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Heat treatment of gray cast iron can only alter its matrix structure, not the graphite morphology. Therefore, heat treatment cannot significantly change the mechanical properties of gray cast iron. Moreover, the low plasticity of gray cast iron makes rapid cooling heat treatment methods difficult to implement, thus imposing certain limitations on its heat treatment. Its primary applications are for stress relief and improving machinability.

1. Stress Relieving (Age Hardening) - Low Temperature Annealing. Place the casting in an oven at 100~200°C, gradually heat to 500~600°C, hold for 4~8 hours, then cool slowly.

2. Annealing for Improved Cutting Performance - High-temperature annealing involves heating the casting to 850~900°C, holding for 2~5 hours, and then slowly cooling to 400~500°C before air cooling.

3. Surface Hardening - Grades and Applications of Grey Cast Iron for Increased Hardness and Wear Resistance

1. A total of six brand numbers: HT100, HT150, HT200, HT250, HT300, HT350

1. Chemical Composition: Wc=2.4%~2.8% 2. Manufacturing Method: Ductile cast iron is obtained from a certain composition of white cast iron, which is graphitized and annealed for a long time to produce spheroidal graphite.

3. Graphitization Annealing Process: After holding at 900~1000℃ for 15 hours, the material is slowly cooled in the furnace to below 650℃ and then air-cooled to obtain forgable cast iron with an F matrix.

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Gray cast iron refers to cast iron that contains flake graphite. Because the fracture surface is dark gray when it breaks, it is named gray cast iron. Its carbon content is

Grey cast iron can be considered as the matrix of carbon steel with flake graphite. Depending on the different matrix structures, grey cast iron is divided into three categories: ferrite matrix grey cast iron; spheroidite-ferrite matrix grey cast iron; and spheroidite matrix grey cast iron.

The mechanical properties of gray cast iron are related to the matrix structure and the shape of graphite. The flake-like graphite in gray cast iron severely disrupts the matrix, easily causing stress concentration at the sharp corners of the graphite, resulting in significantly lower tensile strength, ductility, and toughness compared to steel. However, its compressive strength is comparable to steel, making it the worst in terms of mechanical properties among commonly used cast irons. Additionally, the matrix structure also affects the mechanical properties of gray cast iron. Gray cast iron with ferrite matrix has coarse graphite flakes, lower strength and hardness, and is less commonly used. Pearlite matrix gray cast iron has finer graphite flakes, higher strength and hardness, and is primarily used for manufacturing important castings. Ferrite-pearlite matrix gray cast iron has slightly coarser graphite flakes than pearlite gray cast iron and inferior properties. Therefore, pearlite matrix gray cast iron is more commonly used in industry.

Cast iron is an iron-carbon alloy with a carbon content greater than 2.11%. It is obtained by melting and casting industrial pig iron, scrap steel, and other ferrous and non-ferrous metal alloys at high temperatures. In addition to iron, it contains carbon, which precipitates as graphite in cast iron. Cast iron with flake-like graphite is called gray cast iron or gray iron; with worm-like graphite, it is called spheroidal cast iron; with clump-like graphite, it is called white cast iron or malleable iron; and when it is spherical, it is known as ductile iron.

Gray iron castings are widely used in all major industrial sectors, which require high strength, plasticity, toughness, wear resistance, resistance to severe thermal and mechanical shocks, resistance to high or low temperatures, corrosion resistance, and dimensional stability.

Graphite cast iron, excluding iron, typically contains: carbon content of 3.0% to 4.0%, silicon content of 1.8% to 3.2%, and the total content of manganese, phosphorus, and sulfur not exceeding 3.0%, along with appropriate amounts of rare earth elements and magnesium for nodularity.

The Difference Between Gray Cast Iron and Ductile Cast Iron

Graphite in gray cast iron exists in a flake-like form, while in ductile iron it is spherical. This structural difference results in significant variations in their properties.

（1）Strength and ductility

Gray cast iron has low strength and plasticity (flake graphite splits the matrix, causing stress concentration), high brittleness, and good vibration damping properties. It is mainly used for producing various types of bases and frames that do not require high strength and mainly bear compressive stresses. Spheroidal cast iron: the splitting effect of spherical graphite on the matrix is reduced, and the stress concentration is minimized, thus it has high strength, comparable to medium-carbon steel, and can fully utilize the matrix's properties, with certain plasticity and good toughness. It is often used for manufacturing parts with high strength and toughness requirements and complex shapes (better casting properties than steel), such as engine crankshafts and connecting rods, and similar components.

(2) Heat treatment processing

Ductile iron can generally be heat treated for strengthening, whereas gray iron typically cannot be heat treated to enhance strength (due to the influence of flake graphite). (3) Compared to cast iron, ductile iron has an advantage in terms of strength.

(3) Mechanical PropertiesGraphite cast iron has a tensile strength of 60k, while cast iron's is only 31k. Graphite cast iron has a yield strength of 40k, whereas cast iron doesn't exhibit a yield strength and ultimately breaks. The strength ratio of graphite cast iron is significantly better than that of cast iron. Graphite cast iron is comparable in corrosion resistance to cast iron. The strength of graphite cast iron is comparable to that of cast steel. Graphite cast iron has a higher yield strength, at 40k, while cast steel's yield strength is only 36k.

(4) Corrosion and oxidation resistanceDuctile iron boasts superior corrosion and oxidation resistance compared to cast steel. Due to its spherical graphite microstructure, ductile iron excels in reducing vibration, making it more beneficial for stress relief. One key reason for choosing ductile iron lies in its exceptional properties.

Gray iron castings have gained popularity due to their excellent properties, leading to higher casting efficiency and reduced machining costs for gray iron. Sometimes, gray iron is referred to as "metal of two worlds," indicating it possesses the strength of cast steel and the superior corrosion resistance of cast iron.