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As an applied engineering material,because of its superior and reliable strength, elongation,impact toughness,wear resistance,shock absorption properties,as well as high performance price ratio,Austemperied Ductile Iron（ADI） has a great development and the application of ADI is steadily added in the automotive industry at home and abroad after it came out in 1970’s.To stand heavy destructive load and gouging abrasive wear of the impact load,the back armor plate spring plank not only requires a very high bending fatigue strength and good wear resistance,but also requires better damping properties of reducing the noise.According to the working condition of the back armor plate spring plank,by choosing reasonable chemical compositions and adjusting heat treatment process of the samples,the influence of different austenitizing temperature（860℃、880℃、900℃、920℃）,austenitizing time（60min、90min、120min、150min）,isothermal transformation temperature（370℃、325℃、280℃、235℃）,isothermal transformation time（60min、90min、120min） on the microstructure and properties of the ADI spring bracket was analyzed,the volume fraction of retained austenite in tissue was observed,their mechanical properties was compared and the influence mechanism of them was studied.The main conclusions of this paper are as follows:（1） The optimized content of alloying elements molybdenum,copper,nickel has been determined through many experiments on the basis of choicing the basic components reasonably.Namely,Cu:0.2～0.5%,Ni:0.2～0.4%,Mo:0.15～0.25%.It laid a foundation for obtaining high strength and high toughness austempering ductile iron.（2） By designing casting process and tundish spheroidizing treatment reasonably,the grade of graphite spheroidization is 1～3,the grade of nodular graphite is 6～7 in casting organizations after three inoculations（inoculation in tundish,inoculation out of the iron tank, inoculation with liquid iron）.（3） The organization and mechanical properties are best when the austenitizing at 910℃for 90min and austempering the 350℃for 120min,namely,matrix structure are the upper bainite and retained austenite,the tensile strengthσb≥1000 MPa,yield strengthσ0.2≥700 MPa,elongationδ≥6.0%,hardness（HRC）=30～36.
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