Factors Influencing the Quality of Ductile Iron Pipes

(1) Carbon Equivalent: An excessively high carbon equivalent, especially with high silicon content, affects the spheroidizing of graphite. Tests indicate that thick-walled castings may develop flowered graphite when the carbon equivalent exceeds the eutectic composition. However, increasing the carbon content enhances the recovery rate of magnesium in the iron melt. Therefore, the production principle for most high-carbon, low-silicon grades typically controls the silicon content around 2%. Moreover, the selection of carbon equivalent based on casting wall thickness: for tube walls with thicknesses of 6.5 to 76 millimeters, a carbon equivalent of 4.35% to 4.7%; for wall thicknesses greater than 76 millimeters, a carbon equivalent of 4.3% to 4.35%.

(2) Sulfur: When the sulfur content in the iron melt is too high, sulfur reacts with magnesium and rare earth elements to form sulfides, which due to their low density, float to the surface of the iron melt. These sulfides then react with oxygen in the air to produce more sulfur, which returns to the iron melt, repeating the process, and subsequently reducing the magnesium and rare earth element contents. When the sulfur content in the iron melt exceeds 0.1%, even the addition of a large amount of desulfurizing agents cannot fully spheroidize the graphite.

(3) Rare Earth and Magnesium: Insufficient rare earth and magnesium content often leads to poor spheroidization or spheroidization hesitation. Generally, factories require the participation amount of spheroidizing agent to be between 1.8% and 2.2%.

(4) Thicker casting wall can also lead to poor spheroidizing and dross defects. The primary reason is that molten iron remains in the mold for an extended period, causing magnesium vapor to rise and reduce magnesium content. During the eutectic reaction, the large amount of graphite generated releases latent heat, which re-melts the austenite shell. The protruding graphite from the shell deforms and grows, forming non-spherical graphite.

(5) Temperature: If the molten iron temperature is too high, severe oxidation occurs, as magnesium and rare earth elements readily react with oxides, leading to a decrease in their content. Additionally, high temperatures will increase the burn and evaporation of magnesium. Conversely, if the molten iron temperature is too low, the balling agent cannot melt and be absorbed by the molten iron, instead floating to the surface where it burns or oxidizes.

(6) Dwell Time: The magnesium content in the molten iron decreases with the addition of dwell time after the孕育 treatment, primarily due to oxidation and evaporation of sulfur, magnesium, and rare earth elements. Generally, the dwell time does not exceed 20 minutes.

(7) Sprue design is unreasonable, leading to excessively long pouring time, iron splash, and participation from the air, which causes severe issues with magnesium and rare earth oxides.