To improve the overall performance of ductile iron parts, it is necessary to eliminate the points of white-mouth annealing, normalizing, quenching, and ductile iron annealing, which will be described in detail below.

1. Eliminate white holes annealing

In the process of ductile iron, once the cooling rate is too fast, white holes will appear on the surface of ductile iron castings, resulting in problems such as the inability to cut and process the iron castings. In order to eliminate white-hole annealing, the following points need to be paid attention to:

a. Reheat to reduce the hardness of the iron castings, and heat to the eutectoid temperature to keep the temperature at about 890℃;

b. After heating to a certain temperature, keep it for 2h, use this method to anneal;

c. Using cementite to decompose graphite to cool down the casting slowly. When it drops to about 450℃, it will be air-cooled out of the furnace. It is necessary to pay attention to the temperature to ensure that the temperature is about 4500C and avoid excessive temperature. For example, when the temperature is about 750℃, the eutectoid temperature If the cooling rate is too slow, the cementite will be converted to graphite and the strength of the casting will be reduced.

2. Annealing of ductile iron

Annealing of ductile iron is the main way to improve the toughness of iron castings. Under normal circumstances, in the casting process of ductile iron, the white hole tends to be larger and the internal stress is relatively large. It is difficult to obtain pure ferrite and pearlite, and it is difficult to obtain pure ferrite and pearlite. The strength and toughness are poor. In order to solve this problem and improve the toughness and ductility of iron castings, it is necessary to reheat the iron castings to 900~950°C, and then perform high-temperature annealing after holding for sufficient time. The furnace is cooled to 600°C, and graphite can be precipitated by temperature cooling. It is transformed into ferrite to improve the toughness of iron castings; the iron castings are reheated to about 740°C, and finally the temperature is cooled to 600°C.

3. Normalizing

The purpose of normalizing ductile iron is to realize the transformation from matrix structure to pearlite structure. Reheat ductile iron parts to 900℃ to transform pearlite into austenite, dissolve graphite in austenite, and then transform austenite It is pearlite, which refines the structure and improves the strength of castings.

4. Quenching and tempering treatment

Nodular graphite castings are the main raw materials for making bearings. They require high hardness. The iron castings need to be quenched and tempered at low temperature. The castings are heated to 900°C to cool the austenite in the original matrix to achieve quenching, and then pass 550°C. ~600℃ heating, heat preservation and tempering. After treatment, the hardness and toughness of the casting are improved, and the lubricity and wear resistance of the casting are improved.

5. Austempering treatment of ductile iron

In the process of temperature quenching of ductile iron, the ductile iron is heated to 850°C, and then put into 300°C molten salt for heat preservation to increase the strength of ductile iron.

cast iron box table

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