英文科學(xué)讀本 第二冊(cè)·Lesson 25 Cast-Iron

Lesson 25 Cast-Iron

I want you to help me to think about cast-iron again, said Fred. "First of all, why do we call it castiron, Norah?"

The iron is melted into a liquid, said Norah, "and poured or cast into a mold."

Teacher showed us, said Willie, "that cast-iron is very useful in its own way, but this is only because it is not like other iron. It fuses or melts easily and with less heat than wrought iron."

Yes, said Fred, "and when it is cooling in the mold, it swells out and presses into every corner. It must take the exact shape of the mold.

Cast-iron is just the metal to use for all sorts of articles, where beauty of shape is wanted. We can mold it into any shape we please.

But, Willie joined in, "these cast-iron things won't stand much hard wear and tear, for cast-iron is very brittle, and breaks very easily."

Cast-iron is used for making fenders, fire-grates, railings, iron bedsteads, brackets, saucepans, and kettles, said Fred. "These things will last a long time if fairly used."

Teacher showed us too, said Willie, "that castiron will bear almost any amount of downward pressure. It will neither twist out of shape nor break. This is why the great pillars to support bridges and arches are always made of cast-iron. The stands for our desks at school are made of cast-iron. They will bear any amount of pressure, but they would snap in two with a blow, because they are so brittle."

Can you tell us, Norah, asked Fred, "something we can't do with cast-iron?"

Well, said Norah, "as the cast-iron is so brittle, it could not be beaten out into thin sheets. It would break with the blows of the hammer."

Quite right, said Fred, "cast-iron is not malleable."

And as it will not hold together, it could not be drawn out into wire. It would break apart with the pulling.

Right again. Cast-iron is not ductile, and cannot be made into wire.

SUMMARY

Cast-iron melts with less heat than other iron. It is poured, in the liquid state, into molds, and takes their exact shape. Castiron is very brittle. It is neither malleable nor ductile.