Page 31 - 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

STRUCTURAL STEELS, STEELMAKING, AND FABRICATION 1.29

higher creep strength than carbon steels and are used where higher strength is needed for elevated-
temperature service. Also, chromium is an important constituent of stainless steels.
Columbium in very small amounts produces relatively larger increases in yield point but smaller
increases in tensile strength of carbon steel. However, the notch toughness of thick sections is appre-
ciably reduced.
Copper in amounts up to about 0.35% is very effective in improving the resistance of carbon
steels to atmospheric corrosion. Improvement continues with increases in copper content up to about
1% but not so rapidly. Copper increases strength, with a proportionate increase in fatigue limit.
Copper also increases hardenability, with only a slight decrease in ductility and little effect on notch
toughness and weldability. However, steels with more than 0.60% copper are susceptible to precip-
itation hardening. And steels with more than about 0.5% copper often experience hot shortness dur-
ing hot working, and surface cracks or roughness develop. Addition of nickel in an amount equal to
about half the copper content is effective in maintaining surface quality.
Hydrogen, which may be absorbed during steelmaking, embrittles steels. Ductility will improve
with aging at room temperature as the hydrogen diffuses out of the steel, faster from thin sections
than from thick. When hydrogen content exceeds 0.0005%, flaking, internal cracks or bursts, may
occur when the steel cools after rolling, especially in thick sections. In carbon steels, flaking may be
prevented by slow cooling after rolling, to permit the hydrogen to diffuse out of the steel.
Manganese increases strength, hardenability, fatigue limit, notch toughness, and corrosion resis-
tance. It lowers the ductility and fracture transition temperatures. It hinders aging. Also, it counter-
acts hot shortness due to sulfur. For this last purpose, the manganese content should be three to eight
times the sulfur content, depending on the type of steel. However, manganese reduces weldability.
Molybdenum increases yield strength, hardenability, abrasion resistance, and corrosion resis-
tance. It also improves weldability. However, it has an adverse effect on toughness and transition
temperature. With small amounts of molybdenum, low-alloy steels have higher creep strength than
carbon steels and are used where higher strength is needed for elevated-temperature service.
Nickel increases strength, hardenability, notch toughness, and corrosion resistance. It is an impor-
tant constituent of stainless steels. It lowers the ductility and fracture transition temperatures, and it
reduces weldability.
Nitrogen increases strength, but it may cause aging. It also raises the ductility and fracture tran-
sition temperatures.
Oxygen, like nitrogen, may be a cause of aging. Also, oxygen decreases ductility and notch
toughness.
Phosphorus increases strength, fatigue limit, and hardenability, but it decreases ductility and
weldability and raises the ductility transition temperature. Additions of aluminum, however, improve
the notch toughness of phosphorus-bearing steels. Phosphorus improves the corrosion resistance of
steel and works very effectively together with small amounts of copper toward this result.
Silicon increases strength, notch toughness, and hardenability. It lowers the ductility transition
temperature, but it also reduces weldability. Silicon often is used as a deoxidizer in steelmaking (see
Art. 1.23).
Sulfur, which enters during the steelmaking process, can cause hot shortness. This results from
iron sulfide inclusions, which soften and may rupture when heated. Also, the inclusions may lead to
brittle failure by providing stress raisers from which fractures can initiate. And high sulfur contents
may cause porosity and hot cracking in welding unless special precautions are taken. Addition of
manganese, however, can counteract hot shortness. It forms manganese sulfide, which is more refrac-
tory than iron sulfide. Nevertheless, it usually is desirable to keep sulfur content below 0.05%.
Titanium increases creep and rupture strength and abrasion resistance. It plays an important role
in preventing aging. It sometimes is used as a deoxidizer in steelmaking (see Art. 1.23) and grain-
growth inhibitor (see Art. 1.20).
Tungsten increases creep and rupture strength, hardenability and abrasion resistance. It is used
in steels for elevated-temperature service.
Vanadium, in amounts up to about 0.12%, increases rupture and creep strength without impairing
weldability or notch toughness. It also increases hardenability and abrasion resistance. Vanadium some-
times is used as a deoxidizer in steelmaking (see Art. 1.23) and as a grain-growth inhibitor (see Art. 1.20).

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