The different forging processes used to produce near net shapes
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 a cast ingot, which is heated to its plastic deformation temperature, then forged between dies to the desired shape and size.
During this hot forging process, the cast, coarse grain structure is broken up and replaced by finer grains, achieved through the size reduction of the ingot. This produces a sound central region to the forged product and gives excellent overall structural integrity. Mechanical properties are therefore improved through the elimination of the cast structure, resulting in enhanced density and improved homogeneity. Forging also provides a means for aligning the grain flow to obtain the best desired directional strengths.
The two main processes in open die forging are press forging and hammer forging. Both involve the shaping of heated metal parts between a top die attached to a ram and a bottom die attached to a hammer anvil or press bed. Some key differences between the processes are discussed below:
Hammer forging is limited by size. Weights of 500-650kg are commonly seen as the upper limit for this process. Above this size the advantages of press forging take over, this being due to the almost unlimited amount of power that can be generated from hydraulics.
Hammer forging involves hitting the material at high velocities, resulting in finer grain structures and better mechanical properties than achieved though the slower press forging process. However the increased deformation and control achieved though press forging will give the material better through working and consistency of properties.
Hammer forging can work to nearer net shape with smaller forging allowance, therefore on high cost or difficult to machine alloys there can be significant advantages in the hammer forging process.
Depending upon the alloy type and size, bars will be produced by forging from individually cast ingots (with further extrusion or hot rolling possible if required). Bar stock is a convenient form to hold product in, allowing immediate availability of many grades in a range of diameters, without a prohibitive lead time. It is well-suited for applications where there will be subsequent machining, as that also provides some flexibility on the exact bar diameter and length needed. However, for some components, forging is an attractive process to achieve improved properties in a part this is closer to the final shape.
Near-Net Shape Forging
Forging to near net shape not only provides savings in material usage but can also dramatically improve structural integrity through the control of product grain flow.
Rough forging a heated billet between flat dies to the maximum diameter dimension.
A « Knife » tool marks the starting locations.
Drawing down the first step to size.
The second step is drawn down to size
Swaging the rough forging for a smoother surface finish and to keep allowance to a minimum.
Forging produces predictable and uniform products with:
Refined grain size and flow characteristics through mechanical hot deformation
Superior metallurgical and mechanical qualities, together with increased directional strength
A higher degree of structural integrity
Directional alignment of the microstructure has been achieved through deliberately orienting the forging process in the direction requiring maximum strength. This also yields higher ductility and greater resistance to impact and fatigue compared with a forged bar that is subsequently machined.
Open Die Forging
Starting stock cut to size by weight is first rounded, then upset to achieve structural integrity and directional grain flow.
Work piece is punched, then pierced to achieve starting « doughnut » shape needed for ring rolling process.
Completed preform ready for ring production.
A « pin » or mandrel is placed through the preform allowing the ring to be opened out. This process is sometimes called « Becking out ».
The rings thickness is controlled by forging under a flat die and bed intermittently with the Becking process.
This process is often used in the production of seamless forged rings. Seamless rings can be produced in configurations ranging from flat, washer-like parts to tall, cylindrical shapes. The simplest, and most commonly used shape is a rectangular cross-section ring, but shaped tooling can be used to produce seamless rolled rings in complex, custom shapes with contours on the inside and/or outside diameters.
The process starts with a circular pre-form of metal that has been previously upset forged to give structural integrity and directional grain flow, then pierced to form a hollow ring. The pre-form is then placed over the mandrel roll.
Ring rolling process begins with the idler roll applying pressure to the preform against the drive 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. The axial rolls control the height of the ring as it is being rolled. The process continues until the desired size is achieved.
The process continues until the desired size is achieved.