IRON CONTD hand the same term is often used to imply the softening or annealing only of the tool.
In the act of hardening, steel becomes specifically lighter to an extent varying with the temperature attained before cooling, the composition of the steel, and its form and dimensions, and according as it has been rolled or otherwise worked before treatment; thus Elsner found a specimen of cast steel had the specific gravity 7’9288 before hardening, and 7 ’ 6578 after (both at 11° C.), indicating an expansion in volume of about 3 - 5 per cent. Similarly Hinman found expansions of 2 - 6 to 3 - 5 per cent, for blister steel; and Hausmann found for a very hard steel and for a soft welding steel expansions of l - 7 and 0'8 per cent, respectively. The expansion, however, is not uniform in all directions; thus Caron found that a hammered steel bar 20 centimetres long and 1 centimetre square in section altered after repeated hardenings to the following extent in percentages of the original dimensions :— [table]
whilst with rolled steel there was an increase in length of 2’25 per cent, and no change in the other dimensions.
On thoroughly annealing hardened steel it recovers the original, dimensions possessed before hardening; evidently therefore when a large steel tool is hardened, since the inner portion cannot possibly become cooled at the same rate as the outer film, a kind of strain must be developed amongst the particles; for, whilst the rapid chilling of the outer portion causes that part to occupy a larger volume than it otherwise would, this is not effected to so large a relative extent with the inner portions. The opposite result is necessarily produced during annealing. Accordingly, in the process of tempering steel goods, cracking and splitting or “buckling” and otherwise getting out of shape are inconvenient results that are apt to happen, especially when all parts of the surface are not equally and simultaneously heated or chilled, a result difficult to bring about with articles of certain shapes; the hardening process may in such cases be more safely effected by heating the article up gradually in a fluid bath such as melted lead (or in some cases oil) and then chilling in a cool fluid bath, preferably of oil; whilst the annealing may be performed by allowing the fluid in which the article is slowly heated up to cool down again spontaneously when the requisite temperature is attained. For special articles, c.y., saws, a particular manipulation is requisite to avoid buckling; in some cases the annealing of a flat plate, e.g., a circular saw, is effected between two solid flat masses of iron, which keep the plates from getting out of shape whilst cooling. In some instances when the point only of an article is required to be tempered, e. g., certain kinds of chisels, the whole mass is heated red hot and the point dipped into water or oil to harden it; on taking out, the heat from the body of the hot article is rapidly conducted to the point; as soon as its temperature is sufficiently raised the body is somewhat cooled by a short immersion so as to reduce the temperature of the whole approximately to uniformity, and then the article is allowed to cool altogether slowly; so that the hardening and annealing are effected by one heating only. For articles the temper of which requires to be somewhat exactly determined, the temperature attained is judged of by the tint of the iridescent film of oxide which forms on the surface (previously brightened) during the heating; the higher the temperature the thicker the film, the colours and temperatures being approximately as follows :— [table]
For instance, if a saw is to be tempered at a full blue it is heated over a clear fire or a mass of red hot iron or in a sand bath until the tint appears on the brightened surface, when it is allowed to oool. In many cases the tempering of tools is effected with sufficient accuracy without brightening them, by coating them with tallow, oil, or some similar composition, and then heating over a lamp or amass of hot metal until the tallow, &c., begins to decompose; with sufficient experience as to the size of the article, the mode of heating, and the character of the decomposition set up, much the same results are arrived at as when the colour is watched on the brightened surface. In some instances the requisite temper is given by one operation only, the point of the article to be hardened (a drill, say) being coated with tallow and heated in a flame until the tallow decomposes rapidly, and then plunged into a mass of tallow to chill it; the point is thus rendered harder than the body of the drill. By heating the point of a small tool nearly white hot and plunging it into a stick of sealing wax for a second, and then into another part of the wax, and so on till cold, steel may be made so hard as to be readily capable of boring into similar steel hardened in the ordinary way.
Instead of determining the temperature of the object to be annealed by the tint developed on it, various fusible alloys may be used, a series of such being placed in the annealing bath, so that by observing which are fluid the temperature is known approximately. Parkes gives the following compositions of lead-tin alloys for this purpose :— [table]
The first five of these alloys consequently correspond to the various yellow shades; the next three to brown, brown and purple spots, and purple; and the last four to the blue shades.
