How spheroidal graphite irons is obtained: composition, processes and treatments

To produce spheroidal graphite iron, the composition of the elements must be correctly chosen before carrying out the inoculation and spheroidization treatments. Inoculation aids the formation of graphite; spheroidization, which consists of a calibrated addition of magnesium, is used to ensure that the newly-formed graphite grows in shape and becomes spheroidal.

Composition choice

Obtaining spheroidal graphite is typically associated with the presence of silicon, which is the main element used as a graphitizer and is used in percentages ranging from 2.5 to 4.0%, and of magnesium which influences the growth mechanism of graphite, promoting the spherical morphology if added in the appropriate amounts (0,04-0.07%) to avoid a series of negative side effects. Silicon also performs a ferritising function (i.e. it aids the formation of ferrite, resulting from the decomposition of austenite). The most commonly used element is copper to create a pearlitic matrix.

Inoculation treatment

To counteract a number of kinetic factors that result in the unwanted formation of cementite during solidification, the inoculation treatment facilitates the graphite nucleation mechanism by providing heterogeneous nucleating agents (addition of ferroalloys or specific compounds).

Spheroidisation treatment

This treatment is carried out by adding a calibrated amount of magnesium and counteracting evanescence (loss of magnesium through evaporation, oxidation, formation of unwanted compounds) with various measures (ladle, wire, in-mould).

Spheroidal graphite iron: microstructural characteristics

As far as the microstructural characteristics of a spheroidal graphite iron are concerned, you need to consider, in addition to the presence of graphite spheroids (the quality of which can be assessed by determining the percentage of nodularity through image analysis interfaced with the metallographic microscope), the typical presence in the matrix of ferrite or pearlite, or both, in varying quantities depending on the composition and cooling conditions.

GJS/400-18 or high-silicon cast irons [1]
(ferritic matrix)

GI5/600-3
(ferritic-pearlitic matrix)

GI5/800-2
(pearlitic matrix)

Spheroidal graphite irons: mechanical features

Spheroidal graphite ensures, high ductility compared to other categories of cast iron. The combination of the quantities of ferrite and pearlite in the matrix determines the mechanical characteristics. A fully pearlitic matrix maximises the tensile strength (up to 700-800 MPa), with sufficient levels (a few percentage points) of elongation at break. A fully ferritic matrix maximises the ductility (15-18%), at the expense of tensile strength (about 400 MPa).

Application characteristics of spheroidal cast iron

Spheroidal Graphite Cast Irons (SGCI) are a family of materials that compete favourably with steel in structural applications. Their microstructure, in addition to graphite, consists of a matrix that combine ferrite and pearlite, and allows the mechanical behaviour to be “modulated”. As a result, spheroidal graphite iron is a widely used material today that has found its way into many different fields of application. Thanks to the new grades of ferritic spheroidal cast irons strengthened in solid solution, also known as high-silicon spheroidal cast iron (see EN 1563), the fields of application have increased. Spheroidal graphite iron, in its various grades, is used to produce mechanical components for industrial applications, agricultural and earth-moving machinery, off-road and on-road vehicles, railways, hydraulics and oil-hydraulics. Some of the most common applications using spheroidal graphite iron components include:

Components for earth-moving and farm machinery

Sprocket

Sprocket

Components for planetary and worm gearboxes

1, 2 or 3 stage planetary gear train

Planet gear hubs

Components for automotive, off-and on-road vehicle suspension systems

Suspension arms

Steering knuckles

Components for railway carriages

Bearing support

Brake boxes

Components for hydraulic motors and pumps

Motor bodies

Pump heads

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Infosheet 3

Material properties – Ductile Iron Structural grades

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