Gray cast iron is the most widely used cast iron at present. It is named for its gray section color. In addition to the highest content of Fe, there are also C, Si, Mn, P, s and other elements in gray cast iron; Among them, C, Si and Mn are the elements used to adjust the structure, P is the element used to control, and S is the element that should be limited. In the current industrial application, the content of each element of gray cast iron is: WC = 2.7% – 3.6%, WSI = 1.0% – 2.5%, WMN = 0.5% – 1.3%, WP ≤ 0.3%, WS ≤ 0.15%.
According to the different microstructure and composition of gray cast iron, it can be divided into six brands: HT100, HT150, HT200, HT250, HT300 and ht350. Different grades of gray cast iron have different chemical compositions and different contents of various elements. The application scope of gray cast iron is also different according to different brands. Gray cast iron is generally used for simple and unimportant parts such as cover, cover, oil pan, hand wheel, handle, support, base plate and heavy hammer.
Gray cast iron is widely used in industrial production, which is characterized by:
(1) Mechanical properties: due to the existence of graphite in gray cast iron, its mechanical properties are relatively lower than those of steel. The existing form of graphite in gray cast iron has a great impact on its mechanical properties. The more flake graphite, the wider the distribution range and the larger the size, the greater the negative impact on the metallographic structure, and the worse the strength, hardness and shaping of cast iron.
(2) Other properties: the presence of graphite inside gray cast iron will reduce some mechanical properties of cast iron, but graphite can also bring some excellent properties to gray cast iron:
① Good casting performance. Compared with industrial steel, the structure of carbon equivalent in gray cast iron is close to eutectic composition, which makes the melting point of gray cast iron lower than that of steel and the fluidity of molten metal better than that of steel. In the solidification process of cast iron production, the graphite matrix will expand and self feed the casting, which can effectively eliminate the defects of shrinkage cavity and porosity in the casting.
② Good abrasion resistance. Graphite is a lubricant. The existence of flake graphite in gray cast iron makes the parts have a certain lubricating ability. Moreover, flake graphite will fall off on the surface of cast iron and produce pores. These pores can well absorb and store other lubricants or lubricating oil, which can reduce the wear of parts in the process of use. Therefore, gray cast iron is often used in the production of easily worn diesel engine cylinder block, lathe guide rail and other castings.
③ Strong vibration damping. Graphite has damping performance. Due to the existence of graphite, it can cut off the transmission of vibration in iron castings and convert the vibration energy. Therefore, the damping performance of gray cast iron is better than that of steel. Gray cast iron is also used in the manufacture of pressure bearing and earthquake resistant parts such as box, machine tool underframe and bed.
④ Good machinability. Flake graphite in gray cast iron cuts off the continuity of the casting matrix, so that the cutting performance of iron castings is good, and the graphite has lubrication effect, which can prolong the service life of the tool.
⑤ Small notch sensitivity. Different from steel, the existence of graphite in gray cast iron cuts a large number of notches into the casting matrix, making the effect of external notches (such as oil holes, keyways, knife marks, etc.) on gray cast iron relatively weak. Therefore, the notch sensitivity of cast iron is smaller than that of steel (steel often forms stress concentration and mechanical properties are reduced due to the role of external notches).
Due to its low price, excellent performance and convenient production, gray cast iron is widely used in the production and manufacturing at this stage.