Learn About Graphite In Cast Iron

As a kind of engineering material with a long history and wide application, the performance of cast iron depends largely on the graphite form inside. Graphite is like a double-edged sword in cast iron. It not only gives cast iron unique properties, but also has a complex effect on its properties.

What is graphite in cast iron?

Grafit is an allotrope of carbon that exists as an independent phase in cast iron. Cast iron is an iron-carbon alloy with a carbon content greater than 2.11% (generally 2.5-4.0%). And during its solidification process, the carbon element will be precipitated in the form of graphite. The form, size and distribution of graphite in cast iron play a decisive role in the performance of cast iron.

Graphite in cast iron overview

Grafitin temel özellikleri

Graphite has a typical layered crystal structure with weak van der Waals forces binding the layers together. This structure gives graphite many unique properties. Its texture is soft, the Mohs hardness is only 1-2, and there is a sense of smoothness. Graphite has good electrical and thermal conductivity, and its electrical conductivity is lower than that of general metal materials. But it is an excellent conductor in non-metallic materials. In addition, the chemical properties of graphite are stable. And it is not easy to chemically react with other substances at room temperature, and has good corrosion resistance.

Importance of graphite in cast iron

Graphite plays a key role in cast iron. From the point of view of mechanical properties, the presence of graphite is similar to the formation of a large number of tiny “crack sources” in the metal matrix. This will reduce the strength and toughness of cast iron. However, graphite also gives cast iron some special properties. For example, the lubrication effect of graphite makes cast iron have good anti-friction and wear resistance. It is of great significance in the manufacture of parts such as plain bearings and machine tool guides. The thermal conductivity of graphite helps the cast iron to dissipate heat evenly during the heating process and improve its thermal stability. Moreover, the presence of graphite can also make the cast iron obtain good casting properties. Such as reducing the surface tension of liquid iron, improving its fluidity. This is conducive to the casting of complex shaped parts.

Different types of graphite in cast iron

Spheroidal graphite cast iron (Nodular graphite cast iron)

Nodular graphite cast iron and spheroidal graphite cast iron refer to the same material. And there is no essential difference between the two, only in the name of the difference.

Formation mechanism of spherical graphite

Adding nodulating agent (such as magnesium, rare earth, etc.) and inoculant to liquid iron is the key. Spheroidizing agent reduces the surface tension of carbon in liquid iron and makes graphite grow spherically. The inoculant increased the number of nucleation of graphite, refined and homogenized the graphite balls. And solidified the iron liquid into spherical graphite at a suitable temperature.

Properties of spheroidal cast iron

Excellent mechanical properties, tensile strength of more than 400MPa-1000MPa, far beyond gray cast iron. Elongation of about 2%-20%, toughness, good ductility, can withstand impact and deformation, can partially replace cast steel. Excellent wear resistance, it has high fatigue strength, good durability.

Microstructure of spherical cast iron

Composed of spherical graphite evenly distributed in a metal matrix (ferrite, pearlite or mixed structure). The toughness and plasticity of ferrite matrix are good. The pearlite matrix has high strength and hardness. Small and evenly distributed graphite balls can improve the overall properties of the material.

Sg iron vs cast iron

Compared with gray cast iron, the mechanical properties are greatly improved. Gray cast iron flake graphite seriously cut the matrix, resulting in low strength and toughness. The spheroidal graphite of nodular cast iron has little cleavage to the matrix. Although nodular cast iron needs more precise control of spheroidization and inoculation treatment, it has good fluidity and filling. It can meet the production of complex castings. In application, you can use ordinary cast iron for strength and toughness requirements are not high, such as machine tool bed. Sg iron is used to manufacture key components in automotive, machinery, aerospace and other fields.

Gray cast iron

Microstructure of gray cast iron

Composed of flake graphite and metal matrix (ferrite, pearlite or mixed structure), flake graphite weakens matrix continuity, resulting in poor mechanical properties.

Graphite flakes in gray cast iron

The shape is irregular, in flake or strip. Its size, quantity and distribution affect the performance. Large graphite flakes reduce strength and toughness, and fine and uniform distribution can improve performance. Graphite flake gives gray cast iron good shock absorption. It is suitable for the manufacture of machine tool bed, engine cylinder block and other parts that need shock absorption.

Graphite flake size in cast iron

Measured by length and thickness, the length is dozens to hundreds of microns, and the thickness is several to tens of microns. The size is affected by the casting process and chemical composition. The high carbon equivalent and slow cooling speed make the graphite sheet grow larger, and the addition of alloying elements (such as silicon and manganese) can be refined.

Compacted graphite cast iron

Formation mechanism of wormlike graphite

It is necessary to strictly control the composition of liquid iron and the treatment process. Add an appropriate amount of vermiculator (magnesium, rare earth and other elements complex) and prepare it with inoculation. The vermiculator makes the graphite grow into a vermicular shape with a smooth end. Acting between a nodulator and no treatment, and forming a vermicular graphite under suitable solidification conditions.

Properties of compacted graphite cast iron

Performance between ductile iron and gray iron. Tensile strength 300MPa-500MPa, higher than gray cast iron. Toughness and ductility are better than gray cast iron but slightly worse than nodular cast iron. Good thermal conductivity, close to gray cast iron, wear resistance and thermal fatigue resistance, can work under high temperature, alternating load.

Microstructure of compacted graphite cast iron

Composed of wormlike graphite evenly distributed in a metallic matrix (ferrite, pearlite or mixed structure). The size, quantity and distribution of compacted graphite affect the properties of the material and reasonably control the availability of fine compacted graphite cast iron.

Cast iron microstructure analysis

Analytical methods

There are various methods to study the microstructure of cast iron. It is usually use metallographic microscope. After polishing and corroding cast iron samples, you can observe graphite morphology, size, distribution and metal matrix structure characteristics. Scanning electron microscopy (SEM) has higher resolution and can see fine structures such as the interface between graphite and matrix. Electron probe microanalysis (EPMA) can quantitatively analyze the elements of cast iron and determine the chemical composition of different phases. And you can use x-ray diffraction (XRD) to analyze the crystal structure of each phase.

Relationship between microstructure and properties

The microstructure of cast iron directly determines its performance. The morphology, size and distribution of graphite significantly affect the mechanical properties. The spheroidal graphite has the smallest effect on matrix cleavage, and the strength and toughness are higher. The flake graphite of gray cast iron reduces the strength and toughness, but it has good shock absorption. Compacted graphite makes vermicular cast iron something in between. The structure of the metal matrix is also very important, the toughness of the ferrite matrix cast iron is good. And the strength and hardness of the pearlite matrix are high. By adjusting the döküm process and chemical composition to control the microstructure, the cast iron materials can be obtained to meet the different performance requirements.

Sonuç

Graphite in cast iron has various forms, and different forms give different properties to cast iron. By studying the microstructure of cast iron by various analytical methods, we can clarify the relationship between microstructure and properties. In the future, with the development of materials science and technology, the research on graphite in cast iron will be more in-depth. And it is expected to develop cast iron materials with better performance and wider application.