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. 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 velocities encountered in jet power units. These alloys are chemically similar to the previously mentioned steels, but may also contain cobalt, copper, and columbium in varied amounts as alloying elements.

The term nonferrous refers to all metals that have elements other than iron as their principal constituent. This group includes aluminum, titanium, copper, magnesium, and their alloys; and in addition, such alloy metals as Monel and Babbitt.