CHAPTER VI.
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.
Fig. 100. Blacksmith’s Tongs.
Fig. 101.
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.
Fig. 104. Fig. 105.
At this time the stock required for the eye is beyond the outer edge of the anvil. See [Figure 106].
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.
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.
