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What Is Grey Cast Iron?

What Is Grey Cast Iron?


The Chemical Composition of Grey Cast Iron


Carbon, silicon and manganese in cast iron are the elements that mediate the texture. Phosphorus is the element used for control. Sulfur is the element that should be restricted. Currently, the chemical composition of grey cast iron is generally ranges from wC=2.7%~3.6%,wSi=1.0%~2.5%,wMn=0.5%~1.3%,wP≤0.3%,wS≤0.15%.


The texture of Grey Cast Iron


Grey cast iron is cast iron formed when both the first stage and the second stage graphitization process are fully performed. Its microstructure is characterized by the distribution of flake graphite on various matrix structures. Due to the different extent of graphitization in the third stage, grey cast iron with three different matrix structures can be obtained:

a) iron wire cast iron; b) pearlite grey cast iron; c) iron-body pearlite grey cast iron


Property of Crey Cast Iron


Mechanical properties: The tensile strength, plasticity, toughness and elastic modulus of grey cast iron are much larger than the number of low graphite sheets of the corresponding matrix. The larger the size, the more uneven the distribution, the more severe the splitting effect and stress concentration on the substrate. The lower the strength, ductility and toughness of cast iron. Since the compressive strength σbc, hardness and wear resistance of gray cast iron are mainly depend on the matrix, the presence of graphite has slight effect on it, so when the compressive strength of grey cast iron is generally 3 to 4 times of its tensile strength, on that condition, the pearlite grey cast iron has higher strength, hardness and wear resistance than that of other two matrix structures.


Other properties: Although graphite reduces the tensile strength, ductility and toughness of cast iron, it is also due to the presence of graphite that gives cast iron a range of other excellent properties.


Good casting performance: As the carbon equivalent of grey cast iron is close to the eutectic composition, so the melting point is low, the fluidity is good when compared with steel, and the graphite has a larger specific volume of graphite during the solidification process, partially compensating for the shrinkage of the matrix, thereby reducing the shrinkage of gray cast iron, so grey cast iron can cast castings with complex shapes and thin walls.



Heat Treatment of Grey Cast Iron


1. Elimination of internal stress annealing


It is used to eliminate the casting stress caused by uneven cooling during the solidification process of the casting, and to prevent deformation and cracking of the casting. The process is to heat the casting to 500-600 ° C, and after the heating for a period of time, the furnace is slowly cooled to 150-200 ° C or less to be air-cooled, and sometimes the casting is placed in a natural environment for a long period of time, so that the internal stress of the casting is relaxed. The method is called “natural aging”, and large grey cast iron castings can use this meathod to eliminate casting stress.


2. Surface hardening


The purpose of surface quenching is to improve the surface hardness and wear resistance of gray cast iron parts. In addition to induction heating surface quenching, cast iron can also be surface hardened by contact resistance heating.


Grade and Application of Grey Cast Iron


According to the provisions of GB/T 9439-1988, the grey cast iron is divided into six grades according to the tensile strength of a 30 mm diameter single casting test bar. The grade of grey cast iron is represented by the "HT" ("grey iron" Chinese pinyin prefix) and the minimum tensile strength σb (using the tensile strength of the φ30mm test bar). For example, the grade HT250 indicates a gray cast iron having a minimum tensile strength value of 250 MPa for a φ30 mm test bar. When designing a casting, the cast iron grade should be selected according to the main wall thickness or average wall thickness at the force of the casting.


Type Grade Thickness /mm Mechanical Property Application
σb/MPa≥ HBS
Ferritic grey cast iron HT100 2.5~10 130 10~166 Applicable to unimportant castings with small load and no special requirements for friction and wear, such as protective cover, cover, oil pan, hand wheel, bracket, bottom plate, heavy hammer, small handle, etc.
10~20 100 93~140
20~30 90 87~131
30~50 80 82~122
Ferrite - pearlite gray cast iron HT150 2.5~10 175 137~205 Castings with medium load, such as machine base, bracket, box, tool holder, bed, bearing housing, work table, pulley, end cover, pump body, valve body, pipeline, flywheel, motor seat, etc.
10~20 145 119~179
20~30 130 110~166
30~50 120 105~157
Pearlitic gray cast iron HT200 2.5~10 220 157~236 Cylinder, gear, frame, flywheel, bed, cylinder block, cylinder liner, piston, gear box, brake wheel, coupling plate, medium pressure valve body, etc.
10~20 195 148~222
20~30 170 134~200
30~50 160 129~192
HT250 4.0~10 270 175~262
10~20 240 164~247
20~30 220 157~236
30~50 200 150~225
Breeding cast iron HT300 10~20 290 182~272 Important castings with high load, wear resistance and high air tightness, such as heavy machine tools, shears, presses, automatic lathe beds, stands, frames, high-pressure hydraulic parts, piston rings, gears with high force, Cam, bushing, crankshaft, cylinder block, cylinder liner, cylinder head of large engine
20~30 250 168~251
30~50 230 161~241
HT350 10~20 340 199~298
20~30 290 182~272
30~50 260 171~257




Pub Time : 2018-11-28 10:30:12 >> News list
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