Material
selection is one of the most crucial decisions made in the
design, manufacture, and application of large structural
components. Material selection naturally influences the entire
performance of the design, and thus it is critical that informed
decisions are made during the design stage. Steel castings and
steel forgings are two alternatives for large structural
components. For many design engineers it is often assumed that a
forging is a better product because it is formed or worked
during the manufacturing process. It also assumed that castings
are inferior because they may contain porosity. Nothing could be
further from the truth. Each process has its advantages and
disadvantages. It is just as possible to produce an inferior
product whether it is a forging or a casting. This paper will
present an honest evaluation of castings and forgings, so that
those in the design community can make an informed choice.
Introduction
This paper will concern itself with the differences between
forged and cast steels in heavy sections. Heavy sections will be
interpreted to mean parts in excess of 10 tons and a minimum
metal section of 200 mm (5”). All steel products, whether they
are cast or wrought (forged), start from a batch of molten steel
that is allowed to solidify in a mold. The difference is that a
wrought product is mechanically worked by processes such as
rolling or forging after solidification, while a casting is not.
Melt Shop Practice
The process of steel making is essentially the same for both
wrought and cast steels. Liquid steel is principally an alloy of
iron and carbon. Other metals such as chromium, nickel,
manganese, and molybdenum are added as alloying agents to impart
particular properties to the steel. The raw materials used to
make steel also contain undesirable elements such as phosphorus
and sulfur, which form inclusions in the steel that can never be
completely removed from the steel. Thus the quality of both
forgings and castings is dependent upon the quality of the
molten steel that is poured into the mold.
Since most forge shops purchase their steel ingots, they are
dependent upon the steel mill to control the quality of the raw
material that is used in their product. This also limits forge
shops to supplying the standard alloy grades that the steel mill
offers. Conversely, steel foundries have to both make and pour
their own steel to produce a casting, and thus have full control
of the metal that is used to produce the casting. This also
allows the foundry to supply virtually any alloy grade that the
customer may want.
Liquid steel has a high affinity for oxygen, and it will form
oxide inclusions that can also become trapped in the final
product. Molten steel must be handled properly to minimize the
formation of re-oxidation products. Once the steel is refined in
the melting furnace it is tapped into a ladle, which is a
refractory lined vessel made to handle molten steel. Good steel
making practice dictates the use of a bottom pouring ladle. The
reason for this is that a slag layer is developed on top of the
molten steel by use of fluxes. This slag layer is less dense
than steel, and thus floats on top while at the same time
forming a protective barrier from the atmosphere. This
protective barrier is maintained since the steel is poured from
the bottom of the ladle. The bottom pouring technique is used
for both steel castings and for steel ingots.
One important distinction between wrought and cast steels is the
de-oxidation practice that is used. Wrought steels are typically
“aluminum killed,” which means that a small amount of aluminum
is added during the melting process for the purpose of removing
oxygen from the steel. While very effective at removing oxygen,
the aluminum forms microscopic aluminum oxide particles, which
are abrasive during the machining process. Some steel casting
shops de-oxidize with calcium, which also removes the oxygen but
produces a softer, more machinable inclusion.
Forging Process

Wrought or forged materials by definition are made from cast
ingots, which are then mechanically worked after solidification.
Ingot castings are the raw materials from which all wrought
products such as forgings, plate, and barstock are produced, and
they are nothing more than a casting that is produced by pouring
the liquid steel into a reusable metal mold. The cast ingot
structure consists of different zones that contain porosity and
segregation.
After solidification the ingot is hot forged into the desired
shape using a hammer, press, or ring-rolling machine. As the
forging is hot worked into shape, the inclusions, porosity, and
grains within the steel ingot are forced to flow in the
direction the part is being worked. This imparts directionality
to the finished part. According to the forging industry, this
grain flow makes forgings superior to castings. However, the
fact is that although the mechanical properties of a forging are
higher in the longitudinal direction (direction of working),
they are significantly lower in the transverse direction, or
perpendicular to the grain flow. Thus, when using a forging the
design engineer needs to evaluate the loading characteristics in
both the transverse and longitudinal direction.
Large forgings are hammered or pressed into rough shapes, which
then require extensive machining or welding to other components
to produce a more complex shape. This adds to the cost of the
overall product. Large forgings are limited as to the amount of
mechanical working that can be done.
The forging industry typically refers to the term “reduction
ratio,” which is the ratio of cross-sectional area before and
after forging and is used as a means to specify the quality of
the forging. The typical standard for very large forgings is to
require a minimum of three reductions. It is recognized by the
forging industry that excess hot working can impart too much
directionality into the part.
Forgings are subject to process variables and have the same
potential for defects as any manufacturing process. For example,
a large forging may actually burst or crack internally during
forging if not heated properly.
Casting Process
Most steel castings are produced in expendable sand molds. The
mold is produced by forming sand around a pattern, which is a
replica of the finished part. Molding sands are mixed with
materials that will allow it to hold the desired shape after the
pattern is removed. Holes or cavities are created by assembling
sand cores in the mold. The pattern equipment also includes the
gates and risers which are needed to produce a quality casting.
The gating system is designed to allow the metal to flow into
the mold in a controlled manner. Risers are reservoirs of molten
metal which allow the casting to solidify without shrinkage
porosity.
Post solidification processing includes sand removal or
shakeout, removal of gates and risers, inspection, weld
upgrading, and heat treatment. The main advantage of the casting
process is its versatility. Castings are best suited for complex
geometries that cannot be easily produced by the forging
process.
