Phase Quantification of Regular and Turbo-stratified Graphite in Cast Iron by X-ray Diffractometry/Rietveld Refinement

A method to quantify regular graphite and turbo-stratified graphite (TS-graphite) forming the free carbon in ductile cast iron was developed using X-ray powder diffractometry. Iron in accurately sampled 2-4 g of the ductile cast iron powder was decomposed using hydrochloric acid without heating, and the residue was centrifugally separated. Graphite, TS-graphite, wustite (FeO), and amorphous silicate were identified in the residue. Calibration-standard mixtures were prepared by mixing corundum (alpha-Al2O3), griceite (LiF), and graphite powder which showed the most similar full-width at half-maximum value of (002) line to that of graphite in the ductile cast iron sample. The calibration curve of graphite showed good linearity, with a range of 0-200 mg corresponding to the 0-5.0 mass% of graphite in the 4.00 g of the cast iron sample. The quantitative value of total graphite in the ductile cast iron sample was 3.17 mass% with 8.0% of relative standard deviation for n = 5. The peak area ratio of graphite and TS-graphite was calculated using Rietveld refinement by assuming the same atomic scattering factor for each graphite measurement. The concentration ratio of graphite and TS-graphite were 67.8 and 32.2%, respectively. The analytical values of graphite were in good agreement with those of the NIST gray cast iron standard material, with concentration ratio of 53.7% and 46.3% for graphite and TS-graphite, respectively.