Microstructures And Mechanical Properties Of A Ductile Iron Treated By A New Austempering And Partitioning Process

Austempered ductile iron(ADI),as a kind of high-strength ductile iron(DI),has been widely applied in the field of vehicles,mining and energy and played an increasingly important role in industrial structural materials.Besides,its excellent mechanical properties give it an edge to break the monopoly of cast steel,forged steel,welding,powder metallurgy and forged aluminum.Based on a quenching and partitioning process and the transformation of a nano-scaled bainite at low temperature,a new austempering and partitioning process used for making commercial unalloyed nodular cast irons with enhanced properties has been developed in this paper.The effects of the process on the microstructural characteristic and the mechanical properties of DIs were carried out in the tensile,fatigue and fatigue tests.The micrograph and the fractured surface morphologies have been systematically investigated by means of optical microscopy(OM),scanning electron microscopy(SEM),transmission electron microscopy(TEM).By controlling the quenching speed at 160?/s,the new austempering and partitioning process took 220? close to the Ms point as the partition reference temperature.The microstructure of the matrix was composed of prior martensite(PM),bainitic ferrite(BF),retained austenite(RA)and quenched martensite(QM,existed at certain partition time)when partitioned at 190?,220? and 250? for different time.The microstructure was composed of PM,BF,RA and QM in a relatively short partition time and of PM,BF,RA in a relatively long partition time.The amount of RA decreased with the increase of partition time,with the shape transforming from block to thin film and dispersing evenly.While the amount of BF increased gradually until stable,the size changed from thick to thin and then from thin to thick,and the speed increased with the temperature raising from 190? to 250?.Compared with the normalized samples,the hardness value of the treated samples increased by 44%-68%and fluctuated greatly,the value of which reached its highest and lowest at 190? and 250? respectively.Generally speaking,the hardness first increased,then decreased,and finally tended to be stable according to the partition time.However,martensite was formed when the partition temperature was lower than 190? of Ms point,thus resulting in extremely high hardness.The tensile strength and yield strength were the highest at 220?and the lowest at 190?,presenting a general trend which first increased and then decreased.The speed of reaching the peak accelerated with the increase of temperature,which corresponded to the transformation speed of microstructure.The elongation and elongation product were the highest at 220? and the lowest at 190?.After partitioned at the above temperature for different time,the tensile strength,yield strength,elongation and strong extension product of unalloyed ADI samples were the highest,achieving 1615.1 MPa,1408.1 MPa,5.03%and 13.12×106 MPa2·%,respectively.The strong prolongation product first increased and then decreased except partitioning at 250?.The speed of reaching the peak accelerated with the increase of temperature when strong prolongation product was partitioned at the same temperature.The fatigue life of ADI was investigated by a pull-stop fatigue test with the maximum stress amplitude of 600 MPa which was partitioned at 220?for 240 min.The result showed that fatigue life above 25000 times,which was equivalent to that of high strength carburized alloy steel.When(PM+(BF+RA(+C))nano)multiphase microstructure was formed in the matrix,PM improved the strength and hardness in hardened phase.(BF+RA(+C))nano forming nucleates around PM can absorb more plastic strain and alleviate stress concentration.The multiphase microstructure has synergistic strengthening and toughening effect,which is conducive to the improvement of mechanical properties of materials.