ith the same carbon content and say 5 per cent nickel,
the structure would still be pearlitic, but much finer and denser,
therefore capable of withstanding shock, and having greater dynamic
strength. With about 0.2 per cent carbon and 8 per cent nickel, the
steel is nearing the stage between pearlite and martensite, and
the structure is extremely fine, the ferrite and pearlite having
a very pronounced tendency to mimic a purely martensite structure.
Steel with 0.2 per cent carbon and 15 per cent nickel is entirely
martensite. Higher percentages of nickel change the martensitic
structure to austenite, the steel then being non-magnetic. The
higher percentages, that is 30 to 35 per cent nickel, are used
for valve seats, valve heads, and valve stems, as the alloy is a
poor conductor of heat and is particularly free from any tendency
towards corrosion or pitting from the action of waste gases of
the internal-combustion engine.

Nickel steels having 3-1/2 per cent nickel and 0.15 to 0.20 per
cent carbon are excellent for case-hardening purposes, giving hard
surfaces and tough interiors.

To obtain the full effect of nickel as an alloy, it is essential
that the correct percentage of carbon be used. High nickel and
low carbon will not be more efficient than lower nickel and higher
carbon, but the cost will be much greater. Generally speaking,
heat-treated nickel alloy steels are about two to three times stronger
than the same steel annealed. This point is very important as many
instances have been found where nickel steel is incorrectly used,
being employed when in the annealed or normal state.

CHROMIUM

Chromium when alloyed with steel, has the characteristic function
of opposing the disintegration and reconstruction of cementite.
This is demonstrated by the changes in the critical ranges of this
alloy steel taking place slowly; in other words, it has a tendency
to raise the _Ac_ range (decalescent points) and lower the _Ar_
range (recalescent points). Chromium steels are therefore capable
of great hardness, due to the rapid cooling being able to retard
the decomposition of the austenite.

The great hardness of chromium steels is also due to the formation
of double carbides of chromium and iron. This condition is not
removed when the steel is slightly tempered or drawn. This additional
hardness is also obtained without causing undue brittleness such as
would be obtained by any increase of carbon. The degree of hardness