Analysis and prevention of broken teeth of nodular cast iron gears

The casting is molded in a box of 2 pieces, with 550 kg spheroidizing treatment package, 10 boxes and 3 groups of U-shaped test blocks poured in a package, which is required to be finished within 10 minutes, generally within 8 minutes. However, from the above metallographic photos, it can be determined that the cause of gear fracture is poor spheroidization or spheroidization recession. From the number of customer feedback, it should be caused by the spheroidization recession.

1. Inoculation treatment

When 75sife inoculant is used, the general dosage is 0.73%, about 2 / 3 of the inoculant is used to cover the nodularizing agent, and the remaining 1 / 3 is added during the second tapping. Due to the long time interval between inoculant addition and pouring, it is easy to cause inoculation decline and spheroidization decline.

2. Spheroidizing temperature

The tapping temperature of molten iron is required to be higher than 1 450 ℃, but too high temperature will lead to greater burning loss of nodularizing agent and inoculant, reduce the effect of nodularizing inoculation, and easily lead to premature decline of nodularizing inoculation.

3. Pouring time

The pouring time is prolonged due to the fire escape, which leads to the spheroidization of the casting in the later stage.

4 Preventive measures

(1) Strengthen inoculation control [4-10]. The particle size of inoculant should be controlled at 5 ~ 15 mm, and the amount of inoculant should be increased to 0.9% ~ 1.1%. Among them, 40% of inoculant is used to cover the nodularizing agent, 40% of inoculant is put into the tapping tank to be added with the molten iron flow during the secondary tapping, and the remaining 20% of inoculant is put on the molten iron for floating Si inoculation after the molten iron is fully stirred and slagged. Basife inoculant with particle size of 0.2 ~ 0.7 mm and dosage of 0.1% was used to inoculate with flow through self-made inoculation funnel.

(2) The spheroidizing temperature should be controlled at 1 470 ~ 1 500 ℃.

(3) The pouring time should be controlled within 10 min. When molding, tamp the sand mold, and the box can be closed only after the damage of the parting face of the sand mold is repaired, so as to avoid the extension of pouring time caused by fire escape.

5. Inspection control

(1) U-shaped test block and metallographic test block are poured at the end of each ladle.

(2) Each batch of castings is not only required to fully inspect the appearance, but also required to be 100% knocked by a small hammer with a clear and pleasant sound before release.

3.3 production verification

By adopting the above improvement measures, the metallographic structure of the casting is obviously improved, the spheroidization level is 2 ~ 3, and the volume fraction of pearlite in the matrix structure is 55% ~ 65%, as shown in the figure. The mechanical properties of the castings also meet the standard requirements. In the past half a year, 332 products have been submitted, and no customer feedback on quality problems has been received.

(a) Graphite morphology
(b) Matrix structure

Through the analysis of the pictures and objects fed back by customers, it is determined that the cause of gear broken is spheroidization recession. According to the actual production situation of our company, this paper analyzes the causes of spheroidization decline from the aspects of inoculation control, pouring temperature and pouring time, and puts forward the solving measures. After more than half a year of production practice, the problem of gear tooth breakage has been solved.