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In the iron-copper system, copper can dissolve with molten iron to form a liquid solid solution in a large concentration range. At 1477°C, the solubility of copper in γ-Fe is 8%, and the temperature drops, and the solubility even increases. For example, at 1094°C, the solubility can reach 8.5%. When the temperature is below 650℃, the solubility is close to the fixed value, about 0.35%. At room temperature, the solubility of copper in iron is very small. Regardless of whether it is liquid or solid, copper-rich phases are formed when the amount of copper exceeds the solubility at various temperatures. The solubility of copper in molten cast iron is about 3-5%. The elements such as manganese, aluminum and nickel in cast iron increase the solubility of copper, while magnesium reduces the solubility of copper. With magnesium and copper exceeding 2%, a yellow copper-rich phase containing 97% copper is precipitated. When the copper content increases to 3.8%, the amount of the copper-rich phase increases greatly.

The Influence of Copper on the Eutectic Transition Temperature of Cast Iron
Copper is a weak graphitizing element, which can slightly reduce the tendency of white mouth. Copper reduces the eutectoid transformation temperature. For every 1% increase in copper, the eutectoid temperature drops by 6-10°C, which stabilizes austenite. Copper can also increase the amount of pearlite, refine the eutectic cluster, and make it easier to obtain the pearlite structure of the cast iron during normalizing. Therefore, copper has the effect of promoting the formation of pearlite from austenite and hindering graphitization during the eutectoid transformation. Copper can also strengthen ferrite and increase its strength. When the copper content is below 2.0%, there is no significant effect on the spheroidization rate, the number of balls, and the size of the ball diameter, and the spheroidization rate can be guaranteed to reach more than 90%. When the copper content exceeds 2%, the spheroidization effect will be affected, the spheroidization rate will decrease sharply, and the number of balls will decrease sharply. When the copper content reaches 2.5%, agglomerated graphite and yellow copper-rich phase appear. The melting point of the copper-rich phase is lower than that of the molten iron. After the graphite is precipitated, the copper-rich phase may enclose a part of the graphite, so that the graphite gradually changes from a spherical shape to a lumpy shape.

The despheroidization of copper is related to other elements
For example, magnesium spheroidal graphite cast iron with 0.04% titanium, if agglomerated graphite appears at 0.94% copper, adding 0.02-0.03% cerium can counteract the de-spheroidization effect of copper. The influence of copper on the spheroidization effect is related to the size and thickness of the casting. Adding 3-4% copper to castings with a wall thickness of more than 300mm can eliminate the clumpy graphite in the core of the casting. It should be pointed out that this effect of copper is only effective when it is added together with rare earth elements. Copper promotes the formation of pearlite in ductile iron, which is 10 times more powerful than nickel, but only 1/10 of tin. In the experiment, adding about 0.5% of copper can make the amount of pearlite in the φ25mm ductile iron test bar reach 90%; adding 1.5% of copper, the pearlite content is close to 100%; and when it exceeds 1.5%, ferrite will appear instead. The good effect of copper allows people to obtain as-cast pearlitic ductile iron without heat treatment.

The influence of copper on the properties of gray cast iron
When the amount of copper added is less than 0.5%, it will increase the tensile strength, yield strength, and increase the elastic modulus, but it is not linear. Gray cast iron has the highest strength when its copper content is about 0.8%. As the copper content increases, the impact toughness of gray cast iron decreases. Copper affects the mechanical properties of ductile iron by promoting the formation of pearlite and strengthening the metal matrix. Adding a small amount of copper (less than 0.5%) the content of pearlite increases sharply, the tensile strength, yield strength and hardness also increase sharply, and the elongation decreases sharply. Adding more than 0.5%, the matrix has all been pearlite, but the strength and hardness continue to rise, and the elongation rate no longer drops. This is because copper strengthens the metal matrix. Copper has the function of refining graphite balls and can improve the fatigue strength of copper-containing ductile iron. The solid solubility of copper in cast iron at room temperature is 0.35%. The addition of more than 0.5% copper changes the diffusion rate of carbon atoms during the austenite and eutectoid transformation, promotes the formation of pearlite and refines the pearlite. When the casting is cooled, especially when aging, copper precipitates out of the solid solution to strengthen the matrix and significantly increase the microhardness of the pearlite. Increasing the copper content can increase the wear resistance of the casting. When performing stress relief annealing, the copper will not only reduce the hardness, but also increase the HB10 degree.

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