Marking Tongs—Pig Iron—Puddling—The Bessemer Process—The Open
Hearth Process—Crucible Steel—The Cementation Process—Tempering.
_Exercise No. 16._
In forging tongs, stock ⅞-in. square of Norway or Swedish iron may be
used, as it is much easier for a beginner in welding the handle on to the
jaws. Soft steel may be used later on if desired. Figure 100 shows the
drawing of a finished pair of flat tongs. Figure 101 shows the size of
stock used and the dimensions of the rough forgings. It is not intended
that the dimensions given are to be accurately followed, but they are
given as an idea of what may be forged from this size of stock. In
forging the jaws, no helper is required to handle a sledge hammer after
the piece is cut from the bar for the reason that it is time lost for the
one who handles it, besides one man can do it.
[Illustration: Fig. 100. Blacksmith’s Tongs.]
[Illustration: Fig. 101.]
[Illustration: Fig. 102. Fig. 103.]
In forging the jaws a heavy hand hammer is used, and the bar is heated to
the welding heat, or near it. One and one-eighth inch of the bar is set
on the inner edge of the anvil and the lip is hammered as shown in Figure
102. The lip must not be turned and hammered on its edge. Let it get as
wide as it will, and do not hammer it too thin. After the shoulder has
been started for the length of the lip, it must not be moved. A common
fault is to start the shoulder and then to find that the lip is not long
enough and proceed to make another shoulder. The result of the second
shoulder is that when nearly finished a crack will be discovered. The
reason that second shoulder starts a crack is that the metal stretched
over the first shoulder. This is called a cold shut. See Figure 103.
Another common fault is to lower the bar when making the lip. This pulls
the lip on an angle with the bar and when it is straightened, another
crack is formed in the corner. See Figure 104. The bar must be on the
same plane with the anvil face at all times. When the lip is made, the
bar is turned to the left, setting it on the outer edge of the anvil and
hammering to form the shoulder for the eye. See Figure 105. It is then
turned again to the left hand and hammered down for the last shoulder.
[Illustration: Fig. 104. Fig. 105.]
At this time the stock required for the eye is beyond the outer edge of
the anvil. See Figure 106.
[Illustration: Fig. 106. Fig. 107.]
The rough forging should always be made a little larger than the finished
tongs; finishing it to size when the handle is welded on. When both jaws
are forged, they are cut in the center and the handles are welded on.
When the handles are well upset and scarfed, the shanks of the jaws are
drawn to equal size. Care must be taken in having the scarfed ends equal
in size or a poor weld will result. The handles at the weld are drawn
square with the corners tapered off. The jaws are now drawn and fitted
to size. Notice that the lip tapers on the edge, also on the flat part.
A small flute is fullered lengthways on the inside of the lip so that
round as well as flat iron may be held. The hole is next punched thru the
eye with a hand punch. A piece of ⅜-in. rod of soft steel is cut to the
proper length and used for a rivet. It is heated and inserted into the
holes in the jaws and hammered on both sides with hard blows. The jaws
of the tongs are now heated red and worked back and forth to loosen the
rivet in the eye. The jaws are fitted to the size of the stock they are
to handle as in Figure 107. The regular stock rivets should not be used
in tongs. The ⅜-in. round piece headed from both sides fits the holes
thru the eye best.
[Illustration: Fig. 108.]
In making tongs to hold a larger piece of stock, the square bar should
have an offset. The jaws should then be forged as in Figure 108. Notice
where the hammer strikes the bar to offset it.
In forging tongs, the handles should be welded to the jaws to give
practice in welding.
_Pig Iron._
Pig iron is made by smelting the iron ore in a blast furnace. The ore is
charged in a furnace mixed with lime stone as a flux, and melted by using
coke or coal as fuel. The resulting metal is called pig iron. It contains
from three to five per cent of carbon, two to four per cent of silicon
and various small amounts of sulphur, phosphorus and manganese.
_Puddling._
Wrought iron is made by melting the pig iron in a puddling furnace; about
one-half ton is charged at a time. After it is softened, it is stirred
with large iron hooks by the puddler and his helper. It is kept kneaded
to expose every part to the action of the flame, so as to burn out all of
the carbon. All the other impurities separate from the iron and form what
is known as the puddle clinker.
Pig iron melts at about 2100° F., steel at 2500° F., and wrought iron
at 2800° F., so the temperature of the puddling furnace is kept high
enough to melt pig iron but not hot enough to keep wrought iron in a
liquid state. Consequently, as soon as the iron becomes pure it forms a
spongy mass. This mass of sponge is divided into lumps of about 100 or
150 pounds which are taken to a squeezer and formed into blocks. In the
operation of squeezing the greater proportion of impurities left in the
iron after the puddling, are removed. While these blocks are still hot
they are rolled into flat musk bars. The bars are now cut and heated
to white heat in a furnace, taken to the rolls, welded and rolled into
merchant bars. In the welding and rolling the cinder coated globules of
iron are forced close together as the iron is welded. This gives the
iron a fibrous structure increasing its strength.
