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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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