43. Strength and Tenacity of Iron and Steel.— Independently of the additional strength communicated to iron by the presence of small quantities of carbon, producing steels or so-called steels of various kinds, a considerable increase in this property is brought about by the removal of portions of cindery matter by fusion. Wrought iron that has simply been welded, reheated, and rolled a given number of times is, caeteris paribus, less tenacious than iron from which complete fusion has removed the films of silicate, &c., enveloping the component layers and fibres of the welded material, and preventing their complete union together. Thus the best qualities of weld iron, containing usually about 0 - 2 to 0 - 3 per cent, of carbon, possess a tensile strain of from 20 to 30 tons per square inch, whilst mild “steels” and ingot irons are usually considerably higher in this respect, viz., from 30 to 40 tons per square inch; harder steels of higher carbon percentage are proportionately stronger. The following numbers may be quoted, representing tensile strains in tons per square inch. [table]
Steel wire of as high a breaking strain as 206,170 lb, or 92 tons, per square inch section, has been drawn by Johnson and Nephew (Manchester), whilst wire of tensile strength of 70 to 80 tons per square inch is readily procurable. According to Collingwood, galvanizing increases the strength of freshly drawn wire by about 15 per cent., whilst ungalvanized wire has been found to gain nearly 5 per cent, by merely being allowed to remain at rest for a week or so; the increase of strength in each case appears to be simply due to internal adjustment of the strains produced during drawing.
It does not necessarily follow that the resistance to percussive force exhibited by a given sample of metal will be in the ratio of its tensile strength. Thus phosphorus when present together with only minute quantities of carbon (forming the so-called “phosphoric steels ”) does not very materially decrease the tensile strength, such steels containing 0 ’ 3 per cent, of phosphorus being often but little inferior to soft non-phosphorized steels in this respect; but when tested by a “falling weight” (a mass of known weight falling once or oftener from a known height upon the centre of the bar or rail firmly gripped in supports a known distance apart) the phosphorized metals generally show themselves considerably inferior to the non-phosphorized ones. The same remark applies to silicon. According to
Dudley the effect of phosphorus, silicon, and carbon in hardening iron and making it less capable of resisting percussion are nearly in the proportions of 3, 2, and 1 relatively to one another. The presence of manganese diminishes this deleterious effect of non-metals; whilst, if more than minute quantities of carbon be present, the tensile strength as well as the resistance to percussion is greatly diminished by the additional presence of phosphorus or silicon in proportions beyond certain small limiting amounts. Accordingly it is the usual practice to test rails, bars, &c., not only by the determination of the breaking strain for tensile force (measured by pulling asunder, preferably by hydraulic power, a bar turned to known definite dimensions, and made into the shape of fig. 65), but also by a falling weight,—a “monkey” (somewhat like a pile driver) being raised to a known height and let drop upon the rail.
The particular tests applied in different instances vary much; for instance, some little while ago the official falling weight test for Bessemer rails at Gratz (Austrian South Railway Company’s Works) was to permit a weight of 1000 kilos (about a ton) to fall from a height of 15 feet upon the centre of the rail supported by two rests 3 feet apart, any amount of bending being allowed, but not fracture, whilst a test for elasticity or resistance to permanent deflexion was applied by placing a weight of 17,500 kilos on the middle of the rail similarly supported. The North-Eastern Railway (England) similarly at one time tested rails by allowing a weight of 1800 lb to fall from 4 feet height, the number of blows requisite to produce rupture and the permanent bending produced by each being noted. In other instances the test applied has been a ton weight falling a greater height, such as 20 feet or even 30 feet, the rail being required to stand one such blow only, or a succession, the particular details of the test to be applied being usually specified in each particular case; thus the Midland Railway Company has tested steel rails by allowing a weight of 1 ton to fall three times from a height of 12 feet, the supports being 4 feet asunder.
In just the same way as regards the determination of tensile strength, the dimensions of the piece to be tested (6 inches, 8 inches, 10 inches long, &c.) are usually specified, and the strain which the metal will just stand without becoming permanently elongated {limit of elasticity) determined, as well as the total strain requisite to produce rupture, together with the “ductility” or amount of permanent extension of the test piece and the diminution in section of the bar at the point of rupture. Thus for instance the following numerical data were obtained by Kirkaldy with a particular specimen of West Cumberland Bessemer steel plate, three pieces being tested, each 10 inches long, and respectively ∣, ⅜, and ∣ inch in thickness. [table]
These numbers illustrate, amongst other things, the effect upon the final values produced by variations in the diameter of the test pieces; the shorter and thicker the piece the greater in amount is the permanent extension. In calculating the rupturing strain per square inch, the dimensions of the metal as originally employed before permanent alteration was brought about are employed; by taking the diminished area at the point of fracture as the section, a much higher value is obtained as the tensile strength per unit area of the extended metal. Tempering steel greatly increases its breaking strain and limit of elasticity, but decreases the permanent extension; thus the following values represent certain results obtained with Creuzot steels of the A class, in tons per square inch (see Engineering, 1875, p. 119). [table]
In making a contract for the supply of steel of a particular quality, the details of the tests of strength to be applied should consequently be duly set forth; for instance, a short time ago the French Government required certain steel navy tubes, of which the limit of elasticity and breaking strain tested in a particular way were respectively to be 21 and 38 tons per square inch. Similarly in the construction of the Mississippi great bridge the cast steels used were contracted to be of the following qualities :—“To be of crucible cast steel; the staves of the tubes to stand a compressive strain of 60,000 and a tensile strain of 40,000 lb per square inch section without permanent set, and to stand a tensile strain of 100,000 lb per square inch without fracture. Modulus of elasticity to be between 26 and 30 million lb, preferably nearer the lower limit, and as constant as possible; bars of the same modulus to be selected for the tubes, so that each side shall have same power of resistance; each bar to be tested and modulus stamped on it. Steel pins, rods, bolts, eyewashers, rivets, &c., and the ⅛ inch steel plates for enveloping the staves to stand a tensile strain of 40,000 fb per square inch without permanent set, and an ultimate tensile strain of 100,000 lb without fracture.”
