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Process Control in the Production of Ductile Iron Castings

1. equipment selection

1.1. The selection of melting equipment follows the principle of high efficiency and low consumption after meeting production needs. The advantages of induction electric furnace are: fast heating speed, high thermal efficiency of the furnace, low oxidation burning loss, and low gas absorption. Thus, the electric furnace melting in intermediate frequency can avoid to increase the amount of sulfur and phosphorus, so P is less than 0.07% and S is not more than 0.05% in molten iron.

1.2. The determination of nodulizing bag is to improve the absorption rate of nodulizer, and increase the nodulizing effect, the nodulizing bag should be deeper than normal molten iron bag. The height/diameter ratio of nodulizing bag is 2:1.

2. raw material selection

2.1. The conditions of nodulizer effection are high carbon, low silicon, and a large breeding capacity. In order to stabilize the chemical composition, effectively control white iron elements and anti-nodulizing elements, and ensure the quality of hot metal melting, we should use ZhangGang Z14 pig iron, its chemical composition: C>3.3%, Si: 1.25%-1.60%, P≤0.06%, S≤0.04%.

2.2. Nodulizer selection should be based on different melting equipment, and that is the iron tapping temperature and purity of molten iron (such as sulfur content, oxidation, etc). The nodulizer commonly used in China is the rare earth Mg-Si-Fe nodulizer. The high silicon content in alloys can reduce the reaction intensity, while increase the breeding function. Due to relatively high temperatures, the nodulizer is FeSiMg8Re7 in electric furnace production.

Table 1. Chemical Composition of FeSiMg8Re7 Nodulizer

Item

Tapping temperature /Centigrade

S /%

Nodulizer composition /%

Mg

Re

Si

Electric furnace

1420-1480

≤0.04

7.0-9.0

6.0-8.0

≤44.0

3. melting furnace control

3.1. Chemical composition of molten iron should be high carbon, low silicon, low sulfur, and low phosphorus. The reasonable control of the sulfur content is an important condition in the production of ductile iron.

3.2. The added amount of nodulizer in nodulizing and breeding treatment should be based on molten iron composition, casting wall thickness, and absorption rate. The key to nodulizing reaction control is magnesium absorption rate. High temperature, severe reaction, short time, more magnesium burning loss, the nodulizing effection is poor, while low temperature, smooth reaction, long time, high magnesium absorption rate, the nodulizing effection is good. Therefore, the pouring temperature should control in 1420-1450 ℃.

Table 2. Chemical Composition of Ductile Iron %

Cast iron degree

C

Si

Mn

S

P

QT400-18 before nodulizing treatment
QT400-18 after nodulizing treatment

3.6-4.1

0.9-1.2

≤0.4

≤0.05

≤0.07

3.5-4.0

2.6-3.2

 

≤0.02

 

QT450-10 before nodulizing treatment
QT450-10 after nodulizing treatment

3.6-4.1

0.9-1.2

≤0.4

≤0.05

≤0.07

3.5-3.9

2.5-3.0

 

≤0.02

 

QT600-3 before nodulizing treatment
QT600-3 after nodulizing treatment

3.6-3.9

0.9-1.2

0.6-0.8

≤0.05

≤0.07

3.5-3.8

2.2-2.6

 

≤0.02

 

QT700-2 before nodulizing treatment
QT700-2 after nodulizing treatment

3.6-3.9

0.9-1.2

0.6-0.8

≤0.05

≤0.07

3.5-3.8

2.1-2.5

 

≤0.02

 

3.3. Blast furnace tests generally adopt triangular specimen to determine the breeding and nodulizing affection. Cool triangular specimen to dark red after pouring, then cut and observe the fracture. Silver fracture, white tip, loose center, calcium carbide smell after cutting, and knocking sound like steel, the nodulizing affection is good.

3.4. The pouring of molten iron is easy to recession, therefore, molten iron should be poured as soon as possible after nodulizing treatment, usually within 15 min, and there will be no nodulizing recession.

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