How To Buy Forgings

July 25, 2009

How To Buy Forgings

Close cooperation between buyers and producers of forgings has always been a vital part of achieving the best possible product at the best possible cost. With recent major advances in forging methods and materials improvements, this collaboration is more critical than ever before. By keeping abreast of these advances, and working closely with the forger, the engineer or buyer can ensure delivery of high-quality products with important cost savings.

Despite its long history and the many technological developments that have taken place in recent years, forging still involves a good deal of artistry. Even as product designers and industrial buyers learn more about shaping of metals, there is still much to be gained from bringing the forger into the design and specification phases of product development.

Of course, such basic questions as whether a given part can or should be forged must be addressed at an early stage. There are many instances when any of several processes can be used to produce the component in question.

Once it has been determined that a product or component requires the strength, toughness, dimensional accuracy and overall integrity of forging, there is still the question of which forging process – open die, impression die, ring rolling, etc. – is most appropriate. Usually, this decision is straightforward, based on part size, configuration and quantity required. However, to help in those situations when the choice is not so clear cut, the forging buyer should have at least a general knowledge of methods and equipment used in the industry.

Besides a general knowledge of forging, the buyer should also have a clear idea of what he or she specifically requires and how readily his or her needs can be met by individual forgers. Capabilities can vary dramatically from one company to another. For instance:

–Does the forger have experience in applications similar to the one being considered?
–Is design assistance offered?
–Does the forger have the equipment required to produce the part?
–Is the forger able to provide related services like heat treating, machining, testing and so on?
–Is the forger accustomed to producing the volume required?
–Does the company specialize in long runs, short runs or quick delivery?

The answer to these and other questions will help narrow the field to a few qualified forgers. Then, the buyer can begin to take advantage of the valuable technical and design assistance available from these forging experts.

An experienced and capable forging company engineer should be able to make design suggestions to consolidate components, simplify processing, reduce required machining, speed delivery and so on. It may be possible to achieve forging’s highlevel performance benefits without significantly increasing material or production costs over those associated with other processes.

The key is to get the forger involved early. The benefit derived from consultation will vary with the complexity of the part and the forging process involved. For instance, impression die forgings may benefit somewhat more dramatically than open die products. However, the ideal first step toward getting the most from a forged part is to form a team consisting of the product designer, the purchasing manager and, possibly, a quality-control or manufacturing representative. Then this team should sit down with a technical representative of the forging company while the product or component design is still being evaluated.

The focal point in these early meetings with candidate forgers should be an engineering drawing. The part print should be fully detailed, showing finished dimensions and tolerances. If the forging is to be delivered in a rough-machined or asforged state, the required machining envelope should be clearly specified. In many cases, it can be advantageous to provide a drawing that shows how the forged part will mate with other components in the finished assembly.

Another critical part of these early design meetings should be the service requirements of the application. The forger needs complete information on how the forging will be used, the operating environment, and critical mechanical properties. A thorough understanding of service stresses – load-bearing, power transmitting, impact, hydraulic pressure, high or low temperatures, corrosive conditions – and the stress location can allow the forging engineer to make design and process suggestions that can result in an improved product and reduced manufacturing costs. For instance:

–Material Selection…Often, alternative carbon- and alloy-steel grades can produce similar mechanical properties, depending on forging design, heat treatment, and so forth. Specifying property levels beyond those actually required by the application can significantly increase costs. The best economy is achieved when tensile, hardness, impact and other mechanical properties are realistically based on the service requirements of the component being designed. Once these realistic property levels are established, the forger can help select one material from among the alternatives to achieve the optimum combination of performance, forgeability, heat treatability, machinability and economy.

–Part Configuration…Special preforming operations, reheats or additional dies and equipment may or may not be required to achieve the specified part configuration and the desired grain flow pattern. Almost always, a knowledgeable forger can work with a product design and achieve material and production economies with no loss of part performance. Sometimes, slight changes in part shapes can simplify forging requirements, reduce die costs, and speed production.

The forging engineer studies a new design from the standpoint of its tooling and processing requirements. Reduced draft angles or sharper radii, for instance, can sometimes reduce machining requirements without affecting part function. If a simpler die can be used or if the parting line can be adjusted to allow use of a flat top die, it may be possible to produce the part more economically.

–Dimensional Tolerances…The ability of forgers today to produce asforged shapes to tight tolerances is improving, and most companies are striving to develop their net- and near-net-shape forging capabilities. At present, however, there is some considerable cost involved in holding tight as-forged tolerances. The wise buyer will ask the forger to help evaluate the trade-offs between reduced machining and increased die and processing costs.

In open die forging, particularly nearly all forgings require some machining. Determining where and how much machining stock “envelope” should be specified is a complex decision best made in concert with the forger. But no matter what tolerances are set, it is important to include them along with all dimensions of the part drawing given to the forging engineer. Based on this information, and on his or her experience and the experience of the supplier, the forger can accept or request modification to the specifications to achieve more cost-effective production.

–Applying Guidelines…Over the years, the forging industry has developed a system of dimensional tolerance guidelines that set limits on size (length, width and thickness), die match and straightness. Guidelines for impression die applications, for instance, are found in the Forging Industry Association’s Tolerances for Impression Die Forgings, Hammer, Press and Upsetter.

