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Briefly introduce the hardness of gray iron

Gray iron is a kind of pig iron, also known as cast iron pig iron. The carbon content is about 3% and the silicon content is about 2%. This part is dark gray and that is why it is called gray iron.

Carbon is present in the iron alloy in the form of flake graphite. It is very soft and can be cut. It is easy to move when hot and has good casting performance. More wear resistant. Poor strength and ductility. Due to its soft and lubricated graphite, this pig iron has good cutting, wear and casting properties. However, due to the presence of scaly graphite, its tensile strength is lowered, making it impossible to forge, and can only be used to manufacture various castings such as casting machine tools, iron pipes, etc. It is easy to rust when in touch with water.

Gray iron: high carbon content (2.7%~4.0%). Carbon mainly exists in the form of flaky graphite, and the fracture is gray, referred to as gray iron. Low melting point (1145~1250 °C), small shrinkage during solidification, compressive strength and hardness close to carbon steel, good shock absorption performance. It is used to make structural parts such as machine tools, cylinders and boxes.

Generally, there are the following heat treatment methods:
(1) Stress relief annealing heat treatment;
(2) Graphitization heat treatment;
(3) Matrix heat treatment changes.

When the temperature is below than 600 ° C, we should do stress relief annealing. In order to eliminate deformation or crack caused by internal stress, the casting is slowly heated (60~100 °C / h) to 500 ~ 550 °C for a period of time (every 10mm 2h). Then use the furnace to cool slowly (20~40 °C / h) to 150~200 °C air cooling

There are many kinds of gray iron

For example, HT250 has an annealing hardness of about 209HB and a tensile strength of 250MPa.

HT300 has an annealing hardness of about 231HB and a tensile strength of 300MPa.

HT300 has an annealing hardness of about 253HB and a tensile strength of 350MPa.