_Bessemer Process._
In making steel by the Bessemer process, the pig iron is put into a large
pear shaped vessel called the converter. The bottom is double; the inner
casing is perforated with holes called tuyeres, to admit air forced
under pressure. From ten to fifteen tons of molten iron at one time are
poured into the converter while it is lying on its side. The compressed
air is now turned into the double bottom as the converter rises to a
vertical position. The air has sufficient pressure to prevent the metal
from entering the tuyeres, and it passes up and thru the metal, burning
out the carbon. After the blast which lasts about ten minutes, the metal
is practically liquid wrought iron. The converter is now laid on its
side and the blast is shut off. A certain amount of molten spiegeleisen
(white cast iron containing much carbon or ferromanganese) is added so
as to give the steel the proper amount of carbon and manganese to make
it suitable for its purpose. The steel is then poured into ingots and
rolled into rails, girders, etc. Carbon is pure charcoal; manganese is a
chemical element very difficult to fuse, but easily oxidized.
_Open Hearth Process._
The open hearth process of steel manufacturing is similar to the puddling
process. The carbon is removed by the action of an oxidizing flame of
burning gas. The furnace has a capacity of forty or fifty tons and is
heated with gas or oil. The gas and air needed for its combustion are
heated to a temperature of over 1000° F. before entering the combustion
chamber, by passing thru so-called regenerative chambers. Owing to the
preheating of the gas and air a very high temperature can be maintained
in the furnace so as to keep the iron liquid after it has parted with
the carbon. The stirring up of the metal is not done with hooks as in
puddling furnace but by adding certain proportions of iron scales or
other oxides the chemical reaction of which keeps the metal in a state
of agitation. With the open hearth process the metal can be tested from
time to time. When it contains the proper amount of carbon it is drawn
off thru the tapping hole at the bottom of the hearth, leaving the slag
at the top. As steel is produced in a liquid form, from which impurities
have been removed in the form of slag that rises and floats at the top,
the metal is homogeneous and practically without grain. Wrought iron will
outlast steel when exposed to the weather.
Crucible steel, or tool steel, also called cast steel, is made by
using high grade, Swedish, wrought iron and adding carbon which is low
in phosphorus content. The oldest method is called the “Cementation
Process.” The iron bars were packed in air-tight retorts with powdered
charcoal between them. They were put in a cementation furnace, heated
red and kept at this temperature for several days. The bars, in this
way, absorbed the carbon from the charcoal. The carbonized bars (called
“blister steel”) were then cut into small pieces, remelted in a crucible,
poured in ingots and rolled into bars.
The newer method is to melt small pieces of Norway or Swedish iron base
with charcoal in a graphite or clay crucible. It is then poured into
moulds and made into ingots, after which it is forged or rolled into
bars.
The crucible process enables the manufacture of steel to almost exact
analysis and insures a clean and pure material. It also absorbs the
carbon much faster than steel made the old way.
In the school forge shop, the tool steel used should be of an inexpensive
kind. High priced steel should not be used as more or less is wasted by
the pupils in working. A carbon steel should be used for all forge shop
tools. About 75 to 95 point is suitable. High-speed tool steel should be
used only to give the pupils instruction in its handling and use, and to
familiarize them with the different kinds of steel and their treatment.
To the steel maker, temper means the percentage of carbon in the steel.
The word point means one-hundredth of one per cent, thus 10 point carbon
means ten one-hundredths of one per cent. One hundred and fifty point
carbon contains one and one-half per cent. This is about as high as is
generally made. One hundred and fifty point is known as high temper; low
temper is about 40 point. Steel containing less than 40 point does not
harden to advantage and is classed with machinery steel. There is a range
of tempers between high and low point which are used for different kinds
of tools.
In the forge shop the term _temper_ means the degree of hardness given
to a piece of tool steel. As an example, a piece of steel is heated to
a dark red color and cooled in water or oil. This is called hardening.
If this piece is too hard for the purpose intended, it is then tempered
to reduce some of its hardness, and to give the steel elasticity and
strength. In doing this, it is subjected to heat, (the more heat the
softer the piece becomes). In the forge shop, in tempering steel, the
metal is polished bright after hardening. If it is a small piece, it is
then held on or near a piece of hot iron. As the piece becomes heated,
the steel heated in the air assumes colors; at first a very faint yellow
and gradually darker, until all of the color has disappeared leaving the
steel without any trace of hardness.
These different colors as they appear on the surface of hardened steel
represent different degrees of hardness. The following simple list of
colors applies to the different tools and carbon to use:
Light straw—430° F. Lathe tools—130 point carbon.
Dark straw—470° F. Taps and dies—120 point carbon.
Purple gray—530° F. Chisels and blacksmiths’ tools, 75 to 95 point carbon.
Of course there are other colors than these. As the heat advances every
few degrees the color keeps changing to a darker which indicates the tool
is becoming softer.
The hardening heat is about 1300 to 1400 degrees Fahrenheit, or a cherry
red. About 400 degrees Fahrenheit relieves the strain in a hardened piece
of steel; 600 degrees leaves a trace of hardness and is about right for
springs.
In order to know the results of heating and cooling steel one should
take a small bar and cut nicks in it with a chisel every half inch. The
bar is then heated from a white heat at the end to a very dark red some
inches back. It is then cooled in water, the pieces broken and the grain
noted. The heat that leaves the steel file hard and a very fine grain is
the hardening heat of that steel. The hardening heat is a dark red. The
hotter it was when cooled the coarser the grain shows on the end of the
broken pieces.
In further demonstrating hardening and tempering of tool steel, the
making of a flat cold chisel will be considered. The principles involved
are about the same in all hardening and tempering.
[Illustration]