THE MECHANICAL TREATMENT OF STEEL
Molten steel is practically always poured into upright molds of cast
iron which shape it into long slightly tapering blocks of metal of
square or rectangular cross-section. After the ingot mold has been
stripped off, the still red-hot ingot cannot well be taken directly to
the rolls, for, while the exterior parts may have the proper temperature
for rolling, the interior of the ingot may still be liquid. The ingot,
throughout, should be uniform in temperature when it is rolled. It is
therefore put into a closed pit or furnace of proper temperature where
the center of the ingot can be cooling while the outer portions are kept
hot or are reheated if necessary, until all is ready for the rolling
operation.
It would take a “steel man” a long time to tell you all of the
unfortunate things that can and do happen to such blocks or ingots of
steel which influence their applicability to the purposes for which they
are intended. You must have learned of the most serious of
these—“pipes,” “cracks,” “segregation,” etc., through reports of
investigations of broken railroad rails and accidents caused thereby. A
word or two regarding these:
In the ingot mold the outside of the steel ingot is, of course, the
first to solidify. It may be hours after the freezing of the outer crust
before the interior is able to cool sufficiently that it, too, can set.
As steel, like most other metals and alloys, occupies less space when
“frozen” than it does when molten, there must occur a hollow space in
the interior since the crust is solid and cannot contract much. This
hollow space usually takes the form of a more or less elongated cavity
extending along the axis of the upper quarter of the ingot. It is called
a “pipe.”
[Illustration:
PIPE AND BLOWHOLES IN AN INGOT OF STEEL
]
Then, too, the metalloids of the steel do not always stay where they
belong. Even if the steel has been of a uniform chemical composition
when poured, the interior portions of the ingot after cooling will be
found to have a greater amount of sulphur, phosphorus and carbon than
parts which are nearer the surface. Such gathering together of
constituents of the steel is known as “segregation.”
With the development of the steel industry and the demand for greater
and greater tonnages, ingots have been made larger and larger. Piping,
segregation, etc., are very naturally accentuated in the large masses of
steel.
Much “gray matter” has been expended in attempting to overcome these and
other defects to which large steel ingots are liable. Covering the
molten ingot top with charcoal; filling in before complete
solidification with additional molten metal; and keeping the ingot top
molten by application of powerful gas flames have been, perhaps, the
most useful methods.
But, even so, piping and segregation have not been completely prevented,
though great improvement has resulted.
The usual way around the difficulty is to make certain that only the
bottom (or best half) of each ingot is used for the most important
products, such as locomotive and car axles, firebox and boiler plates,
rails, etc. The next or third quarter or a little more is utilized for
products which go into less exacting service. These may be plates for
ordinary water tanks, for flooring, for ship plates, etc. The top part
which contains the pipe is cut off and goes back to the furnace to be
remelted. It is termed “discard.”
The big steel makers themselves shape most of their steel into such
finished products as rails, plates, rods, and wire. Some of it is by
them reduced from the ingot into intermediate “blooms,” “billets,”
“bars,” etc., and sold in this form for the manufacture of axles, drop
forgings and the hundreds of products which we each day see.
It is a very fortunate circumstance that at a cherry-red or white heat
the carbonless irons and most of the steels can be quite easily
fashioned into products. As is well known to us the most usual methods
of mechanically shaping these metals while hot are by hammering, by
rolling and by forging in a press.
With sufficient power and proper appliances, soft and medium steel to a
considerable extent can be fashioned cold, but, of course, in this
condition its resistance to reshaping is immensely greater. The cold
treatment of these metals is usually some form of tube or wire drawing.
Certain other methods such as extrusion, spinning, etc., are also in
use, and, through them, some otherwise difficultly formed products are
made.
In one of the earlier chapters we saw that annealing refines (make
finer) the grain of a steel casting and improves its physical
properties. Annealing for refining purposes is practiced with other
steel products also, and with just as effective results.
