What
is forging? (view clip)
Forging is a process where metal is pressed, pounded or squeezed
under great pressure into high strength parts known as forgings. The process
is performed hot by preheating the metal to a
desired temperature before it is worked. It is important to note that the
forging process is entirely different from the casting (or foundry) process,
as metal used to make forged parts is never melted and poured (as in the
casting process).
Why use forgings and where are they used?
The forging process can create parts that are stronger than those
manufactured by any other metalworking process. This is why forgings are
almost always used where reliability and human safety are critical. But
you'll rarely see forgings, as they are normally component parts contained
inside assembled items such a airplanes, automobiles, tractors, ships, oil
drilling equipment, engines, missiles and all kinds of capital equipment -
to name a few.
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How forgings compare to castings
Forgings are stronger. Casting cannot obtain the strengthening effects of
hot and cold working. Forging surpasses casting in predictable strength
properties - producing superior strength that is assured, part to part.
Forging refines defects from cast ingots or continuous cast bar. A casting
has neither grain flow nor directional strength and the process cannot
prevent formation of certain metallurgical defects. Pre-working forge stock
produces a grain flow oriented in directions requiring maximum strength.
Dendritic structures, alloy segregation's and like imperfections are refined
in forging.
Forgings are more reliable, less costly.
Casting defects occur in a variety of forms. Because hot working refines
grain pattern and imparts high strength, ductility and resistance
properties, forged products are more reliable. And they are manufactured
without the added costs for tighter process controls and inspection that are
required for casting. Forgings offer better response to heat treatment.
Castings require close control of melting and cooling processes because
alloy segregation may occur. This results in non-uniform heat treatment
response that can affect straightness of finished parts.
Forgings respond more predictably to heat
treatment and offer better dimensional stability. Forgings' flexible,
cost-effective production adapts to demand. Some castings, such as special
performance castings, require expensive materials and process controls, and
longer lead times. Open-die and ring rolling are examples of forging
processes that adapt to various production run lengths and enable shortened
lead times.
How forgings compare to
weldments/fabrications
Forgings offer production economies, material savings. Welded fabrications
are more costly in high volume production runs. In fact, fabricated parts
are a traditional source of forging conversions as production volume
increases. Initial tooling costs for forging can be absorbed by production
volume and material savings and forging’s intrinsic production economics
lower labor costs, scrap and rework reductions and reduced inspection costs.
Forgings are stronger. Welded structures are not usually free of porosity.
Any strength benefit gained from welding or fastening standard rolled
products can be lost by poor welding or joining practice. The grain
orientation achieved in forging makes stronger parts.
Forgings offer cost-effective
designs/inspection. A multiple-component welded assembly cannot match the
cost-savings gained form a properly designed, one-piece forging. Such part
consolidations can result in considerable cost savings. In addition,
weldments require costly inspection procedures, especially for highly
stressed components. Forgings do not. Forgings offer more consistent, better
metallurgical properties. Selective heating and non-uniform cooling that
occur in welding can yield such undesirable metallurgical properties as
inconsistent grain structure. In use, a welded seam may act as a
metallurgical notch that can lead to part failure. Forgings have no internal
voids that cause unexpected failure under stress or impact. Forgings offer
simplified production. Welding and mechanical fastening require careful
selection of joining materials, fastening types and sizes, and close
monitoring of tightening practice both of which increase production costs.
Forging simplifies production and ensures better quality and consistency
part after part.
How forgings compare to machined bar/plate
Forgings offer broader size range of desired material grades. Sizes and
shapes of products made from steel bar and plate are limited to the
dimensions in which these materials are supplied. Often, forging may be the
only metalworking process available with certain grades in desired sizes.
Forgings can be economically produced in a wide range of sizes from parts
whose largest dimension is less than 1 in. to parts weighing more than
450,000 lbs.
Forgings have grain oriented to shape for
greater strength. Machined bar and plate may be more susceptible to fatigue
and stress corrosion because machining cuts material grain pattern. In most
cases, forging yields a grain structure oriented to the part shape,
resulting in optimum strength, ductility and resistance to impact and
fatigue. Forgings make better, more economical use of materials. Flame
cutting plate is a wasteful process one of several fabricating steps that
consumes more material than needed to make such parts as rings or hubs. Even
more is lost in subsequent machining. Forgings yield lower scrap; greater,
more cost-effective production.
Forgings, especially near-net shapes, make
better use of material and generate little scrap. In high-volume production
runs, forgings have the decisive cost advantage. forgings require fewer
secondary operations. As supplied, some grades of bar and plate require
additional operations such as turning, grinding and polishing to remove
surface irregularities and achieve desired finish, dimensional accuracy,
machine-ability and strength. Often, forgings can be put into service
without expensive secondary operations.
How forgings compare to
powder metal parts (P/M)
Forgings are stronger. Low standard mechanical properties (e.g. tensile
strength) are typical of P/M parts. The grain flow of a forging ensures
strength at critical stress points. Forgings offer higher integrity. Costly
part-density modification or infiltration is required to prevent P/M
defects. Both processes add costs. The grain refinement of forged parts
assures metal soundness and absence of defects.
Forgings require fewer secondary operations.
Special P/M shapes, threads and holes and precision tolerances may require
extensive machining. Secondary forging operations can often be reduced to
finish machining, hole drilling and other simple steps. The inherent
soundness of forgings leads to consistent, excellent machined surface
finishes. Forgings offer greater design flexibility. P/M shapes are limited
to those that can be ejected in the pressing direction. Forging allows part
designs that are not restricted to shapes in this direction. Forgings use
less costly materials. The starting materials for high-quality P/M parts are
usually water atomized, pre-alloyed and annealed powders that cost
significantly more per pound than bar steels.
How forgings compare to reinforced
plastics/composite (RP/C)
Forgings offer greater productivity. New advanced-composite part designs may
often require long lead times and substantial development costs. The high
production rates possible in forging cannot yet be achieved in reinforced
plastics and composites.
Forgings have established documentation. RP/C
physical property data are scarce and data from material suppliers lack
consistency. Even advanced aerospace forgings are established products with
well-documented physical, mechanical and performance data. Forgings offer
broader service temperature range. RP/C service temperatures are limited and
effects of temperature are often complex. Forgings maintain performance over
a wider temperature range. Forgings offer more reliable service performance.
Deterioration and unpredictable service performance can result from damage
to continuous, reinforcing RP/C fibers. Forging materials out-perform
composites in almost all physical and mechanical property areas, especially
in impact resistance and compression strength.
