Castings or Forgings? Part 1 of 3
A Realistic Evaluation
An honest presentation of the advantages and limits of both steel component forms allows users to make informed choices when considering a forged or cast design.
Malcolm Blair and Raymond W. Monroe,
Steel Founders’ Society of America, Crystal Lake, Illinois
Much has been said and written recently by the forging industry about the advantages of forged products over castings. Such articles with headlines such as “Upgrade to Forgings” intend to imply that castings are in some way inferior and less reliable. It should be recognized that castings and forgings start from very similar beginnings and castings can have some very distinct advantages over other product forms, including forgings.
Most steel components start as castings: metal that has been melted, poured into a mold and solidified. In the casting process at the foundry, because the mold has the shape of the desired component, all that remains to be done after casting are the various finishing operations.
With forgings, the first shape is an ingot or continuously cast billet. Ingots are large, usually rectangular in form and weigh up to several tons. Ingots or continuously cast billets are forged into shapes by hammers or presses. Extensive machining to final configuration usually is required, and welding also may be necessary before finishing operations can begin.
Table 1 shows typical casting and forging procedures for producing a 10-in. valve. Although no two foundries are identical, all would essentially follow the same procedures identified in Table 1.
For forging shops, however, two alternatives are possible: forging two halves and welding them together, or creating the internal channel from a solid billet by piercing and machining. Piercing consists of punching a hole through the piece. The hole then is tapered in shape, but may require extensive machining to bring it into tolerance.
Although there is a substantial area of overlap, forgings tend to be used exclusively in some applications and castings in others. In determining which process to select, the crucial considerations are the ability to satisfy the design criteria and the ultimate cost of the component.
In forging, metal is moved while it is still in the solid state. Because the forging billet is solid, substantial force is required to change its shape to the desired configuration. Because of this, the required force increases as section size increases. In practical terms, there is a limit on size and section thicknesses produced by forging.
This doesn’t mean that very heavy sections are never forged. But when they are, relatively little deformation or reduction in cross-section occurs. In other words, the surface of the part merely is moved from one place to another.
In contrast, in the casting process the metal starts as a liquid and flows into the desired shape. Therefore, it is practical to cast components of large sizes and section thicknesses. For extremely large components, cast/weld construction is generally preferable to forged/weld construction. The reason is that fewer parts are typically involved, and because steel castings tend to have better weldability than steel forgings.
In forging, solid metal is forced into the die cavity. In casting, liquid metal is poured into the mold cavity. Liquids can flow almost anywhere. Therefore, as complexity of shape increases, the practicality of forging decreases. Castings can accommodate great complexity of shape.
The question of composition has two parts: what is obtainable from foundries and forging shops, and what is or isn’t castable or forgeable?
Forgings are produced from billets obtained from a steel mill and in compositions produced by the mill. Mills tend to produce limited grades of steel and special orders can be prohibitively expensive. Because steel foundries are more flexible, the number of chemical compositions obtainable from steel foundries is virtually unlimited.
Although a single foundry cannot supply every conceivable alloy, it is always possible to obtain a unique composition to meet a unique requirement from a variety of foundries at lower cost than competitive product forms.
The presence of controlled amounts of ferrite in certain stainless steels leads to increased corrosion resistance, higher crack resistance and better weldability. Ferrite occurs normally in most cast stainless steels, with the ferrite level controllable to produce the desired combination of characteristics. However, ferrite impairs hot working properties and is normally not present in forged components.
The important class of work-hardenable steels also are not forgeable. Work-hardenable steels are generally high-manganese (approximately 13% Mn) alloys that become harder the more they are worked. Thus, they are ideal for dipper teeth, compactor feet and other earth-moving and excavation applications.
Check back next week to read part 2.
We sincerely hope you found this article informative and valuable. We welcome your comments or suggestions regarding this article or any other subjects you would like to see us write about.
The Federal Group USA
By Robert Levy – CEO TFGUSA
© Copyright 2017