Classification of graphite distribution in grey cast iron

Graphite in grey cast iron is flaky, and its distribution shape can be divided into a, B, C, D, e and F. Different distribution shapes have different effects on the mechanical properties, service properties, physical properties and technological properties of gray cast iron.

The graphite distribution shape of gray cast iron is described in the previous relevant standards [1-2]. In 2009, the new standard GB / T 7216-2009 metallographic examination of grey cast iron was issued and implemented. The standard mainly adopts the type I flake graphite part of ISO 945-1:2008 “microstructure of cast iron part I: classification and visual inspection of graphite”. The old standard, especially the type and connotation of graphite distribution shape, has been greatly modified.

(1) both C-type and F-type graphite are primary graphite, and graphite is precipitated from molten iron under hypereutectic composition. The characteristics of C-type graphite are long, thick and straight. F-type graphite only appears in castings as thin as single casting piston rings. The walls of single casting piston rings are very thin. CE needs to be improved. Hypereutectic composition is used. Under the condition of thin-wall rapid cooling, F-type graphite is produced. This primary graphite bifurcates in the shape of star (or spider spider). The claw of spider-like graphite is straight, and the claw end is slightly bent.

(2) graphite of a, B, D and e exist in hypoeutectic cast iron. They are eutectic graphite produced in the process of eutectic. The pearlite content in the structure of high-strength gray cast iron with HT250 or above should be more than 98%, and the graphite should be distributed uniformly without direction, i.e. type a graphite. In the case of A-type graphite as the main body, some B-type graphite will appear in the local field of vision. This is due to the condition of increasing supercooling degree in some micro areas of eutectic iron with low macro supercooling degree, resulting in supercooled graphite. Once the supercooled graphite appears, it will release latent heat of crystallization, which will reduce the degree of supercooling. Therefore, the graphite with the same heat dissipation direction is crystallized around the fine supercooled graphite at a slightly higher temperature. In this way, the graphite produced successively gathers into chrysanthemum like distribution, which is called B-type graphite. It is not surprising that the appearance of B-type graphite does not affect the tensile strength of cast iron. D-type and E-type graphite are produced in the later stage of eutectic. D-type graphite is a kind of supercooled graphite formed under a large degree of supercooling. The graphite flakes are small and curly, and there is no directional distribution between austenite dendrites. Under the condition of low CE and high cooling rate, primary austenite dendrites have been developed during eutectic solidification. Eutectic graphite can only precipitate between the secondary branches of dendrites to form directional distribution and E-type graphite. E-type graphite is not supercooled graphite, not short.