Page 30 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

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PROPERTIES OF STRUCTURAL STEELS AND EFFECTS OF STEELMAKING AND FABRICATION

1.28 CHAPTER ONE

1.21 ANNEALING AND NORMALIZING

Structural steels may be annealed to relieve stresses induced by cold or hot working. Sometimes,
also, annealing is used to soften metal to improve its formability or machinability.
Annealing involves austenitizing the steel by heating it above the A 3 temperature line in Fig. 1.13,
then cooling it slowly, usually in a furnace. This treatment improves ductility but decreases tensile strength
and yield point. As a result, further heat treatment may be necessary to improve these properties.
Structural steels may be normalized to refine grain size. As pointed out in Art. 1.20, grain size
depends on the finishing temperature in hot rolling.
Normalizing consists of heating the steel above the A 3 temperature line, then cooling the metal in
still air. Thus the rate of cooling is more rapid than in annealing. Usual practice is to normalize from
100 to 150°F above the critical temperature. Higher temperatures coarsen the grains.
Normalizing tends to improve notch toughness by lowering ductility and fracture transition tem-
peratures. Thick plates benefit more from this treatment than thin plates. Requiring fewer roller passes,
thick plates have a higher finishing temperature and cool slower than thin plates, thus have a more
adverse grain structure. Hence the improvement from normalizing is greater for thick plates.

1.22 EFFECTS OF CHEMISTRY ON STEEL PROPERTIES

Chemical composition determines many characteristics of steels important in construction appli-
cations. Some of the chemicals present in commercial steels are a consequence of the steelmak-
ing process. Other chemicals may be added deliberately by the producers to achieve specific
objectives. Specifications therefore usually require producers to report the chemical composition
of the steels.
During the pouring of a heat of steel, producers take samples of the molten steel for chemical
analysis. These heat analyses are usually supplemented by product analyses taken from drillings or
millings of blooms, billets, or finished products. ASTM specifications contain maximum and mini-
mum limits on chemicals reported in the heat and product analyses, which may differ slightly.
Principal effects of the elements more commonly found in carbon and low-alloy steels are dis-
cussed below. Bear in mind, however, that the effects of two or more of these chemicals when used
in combination may differ from those when each alone is present. Note also that variations in chem-
ical composition to obtain specific combinations of properties in a steel usually increase cost,
because it becomes more expensive to make, roll, and fabricate.
Carbon is the principal strengthening element in carbon and low-alloy steels. In general, each
0.01% increase in carbon content increases the yield point about 0.5 ksi. This, however, is accom-
panied by increase in hardness and reduction in ductility, notch toughness, and weldability, raising
of the transition temperatures, and greater susceptibility to aging. Hence limits on carbon content of
structural steels are desirable. Generally, the maximum permitted in structural steels is 0.30% or less,
depending on the other chemicals present and the weldability and notch toughness desired.
Aluminum, when added to silicon-killed steel, lowers the transition temperature and increases
notch toughness. If sufficient aluminum is used, up to about 0.20%, it reduces the transition tem-
perature even when silicon is not present. However, the larger additions of aluminum make it diffi-
cult to obtain desired finishes on rolled plate. Drastic deoxidation of molten steels with aluminum or
aluminum and titanium, in either the steelmaking furnace or the ladle, can prevent the spontaneous
increase in hardness at room temperature called aging. Also, aluminum restricts grain growth during
heat treatment and promotes surface hardness by nitriding.
Boron in small quantities increases hardenability of steels. It is used for this purpose in quenched
and tempered low-carbon constructional alloy steels. However, more than 0.0005 to 0.004% boron
produces no further increase in hardenability. Also, a trace of boron increases strength of low-carbon,
plain molybdenum (0.40%) steel.
Chromium improves strength, hardenability, abrasion resistance, and resistance to atmospheric
corrosion. However, it reduces weldability. With small amounts of chromium, low-alloy steels have

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