Method for improving machinability of gray cast iron

The tensile strength of gray iron castings has been increased from 200 MPa to 250 MPa or even 300 MPa. How to ensure that high-strength gray iron can be obtained while ensuring complex structure and processing technology, such as good cutting performance of engine cylinder block, has become another important problem in tackling the key problems of high-strength gray iron. The problem of poor machinability of high-strength gray cast iron is common in high-performance castings such as engines in China, and it is also the biggest gap between China and foreign castings.

The poor machining performance of domestic parts and imported parts is not “cutting” in the traditional sense, but the problems of high tool consumption, long cutting time and poor surface finish reflected in high-speed rough machining and finishing processes. The main reasons are:

Due to the large differences in different parts of complex castings such as cylinder block, it is more difficult to control the material uniformity of sections with different wall thickness. The thick wall is cooled slowly, the structure is easy to be coarse, and the strength and hardness are low, while the thin wall is cooled quickly, the white mouth structure is easy to appear, the hardness is too high, or supercooled graphite and high content ferrite structure appear, and the hardness is too low; In order to improve the strength of castings, alloying is usually used. The addition of alloy or trace elements such as titanium in the charge will affect the processability and so on.

In view of the above problems, if you want to improve the processability of gray cast iron, you must first improve the bad heritability of molten iron. The ways are to change the charge, use a variety of charges together, molten iron overheating, etc. among them, increasing the molten iron overheating temperature is a simple and effective measure. The second is inoculation treatment. The inoculant dissolves into the molten iron to promote the formation of graphite core and improve the graphitization ability of molten iron. It can minimize the white mouth tendency of thin-wall part, improve the microstructure of gray castings and refine grains. In this way, the graphite obtained in gray cast iron is small and fine, bent and evenly distributed, which greatly improves the machinability of gray cast iron.

Therefore, it is a very effective measure to improve the machinability of gray castings by improving the quality of inoculation treatment. The effects of different inoculants on the processability and mechanical properties of the materials were studied.