Role of Nodulizers in Cast Iron？

Nodulizers are critical additives in cast iron production, whose core function is to induce the transformation of graphite in molten iron from a flaky morphology to spherical nodules, thereby significantly improving the properties of cast iron. The specific roles and underlying principles of nodulizers in cast iron are as follows:

I. Promoting Graphite Nodulization to Improve Mechanical Properties

Mechanism:In ordinary gray cast iron, graphite exists in flaky form, which disrupts the metal matrix and creates stress concentration points, leading to high brittleness and low strength.

Nodulizers (such as rare earth elements, magnesium, calcium, etc.) adsorb on the growth front of graphite crystals, inhibiting unidirectional growth (e.g., flaky growth) and promoting isotropic growth, ultimately forming spherical graphite nodules.

Performance Enhancements:Spherical graphite significantly reduces the disruption to the matrix, drastically improving the tensile strength, plasticity, and toughness of cast iron (e.g., the tensile strength of ductile iron can reach 400–700 MPa, far exceeding 100–250 MPa for gray cast iron).Spherical graphite minimizes stress concentration, enhancing the material’s resistance to fatigue fracture.

II. Stabilizing Graphite Morphology and Suppressing Undesirable Structures

Prevention of Flaky Graphite Formation:Impurities such as sulfur and oxygen in molten iron tend to promote flaky graphite growth. Elements like magnesium and rare earths in nodulizers react with sulfur and oxygen to form stable compounds (e.g., MgS, Ce₂O₂S), reducing impurity content and inhibiting flaky graphite formation.

Suppression of Cementite (White Structure) Precipitation:Certain nodulizers (e.g., rare earths) hinder the formation of cementite in iron-carbon alloys, promoting graphitization and avoiding the formation of hard and brittle white structures, thus improving machinability.

III. Influence on Casting Process Performance

Molten Iron Fluidity:Although nodulized molten iron has slightly increased viscosity, the spherical graphite morphology ensures more uniform solidification shrinkage, helping prevent defects like shrinkage cavities and porosity, and improving casting quality.

Sensitivity to Cooling Rate:Nodulizers reduce the sensitivity of cast iron to cooling rate, enabling spherical graphite formation in both thick and thin sections of castings, and avoiding microstructural variations caused by uneven cooling (e.g., coarse graphite in thick sections or white structures in thin sections).

Ⅳ. Precautions for Using Nodulizers

1.Control of Residual Content:

Insufficient nodulizer addition may cause poor nodulization (e.g., nodular or vermicular graphite), while excessive addition may increase the tendency for white structures or graphite distortion (e.g., fragmented graphite). Residual content must be precisely controlled through pre-furnace inspection (e.g., triangle test pieces, metallographic observation) (e.g., magnesium residual typically controlled at 0.03%–0.08%, rare earth residual at 0.02%–0.05%).

2.Processing Technology:

Nodulization treatment must be carried out immediately after tapping molten iron (to prevent magnesium volatilization) and combined with inoculation treatment (e.g., adding ferrosilicon alloy) to promote fine and uniform graphite distribution.

3.Impurity Control:

Strictly control sulfur and phosphorus content in molten iron (S ≤ 0.02%, P ≤ 0.08%) to avoid excessive consumption of nodulizers or nodulization failure.