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The Iron the Industry Uses

21st July 1944, Page 28
21st July 1944
Page 28
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Page 28, 21st July 1944 — The Iron the Industry Uses
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The Life Story of what is Still the Most Important Material for Engineering Structures. Relationship Between Production Details and Final Characteristics is Discussed

By A. W. Haigh,

A.M.I.Mech.E., A.M.I.A,E.

WHILST it must be granted that the introduction of highly specialized materials has greatly facilitated the realization in practice of the commercial-motor designers' ideas,' it has brought with it certain responsibilities and liabilities. Development over the past 20 years has been most marked in connection with the special steels.

Prior to the outbreak of war, when spare parts were readily obtainable and when skilled labour and long experience were readily available, potential dangers of specialization were not so apparent. Now, however, with the replacement of worn or damaged parts a matter of some difficulty, and with a great shortness of skilled labour, " home repair " even of a most intricate nature, frequently becomes a xratter of necessity and not of choice.

The success, or otherwise; of the venture. will clearly depend on the widest possible knowledge of the materials being handled, Of no material is this statement more true.

than that of steels, particularly in connection with welding and what might be called " semi-forging" operations, such as hot-straightening. Here, a knowledge of the chemistry and physics of steel

manufacture and behaviour can do much' to prevent the use of wrong—

even dangerous—methods of restoration of damaged or, broken parts. With this object in mind, the author presents abrief and simplified account dealing with steel manufacturing methods and the various types of material produced by each.

• Carbon Content and Mechanical Properties

All carbon steels contain carbon, manganese, silicon, sulphur and phosphorus, the proportions of which elements regulate the properties of the metal. Carbon must be considered as exerting a dominating. influence on the mechanical properties of the material: as its quantity increases the hardness and tensile strength increase up to a carbon content of 0.85 per cent.

From this figure to a content of 1.5 per .cent, the hardness increases, but the tensile strength falls somewhat, whilst above 1.5 per cent, the metal is approaching closely to cast iron, so that steels of this nature are extremely rare. Carbon exists in steel in the combined form, and it is due to this fact that varying physical properties can be. induced by heat treatment, which, in essence, enables physical and chemical equilibrium , to be controlled.

During the manufacture of steel, manganese is used as a deoxidizing and purifying agent. The Oxides of manganesecollect in the slag and can be drawn off separately. It also'combines with sulphur and decreases the harmful effect of this element. It is present in solid solution in the iron, and chernic

ally combined also,with iron and carbon.

Silicon also acts as a deoxidizing and cleansing agent and remains in ' solid solution with the iron but, in the quantities usually present, has no appreciable effect on the mechanical properties of the steel.

Sulphur occurs in steel either as iron sulpbirle or manganese sulphide. The former occurs at the grain boundaries of the steel and, if present in sufficient quantity, causes what is known as " hot shortness," that is, as the sulphide has a low melting point, a lack of cohesion arises between the grains on heating the steel and, in consequence, trouble is encountered when forging or other hot-working processes are attempted.

Manganese sulphide, on the other

hand, melts only at temperatures exceeding those, for hot working and is, therefore, the more desirable form in which sulphur should be included. In order to insure that the sulphur does exist as manganese sulphide it is necessary for the manganese content of the steel to be at least five times that of the sulphur content.

Sulphur and

Free-cutting -Steels

Sulphur does possess one desirable property, namely, it makes the machining of steel fairly free and such items as small !crews and bolts, which carry little or no load and are free from shock applications, can contain as much as 0.1 per cent. sulphur, otherwise the content is strictly limited to 0.06 per cent, for the ordinary,grades of steel and less for the finer grades. Phosphorus in excessive amounts causes failure of steel at normal tem peratures when it is subject to vibra tion or shock. It combines with the iron and remains in solution in the steel and its content in the better grades of steel is limited to 0.05 per cent., whilst high-grade-steel Contains no more than 0.02 per cent.

