The production process of ductile iron involves the following steps:

Melting qualified iron liquid; Spheroidization treatment; Inclusion treatment; Furnace inspection; Pouring of castings; Cleaning and heat treatment; Quality inspection of castings.

Among these steps, melting the iron liquid and conducting effective spheroidization-inclusion treatment are the key points of the production process. 

I. Selection of Chemical Components.

1. Basic Elements: 

① Carbon and Silicon:

Since the weakening effect of spherical graphite on the matrix is very small, the quantity of graphite in ductile iron has little influence on its mechanical properties. When the carbon content varies within the range of 3.2-3.8%, it actually has no significant impact on the mechanical properties of ductile iron. When determining the carbon-silicon content of ductile iron, the main consideration is to ensure casting performance. Therefore, the carbon equivalent is selected around the eutectic composition.

Because the alloying elements shift the position of the eutectic point C on the phase diagram to the right, the eutectic carbon equivalent is moved to 4.6-4.7%.

The fluidity of the iron liquid with eutectic composition.

It has a high tendency to form concentrated shrinkage cavities (easy for secondary pouring and setting of risers).

The organization density of the cast iron is high.

When the carbon equivalent is too low, the casting is prone to shrinkage porosity and cracks.

When the carbon equivalent is too high, the graphite floating phenomenon occurs, resulting in an increase in the number of inclusions in the cast iron, reducing the performance of the cast iron, and polluting the working environment.

Iron liquid treated with magnesium and cerium has a large crystallization overcooling and a tendency to form white iron. Silicon can reduce this tendency.

In addition, silicon can refine the graphite, improve the roundness of graphite spheres, but it also reduces the toughness of the cast iron and raises the transition temperature between toughness and brittleness.

Therefore, when choosing the carbon-silicon content, the principle of high carbon and low silicon should be followed. Generally, it is believed that when Si > 2.8-3%, the toughness of ductile iron will decrease (especially in cold regions).

Ferrite ductile iron   C: 3.6-4.0%   Si: 2.4-2.8%

Pearlite ductile iron   C: 3.4-3.8%   Si: 2.2-2.6% 

② Manganese: The roles of manganese in ductile iron and its role in gray cast iron are different as follows:

In gray cast iron, manganese not only strengthens ferrite and stabilizes pearlite but also reduces the harmful effects of sulfur.

However, in ductile iron, since the spheroidizing elements have a strong desulfurization ability, manganese no longer plays this beneficial role.

Also, due to the serious positive segregation tendency of manganese, it often tends to accumulate at the grain boundaries of the eutectic, and in severe cases, it can cause the formation of intergranular carbides, thereby reducing the toughness of ductile iron.

The control of manganese content depends on the requirements of the matrix and whether the casting undergoes heat treatment.

For as-cast ferrite ductile iron: Mn < 0.5%

For heat-treated ferrite ductile iron: Mn 0.3 - 0.4%

For pearlite ductile iron: Mn 0.4 - 0.8%

For as-cast pearlite ductile iron: Although manganese can be appropriately higher, copper is usually recommended to stabilize pearlite. 

③ Phosphorus:

Phosphorus in ductile iron has a significant tendency for segregation, easily forming phosphorus eutectics at the grain boundaries, which significantly reduces the toughness of the iron.

Phosphorus also increases the shrinkage tendency of ductile iron.

When high toughness is required for ductile iron, the content of P should be controlled below 0.04-0.06%. If molybdenum is present in the ductile iron, the content of phosphorus should be further controlled, as it is prone to form brittle phosphomolybdenum quaternary compounds at the grain boundaries at this time. 

④ Sulfur

In ductile iron, sulfur has a strong combining ability with the spheroidizing elements, forming sulfides or sulfur oxides. This not only consumes the spheroidizing agent, causing instability in spheroidization, but also increases the number of inclusions, leading to defects in the castings. Moreover, it accelerates the decline rate of spheroidization. In foreign production, the S content in the molten iron is less than 0.02%, but the sulfur content in our country's coke is relatively high. Therefore, only by improving the smelting conditions and performing pre-sulfur removal in the furnace can the sulfur content be reduced. 