The principal difference between a casting and a forging is that
the final part shape is created when the molten metal solidifies
in the mold. Since the sand mold produces the desired finished
shape, all that remains is to process the casting through
various finishing operations in the foundry. This processing
does not alter the directionality of the casting. A steel
casting is homogenous. This means that the mechanical properties
of a casting are the same regardless of the direction of applied
stresses.
It is very important to understand the underlying principles
that dictate how a casting solidifies. As steel cools in the
mold it naturally changes from a liquid to a solid, resulting in
volumetric contraction. Additional feed metal in the form of
risers must be supplied to the casting to make up for this loss
in volume. There also needs to be a pathway for the additional
metal to feed the casting as it solidifies. In this case it is
necessary to add material to allow the molten metal to be
properly fed from the molten riser.
The foundry engineer evaluates the shape of the casting and then
determines how to modify the casting so that solidification
progresses from the thinnest section back through progressively
heavier sections. This progressive, controlled manner of
solidification is termed “directional solidification.”
Directional solidification can only occur if the temperature
gradient is controlled by proper casting design. The temperature
gradient can be modeled using solidification software. Thus the
foundry engineer can validate the casting design by
solidification modeling before the part is actually poured.
All castings naturally begin to solidify at the mold wall,
because that is where the heat is first extracted from the
molten metal. Solidification continues to proceed in the regions
of the casting that are cooling the fastest. Good casting design
practice seeks to make sure that the last part of the casting to
solidify always has a supply of molten metal available to avoid
the formation of shrinkage cavities. Since the last area to
solidify is primarily influenced by part shape, it is critical
that the casting user and the foundry work closely together to
make sure that the part is designed in such a way as to optimize
its castability, while at the same time taking advantage of the
castings processes’ ability to produce the part to a near net
shape.
Mechanical Property Comparisons
As previously stated, the forging process produces a part that
is anisotropic. This means that the mechanical properties of a
forging are better in the longitudinal direction (parallel to
lines of flow) versus the transverse direction (perpendicular to
lies of flow). Conversely, a casting is homogeneous; this means
that the mechanical properties of a casting are the same,
regardless of the orientation of test bar material.
In order to demonstrate this difference a 5” thick test casting
was poured from a typical low alloy cast steel. Equivalent test
material was also cut from a 5” thick plate of rolled 4340
steel. Both test plates were then heat treated in the same
production furnace load. Thus the test materials were equivalent
in all respects of processing, except that one was cast and the
other was wrought. Test bars were removed from the test plates
in the orientation.
Test results demonstrate that the mechanical properties of the
cast plate are essentially the same regardless of test bar
orientation. The mechanical properties of the wrought plate are
lower in both the transverse and through thickness orientations,
especially the ductility (indicated by the percentage of
elongation and percentage of reduction in area), which shows a
significant degradation when compared to the longitudinal
direction. The tensile ductility of the cast material is
significantly higher than for the wrought material in the
through thickness orientation, although lower than in the
longitudinal direction.
The same directionality effects are demonstrated when comparing
the fatigue strength of cast and wrought alloys. The un-notched
fatigue properties of the cast steel test are below that of
wrought steel in the longitudinal direction, but above wrought
steel in the transverse direction. However, the notched fatigue
properties of test bars of cast steel are actually superior to
wrought steel, regardless of orientation. This demonstrates that
cast steel is less notch sensitive than wrought steel. Notched
fatigue properties are a more accurate representation of actual
service conditions because most large parts—whether cast or
forged—would be expected to have some type of a notch.
Non-Destructive Testing
Large, heavily loaded parts are often non-destructively tested (NDT)
in order to verify internal part integrity. The most common
methods are ultrasonic (UT) and radiographic testing (RT).
Common specification pitfalls are to discount the effects of
surface finish and machining when specifying NDT methods. For
example, since UT functions by measuring reflected sounds waves,
it works best on a part that is machined and has two parallel
surfaces. Using UT on an un-machined surface compromises the
sensitivity of the test. RT indications will change appearance
before and after machining since the section thickness is
reduced.
The main benefit of RT is that a permanent record is created.
The acceptance criteria are based upon a comparison against ASTM
reference radiographs, which are rated 1 through 5 (best to
worst). The SFSA (Steel Founder’s Society of America) sponsored
a research project to determine the applicability of the ASTM
referenced radiographs. In essence, the study had experienced
ASNT Level III radiographers evaluate the reference radiographs
in a blind test. This group was able to agree on the best and
worst conditions (levels 1 and 5). However, this expert group
could not agree on which reference radiographs represented the
middle levels of 2, 3, and 4.
Both of these examples demonstrate that each method has its
limitations, and the purchaser and the producer need to
understand these limitations. Application of a stringent NDT
requirement does not necessarily result in a high-quality part.
Summary
The main difference between a steel casting and a forging is
that the forging is mechanically worked after solidification.
This mechanical working imparts directionality, or anisotropy,
to the forging. Castings and forgings are both susceptible to
manufacturing problems and misapplication by the buyer.
In general, a forging is best suited to simple configurations
that can be easily worked in a die or other tooling. It is also
suited to applications in which the principal applied stresses
are the same as the direction of mechanical working. A casting
is best suited to complex shapes, custom or tailored
chemistries, and to applications that are subject to multi-axial
stresses.
Casting buyers need to work closely with foundries at the design
stage in order to insure that the design is able to take
advantage of directional solidification. The poor quality image
of castings is often the result of the buyer not understanding
this process. The casting buyer must also understand that there
are limitations to relying solely on NDT to verify quality.
Quality is best enhanced by using tools such as solidification
modeling at the design stage to insure the production of a
high-quality product.
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