Cast Iron

Gray cast iron grating

Gray cast iron is cast iron with flaky graphite, which is called gray cast iron because of the dark gray color of the fracture when fractured. The main components are iron, carbon, silicon, manganese, sulfur and phosphorus, and it is the most widely used cast iron, whose production accounts for more than 80% of the total cast iron production.

According to the morphology of graphite, gray cast iron can be classified as Ordinary gray cast iron, with graphite in the form of flakes.
Ductile cast iron, with graphite in the form of spheres. Malleable cast iron, with graphite in the form of flocculent masses. Creeping graphite cast iron, with graphite in the form of worms.

Gray cast iron characteristics


Gray cast iron has good casting and cutting properties and good wear resistance. Used in the manufacture of frames, boxes, etc.

The graphite of gray cast iron is in the form of flake, the effective bearing area is relatively small, and the graphite tip is easy to produce stress concentration, so the strength, plasticity and toughness of gray cast iron are lower than other cast iron. However, it has excellent vibration damping, low notch sensitivity and high wear resistance.

Gray cast iron composition


Gray cast iron has high carbon content (2.7%~4.0%), which can be regarded as the matrix of carbon steel with flake graphite. Gray cast iron is divided into three categories according to the different matrix organization: ferritic matrix gray cast iron; pearlite a ferritic matrix gray cast iron; pearlite matrix gray cast iron.
Ferritic gray cast iron is distributed with many and coarse graphite flakes on the matrix of ferrite, which has poor strength and hardness and is rarely applied.
pearlitic gray cast iron is distributed with uniform and fine graphite flakes on the matrix of pearlitic, its strength and hardness are relatively high, and it is often used in the manufacture of important parts such as beds and bodies.
Although the strength and hardness of this kind of cast iron is lower than the former, it can still meet the general body requirements, and its castability and shock absorption are good, and it is easy to melt, so it is the most widely used gray cast iron.
The difference in microstructure of gray cast iron is essentially the difference in the form of carbon in cast iron. The carbon in gray cast iron is composed of chemical carbon (Fe3C) and graphite carbon. When the carbon is 0.8%, it is pearlite gray cast iron; when the carbon is less than 0.8%, it is pearlite-ferritic gray cast iron; when all the carbon exists in graphite state, it is ferritic gray cast iron.

Mechanical properties of gray cast iron


The mechanical properties of gray cast iron are related to the organization of the matrix and the morphology of graphite. The flake graphite in gray cast iron has a serious cut on the matrix, and it is easy to cause stress concentration at the sharp corners of graphite, so that the tensile strength, plasticity and toughness of gray cast iron are much lower than steel, but the compressive strength is comparable to steel, and it is also the cast iron with the worst mechanical properties in common cast iron parts. At the same time, the matrix organization on the mechanical properties of gray cast iron also has a certain impact, ferritic matrix gray cast iron graphite flake coarse, the lowest strength and hardness, so less application; pearlite matrix gray cast iron graphite flake fine, have high strength and hardness, mainly used to manufacture more important castings; ferritic a pearlite matrix gray cast iron graphite flake than pearlite gray cast iron slightly coarser, less performance than pearlite gray cast iron. Therefore, the industrial use is more pearlite matrix gray cast iron.

Other properties of gray cast iron


Gray cast iron has good casting performance, good vibration damping, good wear resistance, good cutting performance, and low notch sensitivity.

Influencing factors of gray cast iron properties


Influence of chemical composition. Production is mainly to control the quality fraction of carbon and silicon. If the mass fraction of carbon and silicon is too low, the cast iron is easy to appear white mouth, and the mechanical properties and casting performance are very low; if the mass fraction of carbon and silicon is too high, the graphite flakes are too much and coarse, and even the graphite floats on the surface of the iron, which reduces the performance and quality of castings. Therefore, the carbon and silicon content of gray cast iron is generally controlled at 2% to 4% C; 1.0% to 2.0% Si; 0.5% to 1.4% Mn.
The effect of cooling rate: Under certain casting process conditions, the cooling rate of the casting has a great impact on the degree of graphitization. Castings of different wall thickness with the increase in wall thickness, the cooling rate slows down, carbon atoms have sufficient diffusion time, it is conducive to the full graphitization process, room temperature organization is easy to form gray iron organization; but thin-walled parts in the cooling process, the cooling rate is too fast, easy to form a white mouth iron organization.

Gray cast iron heat treatment


Heat treatment of gray cast iron can only change the matrix organization, can not change the form of graphite, thus it is impossible to significantly improve the mechanical properties of gray cast iron parts. Heat treatment of gray cast iron is mainly used to eliminate internal stress and white mouth organization, stabilize size, improve cutting performance, increase surface hardness and wear resistance, etc.

Internal Stress Relief Annealing

It is used to eliminate the casting stress caused by uneven cooling during solidification to prevent the casting from deformation and cracking. The process is to heat the castings to 500 ~ 600 ℃, after holding for a period of time with the furnace slow cooling to 150 ~ 200 ℃ below the furnace air cooling, sometimes the castings are placed in the natural environment for a long time, so that the internal stress of the castings to relax, this method is called “natural aging”, large gray cast iron parts can be used to eliminate the casting Stress.

Graphitic annealing


To eliminate the white mouth organization, reduce the hardness and improve the cutting performance. The method is to heat the casting to 850 ~ 900 ℃, holding 2 ~ 5 hours, and then slow cooling with the furnace to 400 ~ 500 ℃, and then out of the furnace air cooling, so that the carburized body in the insulation and slow cooling process decomposition and the formation of graphite.

Surface quenching


Improve the surface hardness and extend the service life. Such as for machine tool guide surfaces and internal combustion engine cylinder liner wall and other gray cast iron working surface, the need for high hardness and wear resistance, surface quenching method can be used. Commonly used methods are high (medium) frequency induction heating surface quenching and contact resistance heating surface quenching.

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