Hardness characteristics of grey cast iron

In steel, the ratio of Brinell hardness to tensile strength is constant, which is about 3. In cast iron, this ratio is very dispersed. At the same hardness, the tensile strength has a range. Similarly, at the same strength, the hardness also has a range, because the strength properties are greatly affected by graphite, and the hardness only reflects the matrix condition. Many factories use the hardness of cast iron to measure its tensile strength, and many materials also put forward σ B and HBS.

It must be pointed out that this estimation is reliable only when the process conditions are stable and the parameters of graphite sheets are similar. The hardness of grey cast iron depends on the matrix. This is because the hardness is measured by pressing a steel ball on the test block. The size of the steel ball is quite large relative to the graphite crack, so the external force is mainly received on the matrix. Therefore, with the increase of the number of pearlite in the matrix, the dispersion becomes larger, and the hardness is improved accordingly. When the metal matrix presents hard constituent phases (such as free cementite, phosphorus eutectic, etc.), The hardness increases accordingly.

Other properties of grey cast iron

Graphite reduces the mechanical properties of gray cast iron, but it brings a series of other excellent properties to gray cast iron.

• Good casting performance when the gray iron castings are formed, they not only have good mobility but also can reduce the solidification shrinkage due to the precipitation of graphite with a large specific volume during the solidification process. It is easy to obtain excellent castings and show good casting performance.

• Good vibration damping graphite can buffer the vibration energy received by the cast iron, weaken the transmission of vibration kinetic energy between grains, and convert the vibration energy into heat energy. Therefore, grey cast iron has good vibration damping.

• Good wear resistance graphite itself is also a good smoothing agent. The graphite scattered on the friction surface can play a smooth role. Therefore, grey cast iron has good wear resistance.

• Good cutting performance graphite plays the role of friction reduction and chip breaking when cutting is stopped. Because the scattered graphite forms micro cavities, which can store oil and maintain the continuity of oil film, the gray cast iron has good cutting performance and small tool wear.

• The low notch sensitivity of flake graphite is equivalent to many small notches, which reduces the sensitivity of castings to notches. Therefore, the adverse effect of poor surface processing quality or structural defects on the fatigue strength of cast iron is much smaller than that of steel.

Classification and application of grey cast iron

In consumption practice, various requests are often made for cast iron castings. Because the uses of various cast iron castings are different, the loads they bear are also different, Therefore, different performance requirements are put forward for cast iron, which is the practical basis for formulating cast iron grades.

There are six grades of grey cast iron, among which HT100, HT150, and HT200 are ordinary grey cast iron, and HT250, HT300, and ht350 are inoculated cast iron. Grey cast iron shall be selected according to different brands: ordinary grey cast iron (such as HT100 or HT150) can be selected for low load and unimportant parts, such as protective shield, small handle, cover plate, and heavy hammer; Inoculated cast iron (such as HT250) is generally used for parts with medium loads, such as engine base, support, box, pulley, bearing base, flange, pump body, valve body, pipeline, flywheel, and motor base; High strength gray cast iron above HT250 shall be used for important parts that accept the large load, such as base, bed, gear, cylinder, flywheel, gearbox, medium pressure valve body, cylinder block, and cylinder sleeve; For important parts with high load, wear-resistance and high air tightness, such as heavy machine tool bed, pressure machine body, high-pressure hydraulic parts, piston rings, gears and cams, high-strength gray cast iron above HT250 is generally selected.

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