(1) Macroscopic microstructure of coarse grains
The coarse grains are easily found after the ingot test piece is eroded. In order to ensure the quality of the product, the uniform large grains are controlled according to the five-level standard. Under normal conditions, the grains of the ingot are equal to or less than the second level. Due to improper casting process, occasionally more than two equal-axis grains appear, or more than two equal-axis grains appear in fine equiaxed, or local large grains appear in fine equiaxed grains, large grains The size is several times or ten times larger than the size.
(2) Large grain microstructure characteristics
Under polarized light, the grain is still as macroscopically observed, the grain is still coarse, but the grain orientation is more obvious, the grain color is more beautiful, the large grain fracture structure is rougher than the small grain port, Dense.
(3) Formation mechanism of coarse grains
The grain size of the ingot is affected by the crystal core in the melt or the casting process. When the crystal core is small, the casting cooling rate is slow, the subcooling degree is small, the number of nucleation is small, and the grain growth speed is fast, the uniformity is large. Grain. When the melt is introduced improperly or introduced into the superheated melt, due to the uneven temperature in the liquid blood, the grain grows rapidly at a high temperature, and local large crystal grains or large crystal regions appear in the ingot.
When the chemical elements of the grain refinement are small, uniform large crystal grains can be produced, and local large crystal grains can also be produced. Locally large plums sometimes cannot appear in the ingot, but appear in the heat-treated material of the processed product.
(4) Measures for coarse grain
A alloy melt can not be overheated in whole or in part to prevent non-spontaneous nuclear dissolution and prevent the reduction of crystal core;
B lower the casting temperature;
C increases the cooling strength and increases the crystallization rate;
D alloy composition and impurity content are properly formulated to increase the grain refiner content.
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