MS-4-P-3014 The microstructure characterization of unalloyed austempered ductile iron

The Austempered Ductile Iron (ADI) is advanced material used increasingly for many tough engineering components in automotive, trucks, construction, agricultural and railway industry. The ADI is produced by austempering of ductile iron, where a unique microstructure - ausferrite (mixture of carbon enriched, stable, retained austenite and ausferritic ferrite) is obtained. By varying austempering parameters the different morphologies of ausferrite and amounts of retained austenite could be achieved. For that reason, characterization of microstructure and its influence on mechanical properties is of great importance.
In this paper, the ADI materials were produced by austenitisation at 900°C/2h and austempering at 300°C/1h, 400°C/1h and 400°C/3h. The microstructure was examined by “Leitz-Orthoplan” light microscope, while fracture mode by SEM JEOL JSM 6460LV, at 20kV. To identify microstructures a standard etching by 3% nital and heat tinting etching (heating in air, at 260°C/5h) was carried out.
The microstructure of ductile iron was mostly ferritic, with spheroidisation >90%, graphite amount 10.9%, nodule size 25÷30 μm and nodule count 150÷200 per mm²; while the mechanical properties were: R_m=473 MPa, R_p0.2=326 MPa, A=22.2%, KO=119 J. The ADI austempered at 300°C/1h posses high strength and low ductility (R_m=1513 MPa, R_p0.2=1395 MPa, A=3.8%, KO=68 J), while ADI austempered at 400°C/1h had low strength and high ductility (R_m=1042 MPa, R_p0.2=757 MPa, A=14.2%, KO=140 J). At 400°C/3h, strength of ADI remained at same level, but ductility decreases (R_m=1060 MPa, R_p0.2=780 MPa, A=9.8%, KO=95 J).
The difference in mechanical properties is due to different microstructures, Fig. 1-2. After 1 hour austempering, microstructure is fully ausferritic. When temperature increases from 300 to 400°C the ausferritic morphology is changing, from needle-like (Fig. 1a) to more plate-like (Fig. 1b), while amount of retained austenite increases from 16 to 31.4%. The 3h austempering time results in decrease of retained austenite to 24.1%, due to decomposition to bainite (mixture of ferrite and carbides), Fig. 1c, 2c. The occurrence of carbides (white in Fig. 2c) makes the material brittle and thus, it should be avoided. The effect of microstructure on fracture mode is presented in Fig. 3, where the fracture is changing from mix mode to fully ductile with increases of retained austenite amount, Fig 3a and 3b. On the contrary, the carbides presence, formed during prolonged austempering time, has opposite effect on fracture mode, i.e. fracture becomes fully brittle produced by quasi-cleavage mechanism, Fig. 3c.
Finally, based on presented results it could be summarized that the optimum mechanical properties of an ADI can be achieved upon achieving appropriate microstructure.