Compared with ordinary carbon steel, the properties of gray cast iron have the following characteristics.
(1) Low mechanical properties. The tensile strength of gray cast iron is relatively low, which is inseparable from the structural characteristics of gray cast iron. Due to the low mechanical properties of graphite, the microstructure of gray cast iron is actually equivalent to steel full of holes or cracks. In tension, the tensile strength of flake graphite is much lower than that of steel due to the division effect of flake graphite on steel matrix and the stress concentration effect.
(2) Excellent wear resistance and shock absorption. Because the graphite in cast iron is conducive to lubrication and oil storage, it has good wear resistance. Similarly, due to the existence of graphite, the shock absorption of gray cast iron is better than that of steel.
(3) Good process performance. Due to the high carbon content of gray cast iron, which is close to eutectic composition, the melting point is lower than that of steel, so the casting fluidity is good. In addition, because graphite is easy to form chips during cutting, the machinability of gray cast iron is better than that of steel.
The grade of gray cast iron expressed by its mechanical properties starts with “HT”, followed by three digits, where “HT” represents gray cast iron and the number is its lowest tensile strength value, such as HT200 φ The tensile strength measured by 30mm single cast test bar is greater than 200MPa (but less than 300mpa).
According to GB 5675-85, gray cast iron is divided into six grades: htl00, htl50, HT200, HT250, HT300 and ht350. Among them, htl00 is ferrite gray cast iron, htl50 is pearlite ferrite gray cast iron, HT200 and HT250 are pearlite gray cast iron, and HT300 and ht350 are inoculated cast iron.