This enables it to take up enough silicon from the walls of the crucible to prevent the evolution of gas during solidification, and the consequent formation of blowholes or internal gas bubbles.
They are known as " piping," " blowholes " and " segregation."
Hence in the act of solidifying it expels any excess of gas which it has dissolved while liquid, and this gas becomes entangled in the freezing mass, causing gas bubbles or blowholes, as at A and B in fig.
Because the volume of the pipe represents the excess of the contraction of the inner walls and the molten lake jointly over that of the outer walls, between the time when the lake begins to ebb and the time when even the axial metal is too firm to be drawn further open by this contraction, the space occupied by blowholes must, by compensating for part of this excess, lessen the size of the pipe, so that the more FIG.
Abundant and larger the blowholes are, the smaller will the pipe be.
But deepseated blowholes like those at B are relatively harmless in lowcarbon easily welding steel, because the subsequent operation of forging or rolling usually obliterates them by welding their sides firmly together.
Blowholes may be lessened or even wholly prevented by adding to the molten metal shortly before it solidifies either silicon or aluminium, or both; even as little as 0.002% of aluminium is usually sufficient.
But, because preventing blowholes increases the volume of the pipe, it is often better to allow them to form, but to control their position, so that they shall be deep-seated.
Silicon equals 1.66, there will be no blowholes; if this sum is less, blowholes will occur, and will be injuriously near the surface unless this sum is reduced to o 28.
He thus finds that this sum should be either as great as 1 66, so that blowholes shall be absent; or as low as o 28, so that they shall be harmlessly deep-seated.