IRON. 



nature as the hardening of steel. There 

 is another fact, which is of great conse- 

 quence, and may perhaps afford the prin- 

 cipal ground for Rinmun's apparent ex- 

 ception to the general rule concerning 

 the hardening of steel. Hardness is that 

 property of bodies by which they resist 

 indendation, and rather break, than bend 

 or suffer contusion. It is the opposite to 

 softness. Tenacity, which is very fre- 

 "quently confounded with hardness, is aot 

 the property by which they resist inden- 

 tation, but separation of their parts. It is 

 the opposite to friability or brittleness. 

 Now that steel, in common language, is 

 said to be the hardest, which unites most 

 eminently the properties of hardness and 

 tenacity. But the effect of heat and sud- 

 den cooling is directly opposite in these 

 two respects. Soft steel is the most tena- 

 cious, but the least hard. The operation 

 of hardening diminishes its tenacity, and 

 increases its hardness progressively to the 

 maximum of heat the steel is formed to 

 bear. There will be no difficulty then 

 in conceiving, that the best state of steel, 

 relative to any particular use, will be at 

 some precise degree between the softest 

 and hardest qualities. Thus, for springs, 

 much tenacity and moderate hardness are 

 required. For chissels and similar tools, 

 which operate by a blow, a greater hard- 

 ness may be admitted. Razors, knives, 

 and such tools as effect the intended pur- 

 pose by a gradual stroke, will be stil] 

 more valuable the harder they are: but 

 even in these the tenacity must not be too 

 much diminished, otherwise the edge 

 will be liable to break. They must all be 

 capable of having the edge turned or 

 bended on one side in the operation of 

 whetting. Files are perhaps of all tools 

 such as require the greatest hardness ; 

 but in these, it is far from being the ut- 

 most the steel is capable of receiving. It 

 is found, that the tenacity of steel is con- 

 siderably increased bycontinued hammer- 

 ing to a certain point. But the whole ef- 

 fect of this hammering is taken off by 

 strong ignition. Good steel by hardening 

 at a white heat may be rendered so brit- 

 tle, that it will break full as easily as a 

 stick of the same dimensions, and its tex- 

 ture is then found to be coarse and large 

 grained. As the subsequent annealing 

 does not restore the effect of the ham- 

 mering, nor bring the grain of the steel 

 to the state it would have possessed if a 

 lower heat had been u^ed at first, it is evi- 

 dent, that the most useful hardness is 

 produced by that degree of heajt, which 

 is just sufficient to effect the purpose. 



And, accordingly, tools made of cast steel,, 

 and intended to sustain a good edge for 

 cutting iron and other metals, are not af- 

 terwards annealed, but the ignition is 

 carefully regulated at first. Annealing 

 ought only to be used where considera- 

 ble softness is required. 



Iron may be hardened to a certain de- 

 gree by ignition and plunging in water, 

 but this effect is confined to the surface ; 

 except, as it very often happens, the iron 

 contains veins of steel. These are no 

 shall impediments to the filing and work- 

 ing this material. It sometimes likewise 

 may happen, that the iron may have un- 

 dergone a process of the nature of case- 

 hardening from the fuel. We have been 

 informed by a workman, that ignited iron, 

 suddenly plunged into the soft leather of 

 a shoe, becomes very hard on its surface, 

 which must arise from an instantaneous 

 effect of this kind. 



The increase of dimensions acquired by 

 steel in hardening is such, that in general 

 such pieces of work as are finished soft 

 will not fit their places when hardened. 

 Rinman found, that bars of steel, six inch- 

 es long, six lines wide, and half an inch 

 thick, were lengthened at least one line 

 after hardening by a whitish red heat, 

 which is about one seventieth of the linear 

 dimensions; and supposing the dilatation 

 to be proportional, Guyton Morveau com- 

 putes the bulks to be as 48 to 49. But 

 the cubes of 72 lines and 73 lines are in 

 proportion more nearly as 47 to 50. Va- 

 rious kinds of steel at different hardness 

 must no doubt greatly differ in this re- 

 spect. The specific gravities, as given by 

 Brisson, p. 366, art. GRAVITY, (specific), 

 afford a much less ratio. Rinman found, 

 by his experiments on two different kinds 

 of fine cemented steel, the specific gravi- 

 ty of which was 7.991, that one after 

 hardening gave only 7.553, and the other 

 7.708. These numbers agree sufficiently 

 near with the experiment of the six inch, 

 bar. Yet he once found Styrian steel aug- 

 mented in density, by hardening, in the 

 ratio of 7.822 to 7.782. Morveau found, 

 with pieces of'stee!28 lines long, that the 

 increase of length by hardening was about 

 the 350th part. 



The fineness of grain in hard steel, as 

 exhibited in its fracture, is various, ac- 

 cording to the quality of the metal, and 

 the temper it has received. The harder 

 the steel tiie coarser the grain. But in 

 like circumstances, fine steel has the 

 closest grain, and is ever the most uni- 

 form in its appearance "Workmen avail 



