Nos. 1 and 2 are used exclusively for foundry purposes. No. 3, for both foundry and rolling mill; finally, Nos. 4 and 5, for the rolling mill only.

Mottled iron is iron with a white background, dotted with spots of graphitic carbon.

Properties. In addition to carbon cast iron may contain about five per cent. of impurities like silicon, manganese, phosphorus, sulphur, etc.

The gray cast iron is used for castings. The darker grades of gray cast iron make the smoother castings, but are more brittle. The lighter grades of gray cast iron make tough castings and very often contain blow holes.

The white cast iron is hard, brittle and difficult to work, while the gray cast iron is soft, tough and easily worked. A fracture of good gray cast iron shows a light blueish gray color, a close grained texture and considerable metallic lustre.

A fracture of poor cast iron presents a mottled surface, with patches of darker and brighter iron, or it may show crystalline patches. Air holes may also be present in fractures of very bad specimens.

The quality of cast iron may be improved by long continued fusion and by repeated melting up to about twelve times. Cold blast pig gives stronger iron, but more expensive than hot blast pig.

Effects of Impurities. Carbon in cast iron decreases the specific gravity and the melting point. Other effects have been given under classification.

Manganese increases hardness and shrinkage, it also increases the percentage of carbon that the iron may hold into combination.

Phosphorus is readily taken up during the smelting process. Less than one per cent. of phosphorus in cast iron is beneficial, as it increases the fluidity and lessens the shrinkage. Over one per cent. of phosphorus seriously weakens the iron.

Silicon in small quantity will usually increase the strength of the cast iron. A large amount makes the iron brittle and weak.

Sulphur comes into the iron from the ores and from the coal used in the smelting process. Sulphur in castings. should not exceed half of one per cent. Sulphur increases chill and shrinkage and decreases the strength, rendering castings unsound.

Common Defects in Cast Iron. Blow holes and honeycomb are defects caused by confined air and may render castings unsound. Cavities and holes are caused by the

collection of foundry dirt and other impurities and by unequal contraction during cooling. Internal stresses due to unequal contraction of the metal during cooling will often cause rupture, especially while the casting is struck a few sharp blows. Internal stresses may be avoided during the casting process by uncovering the thick parts first, so that they may cool just as quickly as the lighter parts. Other things being the same, the longer the cooling period the better the castings.

Following defects are often found in cast iron columns: 1. Unequal thickness, due to the shifting of the core before the metal is poured into the mould.

2. Shrinkage cracks, due to unequal contraction during cooling.

3. Warping and bending, caused by unequal contraction during cooling or by handling the columns before they have sufficiently cooled.

4. Cold shuts. In long castings requiring the metal to be poured from both ends in the same time, it often happens that the metal becomes too much chilled to properly mix and unite. This results in the formation of weak seams, known as "cold shuts."

The surface defects of cast iron are swells, scales, blisters, cold shuts, etc.

Inspection of Cast Iron. The work of inspecting iron in general may be divided as follows:

a. Laboratory examination and tests.

b. Mill inspection.

C. Field inspection.

Laboratory Inspection of cast iron consists in examining several test-bars poured from each melt. These test-bars are poured alternately before and after the casting is poured. About one test-bar for each ton of castings is generally sufficient.

Test-bars for tensile strength are about eighteen inches long, and usually turned down in a lathe in order to remove the exterior scale and enable a careful measurement of the diameter. They are then subject to increased tension in a testing machine until rupture occurs. The elongation of the bar is recorded for the various applied tensile stresses. Testbars for bending cast iron are usually 3 inches wide by 1 inch thick, and are either 14 inches or 26 inches long; they are placed on supports 12 or 24 inches apart, with the narrow side vertical, and loaded on the center until broken. The deflection as well as the breaking load are noted.

The Shop Inspection of structural cast iron, like bases, columns, etc., is very similar to the field inspection, and both will be treated together in a future chapter. We will make mention, however, with regard to the shop inspection of cast iron water pipes.

Shop Inspection of Cast Iron Water Pipes. These are first subjected to a surface examination. Pipes with visible honeycomb and serious sand holes and blow holes are at once rejected. The same thing applies also to pipes with swells, scales and blisters on their interior faces. Honeycomb when not visible is easily located by the dull sound given by the pipe on tapping it with a hammer. The pipe is next subjected to a hydraulic pressure about twice as high as the pressure under which the pipe is to be used. While under pressure the pipe is carefully tapped all over with a hammer to discover air holes, flaws, etc. Each cast iron pipe has stamped on it the weight per foot. The weighing and marking of each piece is also inspected. All defective pipes are rejected.

Advantages of Cast Iron. Cast iron has a high compressive strength, is durable and little affected by corrosion; it can be cast readily in many useful shapes, and is cheaper than steel. Cast iron retains its rigidity at a red heat. For these reasons cast iron is used for column footings, columns, water pipes, bed plates for machinery, boilers, etc.