Practical tests of the capability of metal to stand bending double or through some given angle, or twisting round and round in the cold without fracture are often applied, as are also tests of the capability of being bent hot, forged, welded, &c. A test as to the power of resisting repeated bending strains backward and forward through a given angle is sometimes applied; a particular mechanical arrangement for effecting this has been described by Olrick.
James Price has constructed a machine for testing rails as to durability under rolling wear and tear, consisting of a pair of metal rollers 5 feet in diameter and 16 inches wide, weighing 45 cwts. each, supporting a frame weighing 6⅜ tons, connected with a centre boss and vertical axle, so that the rollers are driven round in a circle, one bearing with 5 the other with 6 tons pressure; the rails to be tested are bent into a circle or preferably a polygon, to equalize the wear of the rollers, which are driven over them at a speed of 13 or 14 miles per hour until the rails are broken or wear out.
It by no means follows that the rail which possesses the greatest tensile strength will resist wear and tear and rolling friction best, although this might be anticipated if, other things being equal, increased tensile strength corresponds to greater hardness; on the other hand, experience does not always indicate that the most carbonized rails last the longest, although the superiority of ingot metal (Bessemer steel, &c.) over weld iron (not fused) rails is well demonstrated; it is probable that the interposed film of cinder between the metallic fibres in the latter case greatly facilitates the destruction and wearing away of the upper surface, just as the accumulation of dust and sand on the rail between the passage of consecutive trains, especially conjoined with moisture, conduces to mechanical abrasion. Price Williams has calculated from the results of various British lines that the average tonnage lives of wrought iron and Bessemer rails (i.e., the traffic in tons requisite to wear away f inch of the head of the rail) are respectively close to 17⅜and 161 million tons, the latter being thus more than nine times as lasting as the former.
Numerous observations have been made on the effect on the strength of iron and steel of punching and dialling holes, of notching and otherwise removing part of the surface, and of shearing, with the general result of indicating that the disturbance produced in the relative positions of the constituent particles by forcibly punching and shearing in the cold materially decreases the strength of a bar or plate (apart from the actual removal of substance); but that drilling does not effect the strength in the same way (see various papers in the Journal I. and S. Inst., Iron, and Engineering during the last few years). Annealing restores the strength to a considerable extent; if the plates be punched whilst red hot the annealing takes place spontaneously.
The strengthening effect upon soft steels and ingot irons of hardening by heating and plunging into oil is often very marked, the tendency to crystallinity observable in large masses of cast metal being thus largely removed. A valuable paper 0 on the causes and effects of hardening iron and steel, by Professor Akermann, is to be found in the Journal I. and 8. Inst., 1879, 504; whilst the Research Committee of the Institution of Mechanical Engineers has recently issued reports containing much information on thiä subject.
Effect of Temperature on the Strength of Iron and Steel. —Many observations on this point have been made by different experimenters, with the general result of indicating that at 0° C. and below the tensile strain and resistance to percussion of iron and steel bars, rods, &c., are substantially the same as at the average ordinary temperatures of 15° to 20°, but that what difference there is is usually in the direction of diminution in strength; the numerical values obtainable are considerably variable with the composition of the metal, Ac.; thus Webster finds that a severe cold of - 15° C. does not affect the tensile strain of wrought iron and steel, although it slightly increases the ductility by about 1 per cent, with iron and 3 per cent, with steel; the power of resisting transverse strain is, however, some 3 per cent, lower, whilst the flexibility and the resistance to rupture by impact are reduced by the following amounts :— [table]
A committee of engineers appointed by the Russian Government for the purpose of examining carefully into this question has recently found that, when the amount of phosphorus, silicon, and carbon in Bessemer and Siemens-Martin steel rails exceeds jointly about 0 ’ 54 (varying from 0 ’ 44 to 0 ’ 67) per cent., the rails are decidedly more brittle at temperatures near to - 20° C. than at the ordinary temperature ( 4- 10° to 20°); whilst the effect of low temperature in producing brittleness is not marked when the phosphorus, silicon, and carbon jointly amount to only about 0 ’ 41 (0 , 37 to 0 ’ 55) per cent. Expressing the amounts of non-metals on the scale proposed by Dudley (3 parts of carbon, 2 of silicon, and 1 of phosphorus being considered as equivalent to one another, so that a rail containing carbon = 0'30, silicon — 0 - 20, phosphorus = 005, would be equivalent to one containing phosphorus = 0 ’ 25), these results may be expressed as follows: when the percentages of carbon, silicon, and phosphorus are jointly equivalent to an average of 0 ’ 19 per cent, of phosphorus, the effect of low temperatures is not marked; but when they are equivalent to an average of 0’31 per cent, of phosphorus, the rails are more brittle at temperatures near - 20° than at ordinary average temperatures near to + 15°.