However, the mechanical shaping of steel while at cherry-red or at a
white heat much more materially refines the grain while helping the
strength and greatly increasing the ductility of the alloy. Steel which
has been hot-forged or rolled is said to have been “hot worked.” Steel
usually is “hot worked,” for “cold-working” methods are not so generally
applicable and the product is more liable to suffer under the more
drastic treatment. The amount of “hot-work,” at proper temperatures,
that low and medium carbon steels will stand with improvement of the
grain and physical properties is considerable.
As we must anyway shape the metal into useful implements and other
products, it is fortunate that the quality of the metal is benefited by
the process.
[Illustration:
No. 69a. PHOTOMICROGRAPH OF COLD-DRAWN STEEL WIRE SHOWING DISTORTION
OF THE CRYSTALS FROM COLD WORKING
Hot Working does not produce distortion
but makes the grain finer. Annealing relieves
this distortion to a great extent.
(_Magnification 70 Diameters._)
]
Forging
[Illustration: [Man]]
Undoubtedly the earliest shaping of ferrous (iron) metals was by
hammering the small balls of metal into bars, spears or swords.
Presumably it was done with stone hammers which later had to give way to
hammers made of iron. These had sufficient hardness to serve the purpose
well.
For hundreds of centuries the shaping of iron, steel and the other
metals into tools and weapons must have been done by such forging
methods. It is not difficult for us to picture the early smiths at their
work, laboriously and yet very skillfully hammering into spear-heads and
sword-blades the lumps of iron or Wootz Steel which they had made in
their crude furnaces.
[Illustration:
THE OLD OLIVER FOOT-POWER HAMMER
]
[Illustration:
AN OLD FORGE HAMMER
]
In much the same way, though on a considerably larger scale and with
heavier and better hammers and tools, was the same work done up to the
time of the invention of Cort’s rolling process—about 1783. Various
styles of hammers were used, some with a spring pole attached to raise
them for the next stroke which was delivered by foot power, others known
as “helve” or “shingling” hammers gave periodical blows as teeth on a
revolving wheel lifted and allowed the hammer heads to fall. The heavier
ones often gave as many as seventy-five and the lighter ones which were
used for “tilting” (forging) shear steel into bars or implements as high
as three hundred blows per minute.
[Illustration:
THE SQUEEZER WAS SOMETIMES USED IN PLACE OF THE HAMMER
]
[Illustration:
THE OLD TILT HAMMER
]
Though Cort’s rolls very materially aided in the shaping of balls of
iron from the puddling furnace into bars, the hammering or forging
method remained the one by which finished iron and steel articles were
made.
About 1835 it happened that a very large propeller shaft for a new ship
was desired. Being so large, no one was found who could forge it until
the matter was put before an English iron-worker named James Nasmyth,
who had a reputation for ingenuity. Nasmyth roughly sketched out an
immense hammer which he proposed to operate by steam. There was no
opportunity to build it, however, for the propeller shaft never was
ordered. But the idea of the steam hammer got to certain French
engineers, who constructed one which Nasmyth came upon during a visit to
a French iron works. Nasmyth realized the importance of his invention,
which, luckily, the Frenchmen had not attempted to patent. A patent was
granted to Nasmyth.
[Illustration:
A BELT-DRIVEN POWER HAMMER OF TO-DAY
]
To most of us the steam hammer, still little changed in essentials, is
quite well known and some of us have witnessed the cracking of an egg
without breaking the egg cup which held it. The adjustment and
regulation of these mammoth hammers is so nice that with almost
successive blows a skillful operator can flatten a piece of iron and
then break the crystal of a watch without otherwise injuring the
timepiece. Needless to say, the steam hammer has proved to be the only
efficient hammering device for forging large pieces.
[Illustration:
TWO BOARD HAMMERS AND TRIMMING PRESS
]
But whether made in the small way of the village blacksmith, by the
larger helve, tilt, Bradley, or by the monster steam hammer, each
forging, unless made in a die, must be considered to be _specially
formed_ and no two pieces, when finished, are exactly alike. They are
always “hand made” articles.
Drop Forgings
Many years ago, what are known as duplicate or interchangeable parts,
therefore, were quite unknown and it is related that parts of the famous
English Enfield rifle were made in various parts of the civilized world,
shipped to the Tower of London and there assembled. But during assembly,
the various pieces had to be filed and carefully adjusted by hand
because no two parts were exactly alike. But the “Yankee tool makers” of
New England solved the problem by forging the pieces of which many
duplicates were necessary in a “die” or impression in a block of steel.
