2 Metals

Before considering particular metals and alloys there are terms applied to all metals which should be understood.

Brittleness. The tendency of a metal to break with little deformation and under low stress. Cast iron is brittle.

Compression. The opposite of tension. It is the ability to stand pressure. Metals such as cast iron which are strong in compression are known as "good load carriers".

Conductivity. The ability to allow the passage of electricity or heat. Silver and copper are very good conductors of heat and electricity.

Creep. This is the slow yielding of metals under a load. This yielding may take months or years as in furnace and steam boiler parts. Creep takes place more often at high temperatures, but soft metals such as lead, tin and zinc suffer from creep at room temperatures. The lead sheets on church roofs thicken towards the eaves.

Ductility. The property which enables a metal to withstand mechanical deformation without cracking particularly when being stretched as in wire drawing.

Fatigue Failure. Metals which have withstood all the normal tests under heavy stress have been known to fracture when a much lighter intermittent stress is applied millions of times. Over one hundred years ago Sir William Fairbairn found that a wrought iron girder would stand a single load of up to 12 tons, but if a load of little more than 3 tons was applied 3,000,000 times the girder would break.*

Impact Resistance. This is the ability of metal to withstand a severe impact without failure. A machine developed by Edwin G. Izod is much used in Britain for testing this kind of resistance. On this machine, a notched metal test piece is broken by a heavy swinging pendulum and the amount of energy required to break it is measured.

Malleability. The property enabling a metal to be hammered or rolled into thin sheets or similar forms. Gold is the most malleable metal as it can be hammered to 0-00000025 inch thick. In this form it is translucent and is known as gold leaf which is used for applying to surfaces for decoration such as in sign writing.

Shear Strength. The ability of a metal to withstand the action of two parallel forces acting in opposite directions as in a guillotine.

Specific Gravity. The ratio of the weight of a substance to the weight of the same volume of water.

Tensile Strength. The maximum pulling stress which a metal can withstand before breaking. A test piece of a known cross-sectional area is stretched until it breaks and the power needed is recorded. In England the tensile strength is stated in tons per square inch. The tensile strength of a piece of metal is known as its tenacity.

Toughness. This denotes a condition intermediate between brittleness and softhess. Or it can be said to be a combination of strength and ductility.

Stress. The term is used to denote the intensity of load applied to a material in relation to the area of its cross section.

* From Metals in the Service of Man by W. Alexander and A. Street.

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...prepared ore, coke and limestone, known as "the charge", are put into the blast furnace by means of the skip.
The coke, in addition to producing heat and the reducing agent (reducing iron oxide to iron), combines with the iron to make a lower melting point alloy.

A preheated air blast passes through the large bustle pipe, which encircles the base of the furnace, and is blown into the charge through the tuyeres (pronounced "tweers"). The oxygen of the air blown in at the bottom causes the coke to burn fiercely. This generates heat and large columns of reducing gas. As the coke burns away the charge descends in the furnace against the stream of gas rushing upwards. The gas and heat act on the ore and together with the limestone bring about the extraction of the iron and its separation from the earthy matter.

The hot gases which cannot escape, because the double bell cones are never both open at the same time, are collected at the top of the furnace and pass through the exhaust ducting to the down pipes and thence, after cleaning, to the coke ovens and cowper stoves. The chequered brick lining of these is heated by the gas and later used to preheat the cold air on its way to the furnace. Each furnace has three stoves one being "on blast" and giving up heat whilst the others are "on gas" and receiving heat.

On the bend of each tuyere there is a small piece of blue glass; by looking through this the interior of the furnace can be inspected.

The smelting process is continuous. Once the furnace is lit it is kept going for months or even a year or more and normally is only stopped when the refractory lining needs attention.

The limestone produces a liquid slag which floats on top of the iron. When the slag has risen almost level with the tuyeres it is tapped off through the slag notch. This slag is used for railway track ballast or coated with tar and used for road making. Some basic slag is used for fertiliser.

The iron is tapped four or five times per day by breaking through the clay plug of the tapping hole thus allowing the molten metal to gush out. The impure iron-carbon alloy is either cast into metal moulds or it is cast into moulds of sand which originally looked like pigs feeding from a sow. For that reason the iron at this stage was called pig iron. (This term now applies to all the iron as it comes from the blast furnace.) Often however the pig iron is run into large refractory lined ladles which take the iron in the molten state to the steel making plant.