Disadvantages of Cast Iron. Cast iron is brittle, has a low tensile strength, and a low ductility and elongation. It is not so homogenous as steel and is therefore less reliable; it also snaps under the action of water in fires, when the iron is red hot. Cast iron in building work gives connections which cannot be riveted, and must be bolted, thus causing lack of rigidity. For these reasons cast iron should not be used where subject to tensile stress, heavy vibration, shocks or impact.

WROUGHT IRON.

Wrought iron is the oldest known form of iron. It has been found in the pyramids of Egypt, and has probably resulted at first from the action of fire upon nearly pure iron

ore.

Definition. Wrought iron is metallic iron which has been manufactured by any process without fusion, and which contains less than 0.25 per cent. carbon.

Manufacture. At present wrought iron is generally manufactured from forge pig by a method known as the

puddling process. The pig iron is subjected to the oxidizing flame of a blast in a reverbatory furnace. Here the iron loses some of its impurities through oxidation, and becomes soft like a paste. Operators known as puddlers, using special rakes, form this iron into paste-like balls called puddle balls or blooms, weighing about eighty pounds each. Each ball is then passed through a squeezer to expel cinder and part of the slag; then the ball is rolled into a "muck bar." Muck bars are cut to length, laid in piles, reheated, and rolled to "merchant bars." These are again cut to length, laid in piles, reheated, and rolled, giving "best iron." If the process is again repeated and the best iron is rolled once more, a grade known as "best best iron," or double refined iron, is produced.

Properties. Wrought iron is a malleable metal, can be forged and welded, and will stand shocks. It can not be tempered, and can not be melted, except with great difficulty. Good wrought iron is tough and has a fine fibrous and close texture; if subjected, however, to repeated shocks and excess loads around the elastic limit, the texture changes from fibrous to crystalline, with a decrease in the strength of the metal.

Best iron is about ten per cent. stronger than the merchant bar, due to the second rolling.

Cold rolling decreases the ductility and the ultimate elongation and increases the elastic limit and the ultimate strength. The strength also increases with a higher percentage of carbon

Annealing decreases the ultimate strength and increases the elongation.

The fracture of good wrought iron is fine, fibrous and close, with small crystals of uniform size and color, and with a silky lustre. The metal has a leaden gray color. The fracture of poor wrought iron shows coarse crystals, loose, open and blackish fibres and blotches of color. Flaws in the fractured surface indicate that the reheating, rolling and welding processes were imperfectly carried out.

Wrought iron high in phosphorus is brittle when cold, hence the name, "cold short." Wrought iron containing sulphur, arsenic and other impurities, is known as "red short," and will crack when bent at a red heat. Red short iron cannot be welded.

Common Defects in Wrought Iron. I. 1. Poor material, shown by a fracture with coarse crystals and loose fibres. Flaws in the fracture, indicating "red short" iron. 3. Bright crystalline fracture and discolored spots, indicating "cold short" iron.

2.

Inspection of Wrought Iron. Tests. The usual tests for wrought iron ore are as follows:

I. Cold bending test: A square bar 3/4′′ on each side and about 15" long is bent cold by means of pressure or with a hammer, to an angle of 90° in a curve whose radius is equal to twice the thickness of the bar. Rivet iron is bent on itself or through an angle of 180° while cold. No cracks should result. Wrought iron breaking under this test lacks both ductility and strength.

2. Hot bending test: Iron which is to be worked hot must bend sharply to 90° at a working heat without fracture. Iron showing cracks under these conditions is "red short," or high in impurities, and cannot be welded.

3. Nicking and bending: Specimens upon being nicked on one side and bent should show a fracture nearly fibrous.

4. Tensile strength is determined with a testing machine from test pieces, usually about 18′′ long by 1" wide, cut from the full-sized bar after the material is rolled. The thickness of the test piece will therefore be the same as that of the finished bar. The various stresses and the corresponding elongations are recorded.

5. In comparing several samples of wrought iron it is sometimes found convenient to multiply the tensile strength of each specimen by the corresponding ultimate elongation. The resulting product is a measure of the work required to rupture the bar. The best specimen will correspond to the highest product obtained in this way.

Advantages. Wrought iron is a durable material, and can be readily worked into a large variety of forms when heated. Pure wrought iron is less affected by corrosion. than steel. Unlike cast iron, wrought iron is malleable, and can therefore be made into plates. It is ductile, or can be made into wires, and it is about equally strong in tension and compression. For these reasons wrought iron may be used in places where subject to alternating compressive and tensile stresses, provided the unit stress is not excessive. It was much used for truss-members, columns, beams, girders, wall anchors, rivets, etc. At present it is largely replaced by soft steel.

Disadvantages. Wrought iron is not as homogenous as steel, cannot be melted without difficulty, and is less stiff than steel, i. e., a wrought iron beam will deflect more than a steel beam of similar length and cross-section under the same load.

Wrought iron is also less strong than steel. For all these reasons it is not used where great strength is required.