The forged pieces, of course, took the exact impression of the “die” and
successive pieces thus made were alike in size and shape. From finished
duplicate parts which went to London from the New England states, the
Enfield rifle was assembled with very little final finishing of the “cut
and try” variety.
Done at first with the die on a blacksmith’s anvil and with a light
hammer, this promising method soon developed expert “die-sinkers” (die
makers), also ingenious men of whom the term “Yankee Tool Makers” is
self-explanatory.
In connection with this work what are known as “drop hammers” came to be
largely used. Of these an important type were the “board hammers,” in
which the heavy steel hammer-head was attached at the bottom end of a
vertical board set between pulleys. As the pulleys squeezed and revolved
against the board it was carried up between them and dropped, when the
pulleys loosened it at whatever height was desired. Rapidly and
periodically ascending and dropping upon the anvil beneath, it quickly
forced the white-hot iron into the “die” upon the anvil, forming what
have since been known as “drop forgings.”
[Illustration:
NASMYTH’S STEAM HAMMER REVOLUTIONIZED STEEL WORKING
]
Commonly the hammer face itself carries the impression of the upper part
of the article to be formed, i.e., there is an upper “die” on the hammer
and a lower one on the anvil.
[Illustration:
MODERN FORGING OF AN AUTOMOBILE CRANK SHAFT
]
“Fins” were of course, left all around where the excess metal was
squeezed out from between the upper and lower dies. It shortly developed
that a second pair of dies shaped for trimming could clean the forging
of this excess metal, which is so well known under the appellation,
“flash.”
Nasmyth’s steam hammer, also, has been used very largely for
drop-forging work.
A “cast” metal is not and cannot be as dense, free from holes,
sponginess or other defects or as strong as “worked” metal. While often
not as cheap as castings as far as cost of production goes, “drop
forgings” are usually considerably superior to them and are to be
preferred. However, it does not pay to make dies unless for many pieces.
One or several “castings” can be made without great expense.
Forging of Large Pieces by Hydraulic Press
Of late years much forging has been done, not by the hammer which gives
such a sudden, superficial blow with shallow working of the piece, but
by hydraulic or other press, which very slowly squeezes the hot piece to
smaller and longer shape. Sir Henry Bessemer was one of the first to
realize the advantages of and make use of the press for steel working.
Unlike the hammer the press exerts a deep working of the piece which can
be seen to flow throughout under the stress rather than in surface only
as occurs under the hammer. This is very desirable as the interior,
which is known to have much coarser grain than outer parts, particularly
needs to be “worked.” In plainer terms the press seems to knead the mass
much as the bread-maker kneads dough, while the hammering method simply
batters down the outside. At a glance an experienced eye can tell from
the appearance of the end of a forging whether it has been pressed or
hammered.
[Illustration:
A MODERN HYDRAULIC FORGING PRESS
]
Pressures as high as 8,000 pounds per square inch are used in hydraulic
presses, though much lower pressures are more common.
Forging vs. Rolling
Though we have not yet considered the rolling mill or its products, we
understand that, in general, only products of regular and uniform
cross-section and of considerable length can conveniently be rolled.
Where they can be obtained of satisfactory shape and size, steel
products formed by the rolling process are highly desirable and are
usually cheaper than those which are produced by the forging process.
Compared with those made by the rolling process, forged products are
usually quite costly in labor and time.
Rolling mills, however, cost immensely more to build and equip than do
plants installing even the steam hammering outfit, so the rolling
process cannot pay except for such articles as are demanded in great
quantities. Articles of irregular and odd shape must, of course, be
forged and here, especially for very small articles, the drop-forging
process is available and highly satisfactory where enough pieces of one
kind and size are wanted to pay for the requisite dies.
Forged articles have another advantage which we should not overlook. The
physical properties which are imparted during forging are somewhat
superior to those which the rolls bestow. The physical properties shown
by the latter are very satisfactory, however.