Building Design and Construction Handbook - Merritt - Ventech!

BUILDING MATERIALS 4.73
rolled products are more homogeneous than ingots. Furthermore, during rolling,
the dendritic structure is broken up. Also, recrystallization occurs. The final austenitic
grain size is determined by the temperature of the steel during the last passes
through the rolls (Art. 4.43). In addition, dendrites and 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.
In addition to the preceding effects, rolling also may induce residual stresses in
plates and shapes (Art. 4.41.1). 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. 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 thicknesses. 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 has greater ductility 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 siliconaluminum-killed
steels.
4.48 EFFECTS OF PUNCHING AND SHEARING
Punching holes and shearing during fabrication are cold-working operations that
can cause brittle failure. Bolt holes, for example, may be formed by drilling, punching,
or punching followed by reaming. Drilling is preferable to punching, because
punching drastically cold-works 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 embrittlement. For the purpose, the hole diameter should be increased by 1 ⁄16 to
1 ⁄4 in by reaming, depending on material thickness and hole diameter.
Shearing has about the same effects as punching. If sheared edges are to be left
exposed, 1 ⁄16 in or more material, depending on thickness, should be trimmed by
gas cutting. Note also that rough machining, for example, with edge planers making
a deep cut, can produce the same effects as shearing or punching.