Castings or Forgings? Part 3 of 3
Continued from Part 2
The Practical Purchaser
Design and Design Modifications—Although castings and forgings have many design criteria in common (the need for generous radii, for example), each has its own recommended practices. When changing from one process to the other, the design should be reconsidered and new drawings made, or problems are likely to be encountered.
Casting and Forging Defects
The modification of a design is a different matter. It is often necessary to alter a design by adding a rib or lug, removing a projection, etc. Here, castings provide a distinct advantage, since the modification of a casting pattern or corebox is relatively easy and inexpensive. But the modification of a forging die to accommodate even slight changes is usually difficult, very expensive and new dies may be necessary.
The manufacturing process also places limitations on the shapes that can be produced. Often, foundries must explain to customers that to induce directional solidification and enable the pattern to be removed from the mold, it is necessary to introduce a taper of about 1.5°. According to the Forging Handbook, this magnitude of taper is typically 5–10 times greater in forged products.
If two components are to be welded together, it is advantageous to have one or both parts be steel castings. Castings are generally more weldable than “equivalent” forgings. This is true not only of austenitic grades of stainless steel with controlled ferrite, but of plain carbon and low-alloy steels as well.
The superior weldability of steel castings was demonstrated in research performed by the Univ. of Tennessee. Five “equivalent” grades of low-alloy casting and forging steels were compared in terms of weldability, or resistance to cold cracking.
Cold (or underbead) cracking occurs after the welded joint has cooled. This can be extremely troublesome since cracks are hidden beneath the weld and aren’t revealed by surface inspection.
In the Univ. of Tennessee tests, cast and wrought grades of 8630 steel plus four other manganese-silicon (Mn-Si) grades were tested to determine the degree of preheating necessary to eliminate underbead cracking.
For each grade, the cast steel required lower preheat temperatures than the wrought steel to achieve crack-free performance. Baseline tests to establish the weldability of the grades without preheating showed the same results. Every cast grade was superior to its wrought “equivalent.”
The problem with the wrought steels has been identified as the elongation of the inclusions. The needle-like inclusions in wrought steels appear to be more likely sites for crack initiation than the round inclusions in casting steels.
Where welding is required, the same considerations apply as previously mentioned. At a given preheat temperature, welds on steel castings tend to be less susceptible to under-bead cracking than welds on steel forgings. Additionally, castings will achieve comparable weld reliability at lower preheat temperatures.
The final cost of a part includes its purchase cost plus the cost of performing any necessary additional operations.
Before assembly operations, for example, it is often necessary to machine the part to the desired shape. This cost can be considerable. When parts are relatively simple, castings and forgings tend to require equal amounts of machining. As parts become more complex, however, castings tend to require less machining.
The forging process tends to reduce surface porosity and discontinuities (and may close up small internal cavities). Surface porosity and discontinuities occasionally appear on steel castings and require weld repair. The resultant surfaces meet the same standard requirements as the casting. The cost of this upgrading procedure is usually much less than the cost of additional machining typically required of forging.
Size and Weight
Steel castings are almost always lighter than their forged counterparts and the redesign from forgings to castings usually results in substantial weight savings. For example, a forged muzzle brake for a cannon anti-recoil system weighed more than 600 lb. The cast version weighed 400 lb and lasted more than three times longer.
Set-Up and Production Costs
The key to the casting process is the pattern. The key to the forging process is the die. Pattern costs are substantially lower than die costs and can be amortized over shorter runs. With higher runs and with simple component configurations, forged components can become economically converted to equivalent castings.
Castings will tend to have a definite advantage over forg-ings when any one of three conditions is met:
- a unique metal composition is required;
- the part is relatively large or complex;
- stresses may be multi-axial
The Competition—How large is “large?” How complex is “complex?” When does a production run become “long?”
These are the gray areas in which neither castings nor forgings show a clear-cut superiority. The solution for the designer is to determine the method of production before finalizing the design. A preliminary design will allow the customer to take advantage of the competitive situation. Invite bids from reputable foundries and forging shops and decide which process to use on a specific, case-by-case basis. The results may vary in different cases, but the competition can only serve to benefit the customer, user or purchaser.
Ask the Producers
Clearly, steel castings have and will continue to maintain an important role in manufacturing. Their strength and ability to be produced to the shape required by designers should ensure that their competitive position will be maintained and improved.
Designers and casting buyers can only obtain the real picture about either process’ performance by developing an open dialogue with their component suppliers. Both parties have the same interest in product design, quality and cost. This dialogue will ensure that the real problems will be identified and real solutions found.
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 Malcolm Blair and Raymond W. Monroe
Steel Founders’ Society of America, Crystal Lake, Illinois