steel - Nouvelle page 1 - Free

MMPDS-01
31 January 2003
2.2 CARBON STEELS
2.2.0 COMMENTS ON CARBON STEELS
2.2.0.1 Metallurgical Considerations — Carbon steels are those steels containing carbon up
to about 1 percent and only residual quantities of other elements except those added for deoxidation.
The strength that carbon steels are capable of achieving is determined by carbon content and, to a
much lesser extent, by the content of the residual elements. Through cold working or proper choice of heat
treatments, these steels can be made to exhibit a wide range of strength properties.
The finish conditions most generally specified for carbon steels include hot-rolled, cold-rolled, colddrawn,
normalized, annealed, spheroidized, stress-relieved, and quenched-and-tempered. In addition, the lowcarbon
grades (up to 0.25 percent C) may be carburized to obtain high surface hardness and wear resistance
with a tough core. Likewise, the higher carbon grades are amenable to selective flame hardening to obtain
desired combinations of properties.
2.2.0.2 Manufacturing Considerations —
Forging — All of the carbon steels exhibit excellent forgeability in the austenitic state provided the
proper forging temperatures are used. As the carbon content is increased, the maximum forging temperature
is decreased. At high temperatures, these steels are soft and ductile and exhibit little or no tendency to work
harden. The resulfurized grades (free-machining steels) exhibit a tendency to rupture when deformed in
certain high-temperature ranges. Close control of forging temperatures is required.
Cold Forming — The very low-carbon grades have excellent cold-forming characteristics when in
the annealed or normalized conditions. Medium-carbon grades show progressively poorer formability with
higher carbon content, and more frequent annealing is required. The high-carbon grades require special
softening treatments for cold forming. Many carbon steels are embrittled by warm working or prolonged
exposure in the temperature range from 300 to 700EF.
Machining — The low-carbon grades (0.30 percent C and less) are soft and gummy in the annealed
condition and are preferably machined in the cold-worked or the normalized condition. Medium-carbon (0.30
to 0.50 percent C) grades are best machined in the annealed condition, and high-carbon grades (0.50 to 0.90
percent C) in the spheroidized condition. Finish machining must often be done in the fully heat-treated
condition for dimensional accuracy. The resulfurized grades are well known for their good machinability.
Nearly all carbon steels are now available with 0.15 to 0.35 percent lead, added to improve machinability.
However, resulfurized and leaded steels are not generally recommended for highly stressed aircraft and
missile parts because of a drastic reduction in transverse properties.
Welding — The low-carbon grades are readily welded or brazed by all techniques. The mediumcarbon
grades are also readily weldable but may require preheating and postwelding heat treatment. The
high-carbon grades are difficult to weld. Preheating and postwelding heat treatment are usually mandatory
for the latter, and special care must be taken to avoid overheating. Furnace brazing has been used
successfully with all grades.
Heat Treatment — Due to the poor oxidation resistance of carbon steels, protective atmospheres must
be employed during heat treatment if scaling of the surface cannot be tolerated. Also, these steels are subject
to decarburization at elevated temperatures and, where surface carbon content is critical, should be heated
in reducing atmospheres.
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