Study On Preparation Of High Strength And Toughness ADI From Ductile Iron Bar

Austempered Ductile Iron(ADI)described a material obtained aftr austempering heat-treatmcni and it was widely used in automotive industry.agricultural machineries and construction industry.The ex cellent performance of ADI from high quality ductile iron and chemical compositioncspheroidization rate and casting defects of ductile iron all affect ADI properties.At the same time,different heat treatment processes will significantly affect the mechanical properties of ADI on the premise of ensuring qualified base materials.Therefore,it is very important for improving the properties of ADI to select the qualified ductile iron and appropriate heat treatment processes.The horizontal continuous casting ductile iron bar possesses dense structure and high graphite nodularity grade and overcomes the casting defects of ordinary sand castin such as shrinkage porosity and sand inclusion.For these reasons,this paper adopts the horizontal continuous casting ductile iron material as base material for heat treatment,the efffect of austempering process on microstructure and mechanical properties of ADI were studied by traditional single step and new two-step austempering experiment.That characteristics of tempering response of traditional and two-step ADI were also investigated.The main conclusions of the present study can be drawn as follows:(1)The austenitizing temperature and holding time will affect the austenitization degree,and the volume fraction of austenite and the carbon content of austenite under the traditional isothermal quenching conditions.It can not only ensure the complete austenitizing of the matrix but also prevent the large acicular ferrite when austenitizing at 900?+90min and austempring at 940?+60min.(2)Austempering temperature has a great influence on the morphology of ferrite and the content of residual austenite.The fine acicular ferrite obtained with higher tensile strength at lower temperatre and the feather ferrite was acquired at higher temperature.The strength and elongation are 1517.7MPa and 1.85%at 280?,and 1096.6mpa and 6.3%at 360?.(3)Fine-grained of ADI can be fabricated by the two-step austempering process with higher tensile strength and high carbon content of residual austenite.ADI has better comprehensive mechanical properties in the process of 900?+60min(austenitizing)+310?(first step austempering)+15min+340?(second step austempering)+60min and 940?+60min+290?+15min+360?+60min.(4)It was found that plate-shaped carbide gradually precipitated from the matrix firstly and then carbide accumulating and growing with the increase of tempering temperature during tempered.The strip-shaped carbide is continuously dissolved in the matrix in granular form when the tempering temperature reaches 480?.The two-step ADI is easier to precipitate carbides than the single-step ADI in the tempering experiment.(5)The hardness value of traditional ADI tempering samples decreases slightly,next increases to the highest value and then gradually decreases with the tempering temperature increasing,while the two-step ADI tempering samples shows a trend of increasing slightly firstly,then decreasing gradually as the tempering temperature increasing.The two-step ADI still has higher hardness than the traditional ADI and the hardness values of the two groups of specimens are equal basically as the tempering temperature reaches 540?.