4 6 



NATURE 



[March 13, 19 13 



materials by cheaper water carriage from the 

 Humber. 



It is next of interest to consider how, during the 

 fourteenth, fifteenth, sixteenth, seventeenth, and half 

 the eighteenth centuries, Sheffield made all its fine 

 steel. It seems almost certain that the nearly pure 

 imported Swedish or Spanish irons were carburised 

 "in the dry way," by cementation in charcoal at a 

 yellow heat. The highly ductile bar iron and the 

 blistered and brittle steel resulting from its cementa- 

 tion-carburisation were described. The blister bar was 

 then made into what for perhaps two hundred and 

 fifty years has been known as "shear steel." 



(The method of producing from blister bar both 

 single and double shear steel was then described.) The 

 origin of the name "shear steel" was due to the fact 

 that British cloth-workers insisted on having this fine 

 quality of steel for their cloth-cutting shears, and this 

 material is still branded with rude representations of 

 clothiers' shears. One pair of shears signifies single 

 shear and two pairs double shear steel. The chemical 

 composition of this steel, which is the purest made, 

 is as follows : — Carbon ioo per cent; silicon, 003 per 

 cent. ; manganese, 0-07 per cent. ; sulphur, 001 per 

 cent. ; phosphorus, 0-015 per cent. With its high 

 reputation built up during centuries this material has 

 naturally had its name branded on inferior kinds of 

 steel. Indeed, bars of steel up to 6 in. in diameter 

 have been sold as "shear steel" at 185. per cwt., the 

 price of the raw material from which shear steel is 

 manufactured. Probably a bar ih in. in diameter 

 marks the advisable limit of size for genuine shear 

 steel, and its average market price is about 45s. per 

 cwt. 



The vear 1740 marked for Sheffield, and indeed for 

 the world, the beginning of an epoch of great metall- 

 urgical importance. Benjamin Huntsman, a well- 

 k"own clockmak"r of Doncaster, found that shear 

 steel, on account of its sometimes varying temper and 

 of its weld-lines, often presented uneven hardness and 

 exasperating flaws when made into clock springs. 

 He consequently determined to make a steel even 

 in texture and free from weld flaws. He experimented 

 successfully, and worked out a method for the pro- 

 duction of sound steel ingots by the fluid or crucib'e 

 process, and so founded in Sheffield an industry, 

 destined to become world-wide, which soon extended 

 the fame of Sheffield steel throughout the 1 h 

 world. 



\ composition typical of crucible cast-steel was 

 then given. It is less pure than shear steel, 

 but sounder, being free from weld-lines. It is said 

 that the famous American, General Sherman, when 

 asked to "spare the good Indians," replied that the 

 only good Indians he had ever met were dead Indians. 

 Be this as it may, it is certain that no steel can be 

 good unless it is properly " killed," or, in other words, 

 " dead melted." 



Fig. 1 shows two crucible steel ingots of 

 identical composition and weight when poured 

 in a "lively" and in a "killed" condition. 

 Ignoring the "pipe," or central contraction cavity, 

 the killed steel is quite solid, whilst the unkilled 

 metal is riddled from end to end with gas cavities or 

 "blowholes," containing, under pressure, hydrogen, 

 carbonic oxide, and nitrogen gases, evolved in thi 

 plastic steel during solidification, and thus rendering 

 the ingot commercially worthless. The sound and 

 hence apparently much smaller ingot has been 

 "killed" bv the presence of a trace (say 001 oer cent.) 

 of metallic aluminium. The scientific explanation of 

 this, the most remarkable phenomenon in the whole 

 range of steel metallurgy, may be found in text-books 

 or in reports of metallurgical lectures, but the present 



NO. 2263, VOL. 91] 



lecturer must confess that he is no nearer a con- 

 vincing solution of this problem than when he began 

 his researches twenty-five years ago. 



It is next necessary to correlate the chemical and 

 micrographic analyses of the plain carbon steel upon 

 which the world depended for its cutting implements 

 from the time of Homer 

 to 1870. 



The structure of pure 

 Swedish iron is usually- 

 contaminated with a 

 little slag. Ignoring 

 this, the mass consists 

 of white allotrimorphic 

 crystals of iron with 

 optically black boun- 

 daries. 



In a micrograph of 

 nearly pure iron contain- 

 ing about 0-4 per cent, 

 of carbon, almost half 

 the mass consists of the 

 dark-etching compound 

 constituent pearlite. 



The structure of 

 nearly pure iron con- 

 taining 089 per cent, of 

 carbon consists entirely 

 of pearlite, a mechan- 

 ical mixture of 87 pel- 

 cent, of iron with 13 per 

 cent, of normal carbide 

 of iron, Fe 3 C. The 

 mass abrasion hardness 

 of normal pearlite is 

 about 4-5 — that is, be- 

 tween fluorspar and 

 apatite on Moh's 

 mineral scale. 



We have next to con- 

 sider the phenomena 

 known as the hardening 

 and tempering of steel. 



Figs. 2 and 3 show 

 yerv clearly the begin- 

 ning, the progression 

 and end of the harden- 

 ing of steel — that is to 

 say, the transformation 

 (during a thermal ampli- 

 tude of perhaps j : C.) 

 of the compound 

 constituent pearlite 

 (2iFe+Fe,C) to the 

 micrographically amor- 

 phous constituent hard- 

 enite, which corresponds 

 to the empirical figures 

 Fe 21 C, in which the car- 

 bide of iron, owing 

 to the quenching, is 

 trapped in some mole- 

 cular association with 

 the whole of the iron. 

 The constituent harden- 

 ite has a hardness of p ]G ti 



7 on Mohr's mineral 



scale — that is to say, it is as hard as quartz, flint, or 

 rock crystal. 



It is a little difficult to realise how much the thermal 

 capability of the mineral pearlite (with a hardness 

 of 4'5) to transform itself into the igneous rock 

 hardenite (with a hardness of ~\ has contributed 

 to the advance of civilisation and fo the material well- 

 being of the human race. But unfortunately it was 



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