TEMPERED MICROSTRUCTURE AND MECHANICAL PROPERTIES OF AUSTEMPERED LOW ALLOYED BAINITIC DUCTILE IRON

Austempered bainitic ductile iron has been widely used in machinery components and parts due to its low fabrication cost, excellent mechanical properties, and abrasive wear resistance. In order to get a fine bainitic matrix, austempering process is usually adopted which consists of austenitizing temperature, austempering temperature and time. For quenched ductile cast iron, tempering plays an important role in subsequent heat treatment process. However, less attention has been paid on the microstructural evolution and mechanical properties of the austempered bainitic ductile iron after tempering treatment. Thus, in this work, 3.55C-1.95Si-0.36Mn-3.58Ni-0.708Cu-0.92Mo-0.65Cr (mass fraction, %) bainitic ductile iron was subjected to austempering and subsequent tempering treatment, and the effect of tempering on microstructures and properties has been investigated by using OM, EP-MA, SEM, TEM and XRD. The microstructural evolution during tempering has been investigated, and mechanical properties and wear resistance have also been measured and analyzed. The results show that microstructural evolution of the bainitic ductile iron during tempering contains recovery and recrystallization softening processes of twin martensite and dislocation substructure, decomposition of retained austenite, dissolution of supersaturated carbon and phase transformation in martensite and transformation in eutectic cementite. With increasing tempering temperature, there is a gradual decrease in micro- and macro-hardness of substrate microstructure and compressive strength of austempered low alloyed bainitic ductile iron. When the bainitic ductile iron was tempered at 450 degrees C, the eutectic cementite has the lowest micro-hardness value due to the precipitation of alpha phase in its slice layer and the compressive ratio is thus higher. The mechanical properties of the austempered low alloyed bainitic ductile iron was even worse when tempered at 600 degrees C. Under the wear condition of dry sand/rubber wheel, the austempered low alloyed bainitic ductile iron possesses the best wear resistance when tempered at 450 degrees C. The worn morphology observation by SEM indicates that the worn surfaces were caused by plastic deformation and micro-cutting. The plastic deformation plays an important role in wear process, while the precipitated and finely distributed Mo2C contributes a lot to the improvement of wear resistance when tempered at 450 degrees C .