Properties and characteristics of gray cast iron

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 construction machinery parts.

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

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

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

(5) Impact toughness: grey cast iron is a brittle material with low toughness and plasticity. Grey cast iron is not recommended to bear impact load.

2. Process performance

(1) Casting properties: ① gray cast iron has good fluidity and can be used to produce thin-walled and complex parts; ② The shrinkage of gray cast iron is reduced due to graphitization expansion during solidification. This property can be fully used in production to reduce shrinkage and residual stress; ③ The casting stress of gray cast iron consists of thermal stress, phase transformation stress and mechanical resistance stress, in which the thermal stress is the main factor, and the casting stress is the main cause of cold crack and deformation of gray cast iron; ④ Due to different wall thickness and component segregation, the structures of various parts of the casting are different, resulting in great differences in strength and hardness of various parts of the casting. This difference is called section sensitivity. The more complex the structure is, the more uneven the wall thickness is, the greater the section sensitivity is.

(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 microstructure, the machinability deteriorates sharply.

(3) Welding repair performance: the welding repair performance of gray cast iron is poor. The welded castings are easy to produce cracks, local defects such as hard spots, pores and large stress. Special material electrodes, preheating and slow cooling are often used to solve them in production.

3. Service performance

(1) Vibration damping: gray cast iron has good damping performance and vibration damping performance. It is widely used in machine tool castings and internal combustion engine castings with vibration damping requirements.

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

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

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