What are the key technical points in gray iron casting production?

What are the key technical points in gray iron casting production?

How to enhance the intrinsic and aesthetic quality of large castings, elevate their technical sophistication, and effectively compete in the market are challenges faced by several domestic casting enterprises. Every stage of the casting production process significantly impacts quality and cannot be overlooked—so, what are the key technical considerations for producing gray iron castings?

What are the key technical points in gray iron casting production?

1. After layered casting with sand cores and sand of varying hardness, the hydrostatic or solidification expansion forces of the molten iron often cause wall movement and core displacement, leading to internal and surface shrinkage defects in the casting. Therefore, to ensure dimensional stability of the castings, it is essential to achieve precise control over the casting's shape.

2. To save molding materials, hollow sand cores are typically used when making cores. These cores are lighter than solid cores, resulting in lower thermal capacity and slower solidification rates, which can lead to sand expansion or core cracking. Additionally, molten iron may seep through cracks in the core or sand core into the hollow sections, potentially causing defects in the casting. To enhance the rigidity of hollow sand cores, wet sand or water-glass sand can be packed inside; alternatively, shell cores can be made partially hollow with internal reinforcements added for extra strength. After core setting, the core will yield a robust, well-formed sand core.

What is the correct pouring temperature for gray iron castings?

1. Defects that may form when the temperature is too low

2. Manganese-sulfur porosity is located beneath the casting surface, with most of the pores appearing just below the skin. After machining, these porosity features often become visible, typically ranging in diameter from about 2 to 6 mm. Occasionally, the pores may contain small amounts of residual material. Metallographic analysis reveals that this defect is caused by MNS segregation and entrapped slag inclusions. The root cause lies in the low pouring temperature and the high levels of Mn and S present in the molten iron. To prevent this type of defect, when using a cupola furnace, continuous removal of air bubbles through porous materials can help reduce sulfur content to between 0.06% and 0.08%. Such controlled sulfur levels, combined with an optimal manganese content (0.5% to 0.65%), significantly enhance the purity of the molten iron, effectively mitigating the occurrence of these defects.

3. After treating the residual liquid, small holes—each measuring 1 to 3 mm in diameter—can be found beneath the surface of the castings at each monitoring point. In some cases, only one or two tiny holes appear. Research conducted in Jinxiang indicates that these small holes and trace amounts of residual liquid do occur, yet no evidence of compositional bias was detected during analysis. The study reveals that this type of defect is closely linked to the pouring temperature: when the pouring temperature exceeds 1380°C, such defects are completely absent in the castings. Therefore, the pouring temperature should be carefully controlled within the range of 1380–1420°C. It’s worth noting, however, that the design of the gating system failed to eliminate this defect entirely. Hence, this flaw can likely be attributed to both the relatively low pouring temperature and the presence of atmospheric gases interacting with the molten iron during the casting process.

4. Gas trapped in the sand core often leads to a weak or insufficient number of vent holes, typically due to poor venting during the sand-core-making process. Since sand cores usually harden within the core box during molding, this can result in inadequate venting capacity. To ensure proper gas escape, casting equipment can drill additional vent holes after the core has fully hardened.