Main casting defects of nodular cast iron

1.Shrinkage cavity, shrinkage porosity and casting defects

Due to the characteristics of paste solidification of nodular cast iron, the produced cast iron often produces casting defects such as shrinkage cavity and porosity due to poor feeding. In order to predict these casting defects before casting production, numerical analogy of casting process was carried out at home and abroad as early as the Indian era. The numerical analogy of casting process is to use the numerical analogy technology to simulate the actual casting formation process in the computer virtual environment, including the mold filling process of metal liquid, cooling and solidification process, stress formation process, judge the influence degree of main elements in the forming process, predict the organization, performance and possible casting defects, and provide the basis for optimizing the process and reducing waste products.

In 1962, forsund of Denmark was the first to use computer to simulate the solidification process of castings. Since then, the United States, Britain, Germany, Japan, France and other countries have carried out research in this field. Since the late 1970s, Dalian University of technology and Shenyang Foundry Research Institute took the lead in carrying out the research on this technology in China, and published research reports respectively in 1980 (Guo Keren et al., digital analogy of solidification process of large castings, Journal of Dalian Institute of technology; Shenyang Foundry Research Institute, electronic computer analogy of solidification heat field of castings, casting, and then invested a lot of manpower in Colleges and universities in China to carry out this research.

During the “sixth five year plan” and “Seventh Five Year Plan” period, the central part of the national key research project has a key research project of computer application in casting. “Sixth five year plan” project is “solidification control of large steel castings” and “Seventh Five Year Plan” project is “CAD of casting process of large steel castings”. The joint research of production, learning and research has been organized, which has greatly promoted the development of this technology in China Huazhong University of technology has been able to provide FT-Star and Huazhu cae-inte 4.0 commercial chemistry software respectively, which have been applied in Sanming Heavy Machinery Co., Ltd. and other units, and achieved good results.

Computer numerical analogy consists of three parts: pre-processing, intermediate calculation and post-processing, including the establishment of geometric model, grid division, determination of solution conditions (initial conditions and boundary conditions), numerical calculation, processing of calculation results and graphic display. The basic methods of numerical analogy are mainly finite difference method, finite element method and boundary element method.

(1) Numerical analogy of solidification process is mainly used for heat transfer analysis of casting process. It includes the selection of numerical calculation method, latent heat treatment, prediction and discrimination of shrinkage cavity, and treatment of heat transfer at the interface of casting and mold.

(2) The numerical analogy of flow field involves the transfer of momentum, energy and mass, which is difficult. The numerical solution techniques used include MAC method, SAMC method, sola-aof method and sola-mac method.

(3) For the analogy of casting stress, this research was carried out relatively late and mainly carried out the stress division of elastic-plastic state, including Heyn model, elastic-plastic model, perzyna model, unified internal variable model, etc.

(4) Organizational analogy is still in its infancy. It can be divided into macro vision, meso vision and micro vision analogy. It can calculate the nucleation number, analyze the primary crystal type, dendritic growth rate, simulate the microstructure transformation and predict the mechanical properties. There are deterministic models, statistical models such as Monte, cellular and automaton, phase field models, etc.

Computer and its application is a rapidly developing technical field. Casting, as one of the important industrial fields, should strengthen investment. The research and development of the application of computer in the field of casting research and production will completely change the state of “open eye type and closed eye pouring” in the past. The application of computer will also promote the application and development of nodular cast iron.

2.Porosity casting defects

In the production process of nodular iron castings, after heat treatment, shot blasting or machining, some holes with a diameter of about 0.5-3mm, spherical, elliptical or pinhole shape and smooth inner wall are often found. These holes are generally distributed 2-3mm under the casting skin, which is the so-called subcutaneous pores.

The formation of subcutaneous pores is due to the high tension on the surface of magnesium containing molten iron, which is easy to form oxide film, which has a certain impact on hindering the discharge of precipitated gas and invading gas. These gases will form pores when they stay under the skin. In addition, the paste solidification characteristics of nodular cast iron make the gas channel blocked earlier, which will also promote the formation of subcutaneous porosity casting defects.