The methods by which steel is made have a considerable bearing on its properties, as each process removes the harmful adulterants contained in the metal to a different degree. Similarly, pig iron, 'grey cast iron, malleable iron, .and wrought iron possess widely different properties. More rarely, harmful contamination may occur during manufacture or working.

Iron,. as we know it, is not a pure metal, and the . extent of these impurities governs, to a certain extent, the characteristics of each type of iron. Table 1 gives the proportions of -the constituents of the various irons, but, in addition to their compositions, the methods by which they are produced and their -subsequent treatment have a considerable effect on their properties. Pig iron is in the state in which it is topped from the blast furnace. Cast iron is a combination of pig iron and scrap remelted in acupola. There are two main. types of cast iron, namely, grey and white; the former is usually softer and more easily machinable than • the latter. In the former case, the carbon is present mostly in the free form as graphite. To produce the harder white iron, rapid cooling is necessary; this maintains the carbon in the combined form, giving an altogether harder material. Malleable .iron -is produced by heat treating special grades of white cast iron until the brittle quality is lost and it becomes ductile enough to be used in applications where .shock . loads are accommodated. At one • time the period of heat treatment was a matter . of days, but modern special inalleables are produced in a few.hours; their. properties resemble those of mild steel-. . Wrought iron IT the nearest approa.ch to low carbon steel of the iron group,

the main difference in composition being the manganese content and the presence of slag in a specially distributed form. It is produced by two methods, puddling and the Aston process. In puddling, pig iron is mixed with scrap and melted and refined in a puddling furnace. Iron Oxide reacts with the silicon, manganese and carbon, oxidizes them and, at the same time, increases the temperature of the metal, but the final temperature is not sufficiently high to melt the iron, which must be removed from the furnace in a‘pasty condition, together with the oxidized impurities which form the slag. The paste is squeezed to remove some of the slag and then rolled into puddle bars. These bars are next cut into short lengths, bundled together, heated to welding temperature and again rolled into bars or sheets;'

If this operation, which removes more of the slag, be repeated., the resulting metal is known. as double refined iron. The quantity of metal which can be paddled at one time is comparatively small and the complexity of the Operations increases its cost. Its properties differ markedly from those of mild steel, particularly with respect to corrosion, and its uses are now limited to a few specialized fields.

The Aston process produces a larger quantity of wrought iron from liquid, not pasty metal, the product being used mostly for ornamental work. and pipes.

Improved Steelmaking Systems The methods by which steel is made are constantly being improved, but they may be listed mainly as the open-hearth, the Bessemer, the electric furnace, and the crucible processes, their popularity being in the order set down.

The proportions of a finished car bon steel are set out in Table 1, from which it will be gathered that sulphur and phosphorus constitute harmful elements. It is the regulation of the carbon, manganese and silicon contents and the removal of the two remaining elements so far as possible which the various steel making processes carry out. • The open-hearth method produces two classes of steel known as " acid " and " basic," which depend on the type of refractory lining used in the -hearth. If good-quality steel is to be produced with an acid lining, pig iron and scrap steel of low phosphorus and sulphur content must be used. This selection of material often leads to the supposition that acid steel is of better quality than the basic product, but this, as will be readilY appreciated, is only possible when the sulphur and phosphorus contents in the initial metal used are less in quantity than those elements in the final product of the basic furnace.

The actual heating of the furnace and the operations required to produce the steel are identical for both acid and basic products, with the exception of the addition of limestone to produce basic slag in the basic process. An extremely high temperature is produced by passing the gaseous fuel and air through regenerators at the ends of the furnace (Fig. 1), which are filled with a chequetwork of firebrick.

At regular intervals the direction of flow of the gas is changed, so that one regeneration is being heated by the gases ,leaving the molten metal whilst the other preheats the fuel. The impurities in the metal are oxidized by reaction with iron oxide initially, then at the end of this process ferromanganese and ferrosilicon are added to deoxidize the metal and bring it to its correct composition. If too much carbon has been removed it is made up by the addition of suitable ferrocarbon alloy, but it is more usual to tap the furnace when the carbon content is correct.

