Ultraservice Low-Carbon Alloy Steels (Quenched and Tempered)The need for high-performance materials with higher strength-toweight
ratios for critical military needs, for hydrospace explorations,
and for aerospace applications has led to the development of quenched
and tempered ultraservice alloy steels. Although these steels are similar
in many respects to the quenched and tempered low-carbon constructional
alloy steels described above, their significantly higher notch
toughness at yield strengths up to 965 MPa (140,000 lb/in2) distinguishes
the ultraservice alloy steels from the constructional alloy steels.
These steels are not included in the AISI-SAE classification of alloy
steels. There are numerous proprietary grades of ultraservice steels in
addition to those covered by ASTM designations A543 and A579. Ultraservice
steels may be used in large welded structures subjected to
unusually high loads, and must exhibit excellent weldability and toughness.
In some applications, such as hydrospace operations, the steels
must have high resistance to fatigue and corrosion (especially stress
corrosion) as well.
Maraging Steels For performance requiring high strength and
toughness and where cost is secondary, age-hardening low-carbon martensites
have been developed based on the essentially carbon-free ironnickel
system. The as-quenched martensite is soft and can be shaped
before an aging treatment. Two classes exist: (1) a nominal 18 percent
nickel steel containing cobalt, molybdenum, and titanium with yield strengths of 1,380 to 2,070 MPa (200 to 300 ksi) and an outstanding
resistance to stress corrosion cracking (normally a major problem at
these strengths) and (2) a nominal 12 percent nickel steel containing
chromium, molybdenum, titanium, and aluminum adjusted to yield
strengths of 1,034 to 1,379 MPa (150 to 200 ksi). The toughness of this
series is the best available of any steel at these yield strengths.
Cryogenic-Service Steels For the economical construction of
cryogenic vessels operating from room temperature down to the temperature
of liquid nitrogen (2195°C or 2320°F) a 9 percent nickel
alloy steel has been developed. The mechanical properties as specified
by ASTM A353 are 517 MPa (75,000 lb/in2) minimum yield strength
and 689 to 827 MPa (100,000 to 120,000 lb/in2) minimum tensile
strength. The minimum Charpy impact requirement is 20.3 J (15
ft ? lbf ) at 2195°C (2320°F). For lower temperatures, it is necessary to
use austenitic stainless steel.
Machinery Steels A large variety of carbon and alloy steels is used
in the automotive and allied industries. Specifications are published by
AISI and SAE on all types of steel, and these specifications should be
referred to for detailed information.
A numerical index is used to identify the compositions of AISI (and
SAE) steels. Most AISI and SAE alloy steels are made by the basic
oxygen or basic electric furnace processes; a few steels that at one time
were made in the electric furnace carry the prefix E before their number,
i.e., E52100. However, with the almost complete use of ladle furnaces,
this distinction is rarely necessary today. Steels are ‘‘melted’’ in the
BOP or EF and ‘‘made’’ in the ladle. A series of four numerals designates
the composition of the AISI steels; the first two indicate the steel
type, and the last two indicate, as far as feasible, the average carbon
content in ‘‘points’’ or hundredths of 1 percent. Thus 1020 is a carbon
steel with a carbon range of 0.18 to 0.23 percent, probably made in the
basic oxygen furnace, and E4340 is a nickel-chromium molybdenum
steel with 0.38 to 0.43 percent carbon made in the electric-arc furnace.
The compositions for the standard steels are listed in Tables 6.2.8 and
6.2.9. A group of steels known as H steels, which are similar to the
standard AISI steels, are being produced with a specified Jominy hardenability;
these steels are identified by a suffix H added to the conventional
series number. In general, these steels have a somewhat greater
allowable variation in chemical composition but a smaller variation in
hardenability than would be normal for a given grade of steel. This
smaller variation in hardenability results in greater reproducibility of the
mechanical properties of the steels on heat treatment; therefore, H steels
have become increasingly important in machinery steels.
Boron steels are designated by the letter B inserted between the second
and third digits, e.g., 50B44. The effectiveness of boron in increasing
hardenability was a discovery of the late thirties, when it was noticed
that heats treated with complex deoxidizers (containing boron) showed
exceptionally good hardenability, high strength, and ductility after heat
treatment. It was found that as little as 0.0005 percent of boron.
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