Transcript hot working - Hacettepe University Department of Mechanical

INTRODUCTION
The ultimate goal of a manufacturing engineer is to
produce steel/metal components with required
geometrical shape and structurally optimized for a
given application. One of the method is the
deformation processing. Deformation processing
exploits the ability of steel to flow plastically without
altering the other properties.
INTRODUCTION
The required forces are often very high. Cast ingots, slabs,
blooms and billets are reduced in size and converted into
plates, sheets, rods and others. These forms experience
further deformation to produce the desired products formed
by processes such as forging, extrusion and other sheet metal
forming. The deformation may be bulk flow in three
dimensions, simple shearing, simple bending, or any
combination of these and other processes. The stresses could
either be tensile or compressive or shear or combination of
them. In this connection the steel chemistry and cleanliness
are important factors for deformation processing.
INTRODUCTION
Some aspect of deformation processing will be
discussed. This is given to appreciate the efforts of
steelmakers or any other metalmakers in producing
quality steels and/or metals.
Deformation processing can be carried out either
under hot or cold condition.
HOT AND COLD PROCESSES (WORKING)
Hot Working
The distinction between hot working and cold working does not
depends solely on the temperature, but rather on the processing
temperature with respect to the material recrystallization temperature.
When the processing temperature of the mechanical deformation of
steel is above the recrystallization temperature, the process is termed as
hot working; otherwise, it is cold working.
For hot working processes, large deformation can be successively
repeated, as the metal remains soft and ductile. The hardness of the
material cannot be controlled after hot rolling and it is a function of
chemical composition and the rate of cooling after rolling. The hardness
is generally lower than that of cold rolling and the required deformation
energy is lesser as well. However most metal will experience some
surface oxidation resulting in material loss and poor final surface finish.
HOT WORKING
• Hot working does not produce strain hardening.
Hence no increase in either yield strength or
hardness occurs. In addition yield strength
decreases as temperature increases and the
ductility improves.
• Hot working can be used to drastically alter the
shape of metals without fear of fracture and
excessively high forces.
HOT WORKING
• Elevated temperatures promote diffusion that can
remove chemical inhomogeneties; pores can be
welded or reduced in size during deformation.
• The dendritic grain structure, small gas cavities and
shrinkage porosity formed during solidification in
large sections can be modified by hot working to
produce a fine, randomly oriented, sphericalshaped grain structure which results in a net
increase in ductility and toughness.
HOT WORKING
• Hot working results in reorientation of inclusions or
impurity particles in the metal with the result that
an impurity originally oriented so as to aid crack
movement through the metal can be reoriented
into a “crack arrestor” configuration.
HOT AND COLD PROCESSES (WORKING)
Cold Working
Cold working processes allow desirable metal qualities that cannot be
obtained by hot working, such as eliminating errors attending shrinkage.
As such, a much more compact and higher dimensional accuracy metal
can be obtained with cold working. Furthermore, the final products
have a smoother surface (better surface finish) than those of hot
working and the strength, hardness as well as the elastic limit are
increased. However, the ductility of the metal decreases due to strain
hardening thus making the metal more brittle. As such, the metal must
be heated from time to time (annealed) during the rolling operation to
remove the undesirable effects of cold working and to increase the
workability of the metal.
COLD WORKING
• Some advantages of cold working are:
No heating is required
Better surface finish and superior dimensional control
are achieved
Strength, fatigue, and wear properties are improved
Directional properties can be imparted
COLD WORKING
• Disadvantages of cold working are:
Heavier forces are required
Strain hardening occurs (may require intermediate
annealing treatment to relieve internal stresses)
Residual stresses may be produced
For cold working, the ductility and the yield point
stress of steel are important.
FOUR BASIC BULK DEFORMATION PROCESSES
• ROLLING-Slab or plate is squeezed between
opposing rolls
• FORGING-Work is squeezed and shaped between
opposing dies
• EXTRUSION- Work is squeezed through a die
opening, thereby taking the shape of the opening
• WIRE AND BAR DRAWING- Diameter of wire or bar
is reduced by pulling it through a die opening
SCHEMATICS
ROLLING SCHEMATICS
FORGING
SCHEMATIC
EXTRUSION SCHEMATIC
FORGING MOVIE
ROLLING MOVIE