubic) and M 7C3(hexagonal) carbides are the most common, but M6C (face-centered cubic) and MC (face-centered cubic) carbides are also observed in certain alloys. Nitrides may be observed in the intermetallic phases: sigma (σ – tetragonal), chi (χ – body-centered cubic) and Laves (η – hexagonal). Much has been published regarding selective etching techniques to differentiate sigma from ferrite, or delta ferrite, and for the various carbides, but little has been published regarding chi and Laves. These two constituents are observed only in a few alloys and under specific conditions.
Although some of these constituents have non-cubic crystal structures, they do not respond well to polarized light, and that mode of observation is not useful for discrimination purposes. Nomarski DIC may be of some use to see which constituents are harder or softer than the matrix, but this is of limited help. The best techniques for identifying phases and constituents are diffraction – either X-ray diffraction of extracted residues or convergent-beam electron diffraction using extraction replicas or thin foils. Both methods are slow and require expensive equipment. WDS or EDS analysis can be very helpful once these methods are used on well-characterized specimens. Electron backscattered diffraction (EBSD) can also be used to identify phases. But, WDS, EDS and EBSD all require particles bigger than the beam diameter (>~2 µm) for analysis. Consequently, there has been a long history of interest in identifying these phases and constituents by light microscopic examination.
Specimens of type 316 austenitic stainless steel were welded with type 312 weld rod, designed to produce about 12-15% delta ferrite in the weld nugget. The plate was welded from each side using a total of four passes. A section was aged at 816°C (1500°F) for 160 hours (and slow cooled to ambient) to convert the delta ferrite to sigma and also to sensitize the specimen by precipitating carbide at the grain boundaries.
The metallographic techniques used and accompanying microphotographs will be revealed next week.