Breakages of axles, crankshafts, pumprods, &c., exposed to strains and vibration appear to be more common in frosty weather than at other seasons of the year; it is considered by many that exposure to vibration, &c., and low temperature simultaneously tend to diminish tenacity and set up a brittle structure in a way not observed when only one of the two causes alone acts; direct evidence on this point is, however, wanting. It is noticeable that large masses of cast metal (cast iron, true steel, or ingot metal) if cooled too quickly are apt to have the internal portions in a high state of tension or strain; for the outer portion, when solidified, prevents the contraction taking place that would otherwise ensue during the solidification and cooling of the inner portion; hence the amount of extra strain requisite to produce rupture is much diminished, so that the want of elasticity of bearings, foundations, &c., connected with the almost rigid ground during hard frosts in certain cases causes the strain applied during use to exceed the reduced amount which the metal can then bear without fracture. Chilled castings, case hardened iron, and tempered steel, moreover, are affected by low temperatures in another way; the outer harder portions do not expand at exactly the same rate as the inner softer parts; and consequently alteration of temperature produces such variations in the internal strain as in some cases to lead to fracture either spontaneously or by the superaddition of the strain due to ordinary use. On the whole it appears that no clear evidence is as yet extant proving that vibration either alone or concurrently with low temperature does actually cause a brittle crystalline structure to be developed; whilst on the other hand thousands of examples are extant of axles, engine beams, connecting rods, tires, girders, &c., continually subjected to vibration, percussive action, and varying strains of all kinds for years, in which no such development of brittleness has taken place; in those cases in which fracture has been thus brought about, the probability is that defective workmanship and the development of internal strain are the true causes of the rupture, and not a gradual alteration in texture.
At 300 s to 350° soft irons and steels become much deteriorated in power to resist percussive action and bending strains, whilst at lower temperatures and at a red heat this peculiar comparative rottenness is not marked. Phosphorized iron appears to be affected to a greater extent than purer varieties, but mild Bessemer and Siemens-Martin steels are by no means exempt from the deteriorating influence. A railway wheel that has become heated through the grease-box taking fire by friction rapidly fired guns, and tools that become much heated in use, &c., may readily attain to a temperature sufficiently high to be much less capable of resisting strain than when cool. A large number of experiments on this and allied points are described by Adamson, Journal I. and S. Inst., 1878, 383, and 1879, 30.
Closely akin to the comparative brittleness developed in iron and steel on the one hand by interspersed films of cinder, and by the presence of phosphorus, &c., and on the other by temperature, is the phenomenon which gives rise to the production by over-heating of what is termed “burnt iron”; according to some the want of strength of burnt iron and steel is due to the formation of oxide disseminated through the mass as cinder is through weld iron, this oxide coating the constituent particles and preventing their adhesion to one another; others, however, wholly dissent from this view. Caron (Comptes Rendus, March 4, 1872) has shown that by simply strongly igniting good qualities of malleable iron either in a smith’s forge, or in porcelain tubes in an atmosphere of hydrogen or of nitrogen, the “burnt” crystalline structure can o be developed under circumstances where no oxidation can occur. Akermann also has been led to the same conclusion, defining “burnt” iron as “iron which, through too long continued or strong heating, has had the opportunity of assuming a crystalline texture, with the brittleness which accompanies it on account of the diminished cohesion of the crystals.”
44. Foundry Operations.— Occasionally for rough castings, such as tuyere nozzles, <kc., the pig iron is used as it comes from the blast furnace, a small side channel leading off a portion of the molten pig flowing to the sand bed containing the pig moulds (§ 16) to some other convenient part of the bed in which the moulds have been prepared; but much more frequently the iron employed for castings is remelted by the founder in a cupola furnace, various kinds of pig being intermixed together according to circumstances. A reverberatory furnace is preferable to a cupola, the metal being less altered by oxidation; but a much greater consumption of fuel is thus occasioned. A very coarse grained iron, No. 1, will, on remelting and running into small moulds, give a much finer grain than the original pig; whilst, on the other hand, a large massive casting which takes a long time to solidify would, if of the same metal, develop a large grain like that of the original pig.
