Study On The Phase Transformation Of QTD1050-6 Austempered Ductile Iron

Austempered ductile iron(ADI)is mainly include bainite and some austenite as the matrix of ductile iron,the performance of the ADI is higher,and its cost compared to ductile iron,cast aluminum,forged steel,etc.Its advantage is low cost,with significant performance and cost advantages of both,so that the scope of its application gradually expanded.In the actual production process,usually based on the required organization and performance to rationalize the heat treatment process.In this paper,the phase transition law of QTD1050-6 Austempered Ductile Iron under different heat treatment conditions,namely different isothermal temperature and different cooling rate,is studied by using expansion method combined with metallographic hardness method.Under the experimental conditions,the isothermal temperature in the range of 300 ?-340 ?in the process,the isothermal transformation of bainite in austempered ductile iron indicates that the ferrite is elongated and needle-shaped and the retained austenite is uniform distributed in the structure.The maximum transformation amount of bainite increases first and then decreases with the increase of isothermal temperature.When the isothermal temperature reaches at 300 ?,the maximum transformation amount of bainite reaches 85.9%,and when the isothermal temperature exceeds 400 ?.The austempering temperature of QTD1050-6 austempered ductile iron can be chosen to be around 300 ?.According to the drawn TTT diagram,the isothermal transition curve of bainite in austempered ductile iron is "C" type,C curve of the " nose " temperature of 380 ?,indicating that the bainite at this temperature to achieve the fastest speed of change.QTD1050-6 ADI in continuous cooling transformation process,according to the temperature expansion curve to determine the start and end of the transformation phase?transition temperature and the corresponding time;under different cooling speeds,there are four types of the main phase : the proeutectoid errite transformation,transformation of pearlite,bainite transformation and transformation of martensite.By drawing the CCT curve of ADI,get the AC3 temperature of ADI is 838 ?,the AC1 temperature is 782 ?,the actual temperature of martensitic transformation is 211 ?;The final results are as follows: the austenitizing temperature range of QTD1050-6 is 870-890 ?,In the process of continuous cooling heat treatment,the cooling rate should be less than 14.4 ?/s.According to the metallographic and hardness test,the continuous cooling transformation process,with the different cooling rate,can receive the different organizations,in the formulation of the ductile iron heat treatment process,according to the required performance,and then select the cooling speed,so as to obtain the corresponding organization,has reached the performance requirements.