Steel Fabrications vs. Castings
Fabrications are assemblies of components that have been welded together to form a larger part. They consist of rolled flat products (sheet/plate) and bars that have been welded together, and these assemblies often may include cast or forged components. On the other hand, castings are made from molten metal molded into one solid piece and don’t have joints. Simply put, castings allow designers to put metal only where it is needed.
Steel castings and fabrications can seem interchangeable because they share many qualities: cast steels and wrought steels have such similar mechanical properties that the American Society of Mechanical Engineers Code doesn’t differentiate between steels on the basis of their manufacturing process, but by their chemical composition. However, noticeable differences do exist between the two that can affect the design and cost-effectiveness of a component.
Wrought products (rolled or forged) exhibit a characteristic known as directionality. This characteristic, also known as anistropy, means that a component has strength and ductility in the working direction but has lower transverse properties.
Cast steel products do not exhibit directionality; rather, they can be described as isotropic. Steel castings can be stressed in any direction without concerns over the lower strength, ductility and toughness that are exhibited in the transverse direction of wrought products. Designers of fabrications must be aware of the directional properties and incorporate them into the component’s design, or it could become overstressed when a load is applied in the transverse direction.
When compared to their wrought equivalents, the mechanical properties of cast steels are approximately the average value of the longitudinal and transverse directions in the wrought product. Transverse properties always are lower than the properties obtained in cast steels, which means that casting offers more design flexibility than fabrications. Properties for wrought products are determined by performing mechanical tests in the longitudinal direction. Tests in the transverse direction usually are performed only when specially requested.
Steel castings are tough and ductile, contrary to the common belief that they are brittle and subject to abrupt failure. Brittleness is a function of metallurgy, not of process, and steels are not brittle alloys. Because steel castings are isotropic, uniformly heat treated and more stress relieved than a fabrication, longer fatigue lives and more deformation without sudden failure is common in castings.
One factor that needs to be considered in any analysis of the design of fabrications using wrought products such as bar plate or tube is that the welds usually are placed in the highest stressed location in the component. Unless the fabricator follows a design code, most welds are placed at high-stressed section changes or design features such as corners, which limit the load-bearing strength of the component.
To combat this, welded fabrications often can be redesigned to one-piece castings. Then, the casting, which welds as well as or better than the fabrication, can allow the designer to locate the welds away from the highly stressed areas.
Often the weldability of the materials used in fabrications is taken as a given. They must be weldable, otherwise they would not be used. In addition, there also is the misconception that wrought steels are easier to weld than cast steels. Cast steels have a lower susceptibility to underbead cracking—the cracking that occurs from the introduction of hydrogen into the liquid metal. The isotropic qualities of the casting provide a good (not too hard) welding surface.
Welds made in the production of castings are almost always stress relieved. Castings tend to undergo heat treatment (a strengthening process) after welding occurs, keeping the component strong even in the weld areas. Fabrications usually are not stress relieved after welding, which can lead to a weaker area around the weld. Field welds can be difficult to stress relieve due to the location or size of the part, but if the component is a casting, it can be designed to place the un-stress-relieved weld in a lower stress location, which improves the component life.
It has been shown that when castings are used at junctions such as a node on an oil production platform, the weight of the connecting area can be reduced by as much as 50%, stemming from the elimination of joints and welds to attain the required strength. In addition, the position of the welds also is moved out of the high stress area and the welds become simpler (circumferential instead of complex and irregular in form).
Placing a weld in a position that makes it easier and simpler to perform also can minimize the nondestructive testing required and the number of discontinuities or stress raisers associated with welding in difficult positions and inaccessible areas.
The Design Viewpoint
In discussions with designers, it is sometimes stated that it is easier to design fabrications from plate and bar because it is easier to visualize a component made as a series of right angled connections. Visualizing a component where there is total freedom of form can be more difficult—it requires the designer to think in three dimensions. Yet this freedom permits designers to design only what the component needs—no extra material, edges or welds.
Conventional fabrication generally is a compromise between material availability, fabrication capability, design codes and engineering requirements. However, creative design of castings often will enable steel sections to be tailored to meet specific engineering loads, thus improving engineering efficiency. This often leads to substantial engineering benefits and weight reduction elsewhere in the surrounding steelwork through the elimination of offset work points and their associated bending moments.
Alignment or dimensional problems due to production are likely to be greater with fabrications than castings, because of the distortion that may occur during manufacture. Straightening operations carried out on components can have a more detrimental effect on fabrications because they will be plastically deformed in the high stress areas. These high stress areas often are associated with the welded joint. Although castings also will be plastically deformed during straightening, the location of the deformation is unlikely to be in a notched area such as a weld bead.
Because steel castings typically are welded into a larger fabricated steel structure, it is important to consider deflection. Allowing steel castings to flex protects the weld joints in the base structure from fatigue failure. The combinations of section length, depth and cross-section in geometry permissible with steel’s high stiffness and high yield stress capabilities can resist fatigue and protect mating sections from early fatigue failure. Allowing a steel casting to flex can reduce the stress concentration in the casting’s weld connections to a base structure.
Identifying an appropriate cast shape opens the door for designers to the opportunity for novel designs and shapes that challenge traditional fabricated concepts. Casting design should simplify the component’s shape as far as possible, satisfying the basic engineering requirement while reducing the overall size of the component where possible. This can be achieved by eliminating or adjusting offset work points, local stiffening and deepened sections in plate, box girders and tubes—all of which are necessary in fabrications to achieve acceptable designs.
When designing, it is important to establish geometry that will be workable during secondary operations. This includes casting designs for welding into a larger fabricated assembly, considering the design of weld-joint geometries (compatible mating of casting and plate thicknesses and stress distribution of weld geometries) and thinking about assembly features.
For example, with fabrications the capability to position and hold several pieces of plate in a weld fixture is more difficult than than forming a mold cavity for casting.
How to Choose
Castings allow designers to buy shape cheaply. If the desired component is mostly steel and the costs involved are mostly material, a fabrication should be used. However, the more components, the more linear inches of welding required, the more machining required per individual component, the more attractive casting becomes. Designs or fabrications that comprise the most pieces and the most welds are ideal candidates for a casting conversion.
In general, castings provide tighter tolerances and better mechanical performances. They allow the designer to shape the component exclusively for the project at hand, with no extra pieces or sections. Complex components and assemblies that can be consolidated into fewer parts become cost-effective as castings. Limiting assembly reduces cost. Castings often weigh less because the geometry can be tailored to the actual component requirements instead of being restricted by the capabilities of bars and sheets.
The cost bases for fabrications and castings are different. An increase in steel thickness, shape complexity and stiffening in a fabrication pushes up costs because of the amount of welding and nondestructive testing necessary. This also can be affected by the increased risk associated with the stress relief of highly restrained, heavy sections. Conversely for castings, castability is enhanced with increased section size, and with optimum design, the cost/ton will reduce with increased weight.
--Malcom Blair and Raymond Monroe, Steel Founders Society of America