Gray Cast Iron Microstructure
Gray cast iron is an iron-carbon alloy mainly composed of iron, carbon and silicon. It has good castability, high strength, good machinability, excellent vibration damping and other advantages, so it is widely used in automobiles, machines, chemical processes and marine engineering. The microstructure of gray cast iron mainly includes particles, phases, morphology and other aspects, which are the determining factors of its performance. Here we analyze the microscopic structure of gray cast iron.
Particles
The particles of gray cast iron mainly include different types of graphite flakes, cementite (Fe3C) and ferrite (α-Fe). The size and distribution of these particles vary with the composition of raw materials and heat treatment technology. Graphite flakes usually exist in the form of flat hexagonal or diamond-shaped flakes, and their size is mainly related to casting process parameters and mold temperature, while ferrite or cementite particles usually exist in the form of block-shaped or lamellar structure with a small size and some needles. Materials with high graphite content are often called “white cast iron” because graphite particles are often easily seen with the naked eye.
Phases
Gray cast iron mainly consists of α-Fe matrix and carbon-containing phases (Fe3C, graphite), and its phases are also related to its composition and heat treatment. As the carbon content is low, there is only limited amount of primary carbides, such as η-carbides, and thus the phases are mainly matrix phases. As the carbon content is increased, primary carbides such as cementite and graphite tend to form, reducing the amount of matrix phases.
Morphology
The graphite morphologies of gray cast iron can be divided into four types: flake-shaped, glomerular, network, columnar graphite and mixed. The size and shape of graphite flake depend on the chemistry, the cooling speed and the pattern of casting structure. Due to its special structure, gray cast iron has the advantages of good damping capacity and some anti-wear ability.
In summary, gray cast iron has different particles, phases and morphologies, which are the basic characteristics of gray cast iron, and are also the determinants of its metallurgical and mechanical performance. Therefore, we need to consider the elements and cooling rate when pouring gray cast iron to obtain good microstructure, which is the prerequisite for obtaining good performance of gray cast iron.
