CONTRIBUTIONS OF METALLURGY 257 



that this was exaggerating the disease in order to use more 

 medicine for its cure ; that to give the ingot this great cross sec- 

 tion was to retard the solidification greatly, and thus to in- 

 crease the differentiation which it is the purpose of forging 

 to palliate. We insisted that a reduction of four to one was 

 excessive. 



Fortunately the needs of the war gained us a hearing. We 

 needed cannons, and as quickly as possible. Clearly a re- 

 duction of two to one takes only half as long as a reduction of 

 four to one. This consideration, backed up by the assurances 

 of the most intelligent experts that the faster solidification of 

 the smaller ingots would result in a better product, at last led 

 to casting the steel in much smaller ingots, calling for a re- 

 duction of only two to one instead of four to one. This is 

 typical of a whole class of cases in which the necessities of 

 war induced the authorities to depart from bad practices which 

 had rested on superstition or ignorance. 



Hadfield's Manganese Steel for Helmets. In using Had- 

 field's manganese steel, an alloy of iron with about 12 per cent, 

 of manganese and 1.25 per cent, of carbon, for the helmets of 

 the American and British Armies, the idiosyncrasies of a very 

 remarkable material were utilized in a striking way. Even 

 in our early attempts to use it, we saw some decades ago that, 

 in addition to its extraordinary combination of hardness with 

 ductility, it had some obscure peculiarity which prevented our 

 foretelling with confidence whether it would fit any new serv- 

 ice proposed. Shortly before the war we found that this 

 peculiarity consisted, at least in part, in its increasing greatly in 

 hardness on even slight plastic deformation, that is on being 

 bent, twisted, compressed, lengthened, or otherwise forced be- 

 yond its elastic limit, so that it takes permanent set. This 

 plastic deformation seems to precipitate an overdue allotropic 

 change of the iron itself, from the gamma or non-magnetic 

 ductile state to the beta or hard brittle state, given to common 

 steel by rapid cooling from above a red heat. Thus a rail 

 of manganese steel when first laid in the track consists 