The open-hearth method produces from 7 to 10 tons of good-quality steel per hour from furnaces of up to 300 tons capacity, but where a higher production rate is required and the quality of the steel is not so important, the Bessemer method, which produces up to 70 tons per hour, can be employed.

Bessemer Converter and its Product This process is carried out in a Bessemer converter (Fig. 2), which consists of a steel shell lined with firebrick, and supported on trunnions to facilitate pouring. Molten pig iron direct from the blast furnace is fed to the converter and air blown through it from ducts entering the container at

the bottom. The oxygen in the air combines with the impurities to form their oxides (the sulphur and phosphorus are not removed), the reactions producing sufficient heat to keep the charge in a molten state. At the end of about 18 minutes practically the whole of the carbon, manganese and silicon have been removed, whilst considerable iron oxide and gas remain in solu tion in the metal.

While the product is being poured, additions of various ferro-alloys are added to deoxidize the metal and to bring it to its correct composition. Because of the rapidity of action and the impossibility of controlling the amount of oxidation, the finished steel is, in general, of,inferior quality to the open-hearth metal.

Both open-hearth and Bessemer steels are subject to the addition of impurities from external sources. Where more rigid control of the constituents of the steel is required for high-class metal, the electric furnace is used. The process of refining is • the same as the open hearth, the impurities being oxidized, the only difference being the method of heating without calling for the aid of gas, and air.

For really high-class work, in which. thecontents of the steel are finely controlled, the crucible process, is used. Here the charge consists of correctly proportioned quantities of wrought iron, steel scrap, ferro-manganese and charcoal to produce carbon steel, whilst other ferro-alloys are introduced when

alloy steel is required. The constituents are added in graphite crucibles, so that no foreign matter can affect their composition whatever be the firing process employed. These precautions naturally slow down production besides limiting the quantity of metal which can be produced, hence, crucible steel is usually reserved for tool and other high-class work.

Steel, as tapped from the furnace, A29

possesses mechanical properties which are directly governed by its constituents, but, by the correct application of heat at controlled temperature and for a given time, these properties pan be changed.

When a steel is heated it gains in temperature steadily until, at a set point, it falters. Further application of heat causes a continued rise in ternperature until a second and final faltering point is reached, after which the increased temperature continues steadily. If the specimen be allowed to cool slowly these faltering points are repeated, but in both instances the temperatures at which they occur are lower than those for heating, as illustrated in the graph.

At these critical temperatures of heating and cooling, definite structural changes take place within the steel, the physical state of the metal, after the changes in carbon steels, depending ou the percentage of carbon. There are• various physical1 constituents of steel which may exist separately or together and which may be produced by heat treatment. As these physical components are the actual causes of the hardness, tougnness, ductility, etc„ of the steel, their characteristics are given below.

Basis of Steel Heat Treatment

There are seven forms which, theoretically, steel can take on beat treatment, namely, ferritic, cementitici austenitic, martensitic, troostitic, sorbitic and pearlitic.

• Fig. 3 illustrates graphically at what heats four of these physical states obtain in carbon steel, tropstitic and sorbitic and martensitic structures being induced by special cooling rates. At temperatures below line AEB, which represents the first critical point, all carbon steels are composed of a mixture of ferrite and pearlite, total pearlite, or a mixture of cementite and pearlite • depending on their carbon content.

It be noted that steel with 0.85 per cent, carbon is purely pearlitic. the reason being that pearlite contains approximately 0.85 per cent, of carbon, so that in steels of this content there is neither a lack nor an excess of carbon. Below a 0.85 per cent. .carbon content all the carbon is used in the formation of pearlite, so that none is left to combine with the remaining iron, and ferrite is formed.. Above 0.85 per cent, carbon the surplus forms a carbide with the iron, which takes the form of cementite.

Line CDEF represents the second critical point, and when the metal is heated to temperatures between this line and ABB the pearlite changes to a u s tenite. , As the temperatures approach those represented by the upper line, the ferrite in the lower carbon steels and cementitic on the higher carbon steels dissolve in the austenite until at temperatures above the line the entire structure of all the steels is austenitic.

