THE HARDENING AND TEMPERING OF STEEL. 
249 
made to elucidate it should have been very few. We believe 
we are right in saying that for some years the subject had the 
joint attention of two such minds as those of Faraday and James 
Nasmyth ; hut the results obtained were not considered suf- 
ficiently important to warrant their publication. It will be 
seen, therefore, that the question is eminently worthy of 
thorough scientific examination. 
We will first state, as briefly as possible, the main facts of 
the case. W e shall confine ourselves to ‘ hard 9 steel, such as 
is used for tools, containing not less than say ^ per cent of 
carbon ; with ‘ mild ’ steel, containing a smaller quantity, the 
phenomena are different. Hard steel, as produced mainly by 
the cementation process, can be hardened by being raised to a 
high temperature, and then suddenly cooled, or ‘ quenched/ in a 
hath of some cold fluid, generally water or oil. The outside is 
always the hardest, but, if the thickness be moderate, there is 
considerable hardening throughout. The colder the hath, the 
greater the hardness of the steel, until it reaches the ‘ ice-brook’s 
temper s of Othello’s sword ; but steel in passing rapidly 
through so long a range of cooling is very apt to ‘fly/ or be- 
come brittle. For tough steel, therefore, the range of cooling 
should be as low as possible. By cooling in oil, the tenacity of 
the steel is supposed to he increased ; and this increase is be- 
lieved to he greater the higher the temperature of quenching : 
hut the difference between water-cooling and oil-cooling is pro- 
bably only due to the fact that the latter is a more gradual 
process. Unless the initial temperature of the steel is above a 
certain point, the hardening does not take place at all. 
It will be seen that the phenomena vary materially with 
the initial and final temperatures of the steel, and to some 
extent with the rapidity of its passage from one to the other. 
It will also he seen that the essential qualities of hardness and 
toughness are opposed to each other ; in other words, the steel 
will be more hard, but at the same time less tough, as the fall of 
temperature is greater and more rapid. Hence it is of the 
utmost importance that the cooling should he as gradual as is 
consistent with giving to the steel that degree of hardness 
which is needful for the work it has to do ; it will then he as 
tough as it can be made to be sufficiently hard. In the bringing 
of any article to this desirable condition consists the process of 
tempering. In practice this is effected by first heating and 
quenching the steel, so as to harden it in excess, then raising it 
again very carefully to a certain temperature, varying according 
to the use for which it is intended, and then quenching it again 
from that temperature, The manner in whioh the eorrect 
temperature is ascertained is very curious and striking. Take 
the case of an ordinary cold chisel, which is usually hardened 
