PeripWhen buyers must select a process and supplier for the production of a critical metal component, they face an enormous array of possible alternatives. Many metalworking processes are available, each offering a unique set of capabilities, costs and advantages. The forging process is ideally suited to many part applications; however, some buyers may be unaware of the exclusive benefits available only from this type of metal forming. In fact, forging is often the optimum process, in terms of both part quality and cost, especially for applications that require maximum part strength, custom sizes or critical performance specifications.
There are several forging processes available, including impression or closed die, cold forging, and extrusion. However, here we will discuss in detail the methods, application and comparative benefits of the open die and seamless rolled ring forging processes. We invite you to consider this information when selecting the optimum process for the production of your metal parts.
A Historical Perspective
Perhaps the oldest mechanical method of metal- working, forging traces its origins back to the 13th century, B.C.
To keep up with the industry’s changing needs, forging has evolved to incorporate the tremendous advances in equipment, robotics, technology, and electronic controls that have occurred in recent years. These sophisticated tools complement the creative human skills, which, even today, are essential to the success of every forging made. Modern forging plants are capable of producing superior-quality metal parts in a virtually limitless array of sizes, shapes, materials and finishes.
At its most basic level, forging is the process of forming and shaping metals through the use of hammering, pressing, or rolling. The process begins with starting stock, usually a cast ingot (or a “cogged” billet which has already been forged from a cast ingot), which is heated to its plastic deformation temperature, then upset or “kneaded” between dies to the desired shape and size.
During this hot forging process, coarse grain structure is broken up and replaced by finer grains. Shrinkage and gas porosity inherent in the cast metal are consolidated through the reduction of the ingot, achieving sound centers and structural integrity. Mechanical properties are therefore improved through reduction of cast structure, voids, and segregation. Forging also provides means for aligning the grain flow to best obtain desired directional strengths. Secondary processing, such as heat treating, can also be used to further refine the part. Forging can create a myriad of sizes and shapes with enhanced properties when compared to castings or fabrications.
The Open Die Forging Process
Open die forging involves the shaping of heated metal parts between a top die attached to a ram and a bottom die attached to a hammer, anvil, or bolster. Metal parts are worked at their appropriate temperatures, ranging from 500°F to 2400°F, and gradually shaped into the desired configuration through the skillful hammering or pressing of the workpiece.
While impression or closed die forging confines the metal in dies, open die forging is distinguished by the fact that the metal is never completely confined or restrained. Most open die forgings are produced on flat dies. However, round swaging dies, V-dies, mandrels, pins, and loose tools are also used depending on the desired part configuration and its size.
Although the open die forging process is often associated with larger, simpler-shaped parts such as bars, blanks, rings, hollows or spindles, it can be considered the ultimate option in “custom- designed” metal components. High-strength, long-life parts optimized in terms of both mechanical properties and structural integrity are today produced in sizes that range from a few pounds to hundreds of tons in weight. In addition, advanced forge shops now offer shapes that were never before thought capable of being produced by the open die forging process.
The Seamless Rolled Ring Forging Process
The production of seamless forged rings is often performed by a process called ring rolling on rolling mills. These mills vary in size to produce rings with outside diameters of just a few inches to over 300 inches and in weights from a single pound up to over 300,000 pounds.
The process starts with a circular preform of metal that has been previously upset and pierced (using the open die forging process) to form a hollow “donut.” This donut is heated above the recrystallization temperature and placed over the idler or mandrel roll. This idler roll then moves under pressure toward a drive roll that continuously rotates to reduce the wall thickness, thereby increasing the diameters (I.D. and O.D.) of the resulting ring.
Seamless rings can be produced in configurations ranging from flat, washer-like parts to tall, cylindrical shapes, with heights ranging from less than an inch to more than 9’. Wall thickness to height ratios of rings typically range from 1:16 up to 16:1, although greater proportions can be achieved with special processing. The simplest, and most commonly used shape is a rectangular cross-section ring, but shaped tooling can be used to pro- duce seamless rolled rings in complex, custom shapes with contours on the inside and/or outside diameters.
Advantages of Open Die and Rolled Ring Forging Part Integrity Advantages
1. Directional Strength
By mechanically deforming the heated metal under tightly controlled conditions, forging produces predictable and uniform grain size and flow characteristics. Forging stock is also typically pre-worked to refine the dendritic structure of the ingot and remove porosity. These qualities translate into superior metallurgical and mechanical qualities, and deliver increased directional toughness in the final part.
2. Structural Strength
Forging provides structural integrity that is unmatched by other metalworking processes. By consolidating the ingot center, forging eliminates internal voids and porosity. The process also breaks up and eliminates the dendritic structure that is inherent in the original cast ingot. Predictable structural integrity reduces the need for part inspection requirements, simplifies heat treating and machining and ensures optimum part performance under field service conditions.
3. Impact Strength
Parts can also be forged to meet virtually any impact requirement. Proper orientation of grain flow assures maximum impact strength and fatigue resistance. The high-strength properties of the forging process can be used to reduce sectional thickness and overall weight without compromising final part integrity.
Part Flexibility Advantages
1. Variety of Sizes
Limited only to the largest ingot that can be cast, open die forged part weights can run from a single pound to over 1,000,000 pounds. In addition to commonly purchased open die parts, forgings are often specified for their soundness in place of rolled bars or castings, or for those parts that are too large to produce by any other metalworking method.