Ferrite, or alpha iron, is soft and ductile and strongly magnetic. It is produced by slow-cooling steels of less than 0.85 per cent, carbon from teniPeratures abOve or within the critical range.

Cementite is purely an iron carbide in carbon steels, but can occur in combination with other elements, such as manganese or chromium in alloy steels, It is the hardest and most brittle of all the constituents, and is produced by slowly cooling steels containing over 0.85 per cent. carbon.

Austenite or gamma iron is only present in unalloyed carbon steels above the critical range. At temperatures below this range it separates out into the constituents shown in Fig. 3. It is particularly susceptible to work hardening and, although soft and ductile in itself, it is extremely difficult to machine and highly resistant to wear, because of the local formation of martensite at the points of application of the load. For this reason an austenitic structure is, extremely valuable for cylinder liners and can he retained at ordinary temperatures in steel containing over 11 per cent. of manganese or 25 per cent, of nickel.

Fig. 3 is concerned only with the slow cooling of steel and therefore martensite, which is a product of rapid cooling by quenching, is not shown there. It is composed of needle-like crystals in an angular arrangement and is more difficult to preserve at ordinary temperatures in low carbon steels; the lower the carbon content, the more rapid must be the cooling to achieve this object. The crystalline structure is extremely hard, the hardness increasing with increase in carbon, but it is also very brittle.

Troostite is produced by cooling at a slightly lower rate than that required for martensite or by reheating steels in which the latter has been retained. It occurs in patches in martensitic structures, has less strength and is both softer and more ductile than martensite, butis, nevertheless, very difficult to machine.

A sorbitic structure is composed of a mass of small particles of ferrite and cementite and can be obtained by regulating the rate of cooling or by tempering hardened steel. Sorbite is a desirable feature in the class of steel used forstructural work, as it is strong and fairly ductile.

Pearlite, the formation of which has already been described, is a layered structure of ferrite and cementite. It is strong and has an elongation of about 10 per cent.

It will be appreciated from the /properties enumerated above that any desired hardness, strength or ductility

of carbon steel can be obtained by suit-,, able heat treatment and quenching. The specific temperatures to which the various steels must be raised, the quenching media and the subsequent heat treatment required are beyond the scope of this article,' but they can be easily obtained where required.

Micro-structure and Mechanical Properties In general, when a hard steel (with out case hardening) is required ; martensitic structure is aimed at Varying degrees of hardness, strengt1 and ductility are prcivided by troostite, sorbite and pearlite which, in addition to martensite, are transition products from the decomposition of austenite. It must be borne in mind, however, that the governing factor in the production of the different physical constituents of steel by heat treatment depends on the percentage of Arbon present.

When a steel is first cooled from the furnace or has been worked it is internally strained and must, therefore, be relieved of thee strains before it can be used. This is accomplished by annealing, which should not be confused with normalizing.

Fig. 4 shows the range of temperatures at which both annealing and normalizing are carried out. The former requires the steel to be raised slowly to the temperature indicated in Fig. 4, which is above the critical temperature, the time varying according to the size of the pieces being treated, then cooled slowly, usually in the furnace. Normalizing temperatures are higher and, after heating the pieces, they are cooled slowly in still air, which reduces the grain size and ensure. a homogeneous structure throughout. Because of the even structure produced, normalizing is often used as a preliminary step for other heat treatment.

Internal stresses are also present in steel after hardening, but in this case, If the temperature be raised above„the critical range the hardness will be lost. The tempering temperature therefore is below the critical mark and the nearer to this point the steel is raised the softer it will become when cooled. Usually, at temperatures below 400 degrees F. the martensitic structure of the hardened steel is not altered. Above this 'figure up to about 700 degrees F. the structure changes from troostite to sorbite, so that if a steel be required to be hard and have little. ductility, martensite is aimed at

in tempering. If less hardness and more ductility be required, troostite is the object.

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