Briefly describe the formation of nodular cast iron

 The Formation of Nodular Cast Iron

Ductile cast iron, also known as spheroidal graphite cast iron (SG iron) or nodular cast iron, is a type of cast iron that is characterized by the presence of graphite. These graphite inclusions provide the iron with greater impact and fatigue resistance, allowing it to be used in a variety of applications.

Graphite Nodules

During the casting process, additives and controlled timing allow a carbon-rich iron melt to separate as graphite nodules rather than flakes. This morphological structure gives nodular cast iron its excellent ductility and strength characteristics.

The ductile form of cast iron, often referred to as nodular iron or spheroidal graphite iron (FCD), is a relatively recent development. It is produced by adding tiny amounts of magnesium or cerium to a cast iron alloy that slows the growth of graphite precipitates during solidification by forming bonds with the edges of graphite planes.

While carbon is the most common ingredient of ductile iron, other ingredients are also used. These include silicon, sulphur, manganese and oxygen. This mix of ingredients allows a more complete absorption of carbon by the iron as the metal cools.

As a result, the resulting cast iron has higher mechanical properties than gray cast iron. It is used for a wide variety of industrial applications, such as crankshafts, gears and disc brake calipers.

However, this graphite phase is very brittle, and tensile stresses propagate cracks along the graphite plates internally. This is one of the major reasons why gray cast iron has poor tensile properties and weak impact strengths.

When the eutectic graphite separates, it becomes embedded in a matrix of austenite with an equilibrium carbon concentration of about 2 wt%. At the malleable iron eutectoid temperature, more of the carbon forms on the surface of the austenite and the austenite gradually decomposes into pearlite.

The resulting ferritic matrix has high ductility and strength, but it is still prone to brittle fracture under certain conditions. When a weakly corrosive environment is present, the graphite can leach out and dissolve the iron, forming a highly brittle metallurgical slag that cannot be machined.

This slag can be difficult to remove without destroying the integrity of the steel, so the use of nodular iron has been a key advancement in iron production. As a result, nodular iron can be produced with better tensile and impact properties than grey cast iron, and can be used in a wide range of industrial applications.

Graphite Phase

During solidification, the majority of the carbon in a cast iron melt will precipitate as graphite or cementite. During cooling, however, the majority of the crystallized iron-carbon phase will decompose back into austenite. The resulting matrix can either be completely ferritic or pearlitic (depending on the temperature at which it is cooled), and the final mechanical properties will depend on how the matrix phase has changed during solidification.

The formation of graphite phases is an important factor in the mechanical properties and microstructure of cast iron. The flexible pipe shape of the graphite particles determines whether the material is ductile or brittle, with different graphite shapes producing different elastic modulus values.

In ductile cast iron (also called nodular or spheroidal graphite cast iron), graphite forms into spherical, or nodular, particles during solidification. These particles are surrounded by an austenite matrix phase and are generally stronger than gray iron, but can also be made tougher with controlled additions of alloying elements like tin or copper to promote the formation of pearlite around the nodules.

Ductile iron is a type of cast iron that is often used for chassis parts and other applications where tensile strengths, impact toughness, and ductility are of primary importance. It is also less expensive to produce and can be shaped into complex sections, unlike steel forming.

The graphite in ductile iron is more spherical than that in gray iron, and the nodules are smaller. Because of this, ductile iron has higher tensile strength and stiffness, and can be harder to machine than gray iron.

Nodular cast iron is also much stronger than gray iron because of the rounded graphite nodules, which are less likely to break when subjected to tensile stress. These nodules are also less likely to break when subjected to impact stresses, which makes ductile iron a better choice for chassis parts.

The morphology of nodular cast iron is affected by many factors, including the chemical composition, the casting-section thickness and design, and the casting conditions. This microstructure can be characterized by ultrasonic testing, which is based on the way that sound waves interact with the material's microstructure. It is possible to make quantitative measurements of nodule density, size and shape using ultrasonic velocity measurement techniques. The ability to make these measurements is a critical part of assessing the ductile iron microstructure in the foundry, particularly when the alloy composition is one that promotes the formation of pearlite around the graphite nodules.