To obtain just the right grain under any given circumstances requires an amount of special knowledge and experience of a peculiar kind only to be gained in the foundry itself, the mixture of brands that answer well for a certain kind of casting not being necessarily at all suitable for one of different size; different mixtures, moreover, are apt to differ more or less in the amount of contraction taking place in solidifying and cooling, so that a somewhat different allowance for shrinkage must be made in different cases; as a rule the moulds are made about 1 per cent, larger in each direction than the casting is intended to measure (one-eighth inch to the foot).
For complex forms, a number of different pieces are required to make up the whole mould, each piece being a metal box or “flask” containing sand mixed with charcoal powder, loam, or similar materials somewhat varying in their nature according to the character of the casting, moistened so as just to cohere together and enable the outline of the casting to be defined by the surface of the mixture. The “pattern” or model being placed in a suitable position, the flasks are separately moulded to the variously shaped parts, and then set up (being bolted or otherwise fixed together) so as to enclose a space shaped precisely to the pattern; the molten metal being then allowed to run into this space, by tapping the cupola and letting the fluid iron pass along a runner or gutter on the sand bed floor of the casting house, gradually fills it up, the seoriae floating on the top; the air which previously filled the mould passes away through orifices left for the purpose, along with steam and gases formed by the action of the hot metal on the materials of the mould. To ensure the casting being free from seoriae, and to give sufficient hydrostatic pressure to enable the impression to be sharp, the liquid metal is allowed to rise to some little height above the top of the casting by making the highest part of the cavity of the mould to be some inches or more below the surface of the floor, so that the metal fills up the tubular hollow above the mould thus formed, making aprojection (“gate” or “git”) above the top of the casting; this is ultimately detached by a hammer or chisel, as are also the ridges formed where the different flasks meet, and any similar protuberances at places where a little of the sand has fallen away from the surface of the mould, thus increasing the dimensions of the cavity at such places.
More simple forms are cast in moulds prepared with a smaller number of flasks, two often sufficing, or for some purposes one only; machinery is employed for moulding such objects as gas pipes, railway chairs, &c., required in large quantities. For certain purposes metal moulds are used, or combinations of metal and sand moulds; owing to the greater conducting power, the iron in contact with the metallic parts of the mould is rapidly solidified; a peculiar hardness is thus communicated to the casting, which is then termed a “chilled” casting. For the best qualities of chills certain particular classes of pig are requisite; thus for the cast iron car wheels used largely in America the brands of pig preferred are certain kinds of cold blast charcoal brown haematite or specular iron; iron smelted from the pure magnetites of Lake Champlain does not chill in the required way so as to produce an outer film of white iron constituting the “tread” of the wheel, passing into a mottled iron with a soft grey inner centre, thus combining the maximum of strength with a hard wearing face. The peculiarity of some of the American cast iron in this respect enables machinery of certain kinds to be constructed in part of that kind of metal with a strength difficult if not impracticable to obtain with the same weight of metal from most British brands. In order to produce the chilling effect a cast iron ring is imbedded in the sand mould so as to embrace the circumference of the wheel to be cast; the metal is consequently rapidly solidified in contact with the iron ring, the “chill,” or poι ’ tion solidified as white iron, penetrating inwards some 0 - 75 inch. It is found that confining the chill to the middle portion of the outer circumference only, and not communicating it to the entire flange and the opposite outer portion of the external surface, gives greater strength without diminishing the resistance to wear; this is effected by narrowing the cast iron chilling ring.
Casting under Pressure.— During the solidification of iron, and especially of steel, after running into moulds, bubbles of gas are often extruded, causing the substances to become vesicular and honeycombed, especially at the upper portion; the gas thus evolved from Bessemer metal was found by Bessemer and Henry to be almost entirely carbon oxide; on casting in a mould from which the air was rapidly pumped out, ebullition was set up, owing tu the rapid evolution of gas, just as ordinary spring water apparently boils under the exhausted receiver of an air pump. Three methods of overcoming this practical difficulty of vesicular structure being set up have been used. The first, or “dead melting,” applied to cast crucible steel, is simply to allow the crucibles to stand for some time in the furnace with the molten metal in them before casting; in this way the iron oxide disseminated through the mass is acted upon by the carbon, and this source of gases eliminated; according to Bessemer, silicon is also introduced (by the action of the metal on the pot-material), the presence of which retards or stops altogether the gas evolution (see § 33). The second method is the application of this principle in a more direct way by adding silicious pig, preferably as silicious spiegeleisen (silico-manganeisen), to the fused metal; this process has been long used on the Continent in one form or another by steel-makers who have attained a high reputation for their cast steel products, e.g., in Krupp’s works, at Terre Noire, Ac. The third method is one which prevents the formation of bubbles of gas by mechanical means, the fluid metal being subjected to powerful compression during its solidification. In 1856 a patent for this was taken out by Bessemer, the mould being closed by a strong cover and hydraulic power employed to force inwards a stout wrought iron plunger; Whitworth’s system of casting “compressed steel” is a more perfect development of this notion.
