Transformation characteristics,microstructure and mechanical properties of austempered ductile iron welds

Abstract

The isothermal transformation of austempered ductile iron (ADI) weld at 370°C for a time period ranging from 0·5 to 1440 min involves three stages. The welding process has the effect of accelerating the subsequent transformation during austempering to bainitic ferrite and high carbon austenite (stage 1) and delaying the transformation to carbide (Fe₃C) and ferrite (stage 3). Within the austempering time range 15–240 min, the ADI welds consisting of bainitic ferrite and retained austenite had superior properties (namely tensile strengths between 1030 and 1060 MPa and elongations between 8·0 and 8·5%). As the austempering temperature increased from 310 to 400°C, the tensile strength of ADI weld decreased from 1141 to 1040 MPa and its elongation increased from 4·0 to 9·1%. Under tensile loading a large number of slip bands appeared in the retained austenite and the austenite/ferrite interfaces behaved as obstacles limiting the movement of dislocations. The superior properties of ADI weld are mainly associated with the presence of large amount of retained austenite (30–32%), grain boundary strengthening effect and solid solution strengthening effect.