Effect of composition on Machinability of gray cast iron

Because the flake graphite in gray cast iron has good chip breaking and tool lubrication, the machinability of gray cast iron is greatly affected by the carbon content, so increasing the carbon content of gray cast iron can improve its machinability. In order to ensure the mechanical properties of gray cast iron, some alloy elements need to be added during melting, and different alloy additions have different effects on gray cast iron.

Copper not only improves the strength of gray cast iron, but also does not increase the white tendency of molten iron. It has become the most commonly added alloy element in the production of high-grade gray cast iron. Copper is a stable pearlite element, which can ensure the mechanical properties of gray cast iron; Copper can also promote graphitization, which is conducive to ensuring the machinability of gray cast iron. The optimum addition of copper is 0.2% – 0.4%. Too much copper will promote the production of a large number of sorbite in the matrix, which is not conducive to cutting.

Chromium can not only strongly promote the formation of pearlite, but also refine pearlite. The strength of gray cast iron can be improved by adding chromium. Chromium is a strong carbide forming element, so the addition of chromium will increase the white tendency of gray iron castings and deteriorate the processability of gray iron castings. In general, as long as the chromium content of raw hot metal does not exceed 0.35%, it will not have an obvious adverse impact on the machinability of gray cast iron.

Manganese can hinder graphitization, increase the content of pearlite and austenite, effectively reduce the eutectoid transformation temperature, reduce the spacing of pearlite sheets and refine the layers; Manganese can be solubilized indefinitely in austenite, and the matrix structure is strengthened. The sulfur element in the casting combines with manganese to form manganese sulfide, a high melting point material. Manganese sulfide can be used as a nucleating agent during molten iron solidification, so that the matrix structure can be uniform and refined. During cutting, manganese sulfide adheres to the tool surface to reduce the wear of the tool.

Titanium is a kind of element that does great harm to the machinability of gray cast iron. In molten iron, titanium has strong affinity with nitrogen and carbon, so titanium nitride or titanium carbide is easy to form. Titanium nitride or titanium carbide particles are dispersed in the matrix, and the wear resistance of the cylinder block can be improved. However, too much titanium compounds are not conducive to cutting and do great harm to fine boring and grinding. Therefore, the titanium content is generally below 0.03% – 0.05%.

Rare earth elements can be neutralized with many impurity elements in gray cast iron to purify molten iron. Especially when reacting with sulfur, the reaction product becomes the nucleation core of graphite, and the morphology and distribution of graphite are improved, which is conducive to improving the mechanical properties and processability of the material. However, the excessive addition of rare earth elements will increase the supercooling tendency of molten iron, resulting in the poor machinability of gray cast iron.

Pearlite can be refined obviously by adding an alloy element to gray cast iron. If a variety of alloy elements are used together, the effect will be enhanced. The combined use of alloy can reduce the amount of alloy and reduce the cost.