Recently the efficiency of blast furnaces has been improved by the injection of oil, fine coal, oxygen or steam into the blast.

Cast Iron

Cast iron is usually made from pig iron in a foundry where it is remelted and refined in a small furnace, not unlike a blast furnace, but which is only about 20 feet tall. A general characteristic of cast iron is that it is brittle and cannot be forged into shape. However there are various kinds of cast iron.

Grey Cast Iron. This is the commonest form of cast iron. When fractured it has a grey appearance. The properties of this iron are regulated to suit various requirements. The higher grades are used for machine beds and other machine parts, and the lower grades for grates, drainpipes, guttering, etc.

White Cast Iron. Is very hard and brittle and is difficult to machine. It has a white appearance when fractured. It is used for machine parts, such as those found in cement works, which have to stand great pressure and often rough usage.

Malleable Castings. These are produced by packing white cast iron components in an oxidising material and heating to red heat for several days and allowing to cool slowly. This causes some of the carbon to be removed by oxidation.
Malleable castings are tougher than ordinary iron castings. They can be machined and are used for machine parts which have to withstand shock.

Wrought Iron

Wrought iron is probably the oldest and, at its best, the purest form of iron. It was produced long before the Christian era and in its purest form contains only a very small amount of carbon and fibrous slag.

Primitive furnaces are still in existence in Africa which reduce iron ore to iron using charcoal as a fuel and goatskin bellows for the air blast. Most of the carbon is removed by oxidation thus raising the melting point of the iron which remains in the pasty stage. The almost pure iron which is obtained is hammered into the desired shape. Before the eighteenth century iron was made in this way in Sussex and the Forest of Dean in furnaces known as bloomeries. Wood charcoal was used as fuel.

In 1794 the puddling furnace, which provided a much cheaper way of producing iron, was introduced by Henry Cort. The small amount of wrought iron produced today is made in furnaces similar to the one used by Henry Cort, (fig. 2). It is a reverberatory type furnace and unlike the blast furnace the metal is not mixed with the fuel. The furnace is charged with pig iron and flux. The fuel is burned in a grate at one end of the furnace and only the hot fumes are in contact with the charge. The heat is reflected from the roof on to the charge to melt it.

Soon after the metal melts the carbon monoxide gas burns on the surface and it appears to boil. It is stirred or puddled by men with long heavy iron bars. The almost pure iron rises to the top and because of its higher melting point remains pasty and separates from the slag. The puddler manipulates the iron and unavoidably some of the slag, into blooms on the end of the iron bar. These weigh about 80 lbs. After removal from the furnace the blooms are rolled then wired together in a faggot and brought to welding heat and re-rolled so that they are united. This disperses the slag throughout the metal and gives it its characteristic fibrous structure.

Wrought iron resists corrosion well and for this reason it is used in boiler making. It has also a good resistance to fatigue and sudden shock and is thus used for chains, hooks and haulage gear. Only a limited quantity of wrought iron is made today having been largely superseded by mild steel.

Mild Steel

In 1856 Sir Henry Bessemer introduced the Bessemer Converter which could make steel quickly and cheaply. The converter is shown in figure 3. The outside casing is made of steel and the whole furnace can be tipped on trunnions through which the air pipe, which is connected to the air holes at the bottom passes.

The refractory lining is either "acid", made from silica bricks, or "basic", made from crushed dolomite rammed with tar.

The original converters had only acid linings, until in 1878 Sidney Thomas and Percy Gilchrist introduced the basic fining thus making it possible to use high phosphorous ores.

A known amount of molten pig iron which may contain 3%_4% carbon is poured into the converter together with some limestone. The air blast is then turned on at low pressure. As the converter is turned on its trunnions to the upright position, the air pressure is increased to 20 to 25 pounds per square inch. This prevents the steel from going into the holes.

The air blowing through the steel burns out the carbon and the other impurities. This is known as "the blow". It causes a spectacular show of sparks which lasts about 20 minutes. When the flame drops it means there is no carbon left, but the blast is kept on to burn out the phosphorus and the flame becomes a dense brown smoke. This stage is known as the "afterblow" and lasts for two or three minutes. The carbon has been eliminated from the metal but it contains oxides and gases which have been formed during the blow and which must be removed. The inclusion of the necessary carbon and the de-oxidation are accomplished by the addition, at the end of the blow, of a calculated quantity of ferromanganese, an alloy of iron, manganese and carbon. The converter is rotated and the steel poured into ladles and then cast into ingots. Mild steel contains less than 0-25% carbon.

