Steel

by William Metcalf

OXYGEN AND NITROGEN.

Oxygen and nitrogen are present in all steel and both are injurious, probably the most so of all impurities.

The oxides of iron are too well known to need discussion or description; they are the iron ores mixed with gangue. They are brittle, friable, hard, and weak, like sandstones. Mixed in steel they can be nothing but weakeners, elements of disintegration. Let any one take a handful of scale—or rust—oxide of iron, in his fingers and crumble it, and it will be difficult for him to imagine how such material could be anything but harmful when incorporated in steel. Langley has shown, and other scientists have confirmed him, that oxygen may exist in iron in solution, and not as oxide; the discovery was attended with the assertion that such dissolved oxygen produced excessive red-shortness. The proof that red-shortness was caused in this way was completed by the removal of the oxygen from some extremely red-short steel; the red-shortness disappeared with the oxygen and the steel worked perfectly.

When steel is melted very low in carbon, by any process, it is certain to be red-short and rotten unless the greatest care be used to prevent the introduction of oxygen. Crucible-steel of 15 carbon or less will as a rule be red-short and cold-short; it will not weld, and is generally thoroughly worthless. The same material melted to contain 18 to 25 carbon will be tough and waxlike, hot or cold. It will weld easily into tubes, and may be stamped cold into almost any desired shape.

Bessemer or open-hearth steel of less than 8 carbon is almost certain to be equally worthless, whereas the same material blown or melted not below 10 or 12 carbon, and re-carbonized not above 20, will be tough and good at any heat under granulation, and equally good and tough when cold.

As to Bessemer steel, the author cannot say whether it would be possible to stop the blow between 10 and 15 carbon or not, but it seems certain that if there be no overblowing red-shortness and cold-shortness may be avoided by carbonizing back to about 15 by the use of manganese or silicon, or both together.

In the open hearth it is always possible to stop the melt at 10 carbon, and to deoxidize the heat so as to avoid shortness, and not to go above 20 carbon. Such steel will be sound and tough; it will weld and stamp perfectly, and will be satisfactory for all reasonable requirements.

The reason of this seems to be simple and plain: In melting or blowing out the last fractions of carbon below 10 to 15 the same quantity of air per second or minute must be used as when burning out the higher quantities, and now there is so little carbon to be attacked that the oxygen necessarily attacks the iron in greater and greater force as the carbon decreases.

This leaves an excess of oxygen in the steel which cannot be removed by the ordinary quantities of silicon, or manganese, or aluminum.

If more manganese or silicon be used, the red-shortness and weakness can be cured largely; but then the carbon is raised considerably, and thus the steel is brought up to where it would have been without this excessive decarbonizing, with the difference that it is not quite so strong.

What good is there, then, in extremely low melting?

It must be admitted that there are tough, good-working steels in the market of carbon < 5, manganese < 20. They are made in small furnaces, worked with great care; the product is expensive, and, unless it is wanted to be welded in place of common wrought iron, it is in no case as good as well-made steel of 12 to 20 carbon; even for welding the latter is superior if the worker will only be satisfied to work at a lemon instead of a scintillating heat.

These special cases do not militate against the general fact that extremely low steel is usually red-short and weak.

The above is written for the consideration of those engineers who think they are going safe when they prescribe low tensile strength and excessive ductility. If these requirements meant the reception of pure, or nearly pure, iron, indicated by the low tenacity and high stretch, then they would be wise; but if they result, as they almost certainly do, in initially good material rotted by overdoses of oxygen the wisdom may not be so apparent.