Forged Gears for Structual Integrity and Product Reliablity

For a primer on the differences between cast and forged gears, who better to turn to than the Forging Industry Association? Read on to learn more.

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For a primer on the differences between cast and forged gears, who better to turn to than the Forging Industry Association? Read on to learn more.

The combination of today’s clean steel melting practices and advanced forging technology has resulted in exceptional performance and reliability in forged components ranging from a fraction of a pound to over 250 tons.

Background
During the forging process a metal workpiece is plastically deformed to consolidate any porosity that remains in the stock, and in turn this creates a preferential grain flow that more closely conforms to the shape of the finished component. The controlled process of deformation that takes place imparts exceptional metallurgical soundness, structural integrity, and mechanical properties to the finished forging. Properties often required—such as impact strength, fracture toughness, and increased fatigue life—all benefit from the forging process Figure 1.

Forging has advantages that few processes can duplicate Figure 2. A few benefits of the forging process are as follows:

• Almost all metals and alloys can be forged. Forging is required on many alloys to ensure a uniform microstructure and to obtain the optimum properties of the alloy;
• Ferrous forgings are generally manufactured from stock produced by clean steelmaking practices, resulting in clean steel forging;
• There are few restrictions on part size;
• Forgings can produce high tolerance features;
• The products are fully recyclable;
• Forgings allow for repeatability and reliability in finished components;
• Forgings typically have relatively low-cost life cycles.

Alloy Selection

Virtually all metals can be forged, making an extensive range of physical and mechanical properties available in products with the highest structural integrity. Certain alloys used for gears must be forged to obtain its properties. Carpenter’s Pyrowear 53 alloy, used in high performance gears, is an example of a gear alloy where forging is required to optimize its properties.

Custom alloy grades, as well as standard grades, are available for forging. Companies design their parts to optimize functionality. As is often the case, alloys that start out as custom grades become a standard grade as usage increases.

Clean Steel Melting Practices

Today’s clean steels have resulted from many improvements in steelmaking practices. Steel cleanliness is the foundation on which high performance can be built. It is generally well accepted that fewer and smaller inclusions translate to longer service life relative to fatigue failure. Continuous advances in internal quality have resulted from the ability to accurately measure the cleanliness over a significant volume of steel. Ultrasonic testing has proved to be a critical tool. It has the ability to sample sufficient volume, and the summed lengths of inclusions per unit volume can be correlated to fatigue life (Ref. 1).

Directionality and Low Sulfur Steel Forgings
With forgings produced from clean steel there is little difference between longitudinal and transverse properties (Figure 2). Transverse and longitudinal fatigue samples of 4140 wrought steel (40 HRC) at three sulfur levels of 0.004, 0.012, and 0.077 percent were tested. As you can see, at low S levels of 0.004 and 0.012, the fatigue performance in the transverse direction is nearly identical to the longitudinal samples. Note that there are two curves for the longitudinal samples for two S levels, but the curves are nearly identical, as S level is not expected to have any effects in the longitudinal direction.

Forged vs. Cast Properties

In comparing a forging to a casting of the same alloy and configuration, the forging’s properties will be superior in every orientation. Large forgings have a completely wrought structure, whereas large castings are porous and have differing microstructure and chemistries throughout the part. The microstructure of a forging is uniform with a fine recrystalized grain structure.

In castings, as molten metal solidifies, the shrinking phenomenon creates fissures, voids, and porosity (see Figure 3). Porosity, in turn, can cause a casting to leak and makes it unusable for holding pressure in applications like pumps, compressors, transmissions, and plumbing fixtures. Shrinkage and porosity in castings result in unacceptable levels of defective product, increased quality and warranty costs, lost good will and, ultimately, lost business Figure 3.

Structural Integrity

When required, the internal soundness of fine-grained forgings is guaranteed both by fixed process procedures and ultrasonic inspection. Dramatic advancements in ultrasonic inspection techniques have enabled the non-destructive testing of nonparallel surfaces. Large castings often require 100 percent x-ray inspection to determine the degree of internal porosity and internal soundness. Localized welding, common in castings to repair casting defects, is not allowed for forged products since this process can affect the structural integrity of the component.

Summary
Forgings provide both surface and internal integrity that optimizes gear properties. The defects commonly found in castings are not found in forgings since the material has been significantly worked to ensure a fine-grained structure and to ensure that there are no voids or porosity.

When your application requires reliability (longer fatigue life), repeatability of mechanical properties (clean steel with beneficial grain flow), and the ability to perform in adverse conditions, a forged part is the choice of informed designers.

References:
1) Craig V. Darragh, “Engineered Gear Steels: A Review,” Gear Technology, November/December 2002

2) Department of Energy Project Fact Sheet, “Model Will Teach Mechanisms of Porosity Formation and Enable Foundries to Implement Preventive Measures,” January 2001

The Forging Industry Educational and Research Foundation
Established in 1961, the Forging Industry Educational and Research Foundation (FIERF) is a 501(c)(3) tax-exempt organization. Operating as a “supporting organization” to the Forging Industry Association, the foundation’s mission is to support the forging industry through technology development and education. Its primary activities involve:

Research and Development
• Providing a medium for forging and related industry collaborative research
• Funding technology development
• Transfering technology to forging industry and users
Technical Education
• Fostering forging curriculum and experiential training in university engineering departments
• Providing scholarships to encourage careers in the forging and related industries
Fundraising
• Seeking support from government, industry, and individual sources to grow programs to fulfill research and education goals

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director of Research and Education, Forging Industry Association, Forging Industry Educational and Research Foundation—has 41 years of technical and management experience in the bearings, nuclear, and forging industries. In his current position he is responsible for developing industry-wide technology programs to increase the global competitiveness of the North American forging industry. He is also a member of the Industrial Heating Equipment Association’s (IHEA) Process Heating Steering Committee. To learn more call (216) 781-6260, send e-mail to info@forging.org, or go online to [www.forging.org].