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What is the classification and designation of cast iron?

What is the classification and designation of cast iron?

Update Time:2020-08-21
Cast Iron Classification And Designation Method
Cast iron is an iron-carbon alloy with a carbon content of more than 2.1%. It is obtained by remelting cast pig iron (part of steel-making pig iron) in the furnace, and adding ferroalloy, scrap steel, and reworked iron to adjust the composition. The difference from pig iron is that cast iron is secondary processing, mostly processed into cast iron parts. Iron castings have excellent castability and can be made into complex parts, and generally have good machinability. In addition, it has the characteristics of good wear resistance and shock absorption, and low price.
Representation of cast iron grades: (according to GB5612-85)
Various cast iron codes are composed of the first uppercase regular letter of the Chinese phonetic alphabet that represents the characteristics of the cast iron. When the code letters of the two cast iron names are the same, they can be distinguished by adding a lowercase regular letter after the uppercase regular letter. When the cast iron of the same name needs to be subdivided, take the first capitalized regular letter of the Chinese phonetic alphabet with its subdivision characteristics and arrange it at the back.
Cast Iron Name
Code Number
Representation Method Instance
Gray Cast Iron
HT
HT100...
Compacted graphite cast iron
RuT
RuT400...
Nodular cast iron
QT
QT400-17...
Black Heart Malleable Cast Iron
KHT
KHT300-06...
White Heart Malleable Cast Iron
KBT
KBT350-04...
Pearlitic malleable cast iron
KZT
KZT350-04...
Wear-resistant cast iron
MT
MT Cu1PTi-150
Anti-wear white cast iron
KmBT
KmBTMn5Mo2Cu
Anti-wear ductile iron
KmQT
KmQTMn6
Chilled cast iron
LT
LTCrMoR
Corrosion resistant cast iron
ST
STSi15R
Corrosion resistant ductile iron
SQT
SQTAl15Si5
Heat-resistant cast iron
RT
RTCr2
Heat-resistant ductile iron
RQT
RQTA16
Austenitic cast iron
AT
...
The group of numbers after the code name in the grade represents the tensile strength value; when there are two groups of numbers, the first group represents the tensile strength value, and the second group represents the elongation value. The two sets of numbers are separated by "one".

Alloying elements are represented by international element symbols. When the content is greater than or equal to 1%, they are represented by integers: generally not marked when the content is less than 1%. Conventional elements (C, Si, Mn, S, p) are generally not marked, and their symbol and content are marked only when they have special functions.

White cast iron: All the carbon in white cast iron is in the form of infiltrated carbon (Fe3c), because the fracture is bright white. Therefore, it is called white cast iron. Because of the large amount of hard and brittle Fe3c, white cast iron has high hardness, high brittleness and difficult processing. Therefore, it is rarely used directly in industrial applications, and is only used for a few parts that require wear resistance without impact, such as wire drawing dies, ball mill iron balls, etc. Mostly used as billets for steelmaking and malleable cast iron.


Gray cast iron; most or all of the carbon in cast iron exists as free flake graphite. The fracture is gray. It has good casting performance, good cutting workability, wear resistance, good wear resistance, plus it has simple melting ingredients, low cost, and is widely used in the manufacture of complex structural castings and wear-resistant parts.


Gray cast iron is divided into ferrite-based gray cast iron, pearlite-ferrite-based gray cast iron and pearlite-based gray cast iron according to different matrix structures.


Because of the presence of flake graphite in gray cast iron, graphite is a component with low density, low strength, low hardness, plasticity and toughness tending to zero. Its existence is like a large number of small gaps in the steel matrix, which reduces the bearing area and increases the source of cracks. Therefore, the gray cast iron has low strength and poor toughness, and cannot be pressed. In order to improve its performance, add a small amount of inoculants such as ferrosilicon and calcium silicon to the molten iron before pouring to refine the pearlite matrix.

Malleable cast iron: Malleable cast iron is made of white cast iron blanks cast from iron-carbon alloys with low carbon and silicon content, and then subjected to long-term high-temperature annealing treatment to decompose the cementite into flocculent graphite. Wrought iron is a white cast iron that has been graphitized.


Malleable cast iron is divided into two categories according to the different microstructures after heat treatment; one category is black core malleable cast iron and pearlescent malleable cast iron. The structure of black-core malleable cast iron is mainly ferrite (F) basic + flocculent graphite; the structure of pearlite malleable cast iron is mainly pearlite (P) matrix + flocculent graphite. The other type is white-core malleable cast iron. The structure of white-core malleable cast iron is determined by the size of the section. The small section is based on ferrite, the surface area of ​​the large section is ferrite, and the core is pearlite and annealed carbon.

The graphite becomes fine and uniformly distributed, and the cast iron is treated by this inoculation. It is called inoculated cast iron.

Nodular cast iron: Add a certain amount of spheroidizing agent (commonly used ferrosilicon, magnesium, etc.) to spheroidize the graphite in the cast iron before pouring the molten iron (spheroidal graphite iron). Because carbon (graphite) exists in the cast iron matrix in a spherical shape, it improves its splitting effect on the matrix, and the tensile strength, yield strength, plasticity, and impact toughness of ductile cast iron are greatly improved. And has the advantages of wear resistance, shock absorption, good process performance, low cost, etc. It has been widely substituted for malleable cast iron and some cast steel, forged steel parts, such as crankshafts, connecting rods, rolls, automobile rear axles, etc.