Effect of Process Parameters on the Microstructures and Mechanical Properties of Iso-Thermally Treated Ductile Iron

Abstract

The effect of process parameters on the mechanical properties of iso-thermally treated ductile irons was investigated in this study. Sets of ductile irons produced by sandwich method from a rotary furnace melt were given iso-thermal austempering treatments using commercial neutral salt baths. The samples were preheated at 350°C and held for 1 h in a muffle furnace, followed by ausenitizing in an ausenitizing salt bath furnace containing a mixture of BaCl2 and NaCI, (in the ratio 3:2) at 900°C and soaking at that temperature for 1 h. The samples were immediately transferred to the austempering salt bath furnace containing a mixture of NaNO3 and KNO2 in the ratio 1:1. Sets of the samples were soaked at 300°C for between 5 to 240 min and were all later cooled in air. The austempering procedure was repeated at 350 and 400°C for a second and third groups of samples, respectively. The results showed that austempering at lower temperature (300°C) produced a relatively high tensile strength of 1400 MPa, after 150 min holding compared with austempering at the higher temperature (400°C) which produced a relatively lower strength of 1200 MPa at the same holding time. However, the optimal processing window for the austempering was established to be the intermediate temperature of 350°C after a holding time of 150 min. At this processing window, austempering yielded the optimum combination of mechanical properties of 1502 MPa UTS, 7.5 % elongation, and impact energy of 108 J. These properties correspond to a microstructure consisting of a plate-like morphology of ausferrite and retained austenite.