Other methods of arriving at the same end have been also suggested, the pressure being derived either from the admission of high pressure steam on the top of the ingot mould, or by the generation of gases by the heat of the metal introduced on some chemical composition in the upper part of the closed mould; thus by employing a mixture of nitre and coal dust, the mould being closed by a screw plug and strongly bound round to strengthen it, a high pressure can be exerted, regulated by varying the amount of mixture used in the first instance. A description of the steam process as employed at the Edgar Thomson Works, Pittsburg, is given by H. R. Jones in the Journal I. and S. Inst., 1879, 477; it appears to be simple and inexpensive as compared with the Whitworth hydraulic arrangements, but generates far less pressure, Whitworth steel being compressed by forces up to 6 to 9 tons per square inch; increasing the pressure up to 20 tons produces little or no further effect, but, caeteris paribus, the ductility of the compressed metal increases with the pressure used when below some 6 tons per square inch. According to Euverte, no practically valuable results were obtained at Terre Noire nor at St Étienne by the application of intense pressure to the open-hearth steéls there made,—indicating probably that the removal of gas bubbles effected by Whitworth’s operation in crucible steel is accomplished chemically in soft so-called steels of low carbonization made with silico-manganeisen.
The modus operandi of pressure in consolidating steel during casting appears from Bessemer’s observations as to the more copious evolution of gas on diminishing pressure to be simply the application of the well-known law of increased solubility of gas in any given medium with increased pressure; under high pressure the fluid metal retains the gas dissolved just as soda water does the carbon dioxide whilst it remains in bottle; but, justas effervescence is produced in this latter case as soon as the pressure is relieved, so in the case of steel would gas bubbles appear under the ordinary pressure which would not have been developed under high pressure. It is, however, somewhat difficult to understand why steel should retain hydrogen and carbon oxide dissolved when at a high temperature and perfectly fluid, and should extrude it on cooling somewhat and when just on the point of solidifying; but the phenomenon is not an isolated one, fused silver behaving in precisely the same way towards oxygen, and thus giving rise to the well-known “spitting” of silver during solidification. The function of silicon in preventing the extrusion of gas may be ascribed either to its communicating the physical peculiarity to the steel of dissolving as much gas at the solidifying temperature as at the higher temperature, ami consequently of preventing the separation of gas during solidification; or else to its chemical reaction upon the dissolved carbon oxide forming particles of silica disseminated through the mass and setting free carbon, so that the metal containing less dissolved carbon oxide can thus retain more hydrogen; experimental evidence is wanting finally to decide the question, but the latter view seems to be highly probable.
45. Protection of Iron from Oxidation l>y Surface Appliances.— One of the greatest inconveniences in connexion with the use of iron and steel for constructive and general purposes is the tendency of the metal to oxidize and rust in the air under ordinary atmospheric conditions, i.e., in presence of free oxygen together with moisture and small quantities of carbon dioxide (and in the case of the air of seaside places of saline spray, and in that of coal-consuming districts of sulphur acids, &c.). Highly polished iron can be kept in perfectly dry air without rusting at all; and in contact with a solution of an alkali, such as caustic soda or ordinary carbonate of soda, the tendency to oxidize is far less than in simple spring water, sea-water, or moist air; in certain cases this property may be utilized for the preservation of bright steel objects kept in stock, a coating of ordinary whitewash (chalk and water, or preferably lime and water) being brushed over them and allowed to dry on; a similar coating of lime or whiting and oil is even more efficacious, especially if the oil be a non-drying one, z.e., one that does not spontaneously take up oxygen and become more or less acidified. Protective coatings of paints of various kinds—tar, melted pitch, Ac.—are generally applied to the exterior of large iron constructions, such as bridges, pillars, girders, rainwater spouts and conduits, railings, and the like; the function of these is more mechanical than chemical, the coating simply preventing the metal from coming in contact with the oxidizing medium; but in some of the paints used the basic character of certain of the materials probably also diminishes the tendency to oxidation. In certain cases the corrosion of iron can be diminished by placing a more active metal in contact with it (e.y., zinc), so that by a galvanic action the oxidation is largely limited to the zinc; by causing the surface of the iron to be closely adherent to the protecting zinc coating (by dipping the brightened metal in fused zinc), a sort of permanent metallic paint coating is obtained, which acts as a preservative in the threefold manner of mechanically preventing contact with air, of galvanically confining the oxidation to the zinc, and of chemically causing the iron to be coated with a basic film of zinc oxide (when the zinc has become slightly oxidized).