2. Variety of Shapes
Shape design is just as versatile, ranging from simple bar, shaft and ring configurations to specialized shapes. These include multiple O.D/I.D. hollows, single and double hubs that approach closed die configurations and unique, custom shapes produced by combining forging with secondary processes such as torch cutting, sawing and machining. Shape designs are often limited only by the creative skills and imagination of the forging supplier.
3. Metallurgical Spectrum
Metallurgical properties can be greatly varied through alloy selection, part configuration, thermal mechanical working, and post-forming processes.
4. Quantity and Prototype Options
Virtually all open die and rolled ring forgings are custom made one at a time, providing the option to purchase one, a dozen or hundreds of parts as needed. An added benefit is the ability to offer open die prototypes in single piece or low volume quantities. No better way exists to test initial closed die forging designs, because open die forging imparts similar grain flow orientation, deformation and other beneficial characteristics. In addition, the high costs and long lead times associated with closed die tooling and set- ups are eliminated.
1. Material Savings
Forging can measurably reduce material costs since it requires less starting metal to produce many part shapes. For example, with a torch cut part (A), all corner stock and the full center slug are lost, even though you pay for the excess material. With a forging (B), the part is shaped to size with minimal waste.
2. Machining Economies
Forging can also yield machining, lead time and tool life advantages. Savings come from forging to a closer-to-finish size than is capable by alternate metal sources such as plate or bar. Less machining is therefore needed to finish the part, with the added benefits of shorter lead time and reduced wear and tear on equipment.
3. Reduced Rejection Rates
By providing weld-free parts produced with cleaner forging quality material and yielding improved structural integrity, forging can virtually eliminate rejections.
4. Production Efficiencies
Using the forging process, the same part can be produced from many different sizes of starting ingots or billets, allowing for a wider variety of inventoried grades. This flexibility means that forged parts of virtually any grade can be manufactured more quickly and economically.
Material Grades and Secondary Processes
Practically all ferrous and non-ferrous alloys can be forged, however each metal has unique characteristics. Some of the most common metals include:
• Alloy Steel
• Carbon Steel
• Stainless Steel
• Tool Steel
• Custom grades
Modern steel mills offer the strongest, cleanest materials in the world. Their diverse capabilities also allow the development of custom-melt materials uniquely suited to any application.
Adding secondary processing to your forging adds value and oftentimes reduces over- all costs. In conjunction with proper alloy selection and forging practices, heat treating, machining, sawing, torch cutting and testing options allow forged parts to be produced to almost any desired shape, size or property.
Of specific note are custom, near net shaped parts not normally associated with the open die process. Combining the creative skills of the forgemaster with the use of simple, loose tooling or secondary processing such as torch cutting, sawing and machining, uniquely shaped parts of the highest quality can be produced.
Applying the Forging Process Applications
Forged metal parts are used in nearly all metalworking industries. Some of the major industry applications include:
• Gear/Power Transmission
• General industrial equipment
• Oil and gas
• Power generation
• Warehouse / Service centers
For advice on whether the forging process is appropriate for a specific part, consult an experienced forging specialist. Figure 7 shows some examples of common forged parts.
Making The Decision
Forgings can deliver better part strength compared to other metalworking processes — such as machined bar, torch cut plate, weldments, fabrications, or castings — due to the elimination of porosity, contoured grain flow, and fine grain size. The ability to forge near net shape parts saves cost, time, machining labor, and material for low quantity orders or special sizes. Forgings are also ideal for high quality, restricted mechanical properties, and safety critical applications. This is why forging is almost always the preferred method where quality and safety are a concern. When selecting a forging supplier you should consider the following:
• HUBS/TOOLED FORGING
• SEMI-CLOSED DIE
• TOURCH OR PROFILE CUT
• COMPLEX FORGED SHAPES
Does the potential supplier offer technical assistance to improve the quality of your finished part and at the same time reduce production time and costs? Does the source utilize advanced metal and part testing technologies? Can the supplier provide application assistance using advanced simulation capabilities utilizing finite elements analysis?
Are experienced metallurgists, several with advanced degrees, available to assist you with your selection and application of optimum alloys for your forged metal part? Does the supplier maintain the largest inventory of metal alloys? Can the supplier also provide custom-melt and specialty alloys?
What are the parameters—in terms of forging sizes, shapes and alloys—of the supplier’s capabilities? Can all your open die and rolled ring forging requirements be met by the supplier? Does the vendor offer near net, semi-closed die and custom shaped forgings? Ask for a current equipment list, and determine whether the source has today’s most advanced forging presses, ring mills, metallurgical capabilities and other systems. Can the supplier also provide in-house secondary services such as machining, torch cutting, heat treating and testing?
Is the supplier ISO 9001:2008, DNV and PED Certified? Is the source an American Bureau of Shipping certified and Lloyds Registry approved forging supplier? Are the supplier’s inspectors Level III Certified to SNT-TC-1A? Is supplier ASNT Certified? Does the source perform chem checks on both incoming materials and outgoing forged parts?
How long has the supplier been in the forging business? What type of forged parts does the source deliver? Has that supplier produced similiar components? Can the potential vendor meet your specific requirements for part quality, delivery and cost?
Used; Forging Industry Association trade publications.