Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/371

 IRON 355 of consecutive trains, especially conjoined with moisture, conduces to mechanical abrasion. Price Williams lias 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 17Jand 161 million tons, the latter being thus more than nine times as lasting as the former. Xumerous observations have been made on the effect on the strength of iron and steel of punching and drilling holes, of notch ing 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 A arious papers in the Journal I. and S. Inst., Iron, and Engineer ing 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 hard ening 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 O on the causes and effects of hardening iron and steel, by Professor Akermann, is to be found in the Journal I. and S. List., 1879, 504 ; whilst the Kesearch Committee of the Institution of Mechanical Engineers has recently issued reports containing much information on this 1 subject. Effect of Temperature on the Strength of Iron and Steel. Many observations on this point have been made by dif ferent experimenters, with the general result of indicating that at C. and below the tensile strain and resistance to percussion of iron and steel bars, rods, &c., are substan tially 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 com position of the metal, &c. ; 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 : Reduction in Power of Resisting Impact. Reduction in Flexibility. Wrought iron Per cent. 3-0 Per cent. 18 Best cast steel 3 5 17 Malleable cast iron 4-5 15 Cast iron 21-0 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 54 (varying from 0-44 to 67) per cent., the rails are decidedly more brittle at temperatures near to - 20 C. than at the ordinary tem perature (+ 10 to 20) ; whilst the effect of low tempera ture in producing brittleness is not marked when the phosphorus, silicon, and carbon jointly amount to only about 41 (0 37 to 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 = 30, silicon = 20, phosphorus = 05, would be equivalent to one containing phosphorus = 25), these results may be expressed as follows : when the per centages of carbon, silicon, and phosphorus are jointly equivalent to an average of 19 per cent, of phosphorus, the effect of low temperatures is not marked ; but when they are equivalent to an average of 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 ten sion 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, &e. , 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 spon taneously 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 tem perature 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 work manship and the development of internal strain are the true causes of the rupture, and not a gradual alteration in texture. At 300 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 deteriorat ing 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 tem perature 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 &quot;burnt iron&quot;; 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 (Comptcs Eendus, 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 &quot;burnt&quot; crystalline structure can O be developed under circumstances where no oxidation can occur. Akermann also lias been led to the same conclusion, defining &quot; burnt &quot; iron as &quot; 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.&quot; 44. Foundry Operations. Occasionally for rough cast ings, such as tuyere nozzles, &c., 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 circum stances. 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.