Types, Characteristics, and
Uses of Alloyed Steels
While the plain carbon type of steel remains the
principal product of the steel mills, so-called alloy or
special steels are being turned out in ever increasing
tonnage. Let us now consider those alloyed steels and
their uses in aircraft.
CARBON STEELS.Steel containing carbon in
percentages ranging from 0.10 to 0.30 percent are
classed as low-carbon steel. The equivalent SAE
numbers range from 1010 to 1030. Steels of this grade
are used for making such items as safety wire, certain
nuts, cable bushings, and threaded rod ends.
Low-carbon steel in sheet form is used for secondary
structural parts and clamps, and in tubular form for
moderately stressed structural parts.
Steels containing carbon in percentages ranging
from 0.30 to 0.50 percent are classed as medium-carbon
steel. This steel is especially adaptable for machining or
forging and where surface hardness is desirable. Certain
rod ends and light forgings are made from SAE 1035
steel.
Steel containing carbon in percentages ranging
from 0.50 to 1.05 percent are classed as high-carbon
steel. The addition of other elements in varying
quantities adds to the hardness of this steel. In the fully
heat-treated condition, it is very hard and will withstand
high shear and wear and have little deformation. It has
limited use in aircraft. SAE 1095 in sheet form is used
for making flat springs, and in wire form for making coil
springs.
NICKEL STEELS.The various nickel steels are
produced by combining nickel with carbon steel. Steels
containing from 3 to 3.75 percent nickel are commonly
used. Nickel increases the hardness, tensile strength, and
elastic limit of steel without appreciably decreasing the
ductility. It also intensifies the hardening effect of heat
treatment. SAE 2330 steel is used extensively for
aircraft parts such as bolts, terminals, keys, clevises, and
pins.
CHROMIUM STEELS.Chromium steels are
high in hardness, strength, and corrosion-resistant
properties. SAE 51335 is particularly adaptable for
heat-treated forgings that require greater toughness and
strength than may be obtained in plain carbon steel. It is
used for such articles as the balls and rollers of
antifriction bearings.
CHROMIUM-NICKEL OR STAINLESS
STEELS.These are corrosion-resisting metals. The
anticorrosive degree is determined by the surface
condition of the metal as well as by the composition,
temperature, and concentration of the corrosive agent.
The principal part of stainless steel is chromium, to
which nickel may or may not be added. The corrosion-
resisting steel most often used in aircraft construction is
known as 18-8 steel because of its content of 18 percent
chromium and 8 percent nickel. One of the distinctive
features of 18-8 steel is that its strength maybe increased
by cold-working.
Stainless steel may be rolled, drawn, bent, or formed
to any shape. Because these steels expand about 50
percent more than mild steel and conduct heat only
about 40 percent as rapidly, they are more difficult to
weld. Stainless steel, with but a slight variation in its
chemical composition, can be used for almost any part
of an aircraft. Some of its more common applications
are in the fabrication of exhaust collectors, stacks and
manifolds, structural and machined parts, springs,
castings, and tie rods and cables.
CHROME-VANADIUM STEELS.These are
made of approximately 0.18 percent vanadium and
about 1.00 percent chromium. When heat treated, they
have strength, toughness, and resistance to wear and
fatigue. A special grade of this steel in sheet form can
be cold-formed into intricate shapes. It can be folded and
flattened without signs of breaking or failure. SAE 6150
is used for making springs; and chrome-vanadium with
high-carbon content, SAE 6195, is used for ball and
roller bearings.
CHROME-MOLYBDENUM STEELS.Molyb-
denum in small percentages is used in combination with
chromium to form chrome- molybdenum steel, which
has various uses in aircraft. Molybdenum is a strong
alloying element, only 0.15 to 0.25 percent being used
in the chrome-molybdenum steels; the chromium
content varies from 0.80 to 1.10 percent. Molybdenum
raises the ultimate strength of steel without affecting
ductility or workability. Molybdenum steels are tough,
wear resistant, and harden throughout from heat
treatment. They are especially adaptable for welding,
and for this reason are used principally for welded
structural parts and assemblies. SAE 4130 is used for
parts such as engine mounts, nuts, bolts, gear structures,
support brackets for accessories, and other structural
parts.
The progress of jet propulsion in the field of naval
aviation has been aided by the continuous research in
high-temperature metallurgy. This research has brought
forth alloys to withstand the high temperatures and
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