2. Alloying Elements: 

Molybdenum: Adding molybdenum to the production of high-strength ductile iron enhances the strength of the cast iron. The dosage is 0.25%. When adding molybdenum to the production of bainitic ductile iron or austenite-bainitic ductile iron, the austenite isothermal segregation curve (C curve) of the cast iron shifts to the right, improving the hardenability. The molybdenum content should be between 0.6% and 0.8%.

However, adding molybdenum to cast iron has a significant positive segregation tendency within the eutectic clusters. When the molybdenum content is between 0.8% and 1%, it is prone to form brittle phases such as molybdenum-containing phosphorus eutectic or carbonitrides at the boundaries of the eutectic clusters. Molybdenum is expensive, so its use should be carefully controlled. 

② Copper: Copper has the effect of stabilizing pearlite.

Some domestic factories use Cu 0.4-0.8% alloyed iron to make automotive crankshafts.

In bainitic alloy iron: Mn 0.2-0.4%, Cu 0.6-0.8% are combined.

Using alloy iron through isothermal quenching can stably obtain a high percentage of bainite structure. 

③ Nickel: Often serves as a strengthening element (similar to copper), typically found in thick-walled components and bainitic structures. 

④ Chromium: Used in pearlitic malleable cast iron. When 0.2-0.3% of chromium is added to the cast iron, it can significantly stabilize the pearlite structure and enhance mechanical properties. However, it is prone to form iron-chromium carbides, so it should be used with caution. 

⑤ Antimony: It is an element that strongly stabilizes pearlite. When its content is between 0.006% and 0.008%, it can effectively increase the percentage of pearlite in the spheroidal iron matrix. 

When producing as-cast pearlitic ductile iron, antimony can be used instead of copper, which is more economically viable.

However, antimony interferes with the graphite spheroidization process.

When Sb > 0.01%, the graphite shape will deteriorate.

Therefore, the content of antimony in ductile iron should be strictly controlled below 0.006 - 0.008%, and accumulation should be prevented. 

3. Trace Elements: 

Spheroidal iron often contains trace elements that are not intentionally added, such as Ti, Pb, Al, Cr, Sn, Sb, etc. These elements have adverse effects on the properties of cast iron, either interfering with graphitization or causing the precipitation of brittle phases at the boundaries of eutectic clusters, or preventing ferrite formation. When 0.01-0.02% Re is added, it can neutralize the harmful effects of these elements. 

II. Melting and Pre-furnace Treatment Techniques for Cast Iron: 

Machined cast iron has high mechanical properties and is based on the good graphite spheroidization condition. The criteria for measuring the spheroidization condition of graphite are the spheroidization rate, graphite spheroid diameter, and the roundness of graphite spheres.

Spheroidization rate: In the representative field of the microscopic structure of cast iron, the ratio of the number of spherical graphite per unit area to the total number of graphite (expressed as a percentage).

Graphite spheroid diameter: The diameter of the representative spherical graphite measured under a magnification of 100 times.

Roundness: A quantitative concept for the roundness of graphite spheres. 

1. Requirements for smelting: 

The iron liquid used for ductile iron should be: at a high temperature, with low sulfur, phosphorus and impurity content.

Since ductile iron requires spheroidization and eutectoid treatment, the iron liquid temperature needs to be reduced by 50-100℃. To ensure the pouring temperature, the iron liquid should be at 1450-1470℃. Also, due to the addition of treatment agents, the eutectoid treatment needs to bring in a large amount of silicon. Therefore, the iron liquid should have a lower silicon content (less than 1.2-1.4%), and the raw iron used for making ductile iron should be low-silicon raw iron. 

Low sulfur content: In addition to the sulfur content of raw materials being low, the sulfur content of coke used in the blast furnace should also be low. This is a combination of blast furnace + induction electric furnace + intermediate desulfurization.

The requirements for raw materials are low sulfur and low phosphorus content, and efforts should be made to reduce the presence of anti-gelating elements. The oxidation degree of the molten iron should be strictly controlled. 

2. Spheroidization treatment 

Spheroidizing elements: Elements added to the molten iron can cause graphite to grow into spherical shapes during crystallization.

Strong: Magnesium, Cerium, Lanthanum, Calcium, Yttrium

Medium: Lithium, Strontium, Barium, Thorium

Weak: Sodium, Potassium, Zinc, Cadmium, Tin, Aluminum

Anti-spheroidizing elements: Certain elements present in the molten iron prevent graphite from growing into spherical shapes during growth.