Graphite in cast iron and its influencing factors
1. Generation of graphite in cast iron
Graphite is a crystalline form of carbon with hexagonal lattice, and its strength, hardness, plasticity and toughness are very low.
It has a great influence on the properties of cast iron. In cast iron, graphite can be precipitated from liquid phase or austenite of cast iron, or decomposed from cementite formed first. The process of carbon precipitation in the form of graphite in cast iron is called graphitization. Graphite in gray iron is mainly precipitated from liquid.
2. Factors affecting graphitization
(1) Influence of chemical composition
The influence of carbon and silicon, they are elements that strongly promote graphitization. Carbon is the basis of graphitization. The higher the content of silicon and carbon, the easier it is to form gray cast iron.
Sulfur strongly hinders graphitization, which is easy to make carbon appear in the form of cementite, promote the white cast iron, and reduce the fluidity of molten iron. Therefore, the sulfur content should be as low as possible, generally 0.15% in Xiaoyu
Manganese hinders graphitization. It can form manganese sulfide with sulfur, so as to weaken the adverse effect of sulfur on graphitization. Therefore, the manganese content can be appropriate, generally 0.6% - 1.3%.
Phosphorus has little effect on graphitization. It can improve the fluidity of molten iron, but it will increase the cold cracking tendency of cast iron. The content should also be controlled, generally 0.3%.
3. Effect of cooling rate
The slower the cooling rate, the more conducive to graphitization. The cooling rate of cast iron mainly depends on the mold material and casting wall thickness. Cooling in sand mold is slower than that in metal; The thicker the casting wall, the slower the cooling.