How to Achieve Vermicular Graphite Shape in Casting Process

Vermicular graphite iron (VGI), also known as compacted graphite iron (CGI), is a high-performance cast iron material with properties between gray iron and ductile iron. Its unique vermicular graphite shape endows it with excellent thermal fatigue resistance, high elastic modulus and superior dimensional stability, making it widely used in automotive, machinery, aerospace and other industries, especially for key components such as engine cylinder blocks, brake drums and machine tool beds. The formation of vermicular graphite shape is not accidental; it requires precise control of the casting process, chemical composition and treatment technology. This article details the key steps and technical points to achieve vermicular graphite shape during the casting process, providing practical guidance for foundry enterprises.

The core of achieving vermicular graphite shape lies inprecise control of the vermicularization treatment, which is the key link to transform the graphite growth mode from flake or spherical to vermicular. Unlike ductile iron which requires complete spheroidization and gray iron which forms flake graphite naturally, vermicular graphite requires a "moderate" treatment effect—too much vermicularizing agent will lead to spheroidization, while too little will result in flake graphite, forming a narrow processing window. The specific implementation steps and key controls are as follows:

First, strict control of base iron composition is the foundation. The chemical composition of the base iron directly affects the effect of vermicularization treatment and the final graphite shape. It is necessary to strictly control the content of key elements: carbon (C) should be controlled at 3.7% - 3.9%, silicon (Si) at 1.9% - 2.1%, which can promote graphite nucleation and growth, and avoid defects such as shrinkage and white spots. Sulfur (S) is a harmful element that affects vermicularization effect; its content must be controlled below 0.02%, because excessive sulfur will consume the vermicularizing agent and lead to incomplete vermicularization. In addition, the content of phosphorus (P) should be ≤ 0.03%, and the content of manganese (Mn) should be 0.2% - 0.4% to avoid adverse effects on casting performance. The base iron should be melted in an intermediate frequency furnace, and the furnace charge should be composed of 70% - 80% packed scrap steel and 20% - 30% recycled materials to ensure the uniformity and purity of the base iron.

Second, accurate vermicularization treatment is the core step. The commonly used vermicularizing agents are magnesium-titanium-cerium (Mg-Ti-Ce) alloys, which can effectively change the growth direction of graphite and promote the formation of vermicular shape. The treatment method usually adopts the "sandwich method" in a preheated ladle: first add a layer of vermicularizing agent at the bottom of the ladle, cover it with a layer of inoculant, then pour the molten iron into the ladle, and stir evenly to ensure full reaction between the molten iron and the vermicularizing agent. The addition amount of the vermicularizing agent must be accurately controlled according to the composition of the base iron and the treatment temperature, generally accounting for 0.2% - 0.5% of the molten iron quality. Too much addition will lead to spheroidization of graphite, while too little will fail to achieve the vermicular effect. At the same time, the treatment temperature should be controlled between 1420℃ and 1450℃; if the temperature is lower than 1400℃, the vermicularizing agent cannot melt completely, resulting in uneven distribution; if the temperature exceeds 1480℃, excessive oxidation of alloy elements will occur, leading to vermicularization failure.

Third, timely and effective inoculation treatment is an important guarantee. After vermicularization treatment, the molten iron is prone to graphite spheroidization or white mouth defects, so inoculation treatment must be carried out immediately. The inoculant is mainly silicon-iron alloy, which can promote graphite nucleation, refine graphite particles, and ensure the uniformity of vermicular graphite shape. The inoculation method adopts post-inoculation, that is, adding the inoculant into the molten iron after vermicularization treatment, with the addition amount of 0.3% - 0.6%. The inoculation must be timely and uniform to avoid the decline of inoculation effect caused by long-term storage of molten iron.

Fourth, strict control of casting process parameters is necessary to ensure the stable formation of vermicular graphite. The pouring temperature should be controlled between 1380℃ and 1420℃; too high temperature will lead to excessive oxidation of molten iron and affect graphite growth, while too low temperature will reduce the fluidity of molten iron and cause casting defects. The pouring speed should be stable and uniform to avoid卷入 gas and inclusions, which will affect the nucleation and growth of graphite. In addition, the mold temperature should be controlled reasonably; too fast cooling will lead to white mouth, while too slow cooling will cause graphite to grow excessively and form flake graphite. After casting, the casting should be cooled slowly to avoid thermal stress and ensure the stable shape of vermicular graphite.

Finally, strict quality inspection is the key to ensuring the qualification of vermicular graphite shape. After casting, metallographic inspection should be carried out on the casting to check the shape, distribution and vermicularization rate of graphite. The qualified vermicularization rate should be above 80%; if the vermicularization rate is not up to standard, the cause should be analyzed (such as improper addition of vermicularizing agent, unqualified base iron composition, etc.) and corresponding measures should be taken to adjust, so as to ensure that the final casting meets the technical requirements.

In summary, achieving vermicular graphite shape in the casting process requires the combination of strict base iron composition control, accurate vermicularization treatment, effective inoculation treatment, scientific casting process parameters and strict quality inspection. Only by mastering these key technical points and controlling each link precisely can we stably produce high-quality vermicular graphite castings, meet the needs of various industrial fields, and enhance the market competitiveness of products.