The Cementation Process

This is the oldest known method of making steel. It was probably first used in India about 1400 B.C. Bars of wrought iron were packed with charcoal in sealed crucibles and kept at red heat for several days. In this process the carbon in the charcoal combines with the iron to form cementite at the sur­face of the bars; for this reason it is called the cementation process. The bars were then bundled together and reheated to welding heat and forged together into a single bar to obtain a more even distribution of carbon. The bar could then be cut into short lengths and welded together again. This was repeated as often as desired. The steel thus obtained was known as shear steel. Small quantities of shear steel are still made. This process was quite good for small articles such as knives and shears, but it was not possible by this method to obtain an even distribution of carbon for large work. The cementation process is similar to the present-day case hardening process (see Chapter 7).

 

The Crucible Process

Benjamin Huntsman who was a clock maker realised that to make good clock springs he needed steel in which the carbon was evenly distributed. In 1740 he developed a method of introducing the carbon to the iron when it was molten. This was done in small quantities in a crucible (because this steel was poured into moulds it was known as cast steel). It is actually an alloying process and is similar in principle to the modern induction process.

Open-Hearth Process

This was developed in 1867 by Charles W. Siemens in co-operation with Pierre Martin, by whom it was first patented.
Today more steel is produced by this method in Britain than by any other.

Figure 4 shows the open-hearth furnace. This is a rever-beratory furnace in which the heat from the burning fuel, after passing over the charge, heats the chequered brickwork of one set of preheating chambers. When this set of chambers is hot the passage of gas and air is reversed so that it now passes through the hot bricks and becomes itself preheated and, after passing over the charge, heats the other set. The direction of the flow of gases is reversed periodically. This is known as the regenerative principle, and by this means a sufficiently high temperature is obtained to treat large quantities of metal and to keep it molten throughout the process.

The furnace is charged with pig iron and steel scrap. When this is melted, iron or millscale is added mainly to remove carbon by oxidation. During this process samples are taken from the furnace and analyses are made of the metal and slag. When the refining is complete either the tapping notch is broken or the furnace is tilted, depending on the type, and the steel is tapped into a ladle. The steel is then teemed from the ladle into ingot moulds. The furnace is tapped about every 12 hours. Some furnaces charged with hot metal are ready for tapping after 8 hours.

By using either a basic or acid lining, according to the type of charge, this furnace produces low and medium carbon
steels. It has a capacity between 150 and 300 tons and is fired by gas or fuel oil.

Electric Furnaces

These are either electric arc (fig. 5), in which the heat is generated by an arc between graphite electrodes and the metal, or induction furnaces. high powered arc furnaces which in the past it took 21 open-hearth furnaces to make.

The high frequency induction furnace (fig. 6) is a hollow vessel with a refractory lining round which is wound a water cooled coil of copper wire. The charge consists of carefully selected scrap to which the necessary alloys are added. When the coil is energised with an electric current an induced current in the charge causes it to heat up and melt. There is very little slag in this process.

Both processes are used for making high grade alloy-steels including stainless steel. For these high grade steels the temperature must be carefully controlled and impurities kept to a minimum.

Since the cost of electricity has not risen as rapidly as the cost of other fuels electric arc furnaces, with a capacity up to 100 tons, are being used on a large scale. One steelworks in Britain is now making 1-35 million tons of ingots in one year with six high powered arc furnaces which in the past it took 21 open-hearth furnaces to make.

Modern Converter Processes

Since Bessemer introduced his converter in 1856 and thereby started the "Steel Age", improvements have been made notably by the introduction of oxygen to the process. By inject­ing oxygen and air, or steam, or carbon dioxide, a greater proportion of scrap can be added to the charge: up to 15% as against a maximum of 5%.

The most recent converter processes are known as the L.D. process, the Kaldo and the rotor.

The L.D. process is named after the initial letters of the Austrian towns Linz and Donawitz where it was first developed. In this process oxygen is blown on to the top of the molten pig iron in the converter, which is vertical (fig. 7). This process is mainly used on pig iron with a low phosphorous content.

