Page 35 - 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 1.33

The final austenitic grain size is determined by the temperature of the steel during the last passes
through the rolls (see Art. 1.20). In addition, inclusions are reoriented in the direction of rolling. As
a result, ductility and bendability are much better in the longitudinal direction than in the transverse,
and these properties are poorest in the thickness direction.
The cooling rate after rolling determines the distribution of ferrite and the grain size of the ferrite.
Since air cooling is the usual practice, the final internal structure and, therefore, the properties of plates
and shapes depend principally on the chemistry of the steel, section size, and heat treatment. By nor-
malizing the steel and by use of steels made to fine-grain practice (with grain-growth inhibitors, such as
aluminum, vanadium, and titanium), grain size can be refined and properties consequently improved.
In addition to the preceding effects, rolling also may induce residual stresses in plates and shapes
(see Art. 1.14). Still other effects are a consequence of the final thickness of the hot-rolled material.
Thicker material requires less rolling, the finish rolling temperature is higher, and the cooling rate
is slower than for thin material. As a consequence, thin material has a superior microstructure.
Furthermore, thicker material can have a more unfavorable state of stress because of stress raisers,
such as tiny cracks and inclusions, and residual stresses.
Consequently, thin material develops higher tensile and yield strengths than thick material of the
same steel chemistry. ASTM specifications for structural steels recognize this usually by setting
lower yield points for thicker material. A36 steel, however, has the same yield point for all thick-
nesses. To achieve this, the chemistry is varied for plates and shapes and for thin and thick plates.
Thicker plates contain more carbon and manganese to raise the yield point. This cannot be done for
high-strength steels because of the adverse effect on notch toughness, ductility, and weldability.
Thin material generally has greater ductility and lower transition temperatures than thick material
of the same steel. Since normalizing refines the grain structure, thick material improves relatively
more with normalizing than does thin material. The improvement is even greater with silicon-aluminum-
killed steels.
(W. T. Lankford, Jr., ed., The Making, Shaping and Treating of Steel, Association of Iron and
Steel Engineers, Pittsburgh, Pa.)

1.25 EFFECTS OF PUNCHING HOLES AND SHEARING

Excessive cold working of exposed edges of structural-steel members can cause embrittlement and
cracking and should be avoided. Punching holes and shearing during fabrication are cold-working
operations that can cause brittle failure in thick material.
Bolt holes, for example, may be formed by drilling, punching, or punching followed by reaming.
Drilling is preferable to punching, because punching drastically coldworks the material at the edge
of a hole. This makes the steel less ductile and raises the transition temperature. The degree of
embrittlement depends on type of steel and plate thickness. Furthermore, there is a possibility that
punching can produce short cracks extending radially from the hole. Consequently, brittle failure can
be initiated at the hole when the member is stressed.
Should the material around the hole become heated, an additional risk of failure is introduced.
Heat, for example, may be supplied by an adjacent welding operation. If the temperature should rise
to the 400 to 850°F range, strain aging will occur in material susceptible to it. The result will be a
loss in ductility.
Reaming a hole after punching can eliminate the short, radial cracks and the risks of embrittle-
1
1
ment. For that purpose, the hole diameter should be increased from / 16 to / 4 in by reaming, depend-
ing on material thickness and hole diameter.
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Shearing has about the same effects as punching. If sheared edges are to be left exposed, / 16 in
or more material, depending on thickness, should be trimmed, usually by grinding or machining.
Note also that rough machining, for example, with edge planers making a deep cut, can produce the
same effects as shearing or punching.
(M. E. Shank, Control of Steel Construction to Avoid Brittle Failure, Welding Research Council,
New York.)

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