“A Comparison of Steel Castings vs. Forgings for Large
Structural Components” or ‘How to Separate Fact from Fiction”
Presented at the 10th Heavy Movable Structures Biennial Symposium October
27, 2004
Abstract:
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.
Fig. 1 Bottom Pouring in a Steel Foundry
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 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 as shown in Fig. 2 below.
Fig. 2 Typical Cast Ingot Structure
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. The
effects of grain flow are shown in Fig. 3.
Fig. 3 Forging Grain Flow Crane Hook
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.
Fig. 4 Internal burst in a large forging
The forging industry typically refers to the term “reduction ratio” which is
the ratio of crossectional 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 just as any manufacturing process. For example a large forging may
actually burst or crack internally during forging if not heated properly
(Fig. 4).
Casting Process
Most steel castings are produced in expendable sand molds. The mold
is produced by forming sand around a pattern, which is 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.
 
Fig. 5 Ring Gear Casting Mold
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. If a region of a casting is
isolated from the riser a shrinkage cavity will form (Fig. 6). In this case
it is necessary to add material to allow the molten metal to be properly fed
form the molten riser.
Fig. 6 Directional Solidification & Design
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, as demonstrated in Fig. 7.
Thus the Foundry Engineer can validate the casting design by solidification
modeling before the part the part is actually poured.
Fig. 7 Solidification
Model of Gear Casting
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 it’s 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) direction
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.
Fig. 8 Test Bar Orientation
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 one was cast and the other
was wrought. Test bars were removed from the test plates in the orientation
shown in Fig. 8.
Test results shown in Table 1 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 % elongation and % 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 fatigue
strength of cast and wrought alloys. Fig. 9 below shows that the unnotched
fatigue properties of cast steel test are below that of wrought steel in the
longitudinal direction but above wrought steel in transverse direction.
However the notched fatigue properties test bars cast steel are actually
superior to wrought steel regardless of orientation. This demonstrates that
cast steel is less notch sensitive than wrought steel.
Figure 9
Fatigue Properties
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 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 mechanical 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.
References
1. ASM Metals Handbook Desk Edition, 1985, pp. 4-43
2. ASM Metals Handbook Vol. 19 Tenth edition, pp. 657-658
3. Fatigue Properties of Cast and Comparable Wrought Steels, Evans, Ebert, &
Briggs, Proc., ASTM, Vol. 56, 1956
4. Analysis of ASTM X-Ray Shrinkage Rating for Steel Castings. Presented at
the 2000 SFSA T&O Conference by Carlson, Ou, Hardin, & Beckermann
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