STEEL AS APPLIED TO ARMOUR. 
229 
and manganese are both most important elements, but do not appear 
to so directly affect the question we are dealing with, that is to say, the 
attainment of the hard face to a plate, though their presence in proper 
proportions is essential to get good steel plates. Manganese and 
carbon together, being added in fact, to all furnace charges before 
tapping, in the form of spiegeleisen, or ferromanganese, etc. 
Nickel, we have lately heard a good deal of as being capable of 
imparting special resisting properties to steel, into which it is intro¬ 
duced in proper proportions—though its introduction has not been 
attended with that success on this side of the Atlantic which is reported 
from America. It appears to toughen steel, but “ nickel steel ” plates 
have been various in their behaviour, the correct method of alloying 
it not appearing to be entirely understood as yet. Steel possesses the 
following property : if it be heated up to a high temperature, and 
then suddenly cooled by plunging it into liquid, such as oil, it is 
“ hardened”—its breaking strain and limit of elasticity are raised, 
but, as might be expected, its ductility is lessened; but if the steel is 
now heated up again to a temperature below that at which it was 
hardened and allowed to cool slowly, its ductility is in a great measure 
restored, and at the same time its breaking strain and elastic limit are 
lowered but little. 
If we examine the fracture of steel in the “ soft ” state, we find it is 
composed of a group of fair-sized crystals, but after hardening, the 
appearance of the fracture is much finer and closer and the crystals 
much smaller; in fact in very hard tool steel it is hard to distinguish 
them. 
It would seem as if, when the steel is heated, these crystals dispose 
themselves in certain ways, or the crystalline form is lost entirely, and 
the rapid chilling re-crystallises the steel so suddenly that only small 
crystals have time to form, and at the same time they are all thrown 
into a state of tension one with the other. By subsequent heating 
and slow cooling the crystals are enabled to dispose and settle them¬ 
selves comfortably, while at the same time the size of them is affected 
but little, if at all, the ductility is thus in a great measure restored, 
and it would appear as if the hardness depends to a certain extent on 
the size of the crystals in a particular steel, and this is but little 
affected by “ annealing ” at a suitable temperature. The higher the 
proportion of carbon, the lower is the temperature to which it is 
requisite to heat the steel in order to harden it, and for each steel 
there is a particular temperature of hardening, heating above which 
does harm. The lower temperature to which it is necessary to heat a 
high carbon steel in order to harden it, is due to the amount of carbon 
present, and has its analogy in the fact that cast-iron, which is iron in 
the maximum degree of carburization, melts at 2786° F., while wrought- 
iron, which has the smallest amount of carbon alloyed with it, will not 
become truly liquid at any ordinary furnace temperature. Wrought- 
iron is usually termed in text-books ff the fibrous form of iron,” and 
this is true so far as it goes, though steel undoubtedly can have its 
crystals arranged by forging, so as to show some of the characteristics 
peculiar to a line of fibre running through a mass of wrought-iron. 
31 
