The rails are then cut to a length, straightened, and finished.
For rails which have to withstand traffic, the upper and lower surfaces of the piles are of superior or better-worked iron.
Wrought iron girders can be rolled with ease up to a depth of about 10 inches. When they are required of greater depth than this, the upper and lower portions are sometimes rolled separately, and then united by inserting a piece of iron containing more carbon, and which is therefore more fusible. This piece is subjected to a fierce heat from blowpipes, and at the same time hammered on both sides, so as to weld the upper and lower portions of the girder together.
"When a bar of wrought iron is heated to redness and quenched in water it becomes permanently shorter than before. This fact is well known to practical men, who sometimes avail themselves of it when a wrought-iron crank, etc., has been accidentally bored out too large for its shaft; by one or more heats it may be reduced so as to be a good fit."1
Cold Short iron is very brittle when cold, and cracks if bent double, though it may be worked at a high temperature.
This defect generally appears in an iron produced from a poor ore, or is caused by an excess of phosphorus.
1 Box on Heat.
Bed Short or Hot Short iron cracks when bent or finished at a red heat, but is sufficiently tenacious when cold. The defect is generally caused by sulphur from the fuel. Red short iron, though useless for welding and for many other purposes, is tougher, when cold, than other iron, and is much used for tin plate.
Arsenic, copper, and several other impurities also produce red shortness.