Prediction of mechanical properties of Austempered Flake Graphite Iron and compacted graphite iron using response...

The austempering process is a unique heat treatment that enables significant modification of the microstructure and mechanical properties of Flake Graphite Iron (FG-30) and Compacted Graphite Iron (CG-300). In this study, FGI and CGI specimens were austenitized at 900 °C and isothermally austempered at three temperatures 270 °C, 320 °C, and 370 °C for holding times of 30, 75, and 120 min, generating Austempered Flake Graphite Iron (AFGI) and Austempered Compacted Graphite Iron (ACGI). A Central Composite Design (CCD)-based Response Surface Methodology (RSM) was employed with austempering temperature and holding time as input factors to construct second-order predictive models for hardness, tensile strength, and elongation. ANOVA-validated RSM models achieved R² values of 0.9767, 0.9980, and 0.8760 for AFGI hardness, tensile strength, and elongation respectively, and 0.9498, 0.9998, and 0.9240 for the corresponding ACGI responses. From the experimental dataset, AFGI austempered at 270 °C maintained the maximum hardness of 415 BHN and tensile strength of 480-491 MPa (elongation 0.2%) at all time periods; the lowest AFGI values occurred at 370 °C/30 min with 229 BHN and 415 MPa. For ACGI, the best mechanical combination was achieved at 270 °C/120 min: 415 BHN, 668 MPa tensile strength, and 3.2% elongation; the highest elongation of 3.4% was recorded at 370 °C/120 min. Across all conditions, ACGI demonstrated a mean tensile strength of 640.62 MPa - 193 MPa higher than the AFGI mean of 447.23 MPa with elongation 10-15 times greater. The six RSM regression equations (Eqs. 2-7) provide reliable, computationally efficient tools for process optimization of austempered cast irons within the studied parameter space. RSM 3D response surface plots (Figs. 15, 16, 17, 18, 19, 20, 21 and 22) visually characterize the hardness, tensile strength, and elongation response landscapes for both AFGI and ACGI over the full temperature-time design space. Numerical optimization using Design-Expert 13 identified optimal austempering conditions of T = 365.5 °C, t = 34.8 min for AFGI (desirability = 1.000) and T = 290.7 °C, t = 47.3 min for ACGI (desirability = 1.000), yielding predicted properties of 235.3 BHN/416.7 MPa/0.220% elongation and 411.9 BHN/650.6 MPa/3.270% elongation, respectively. Application-specific optima are clarified and validated in Sect. 5.8: when hardness and tensile strength are the optimisation goals, maximum AFGI properties occur at 270 °C (426 BHN, 493 MPa predicted; 415 BHN, 491 MPa measured), whereas the 365.5 °C solution maximises only the marginal AFGI elongation. Model predictions at the optima agree with measured data to within ± 3%.