5.2.1 The Open Die Process

Open die forging is a hot forming process that uses standard flat, "V", concave or convex dies in presses. The process is used to form a virtually limitless range of component sizes from a few pounds to over 300 tons. The workpiece is heated to improve its plastic flow characteristics and reduce the force required to work the metal. The workpiece is systematically deformed by a series of strokes from the upper die while being supported on the lower die. The position is changed between strokes by a means such as the manipulator shown in Figure 5-1.

Open die forging processes allow the workpiece freedom to move in one or two directions. The workpiece is typically compressed in the axial direction (direction of movement of the upper die) with no lateral constraint. Lateral dimensions are developed by controlling the amount of axial deflection, or by rotating the workpiece. Some of the most commonly performed operations are upsetting; cogging; drawing; piercing, punching, saddening and hot trepanning; hollow forging; closing in; and ring forging. Following is a brief description of the most common operations.

Some of the latest in programmable press controls are being applied to open die processes. Programmable controls generate greater accuracy, better utilization of stock, and repeatability.

Upsetting Upsetting is working with the axis of the stock in the vertical position under the forging press or hammer. The operation decreases the axial length of the stock and increases its cross section. Upsetting is usually accomplished with flat dies, as shown in Figure 5-2. Flat dies are larger than the cross section of the workpiece. Friction between the dies and workpiece is inevitable, and causes the barreling effect shown in the figure.

Cogging Cogging is the systematic reduction of an ingot to billets or blooms by narrow dies. Narrow dies, shown in

Figure 5-3 exceed the width of the workpiece, but not the length. Narrow dies may be flat, as shown, V, concave or convex. The ingot is reduced and elongated by repeated strokes as it is systematically advanced and sometimes rotated. The process changes the grain structure of the metal as show in Figure 5-4 and consolidates internal ingot defects such as porosity and blow holes.

Drawing or Solid Forging Drawing or solid forging is used to produce a shape with length much greater than its cross section by reducing the section and simultaneously elongating the ingot or bloom, as show in Figure 5-4. It is used to produce stock for further forging operations or end products, such as bars or shafts.

Piercing, Punching and Trepanning In these operations, a punch is forced into a piece of hot steel to form a cavity. Piercing generally implies a blind cavity made by displacement with no removal of metal. Punching implies the use of a solid punch to form a through hole by displacing and removing metal in the form of a slug. Trepanning also forms a through hole, using a hollow punch to remove the central metal as a core. Two piercing sequences are shown in Figure 5-5.

Hollow Forging Hollow forging is used to produce hollow forms by expanding or lengthening on a mandrel. The process, shown in Figure 5-6, begins with a pierced or cupped forging. Wall thicknesses are reduced and length increased by a drawing operation. Long hollow forms use a mandrel, in an operation similar to ring forging, which is shown in Figure 5-7.

Closing-in Closing-in is used to reduce the section on a portion or portions of a hollow forging. The area or areas to be reduced are reheated to forging temperature and reduced using V-tapered, curved or formed dies. A variant is shown in Figure 5-7.

The forging was provided with a step on the outer diameter, which is reduced so that only the bore is reduced at the site.

Ring Forging Ring forging produces rings from pierced blanks by open die forging over a mandrel. The process is shown in Figure 5-8. Slight rotation of the ring on each press stroke reduces the ring wall uniformly and increases both the inside and outside diameters. The height of the ring remains nearly constant, but may require edging.

Combined Processes The above operations are usually combined to produce shapes ranging from simple to complex in a wide range of sizes. The process is specially suited for very large, and sometimes very complex forgings, which are well beyond the range of impression die processes, such as the 60,000 pound as-forged crankshaft shaft shown in Figure 5-9 and the large valve body shown in Figure 5-10.


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Open die forging is a hot forming process that uses standard flat, "V", concave or convex dies in presses. The process is used to form a virtually limitless range of component sizes from a few pounds to over 300 tons. The workpiece is heated to improve its plastic flow characteristics and reduce the force required to work the metal. The workpiece is systematically deformed by a series of strokes from the upper die while being supported on the lower die. The position is changed between strokes by a means such as the manipulator shown in Figure 5-1.

Open die forging processes allow the workpiece freedom to move in one or two directions. The workpiece is typically compressed in the axial direction (direction of movement of the upper die) with no lateral constraint. Lateral dimensions are developed by controlling the amount of axial deflection, or by rotating the workpiece. Some of the most commonly performed operations are upsetting; cogging; drawing; piercing, punching, saddening and hot trepanning; hollow forging; closing in; and ring forging. Following is a brief description of the most common operations.

Some of the latest in programmable press controls are being applied to open die processes. Programmable controls generate greater accuracy, better utilization of stock, and repeatability.

Upsetting Upsetting is working with the axis of the stock in the vertical position under the forging press or hammer. The operation decreases the axial length of the stock and increases its cross section. Upsetting is usually accomplished with flat dies, as shown in Figure 5-2. Flat dies are larger than the cross section of the workpiece. Friction between the dies and workpiece is inevitable, and causes the barreling effect shown in the figure.