The Kaldo process is so named from Professor Kalling its inventor and Domnarvet, the Swedish steel works where it was developed. It is a basic process. The converter, which is inclined at an angle of 200 to the horizontal, slowly revolves about its axis whilst a jet of oxygen is directed on to the surface of the molten metal through a lance (fig. 7). This process takes about 90 minutes and irons with up to 2% phosphorus are used. The rotation of the converter allows the heat to be evenly transmitted to the charge and the quality of the metal obtained can be more closely controlled than by other converter methods. A fairly high percentage of scrap can be used in this process and a wide range of steels obtained.

The rotor process was developed in Germany. It uses a horizontal converter with two nozzles. One jet of oxygen blows into the molten metal as the vessel slowly revolves, and the other jet blows on to the surface of the metal (fig. 7).

Alloy Steel and Alloying Elements

Plain carbon steels have certain limitations such as lack of strength, hardness and high ductility, also non-retention of hardness at temperatures developed in metal cutting.
By using alloying elements special qualities have been imparted to steels to suit them for specific uses. Here are some of the important alloys and alloying elements and the effects they have on steel.

Chromium. This is the chief alloying element in all stainless steels. These contain between 12% and 30% chromium. One of the best known groups of stainless steel is the austenitic, which is generally known as 18/8 stainless steel, because it contains approximately 18% chromium and 8% nickel with additions of titanium, molybdenum and copper. Austenitic stainless steel cannot be hardened by heat treatment but can be work hardened. There are two other groups of stainless steel namely ferritic, which can be hardened only to a small extent, and martensitic, which can be hardened and tempered in a similar manner to carbon steels. A wide variety of stainless steels is used where resistance to corrosion and strength at high temperatures is needed, such as in jet engines. It is also used for surgical instruments, cutlery and kitchen utensils.

Cobalt. It is highly magnetic and is used in cutting tools to increase their hot hardness and in heat resisting nickel base alloys.

High Speed Steel. In 1868 Robert Mushet discovered that certain tungsten alloy steels could "self harden". Later, in 1900, he produced high speed steel which contained between 14% and 18% tungsten. This steel keeps its hardness at high temperatures; for this reason it is used in cutting-tool steels and for hot dies used for hot working of metals.

Manganese. It is usually added to steel in the form of ferro-manganese, an alloy of iron, manganese and carbon. Above 10% added to steel makes it very difficult to machine. Small additions to steel improve the elasticity.

Molybdenum. It has a very high melting point—2,625°C which is exceeded only by four other metals: tungsten, rhenium, tantalum and osmium. It is used in heat resisting steels and where 18% tungsten is used in high speed steel it can be replaced by 9% molybdenum—it is said to have twice the "power" of tungsten.

Nickel. Alloys of iron and between 36% and 50% nickel are used for length standards, pendulum rods and measuring tapes, because they expand and contract very little at room temperatures. It is used in nickel chrome steel which has approxi­mately 4-4% nickel, 1-2% chromium, 0-5% manganese, 0-3% carbon and 0-2% molybdenum and a tensile strength of up to 100 tons per square inch.

Silicon. It is a non-metal. When used in conjunction with manganese it makes excellent steel for car springs and bridges. Silicon improves the elasticity of steel. When up to 4% is added to steel it greatly increases its magnetic permeability.

Tungsten. It has the highest melting point of any metal— 338o°C. It is used in steels which need to keep their hardness at high temperature such as high speed steel.

Vanadium. This is used as a de-oxidising agent in steel manufacture. It also gives steel grain refinement. 0-5% vanadium added to chromium steel makes it easier to forge and stamp and more resistant to shock.

Two metals which are neither iron nor steel but which are so much used in engineering that they must be mentioned are: cemented carbides and stellite.

Cemented Carbides. These are very hard and britde. They are used as tips on cutting tools which are brazed on to carbon steel shanks. Cemented carbides consist of particles of tungsten carbide in a matrix of metal with a lower melting point; this matrix metal is usually cobalt. Cemented carbides are classified as:
1. Tungsten carbides which are used for machining highly abrasive metals such as irons and bronzes.
2. Titanium tungsten carbides which although less hard than tungsten carbides are used to machine steels because they resist the tendency for "chips" to become welded to the tip.

Stellite. This is an alloy of cobalt, chromium and tungsten. It is produced in an electric furnace and cast into shape; it cannot be forged into shape. Since it will retain its hardness and cutting edge even at red heat, it is used for rapid machining of hard metals. Stellite contains about 50% cobalt, up to 33% tungsten and 3% carbon. As it contains no iron it is not a steel and is non-magnetic.