Many other metallic protective coatings can be similarly applied. Of these the most frequently employed is tin, forming ordinary “tinplate,” the manufacture of which dates back a considerable length of ’time. Other coatings can also be applied by means of electricity, at least to small articles; electro-coppered iron goods and nickelized steel articles thus prepared are now frequently manufactured.
It has long been noticed that iron bars, plates, &c., from the forge or rolling mill are coated externally with a film of magnetic oxide formed by the action of the air on the heated metal; during rolling this film becomes somewhat thick and peels off, forming “mill-scale”; when the film is but thin and is closely adherent, it has long been known that the metal does not rast so readily at the parts thus covered up with a protective coating as at points where this coating is removed. Similarly it has been long known that certain natural forms of magnetic oxide of iron, and in particular the “iron sands” of New Zealand, Canada, and elsewhere possess the power of resisting the rusting action of sea-water and atmospheric air conjoined. The idea oι purposely coating iron articles with a film of magnetic oxide prepared in such a way as to resist to the greatest possible extent natural peroxidation, and so closely adherent to the underlying metal as not to be readily detached by ordinary usage, has accordingly been successfully carried out in practice by’ more than one person. Bαrff's process for producing such a coating is based on the reaction occurring between iron and steam at moderately elevated temperatures, viz.,
31 e 4* 41Γ .,O = 1 e∙ 1 O^ + 4 H t ,, hydrogen and magnetic oxide being produced; the articles to be protected, having first been worked into shape, are placed inside a large muffle or chamber of brick heated to a suitable temperature (400° to 600° C.), and subjected to the action of superheated steam for a period of from five to ten hours according to the thickness of the coating required. Bower's process consists in the exposure of the iron articles to be protected to the action of air and carbon dioxide in a chamber or retort heated externally and capable of being closed air-tight; when hot, air is blown in from time to time so as to produce a thin coating of oxide on the surface. A modification of the process consists in alternately oxidizing and reducing the surface, the fuel being burnt inside the chamber; heated air, in excess of that requisite for the combustion, is introduced, and the surface oxidized, so that the outer film is peroxide, magnetic oxide underlying; by shutting off the air supply for a short time the peroxide is reduced and a uniform film of adherent magnetic oxide produced. Both processes can be so worked as to give a highly protective coating to the iron, so that the articles treated will resist ordinary atmospheric influences for long periods of time. The chief weak point in each case (excluding the question of cost) is that it is impossible to apply any amount of force to the treated articles without cracking or stripping off chips of the coating, so that protected iron articles cannot be hammered, rivetted together, bent, brotherwise subjected to mechanical strains, but must be worked to the dimensions and shape (however large) that are ultimately required before treatment; hence the processes become too costly for large girders, &c.
Tin and Terne Plates.— In order to protect thin iron plates from oxidation and to enable them to be readily soldered together so as to manufacture the numerous articles in ordinary use made by the “whitesmith” or “tin-man,” they are coated over with a closely adherent layer of metallic tin, forming tinplate ; when instead of pure tin an alloy of tin and lead is used as a protective coating, the product is known as terneplate. The charcoal plates (§ 23) prepared from a good quality of iron used for the tinning process and properly annealed are thoroughly scoured with sand and water and “pickled” in dilute sulphuric acid alternately until perfectly clean and bright; they are then washed and immersed in a pan full of melted grease until all adherent water has boiled away, and dipped into a bath of melted tin (covered with grease to prevent oxidation) which adheres to the surface more or less completely; the first bath is of less pure tin than that contained in a second into which the plate is further dipped so as to complete the coating; the plate is then taken out, and wiped with a hempen rubber to remove superfluous tin and make sure that the surface is completely covered, and dipped for a third time into another bath, after which it is placed in a grease bath of tallow and palm oil at a moderately high temperature, so that the surplus tin may run off, and then into a cooler grease bath to avoid too sudden chilling, which would impair the face of the coating, and finally taken out and cooled in the air. As the tin in the third bath becomes alloyed with iron from the operation, it is removed into the second, fresh pure tin being used instead; similarly the metal from No. 2 bath is by and by removed to supply the first one, so that the plate is covered with less and less ferruginous tin successively as it passes through the three baths. For terne plates the process is much the same, only an alloy of about half tin and half lead is used instead of pure tin; in consequence, the surface produced is not so brilliant.
