Application of Inoculants in Cast Iron

Inoculants are indispensable key additives in the melting and casting process of cast iron. Their core function is to improve the mechanical properties, machinability, and casting process performance of cast iron by promoting graphite nucleation, refining the structure. They are widely used in the production of various cast irons (gray cast iron, ductile cast iron, malleable cast iron, etc.). The following is an explanation from the aspects of action mechanism, common types, application scenarios, and process key points:

I. Core Action Mechanism

The properties of cast iron (such as strength, toughness, and machinability) are closely related to the morphology, distribution, and matrix structure of graphite. The core function of inoculants is to change the precipitation behavior of graphite by introducing a large number of fine "nucleation sites" (such as silicon compounds, carbides, etc.) into the molten iron:

1.Graphite Refinement: Promote graphite to precipitate in fine, uniform forms (flaky, spherical, etc.), avoiding coarse graphite (e.g., excessively coarse flaky graphite significantly reduces strength);

2.Chill Inhibition: Prevent or reduce the formation of hard and brittle cementite (chilled structure), especially effective for thin-walled castings (which cool quickly and are prone to chilling);

3.Matrix Stabilization: Reduce segregation of pearlite and ferrite, making the matrix structure more uniform, thereby improving comprehensive mechanical properties (such as strength and hardness).

II. Typical Application Scenarios

1.Gray Cast Iron:

Gray cast iron relies on the morphological control of flaky graphite (fine and uniform flaky graphite can balance strength and machinability), and inoculation is a core process. For example:

Engine cylinder blocks and heads: Require high strength (HT250 and above) and good machinability. Inoculation with ferrosilicon or silicon-barium refines flaky graphite and avoids chilling in thin-walled areas;

Machine tool beds: Require shock absorption and wear resistance. Inoculants can stabilize the pearlite matrix and control graphite distribution to balance rigidity and shock absorption.

2.Ductile Cast Iron:

Ductile cast iron requires spheroidizing agents (such as magnesium, cerium) to make graphite spherical, but spheroidization tends to cause "chilling tendency" or coarse graphite spheres. Inoculants (such as ferrosilicon, silicon-strontium) can assist:

Promote spherical graphite nucleation, refine sphere diameter (≤100μm), and improve spheroidization rate;

Inhibit cementite formation, ensuring the matrix is mainly ferrite or pearlite (e.g., high-strength ductile iron for automobile crankshafts and wind power flanges relies on inoculation to refine the structure).

3.Thin-Walled Cast Iron Parts:

Thin-walled parts (such as cylinder liners, hydraulic valves) cool quickly, and are prone to chilling (hard, brittle, and unprocessable) because graphite cannot precipitate in time. Inoculants (such as silicon-strontium) have strong nucleation ability, which can force graphite to precipitate under rapid cooling to avoid chilling.

4.Malleable Cast Iron:

Malleable cast iron needs to be first cast into "chilled blanks", then annealed to decompose cementite into flocculent graphite. Inoculants can refine the grains of chilled blanks, shorten annealing time, and improve the uniformity of flocculent graphite.

Ⅲ. Key Process Points

The inoculation effect is achieved through strict control of process parameters, with core points including:

1.Addition Amount: Adjust according to cast iron composition and casting wall thickness, usually 0.2%-0.8% of the molten iron weight (0.2%-0.5% for ferrosilicon, 0.1%-0.3% for silicon-strontium). Excessive addition may cause graphite flotation (in large thick parts) or strength reduction.

2.Addition Temperature: Should be added when the molten iron has good fluidity (1400-1500℃). Too high temperature will burn the inoculant (silicon is easily oxidized), while too low temperature will result in insufficient dissolution of the inoculant and poor nucleation effect.

3.Addition Methods:

Pouring method: Add inoculant to the bottom of the ladle during tapping, using molten iron impact to disperse it (traditional method, prone to fading);

Stream inoculation: Continuously add inoculant (such as granular ferrosilicon) into the runner during pouring, reducing molten iron residence time and fading;

In-mold inoculation: Place inoculant (such as paste inoculant) locally in the mold gate or cavity to specifically improve local structure (e.g., at hot spots of castings).

4.Anti-Fading Property: The inoculation effect will fade over time (nucleation sites in molten iron gradually dissolve). It is necessary to control the time from inoculation to pouring (usually ≤30 minutes). Stream and in-mold inoculation can significantly reduce fading.

Summary

Inoculants fundamentally solve problems such as "low strength, high chilling tendency, and poor machinability" of cast iron by regulating graphite morphology and matrix structure. They are the core guarantee for cast iron to achieve "high strength, wide wall thickness adaptability, and complex part forming", and are indispensable in the production of cast iron parts in automotive, machine tool, wind power, construction machinery and other fields.