Page 29 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch01.qxd 9/29/05 4:59 PM Page 1.27

PROPERTIES OF STRUCTURAL STEELS AND EFFECTS OF STEELMAKING AND FABRICATION

STRUCTURAL STEELS, STEELMAKING, AND FABRICATION 1.27

Martensite starts to form at a temperature below about 500°F, called the M s temperature. The
transformation differs from those for pearlitic and bainitic steels in that it is not time dependent.
Martensite occurs almost instantly during rapid cooling, and the percentage of austenite transformed
to martensite depends only on the temperature to which the steel is cooled. For complete conversion
to martensite, cooling must extend below the M f temperature, which may be 200°F or less. Like bai-
nite, martensite has an acicular microstructure, but martensite is harder and more brittle than pearlitic
and bainitic steels. Its hardness varies with carbon content and to some extent with cooling rate. For
some applications, such as those where wear resistance is important, the high hardness of martensite
is desirable, despite brittleness. Generally, however, martensite is used to obtain tempered marten-
site, which has superior properties.
Tempered martensite is formed when martensite is reheated to a subcritical temperature after
quenching. The tempering precipitates and coagulates carbides. Hence the microstructure consists of
carbide particles, often spheroidal in shape, dispersed in a ferrite matrix. The result is a loss in hard-
ness but a considerable improvement in ductility and toughness. The heat-treated carbon and HSLA
steels and quenched and tempered constructional steels discussed in Art. 1.1 are low-carbon marten-
sitic steels.
(Z. D. Jastrzebski, Nature and Properties of Engineering Materials, John Wiley & Sons, New York.)

1.20 EFFECTS OF GRAIN SIZE

As indicated in Fig. 1.13, when a low-carbon steel is heated above the A 1 temperature line, austen-
ite, a solid solution of carbon in gamma iron, begins to appear in the ferrite matrix. Each island of
austenite grows until it intersects its neighbor. With further increase in temperature, these grains
grow larger. The final grain size depends on the temperature above the A 3 line to which the metal is
heated. When the steel cools, the relative coarseness of the grains passes to the ferrite-plus-
pearlite phase.
At rolling and forging temperatures, therefore, many steels grow coarse grains. Hot working, how-
ever, refines the grain size. The temperature at the final stage of the hot-working process determines
the final grain size. When the finishing temperature is relatively high, the grains may be rather coarse
when the steel is air-cooled. In that case, the grain size can be reduced if the steel is normalized
(reheated to just above the A 3 line and again air-cooled). (See Art. 1.21.)
Fine grains improve many properties of steels. Other factors being the same, steels with finer
grain size have better notch toughness because of lower transition temperatures (see Art. 1.13) than
coarser-grained steels. Also, decreasing grain size improves bendability and ductility. Furthermore,
fine grain size in quenched and tempered steel improves yield strength. And there is less distortion,
less quench cracking, and lower internal stress in heat-treated products.
On the other hand, for some applications, coarse-grained steels are desirable. They permit deeper
hardening. If the steels should be used in elevated-temperature service, they offer higher load-carrying
capacity and higher creep strength than fine-grained steels.
Austenitic-grain growth may be inhibited by carbides that dissolve slowly or remain undissolved
in the austenite or by a suitable dispersion of nonmetallic inclusions. Steels produced this way are
called fine grained. Steels not made with grain-growth inhibitors are called coarse grained.
When heated above the critical temperature, 1340°F, grains in coarse-grained steels grow gradu-
ally. The grains in fine-grained steels grow only slightly, if at all, until a certain temperature, the coars-
ening temperature, is reached. Above this, abrupt coarsening occurs. The resulting grain size may be
larger than that of coarse-grained steel at the same temperature. Note further that either fine-grained
or coarse-grained steels can be heat-treated to be either fine-grained or coarse-grained (see Art. 1.21).
The usual method of making fine-grained steels involves controlled aluminum deoxidation (see
also Art. 1.23). The inhibiting agent in such steels may be a submicroscopic dispersion of aluminum
nitride or aluminum oxide.
(W. T. Lankford, Jr., ed., The Making, Shaping and Treating of Steel, Association of Iron and
Steel Engineers, Pittsburgh, Pa.)

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