Correlation of measurements of retained austenite in carburized steels by X-Ray diffraction and quantitative metallography

Abstract

The retained austenite contents in carburized samples of EX24, EX32, and SAE 4820 were measured by both X-ray diffraction and automated quantitative metallography. The carburized samples were tempered 1 h at 200 °C prior to determination of the austenite contents. For the metallographic measurements, the specimens were etched in a nital solution which allowed clear distinction among the three principal microconstituents present: retained austenite, bulk martensite, and surface martensite which forms in retained austenite during specimen preparation. It was found that the usefulness of the metallographic procedure was limited to case carbon contents in excess of about 0.6 wt pct. At lower carbon contents, the morphology of the martensite was predominantly nonlenticular, and detection of the austenite in such a matrix was impossible by metallography. Carbon contents in excess of 0.6 wt pt resulted in a predominantly lenticular martensite matrix, so that retained austenite was readily detectable by metallography and good agreement between the X-ray and metallographic measurements was obtained. At high retained austenite contents, it was found that the X-ray measurements tended to detect less retained austenite than was detected by metallography. This was ascribed to the inability of the X-ray technique to differentiate the surface martensite from the bulk martensite. With quantitative metallography, the surface martensite can be distinguished as a separate microconstituent and properly counted as retained austenite.