Cast Steel. 
429 
the desired form, it can then be made so hard as to be ca» 
pable of catting the hardest substances (the precious 
stones excepted), while at the same time it is almost proof 
against being itself worn by friction : but in this state it 
is brittle, like all other hard substances, and for many 
purposes must have this brittleness lessened, and this is 
what is termed by workmen, tempering, and consists in 
giving it certain degrees of heat according to the temper 
desired, which may be produced in any degree until the 
whole effect of hardening is destroyed, and the steel is re- 
duced to its soft state. On each of these operations I 
shall offer a few remarks, proved by long experience j 
and, first, on the choice of steel for such purposes as re- 
quire the best that can be procured for making cutting in* 
struments, such as gravers, punches, turning tools, chisels, 
<kc. &c. to be employed in turning or cutting tempered 
steel, and substances that are too hard to be cut by tools 
made of ordinary steel : for these purposes cast steel is 
undoubtedly the best ; but even this sort differs in quali- 
ty. 
The general mode of choosing such as is most suitable 
for the above purposes is to break a bar, and observe its 
fracture, and to select the closest grained ; but this mode 
is not always certain, owing to the difference made in the 
fracture by the steel being hammered under a greater or 
less degree of heat, steel being much improved by being 
hammered under a low heat, and even when cold ; and 
when overheated, being quite spoiled for the above pur- 
pose. It is owiifg to this circumstance that the best sorts 
of cast steel are incapable of being welded as above men- 
tioned. Another method is, to harden with as low a heat 
as possible a piece of steel, and then to break it, and ob- 
serve its fracture : but this is not wholly to be depended 
upon ; for some steel breaks with a very close grain, and 
yet is not of a good quality. But the surest method is to 
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