What Is Grey Cast Iron?

The Chemical Composition of Grey Cast Iron

Carbon, silicon and manganese in cast iron are the elements that mediate the texture. Phosphorus is the element used for control. Sulfur is the element that should be restricted. Currently, the chemical composition of grey cast iron is generally ranges from wC=2.7%~3.6%，wSi=1.0%~2.5%，wMn=0.5%~1.3%，wP≤0.3%，wS≤0.15%.

The texture of Grey Cast Iron

Grey cast iron is cast iron formed when both the first stage and the second stage graphitization process are fully performed. Its microstructure is characterized by the distribution of flake graphite on various matrix structures. Due to the different extent of graphitization in the third stage, grey cast iron with three different matrix structures can be obtained:

a) iron wire cast iron; b) pearlite grey cast iron; c) iron-body pearlite grey cast iron

Property of Crey Cast Iron

Mechanical properties: The tensile strength, plasticity, toughness and elastic modulus of grey cast iron are much larger than the number of low graphite sheets of the corresponding matrix. The larger the size, the more uneven the distribution, the more severe the splitting effect and stress concentration on the substrate. The lower the strength, ductility and toughness of cast iron. Since the compressive strength σbc, hardness and wear resistance of gray cast iron are mainly depend on the matrix, the presence of graphite has slight effect on it, so when the compressive strength of grey cast iron is generally 3 to 4 times of its tensile strength, on that condition, the pearlite grey cast iron has higher strength, hardness and wear resistance than that of other two matrix structures.

Other properties: Although graphite reduces the tensile strength, ductility and toughness of cast iron, it is also due to the presence of graphite that gives cast iron a range of other excellent properties.

Good casting performance: As the carbon equivalent of grey cast iron is close to the eutectic composition, so the melting point is low, the fluidity is good when compared with steel, and the graphite has a larger specific volume of graphite during the solidification process, partially compensating for the shrinkage of the matrix, thereby reducing the shrinkage of gray cast iron, so grey cast iron can cast castings with complex shapes and thin walls.

Heat Treatment of Grey Cast Iron

1. Elimination of internal stress annealing

It is used to eliminate the casting stress caused by uneven cooling during the solidification process of the casting, and to prevent deformation and cracking of the casting. The process is to heat the casting to 500-600 ° C, and after the heating for a period of time, the furnace is slowly cooled to 150-200 ° C or less to be air-cooled, and sometimes the casting is placed in a natural environment for a long period of time, so that the internal stress of the casting is relaxed. The method is called “natural aging”, and large grey cast iron castings can use this meathod to eliminate casting stress.

2. Surface hardening

The purpose of surface quenching is to improve the surface hardness and wear resistance of gray cast iron parts. In addition to induction heating surface quenching, cast iron can also be surface hardened by contact resistance heating.

Grade and Application of Grey Cast Iron

According to the provisions of GB/T 9439-1988, the grey cast iron is divided into six grades according to the tensile strength of a 30 mm diameter single casting test bar. The grade of grey cast iron is represented by the "HT" ("grey iron" Chinese pinyin prefix) and the minimum tensile strength σb (using the tensile strength of the φ30mm test bar). For example, the grade HT250 indicates a gray cast iron having a minimum tensile strength value of 250 MPa for a φ30 mm test bar. When designing a casting, the cast iron grade should be selected according to the main wall thickness or average wall thickness at the force of the casting.