Properties and characteristics of grey cast iron

Gray cast iron characterized by flake graphite has certain strength, hardness, modulus of elasticity, shock absorption, wear resistance and thermal conductivity, especially with excellent casting performance, good cutting performance and low casting cost. So far, it is still the most widely used cast iron in the world.

According to the statistical report of global casting production released by modern casting magazine of the United States, the global casting production in 2016 was 104.4 million tons, close to the production of 104.1 million tons in 2015. The global casting production increased by 0.4% from 2014 to 2015, and by 0.2% from 2015 to 2016. In 2016, the global casting output grew at a low speed, with the growth rate less than 0.5% for two consecutive years, showing a slow growth trend.

According to the statistics of China Foundry Association, China’s casting output in 2016 reached 47.2 million tons, an increase of 5.4% year on year, accounting for 45% of the total global output. According to the data provided by the Indian Foundry Association, the production of castings in India in 2016 was 11.35 million tons, an increase of 5.4% year on year, and the development speed was faster than that of the United States. Due to the decline of heavy cast iron industry, such as agricultural machinery, oil exploration and mining machinery, the casting production in the United States in 2015 and 2016 was lower than that in 2014. In 2018, casting production in the United States is expected to grow by 2.8% and casting sales revenue will increase by 4.7%.

1. Mechanical properties

(1) Tensile strength: the tensile strength of gray cast iron is generally 100-350 MPa, which can meet the strength requirements of many engineering machinery parts.

(2) Compressive strength: gray cast iron has a very high compressive strength, which is 3-4 times of its tensile strength. It can be equivalent to steel, and is the best choice for pressure parts (such as base).

(3) Hardness: the hardness of gray cast iron can meet the wear-resistant requirements of general engineering machinery parts. The higher the hardness, the better the wear resistance.

(4) Modulus of elasticity: the modulus of elasticity of gray cast iron is not a fixed value, but a variable. There is no straight line in the stress-strain curve, because the existence of graphite makes the gray cast iron produce plastic deformation even under a small stress. The elastic modulus of gray cast iron is related to its strength. The higher the grade is, the greater the elastic modulus is.

(5) Impact toughness: grey cast iron is a brittle material with low toughness and plasticity. It is not recommended to use grey cast iron as a material bearing impact load.

2. Process performance

(1) Casting properties: (1) gray cast iron has good fluidity and can be used to produce thin-walled and complicated parts; (2) the shrinkage of gray cast iron is reduced due to graphitization expansion during solidification, which can be fully utilized in production to reduce shrinkage and residual stress; (3) the casting stress of gray cast iron is composed of thermal stress, transformation stress and mechanical resistance stress, Among them, the thermal stress is the main factor, and the casting stress is the main cause of the cold crack and deformation of gray iron castings; (4) because of the different wall thickness and composition segregation of each part of the casting, the structure of each part of the casting is different, resulting in the great difference in the strength and hardness of each part of the casting. This difference is called section sensitivity. The more complex the structure, the more uneven the wall thickness, the greater the section sensitivity.

(2) Machinability: the flake graphite in the structure of gray cast iron cuts the metal matrix, so it has good machinability. However, with the increase of strength and hardness, the machinability decreases. When cementite appears in the structure, the machinability deteriorates sharply.

(3) Weld repair performance: the weld repair performance of gray cast iron is poor, the weld repair castings are easy to produce cracks, some of which have defects such as hard spots, pores and large stress, etc., which are often solved by special materials such as electrodes, preheating and slow cooling.

3. Service performance

(1) Damping property: gray cast iron has good damping property and damping property, which is widely used in machine tool castings and internal combustion engine castings with damping requirements.

(2) Wear resistance: gray cast iron has good wear resistance under sliding condition, widely used in brake pads, brake drums, cylinder liners, piston rings, machine bed with guide rails and other parts.

(3) Heat fatigue resistance: grey cast iron has good heat fatigue resistance, which is widely used in automobile cylinder, cylinder head, ingot mold and other castings.

(4) Compactness: the existence of graphite destroys the continuity and compactness of the matrix, but it can still maintain a certain compactness through a variety of control factors, so that the gray cast iron is widely used in hydraulic parts, cylinder, pump body, compressor and other pressure and leakage resistant parts.