The front axle is a key component of the shock absorber system of an agricultural machine. It is usually produced by furan resin sand process. The material is qtd1050-6, the single weight of the casting is 190 kg, and the maximum outline size is 1300 mm × 400 mm × 200 mm, the maximum wall thickness is 110 mm, and the structure is shown in Figure 1. In order to ensure the ideal performance after isothermal quenching, the spheroidization rate of ductile iron blank is required to be ≥ 85%, and the number of graphite balls is required to be ≥ 100 /mm2.

During the trial production, it was found that the cross section of the position with the front axle wall thickness of 110 mm was dark gray and accompanied by shadows, as shown in Figure 2. The number of graphite balls at this position was about 50 /mm2, and there was fragmented graphite, as shown in Figure 3. It was difficult to ensure that the ductile iron blank obtained qualified performance after heat treatment.

For ductile iron castings with large wall thickness, it is easy to have defects such as too large and too few graphite balls and fragmented graphite. The graphite morphology of nodular iron castings can be effectively improved by adding graphite type carburizing agent, adopting multiple inoculation to strengthen the inoculation effect, adopting low Re spheroidizing agent, using cold iron forced cooling and other measures. However, for ductile iron with large wall thickness, it is difficult to completely eliminate the above casting defects by using a single measure, and a variety of measures must be adopted at the same time.