Graphite Particles

Graphite particles are formed in a variety of situations during the solidification of cast iron. During the solidification process, a large proportion of the carbon precipitates out of the solution as either graphite or cementite and is then embedded in a matrix of austenite. During further cooling the carbon concentration in the austenite decreases and eventually the phase is converted into pearlite.

The morphology of the resulting nodular graphite particles is critical to their ability to interact with the cast iron matrix and to thereby influence its mechanical properties. The particles act as stress raisers and may prematurely cause localized plastic flow at lower stresses or initiate fracture in the matrix at higher stresses.

In ductile cast iron, the shape of the graphite particles is a significant factor in their ability to promote microcracks in the matrix. Specifically, the malleble box more irregularly shaped graphite particles are, the more likely they are to form microcracks in the matrix. This is because the more irregularly shaped the graphite particles are, the greater their ability to deviate from an ideal spherical shape.

This is particularly true for ductile cast iron with an increased silicon content. In this case, the spherical shape of the graphite particles is significantly reduced with increasing the percentage of silicon. This decrease in sphericity leads to an increase in the number of large particles with large perimeters, as shown in Figure 5.

For coated graphite specimens (Al2O3 coating), weight loss occurred only at temperatures of 500 degC and 600 degC, with no CO gas emission by the analyzer until 700 degC. This is attributed to the Al2O3 coating inhibiting the oxidation induced degradation of graphite.

On the other hand, for uncoated graphite samples, weight loss was appreciable even as low as 800 degC, with CO gas emission by the analyzer saturating at 900 degC. However, this is a sporadically occurring oxidation process, unlike the case of bare graphite where the generation of particles takes place for a prolonged time scale as well.

Nodular graphite is a valuable material for many industrial applications due to its primary properties of elasticity, strength and reactivity. Its secondary properties include a high thermal conductivity and excellent resistance to corrosion.

Graphite Shape Factors

The formation of nodular cast iron is a process that involves the solidification of graphite into nodules. In addition to a range of influencing factors, such as chemical composition and metallurgical treatments, graphite morphology also plays an important role in the nodularisation process.

The different graphite morphologies are characterised by a number of shape factors, which describe the deviation of the graphite particles from a sphere (compactness degree). These parameters have an important role in describing the influence of graphite on the mechanical properties of cast iron, as well as in stress concentration at the graphite-matrix interface.

Graphite can be classified into three basic forms, namely flake, vermicular and spheroidal, depending on the oxidation state of the carbon in the cast iron. During the solidification process, graphite particles can be transformed into nodules of a variety of shapes, including spheroidal and flake shapes.

In thin wall iron castings, the finer graphite microstructure requires a method of measuring that can fully capture this complexity. There brass bush are a number of methods that can be used to measure graphite shape, however, these measurements have limitations.

One important limitation is that graphite shape is pixel dependent and cannot be used indiscriminately. It is therefore necessary to use a combination of graphite shape factors in order to be able to accurately measure the morphological complexities associated with the different graphite shapes found in ductile cast iron.

These complexities are mainly due to the presence of a large number of spheroids and irregular spheroidal graphite nodules in the solidified cast iron. Several shape factors are commonly used to describe these spheroidal graphite nodules, but the most useful descriptor for graphite shape in thin wall iron castings is sphericity.

Sphericity is a much more accurate descriptor of spheroidal graphite than compactness, which may explain why it is more common to use sphericity in the evaluation of spheroidal graphite.

This paper examines the influence of graphite shape factors on the nodularity NG4 and NG5 expressions in a range of wedge castings based on high-Si ductile iron. The influence of the minimum imposed graphite shape factors (RSF and SSF) on these nodularity expressions is discussed in detail. The influence of the minimum imposed graphite shapes on a 3 and 15 mm wedge-casting wall thickness was also investigated.