NATURE 



\_Nov. 14, 1889 



'he dissolved or hardeninof carbon to the combined 

 carbide-carbon. It follows that, if steel be quickly cooled 

 after the change from fi to o. has taken place but before 

 the carbon has altered its state — that is, before the change 

 indicated by the second break in the curve has been 

 reached — then the iron should be soft, but the carbon, 

 hardening carbon ; and as such, the action of a solvent 

 should show that it cannot be released from iron in the 

 black carbide form. This proves to be the case, and 

 affords strong incidental proof of the correctness of the 

 view that two modifications of iron can exist. 



It will be seen, therefore, that, although the presence of 

 carbon is essential to the hardening of steel, the change 

 in the mode of existence of the carbon is less important 

 than has hitherto been supposed. 



The a modification of iron may be converted into the /3 

 form by stress applied to the metal at temperatures below 

 a dull red heat, provided the stress produces permanent 

 deformation of the iron,^ but the consideration of this 

 question would demand a lecture to itself. I am anxious 

 to show you an experiment which will help to illustrate 

 the existence of molecular change in iron. 



Here is a long bar of steel containing much carbon. 

 In such a variety of steel, the molecular change of the iron 

 itself, and the change in the relations between the carbon 

 and the iron, would occur at nearly the same moment. It 

 is now being heated to redness, but if you will look at 

 this diagram (Fig. 9), you will be prepared for what I want 



shown by Spring, even at the ordinary temperature, while, 

 in the case of steel, it must take place far below incipient 

 fluidity — indeed, at a comparatively low temperature, as is 

 shown by the following experiment on the welding of steel. 

 Every smith knows how difficult it is to weld highly 

 carburized hard tool-steel, but if the ends of a newly- 

 fractured f^-inch square steel rod, a (Fig. 10), are placed 



Fig. 9. — The bar of steel, a, i inch in section and i8 inches Ion?;, heated to 

 bright-redness and firmly fixed in a vice or other supp )rt at b. A weight 

 of about 2 pounds is rapidly hung on to ihe free end. and a light 

 pointer, c, is added to magnify the motion ot the bir. It remains per- 

 fectly rigid for a per.od varying from 33 to 40 seconds, and then, when 

 the bar has cooled down to very dull redness, it suddenly bends, the 

 pointer falling from 6 to 8 inches to the position C. 



you to see in the actual experiment. One end of the red- 

 hot bar a will be firmly fixed at b, a weight not sufficient to 

 bendit is slung to the free end, which is lengthened by the 

 addition of a reed, <:, to magnify any motion that may take 

 place. Now remember that as the bar will be red-hot it 

 ought to be at its softest, you would think, when it is freshly 

 withdrawn from the furnace and if the weight was ever to 

 have power to bend it, it would be then ; but, in spite of the 

 rapidity with which such a thin bar cools down in the air 

 and becomes rigid, points of molecular weakness come 

 when the iron changes from 3 to a, and the carbon passes 

 from hardening carbon to carbide-carbon ; at that moment, 

 at a temperature much below that at which it is withdrawn 

 from the furnace, the bar will begin to bend, as is shown 

 by the dotted lines a', c'. It has been found experimentally 

 ihat this bend occurs at the point at which, according to 

 Osmond's theory, molecular change takes place. Mr. 

 Coffin takes advantage of this fact to straighten distorted 

 steel axles. 2 



There is a sentence in the address which has just been 

 delivered before Section G, by Mr. Anderson, which has 

 direct reference to molecular change in iron. He says : — 



"When, by the agency of heat, molecular motion is raised to 

 a pitch at which incipient fluidity is obtained, the particles of 

 two pieces brought into contact will interpenetrate or diffuse 

 into each other, the two pieces will unite into a homogeneous 

 whole, and we can thus grasp the fall meaning of the operation 

 known as ' welding.' " 



It is, however, possible to obtain evidence of inter- 

 change of molecular motion, as has been so abundantly 



^ " Etudes Metallurgiques," par Osmond, p. 6 (Pans : Dunod, 1888.) 

 ^ Trans. American Soc. Civil Engineers, xvi., 1887, p. 324. 



Fig. 10. 



together and covered with platinum foil, b, so as to exclude 

 the air, and if the junction is heated in the flame of a 

 Bunsen burner, f, the metal will weld, without pressure, 

 so firmly that it is difficult to break it with the fingers, 

 although the steel has not attained a red-heat.' 



The question now arises. What is the effect of the 

 presence of other metals in steel, of which much has been 

 heard recently ? (i) Manganese. Osmond has shown that 

 this metal enables steel to harden very energetically, as is 

 well known. If much of it be present, 1 2 to 20 per cent., in 

 iron, no break whatever is observed in the curve which re- 

 presents slow cooling (see line marked " manganese steel" 

 (Fig. 7). That is, the iron never shows such a change as 

 that which occurs in other cooling masses of iron. Then 

 you will say such a material should be hard however it is 

 cooled. So it is. There is one other important point of 

 evidence as to molecular change connected with the 

 addition of manganese to submit to you. Red-hot iron 

 is not magnetic. Hopkinson- has shown that the tem- 

 perature of recalescence is that at which iron ceases to be 

 magnetic. It may be urged that /S iron cannot therefore 

 be magnetized. Steel containing much manganese cannot 

 be magnetized, and it is therefore fair to assume that the 

 iron present is in the /3 form. Hadfield^ has given 

 metallurgists wonderful alloys of iron and manganese in 

 proportions varying from 7 to 20 per cent, of manganese. 

 This core of iron round which a current is passing, 

 attracts the sphere of iron, but if nothing is changed, 

 except by replacing the core of iron with a core of 

 Hadfield's steel, it is impossible to make a magnet of it. 

 [Experiment shown.] 



Prof. Ewing, who has specially worked on this subject, 

 concludes that, " no magnetizing force to which the 

 metal is likely to be subjected in any of its practical 

 applications would produce more than the most infini- 

 tesimal degree of magnetization " in this material. 



It has been seen that quantities of manganese above 7 

 per cent, appear to prevent the passage of {-i iron into the 

 a form. In smaller quantities manganese seems merely 

 to retard the conversion, and to bring the two loops of 

 the diagram nearer together. 



Time will not permit me to deal with the effect of 

 other elements on steel. I will only add that tungsten 

 possesses the same property as manganese, but in a 

 more marked degree. Chromium has exactly the re- 

 verse effect, as it enables the change of hard \-i iron 

 to a soft iron to take place at a higher temperature 

 than would otherwise be the case, and this may explain 

 the extreme hardness of chromium steels when hardened 

 in the same way as ordinary steels. 



There are a few considerations relative to the actual 

 working of steel with which I can deal but briefly, notwith- 

 standing their industrial importance. The points a and 

 b, adopted in the celebrated memoir of Chernoff to which 



' Trans. American Society Mechanical Engineers, ix., iS88, p. 155. 



'^ Prcc. Roy. Soc, xlv., 1889, pp 318, 445, and 457. 



3 Proc. Inst. Civil Engineers, xciii. Part iii., 1888. » 