When tin plates are partially etched by dilute aqua fortis mixed with common salt or sal ammoniac, they acquire a peculiar spangled appearance, owing to the dissolving away of adherent tin, leaving behind a less readily attacked crystalline tin-iron alloy; the “moirée metallique” thus prepared, after varnishing to prevent oxidation, is frequently used for ornamental purposes.
Galvanized Iron.— When perfectly cleansed iron is immersed in melted zinc instead of tin, the zinc adheres to the surface just as tin does in the tinplate manufacture, forming “galvanized iron,” the name being derived from the circumstance that the coating is analogous to that producible by electrical means. Norwood and Bogers apply a thin coating of metallic tin to the iron before dipping it in the zinc bath, by putting in a wooden tank alternately granulated zinc and the cleaned iron plates, the tank containing a dilute solution of chloride of tin, so that the tin is deposited by a kind of galvanic action.
By the ordinary processes of electroplating manufactured iron and steel goods can be covered over with protective coatings of other metals, notably copper and nickel. Similarly by various processes silver and gold coatings can be laid on, especially by means of the application of mercurial solutions of the precious metals (water gilding), the mercury 7 being volatilized by 7 heat and the residual gold or silver made more adherent by burnishing, the process being repeated several times if necessary. “Pyrosilver” goods are prepared by heating the silvered steel whereby the silver sinks, as it were, into the metal; successive coatings are then applied and “burnt in,” until the sinkage ceases; in this way a coating is obtained not readily detachable by wear and tear. Protective coatings of various forms of enamel are often employed for various classes of iron goods, more especially advertisement boards, i.e., sheets of enamelled iron with the lettering done in some differently coloured enamel. A modification of this class of coating has been recently introduced by Dodé, metallic platinum being mixed with the enamelling composition so as to “platinize” the metal and thus add to the durability; according to the inventor, the comparatively high price of platinum does not interfere with the applicability of the process, one platinizing application costing no more than three coats of good paint, and only about a tenth of the expense of nickelizing.
X. Statistics of the Iron and Steel Manufacture.
46.—The following data are abridged from the Journal of the Iron and Steel Institute, 1880. [table] [table] [table] [table] [table]
The following table, abridged from Hunt’s Mineral Statistics of the United Kingdom, illustrates the gradual increase of the pig iron manufacture in Great Britain during the last twenty years or so :— [table]
The following table, abridged from the Statistical lieport of the British Iron Trade Association, gives compendiously a synoptical view of the iron and steel trade of the United Kingdom in 1878 and 1879:— [table]
In America, which comes next to Great Britain in amount of pig iron production, and slightly exceeds that country in make of steel, the following figures illustrate the production of pig iron during the last twenty-five years (in thousands of tons of 2000 lb each) :—- [table]
The following list of American iron and steel works is. abridged from the Directory of the Iron- ∏ r oιVs of the United States, April 15, 1880 :— [table]
The present position of the iron and steel trade generally as compared with that at a period of some quarter of a century ago, may be put in a few sentences thus. An enormous increase in the production of iron and steel from natural sources has occurred; not only have the older iron producing countries, as a rule, shared largely in this increase, but further, in many countries and districts where the amount of iron production was formerly little more than a nominal amount or even absolutely nil, the manufacture has now attained to notable dimensions, whilst there is every prospect of this increase in production continuing, notwithstanding the great fluctuations in trade experienced from time to time in particular districts. This is mainly due to the vastly increased use of iron and steel (or so-called steel = fused iron of a low degree of carbonization) for constructive and other general purposes, and especially to the introduction of the peculiar melted products obtained from pig iron by means of Bessemer’s blowing process conjoined with Mushet’s spiegeleisen-addition improvement; by Siemens’s open-hearth “steel” making process involving the use of gaseous fuel and the regenerative principle; or by various modifications of these methods, such as the Snelus-Thomas-Gilchrist process, by means of which phosphorus is removed to a large extent from the pig iron whilst undergoing decarbonization. By this and various other allied devices, the production of useful kinds of so called “steels” from impure ores, which till recently could only be utilized for inferior cast and wrought iron production, has become possible. On the other hand the increasing use of fused “steels,” prepared by various of these methods for many purposes for which wrought iron was exclusively used some quarter of a century ago, has stimulated the wrought iron department, and has in consequence caused the inven tion of numerous machines for diminishing the cost and increasing the output of puddled iron, and for effecting a greater degree of purification than hand puddling ordinarily produces. Notwithstanding these advances, however, it is evident that the weld iron industry is by no means increasing in magnitude as rapidly as the ingot metal manufacture. So evident is the disparity that it is believed by many competent judges that puddled metal will in a longer or shorter time become practically a thing of the past, and that ingot metal will almost wholly take its place,—especially when the advantages of the latter as regards physical qualities and cheapness of production have become more extensively appreciated. (c. κ. a. w.)
