538 



IRON AND STEEL. 



may be powerfully modified by introducing 

 into the steel certain elements foreign, or ap- 

 parently so, to its constitution. (See Action of 

 Manganese, &c., above.) 



Having shown, now, that the effect produced 

 in a complete manner by the tempering of steel 

 is in part realized by hammering, this effect 

 being a (mechanical) combination of carbon 

 with the iron, M. Caron is led to explain the 

 phenomenon of tempering by the supposition 

 that in cooling rapidly a piece of steel, the 

 metal is in reality submitted to an almost 

 instantaneous compression, and which has a 

 very considerable analogy with the shock pro- 

 duced by the blow of a hammer. This sup- 

 position, also, experiment appeared decidedly to 

 confirm. A bar of steel of known measurement, 

 tempered by suddenly cooling from a red heat, 

 was found reduced in all its dimensions below 

 those which it had at such heat, though these 

 were still greater than those it had before the sin- 

 gle tempering ; in case of repeated temperings, 

 the length soon becomes less than that of the 

 original bar, and goes on' diminishing, though in 

 such case the other dimensions usually enlarge 

 at the same time, and in proportions such that 

 the density may be reduced. M. Caron con- 

 cludes that, while heat has the effect to con- 

 siderably dilate the metal, and to give to the 

 molecules of iron and carbon the requisite 

 mobility to allow of their being brought into 

 union, the rapid cooling in the process of tem- 

 pering has then the effect to bring the unlike 

 molecules so quickly one upon the other, that 

 the action is like that of the shock of a ham- 

 mer, and the consequence to occasion the 

 intimate combination of the two which takes 

 place. But the action of tempering is more 

 effectual, in that the contraction of the metal 

 raally takes place in all directions at once; 

 while, in hammering, the action is exerted 

 only in a single direction. And, what is still 

 more important, the heat due to hammering in 

 part destroys the effect that would result from 

 the blows ; but in tempering no such counter- 

 action is possible, since while the shock is 

 being imparted to the metal the latter has also 

 grown cold. 



Finally, the author has experimented in the 

 way of tempering steel in a great variety of 

 liquids, such as mercury, oil, water charged with 

 different salts or acids, water covered with oil, 

 or holding mucilages or sirups in solution, &c. ; 

 and he has found that the hardness, temper and 

 other effects due to the process appear always 

 to be induced in a degree which is inversely 

 proportional to the square of the time occupied 

 in the cooling of the metal. Technologiste, 

 March, 1863. 



Strength of Wrought Iron and Steel. Mr. 

 David Kirkaldy, of Glasgow, Scotland, has 

 recently published a volume embodying the 

 results of his experiments made upon many 

 hundreds of bars, plates, bolts, angle irons, 

 &c., and the subject has also been discussed 

 upon the reading of a paper embracing these 



results, before the Scottish Engineers' Asso- 

 ciation. 



Mr. Kirkaldy has been led to conclude that 

 the breaking strain alone of iron or steel does 

 not furnish a true indication of its quality ; that 

 a high breaking strain may be due to the iron's 

 being of superior quality, dense, fine, and 

 moderately soft, or simply to its being very 

 hard and unyielding ; and a low breaking strain, 

 either to looseness and coarseness in the tex- 

 ture, or to extreme softness, although in this 

 case accompanied by very close and fine quality. 

 In an age in which iron is becoming so exten- 

 sively used, as in the present, in the construc- 

 tion of machinery, engines, buildings, bridges, 

 ships, &c., it becomes in the highest degree 

 important that the qualities of this material 

 should be thoroughly understood; and espe- 

 cially, that the best modes of determining its 

 strength should be ascertained. The author 

 concludes that the true test of strength for prac- 

 tical purposes, both of iron and steel, obtained, 

 however, only by the actual breaking of a bar, 

 is to be found in a consideration, jointly, of 

 the area of section of the bar given at the place 

 of fracture, with the breaking strain. In other . 

 words, the ratio of strength of two bars is not, 

 as heretofore supposed, a simple one depending 

 on comparison of the breaking strain simply of 

 the two ; it is a compound ratio, into which, 

 besides the comparison of breaking strains, 

 there must enter also the inverse ratio of the 

 areas of fracture. Thus, if two bars part at 

 the same degree of strain, that which before 

 breaking has been drawn to the smaller section 

 at the place of fracture will, for practical and 

 safe loads, prove the stronger. The author 

 accordingly introduces a new standard or unit 

 of measure for the tensile strength of bars, 

 plates, bolts, &c. ; namely, "the breaking 

 strain per square inch of the fractured area of 

 the specimen, instead of the breaking strain per 

 square inch of the original area." As a corol- 

 lary, he submits that the working strain, or 

 safe load, should be proportioned to the indica- 

 tion of the former test, and not to that of the 

 latter and heretofore usual one. 



Mr. Kirkaldy gives the breaking strain of 

 steel lower than Barlow had done ; but the 

 pieces he experimented on were not in so fa- 

 vorable form for exhibiting strength, as were 

 those tested by the latter. In case of wrought 

 iron, of 188 rolled bars the mean strength was 

 25 1 tons per square inch section; of 72 bars of 

 angle iron, 24^ tons; of 167 plates, lengthwise, 

 60,737 Ibs., and of 160 plates, crosswise, 46,171 

 Ibs., mean of both, 21 tons pej square inch. 

 The breaking strain is generally assumed to be 

 about 25 tons for bars, and 20 tons for plates; 

 but a wide difference of opinion exists as to the 

 proper working strain, i. e., the load which 

 shall be safe in practice. This has been vari- 

 ously stated at from a third to a tenth. The 

 circumstance of the quality of the iron, as af- 

 fecting the working strength, has been in the 

 past too much overlooked. The new experi- 

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