Microstructure etching and carbon analysis techniques

The purpose of this program was to investigate etching methods and carbon content analytical techniques for characterizing alloy 690 tubing material. Results from initial etching investigations were used to direct further study towards those techniques which exhibited the greatest potential for providing quick, accurate, and reliable methods for assessing potential SCC performance by microstructural characterization. Follow-up investigations of selected methods provided additional evidence of the effectiveness and reproducibility of the selected etches when used by a variety of investigators. The carbon content results were used to assess the need for improved analytical methods to enable meeting the tight carbon limits required by current specifications. The program identified the bromine methanol etch, followed by examination using a scanning electron microscope, as the most suitable alloy 690 etch. However, the need for a suitable etch using optical microscopy was also identified. A glyceregia etch showed an excellent ability to reveal carbide morphology. Promising grain boundary etches for use with optical microscopy were also identified, including a modified nital etch and a hot oxalic acid etch, but further optimization of one or more of these methods is necessary before they can be fully recommended for use with alloy 690. 2 tabs