Standards also have been developed to apply to such material considerations as chemistry, strength, ductility, impact resistance, conductivity, soundness and grain flow. These have been published by such organizations as ASTM, SAE, and the American Standards Association. Unless there is a good reason to specify a special material or tighter tolerance controls, it is best to follow the established standards to avoid additional costs.

–Surface Finishing…Most forging companies have machining capabilities and some offer extensive finishing services. Many buyers specify that rough machining be done by the forger so that any surface imperfections will be discovered before the parts are shipped. And there is a growing trend toward specifying that the forger also do finish machining, for reasons of economy and to isolate responsibility. The buyer gets a finished, ready-to-install component. Intermediate steps in production are left to the forger.

With the added responsibility, the forger gains some flexibility that can result in overall savings for the buyer. Armed with a drawing showing finished dimensions and tolerances, the forger can design an ideal forging around the finished part. The parting line can be positioned for maximum efficiency. The chief benefit, however, is that the machining envelope can sometimes be reduced to save material and machining time.

–Inspection and Testing…Only those tests needed to establish the mechanical properties and quality required for reliable performance should be specified to minimize the costs involved. While the buyer will normally specify the type of test and acceptance levels required for a forging, the forger can offer good advice on appropriate testing. Tests on representative bar samples are relatively simple. When the specification requires that additional tests be made on the forging itself, costs increase. Non-destructive testing – ultrasonic and magnetic particle inspection – is becoming increasingly important for critical service applications like generator or turbine rotor shafts. Because these tests can be time-consuming and expensive, however, they should be required only when absolutely necessary.

Statistical process/quality control techniques are being applied in many forge shops. Such capabilities may reduce the need for some of the costly testing of individual forgings.

–Delivery…While not necessarily important in early design discussions, it is helpful to discuss production volumes and anticipated shipping schedules with the prospective forger. This information allows him or her to take these factors into consideration when making tooling decisions.

In addition, production-run setup and material acquisition requirements will vary with anticipated volume.

Substantial reductions in material and production costs are attainable through advance planning. And it is almost always more economical to forge and ship a quantity of parts at one time, rather than shipping to a monthly, weekly or daily schedule. However, any economies realized through bulk handling must be achieved through Just-In-Time material- control programs. The forger usually can help reconcile these conflicting objectives.

Even in an industry as mature and well-established as forging, there are continual advances in processing technology and techniques. Some of the developments underway in the industry today may have an impact on the production of many forgings, while others may affect only a narrow segment of the business.

No matter, it is important for the forging buyer to stay abreast of developments so as to be aware of options becoming available through advancing technology. This is another good reason to get the candidate forger involved early in product development discussions. The competent forging engineer will be able to identify situations where new technology and processing techniques can benefit the buyer’s particular project. And because capabilities vary so widely from one forging plant to another, only the forger can determine how cost-effectively a particular new or more advanced procedure can be applied in that plant.

Here are some areas where technology is changing the way forgings are made:

–Net-Shape Forging has been getting a lot of attention primarily because of the potential to dramatically reduce finishing costs. Not only should it be possible to reduce the amount of stock machined away after forging, but the costs associated with machining time will be reduced. In the industry as a whole, however, net-shape forming techniques require further refinement before they become a real alternative for the average forging buyer.

That does not mean there are no benefits to be achieved by studying net-shape techniques. Given a certain latitude in material selection and in forging design, the knowledgeable forger should be able to keep machining and associated costs to a minimum. The more input the forger has in the early stages of product development, the more likely nearnet shapes can be achieved.

–Microalloyed Steels have been used to reduce the need for heat treatment after forging. Small additions of vanadium, columbium and other ingredients can greatly strengthen plain carbon grades of steel. These materials can be used to forge such parts as crankshafts, connecting rods and front axles for trucks – without the need for heat treatment.

This technology is still in its infancy and not all forgers will be able to take advantage of it. But under certain circumstances, and for certain components, it may be possible to reduce or eliminate the costs associated with heat treatment. Close cooperation between forging producer and forging buyer is required to ensure that the performance properties of the finished product suit the application.

–CAD/CAM and Computer Control are having a dramatic effect on many industries. Forging is no exception. From quoting, to die design and production, to billet handling and presses, to heat treating and machining, the computer is affecting the way forgings are produced.

The custom forge plant is essentially a service organization. One of the most important aspects of the service provided is the assistance the forger can give in the design and development of a product to be forged. Today, competition among forgers in the global marketplace is allowing the buyer to demand – and get – ever higher levels of service from the companies vying for the business.

As materials and process technologies advance, it is increasingly important for the forging buyer to involve the forger in decisions that ultimately affect the cost and performance of the part. Through close collaboration with forgers, buyers can gain the greatest benefits from forging industry innovation and can help spur further progress.

This article is adapted from the Forging Industry Association’s booklet “How to Buy Forgings.” FIA, formed in 1913, is the trade association for U.S. and Canadian producers of forgings. Headquartered in Cleveland, it serves 150 forging company members operating more than 200 plants. Its members account for 79% of the custom forged products produced in North America.

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