Methods for refining gray cast iron grains
Inoculation treatment
Use of inoculant: Add inoculant to the molten iron before pouring, such as the commonly used ferrosilicon inoculant. The effective elements in the inoculant (such as silicon) can serve as foreign nuclei to increase the number of nuclei. The amount added is generally 0.2% – 0.8% of the mass of the molten iron, which can effectively refine the grains and improve the graphite morphology.
Inoculation method: The flushing method can be used, that is, the inoculant is placed at the bottom of the ladle. When the molten iron is flushed into the ladle, the inoculant is evenly dissolved in the molten iron. There is also an instantaneous inoculation method, in which the inoculant is added during the pouring process, so that it can provide nuclei in the final stage of pouring and better refine the grains.
Alloying
Adding titanium, zirconium and other elements: Add appropriate amounts of titanium (0.05% – 0.15%), zirconium (0.05% – 0.2%) and other elements to gray cast iron. These elements can form high-melting-point compounds, such as titanium carbides and nitrides, which can serve as non-spontaneous crystal nuclei to refine the grains.
Controlling pouring conditions
Lowering the pouring temperature: Properly lowering the pouring temperature can increase the supercooling of the molten iron in the cavity, which is conducive to the formation and growth of crystal nuclei, thereby refining the grains. However, the pouring temperature cannot be too low, otherwise the molten iron will have poor fluidity and may easily cause defects such as insufficient pouring. Generally, the pouring temperature is controlled at 1300 – 1380℃.
Speeding up the pouring speed: Rapid pouring can make the molten iron flow quickly in the cavity, which will improve the temperature uniformity of the molten iron. At the same time, the convection generated is conducive to the dispersion of crystal nuclei and helps to refine the grains.
Refining the grains of gray cast iron has a number of positive effects on its mechanical properties:
Strength improvement
After grain refinement, the grain boundary area increases. Since grain boundaries can hinder the movement of dislocations, when the material is subjected to external forces, dislocations accumulate at the grain boundaries, and higher external forces are required to make the dislocations continue to move, thereby improving the tensile strength and compressive strength of gray cast iron. For example, after effective grain refinement, the tensile strength of gray cast iron may increase by 20% – 30%.
Toughness improvement
Fine grains enable the material to disperse stress concentration when it is deformed by force. Because stress concentration is more likely to occur at the grain boundaries of coarse grains, and refined grains allow stress to be dispersed and transmitted through more grain boundaries, reducing the degree of local stress concentration, the toughness of the material is enhanced, and it is less likely to brittle fracture under impact loads.
Hardness improvement
Grain refinement will increase the hardness of gray cast iron. This is because when the grains become smaller, the bonding force between atoms is more evenly distributed at the microscopic level, and the ability to resist deformation is enhanced. In some application scenarios where wear resistance is required, the improvement in hardness can improve the wear resistance of gray cast iron.