Cogging Cogging is the systematic reduction of an ingot to billets or blooms by narrow dies. Narrow dies, shown in

Figure 5-3 exceed the width of the workpiece, but not the length. Narrow dies may be flat, as shown, V, concave or convex. The ingot is reduced and elongated by repeated strokes as it is systematically advanced and sometimes rotated. The process changes the grain structure of the metal as show in Figure 5-4 and consolidates internal ingot defects such as porosity and blow holes.

Drawing or Solid Forging Drawing or solid forging is used to produce a shape with length much greater than its cross section by reducing the section and simultaneously elongating the ingot or bloom, as show in Figure 5-4. It is used to produce stock for further forging operations or end products, such as bars or shafts.

Piercing, Punching and Trepanning In these operations, a punch is forced into a piece of hot steel to form a cavity. Piercing generally implies a blind cavity made by displacement with no removal of metal. Punching implies the use of a solid punch to form a through hole by displacing and removing metal in the form of a slug. Trepanning also forms a through hole, using a hollow punch to remove the central metal as a core. Two piercing sequences are shown in Figure 5-5.

Hollow Forging Hollow forging is used to produce hollow forms by expanding or lengthening on a mandrel. The process, shown in Figure 5-6, begins with a pierced or cupped forging. Wall thicknesses are reduced and length increased by a drawing operation. Long hollow forms use a mandrel, in an operation similar to ring forging, which is shown in Figure 5-7.

Closing-in Closing-in is used to reduce the section on a portion or portions of a hollow forging. The area or areas to be reduced are reheated to forging temperature and reduced using V-tapered, curved or formed dies. A variant is shown in Figure 5-7.

The forging was provided with a step on the outer diameter, which is reduced so that only the bore is reduced at the site.

Ring Forging Ring forging produces rings from pierced blanks by open die forging over a mandrel. The process is shown in Figure 5-8. Slight rotation of the ring on each press stroke reduces the ring wall uniformly and increases both the inside and outside diameters. The height of the ring remains nearly constant, but may require edging.

Combined Processes The above operations are usually combined to produce shapes ranging from simple to complex in a wide range of sizes. The process is specially suited for very large, and sometimes very complex forgings, which are well beyond the range of impression die processes, such as the 60,000 pound as-forged crankshaft shaft shown in Figure 5-9 and the large valve body shown in Figure 5-10.


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Open die forging is a hot forming process that uses standard flat, "V", concave or convex dies in presses. The process is used to form a virtually limitless range of component sizes from a few pounds to over 300 tons. The workpiece is heated to improve its plastic flow characteristics and reduce the force required to work the metal. The workpiece is systematically deformed by a series of strokes from the upper die while being supported on the lower die. The position is changed between strokes by a means such as the manipulator shown in Figure 5-1.

Open die forging processes allow the workpiece freedom to move in one or two directions. The workpiece is typically compressed in the axial direction (direction of movement of the upper die) with no lateral constraint. Lateral dimensions are developed by controlling the amount of axial deflection, or by rotating the workpiece. Some of the most commonly performed operations are upsetting; cogging; drawing; piercing, punching, saddening and hot trepanning; hollow forging; closing in; and ring forging. Following is a brief description of the most common operations.

Some of the latest in programmable press controls are being applied to open die processes. Programmable controls generate greater accuracy, better utilization of stock, and repeatability.

Upsetting Upsetting is working with the axis of the stock in the vertical position under the forging press or hammer. The operation decreases the axial length of the stock and increases its cross section. Upsetting is usually accomplished with flat dies, as shown in Figure 5-2. Flat dies are larger than the cross section of the workpiece. Friction between the dies and workpiece is inevitable, and causes the barreling effect shown in the figure.

Cogging Cogging is the systematic reduction of an ingot to billets or blooms by narrow dies. Narrow dies, shown in

Figure 5-3 exceed the width of the workpiece, but not the length. Narrow dies may be flat, as shown, V, concave or convex. The ingot is reduced and elongated by repeated strokes as it is systematically advanced and sometimes rotated. The process changes the grain structure of the metal as show in Figure 5-4 and consolidates internal ingot defects such as porosity and blow holes.

Drawing or Solid Forging Drawing or solid forging is used to produce a shape with length much greater than its cross section by reducing the section and simultaneously elongating the ingot or bloom, as show in Figure 5-4. It is used to produce stock for further forging operations or end products, such as bars or shafts.

Piercing, Punching and Trepanning In these operations, a punch is forced into a piece of hot steel to form a cavity. Piercing generally implies a blind cavity made by displacement with no removal of metal. Punching implies the use of a solid punch to form a through hole by displacing and removing metal in the form of a slug. Trepanning also forms a through hole, using a hollow punch to remove the central metal as a core. Two piercing sequences are shown in Figure 5-5.

Hollow Forging Hollow forging is used to produce hollow forms by expanding or lengthening on a mandrel. The process, shown in Figure 5-6, begins with a pierced or cupped forging. Wall thicknesses are reduced and length increased by a drawing operation. Long hollow forms use a mandrel, in an operation similar to ring forging, which is shown in Figure 5-7.

Closing-in Closing-in is used to reduce the section on a portion or portions of a hollow forging. The area or areas to be reduced are reheated to forging temperature and reduced using V-tapered, curved or formed dies. A variant is shown in Figure 5-7.

The forging was provided with a step on the outer diameter, which is reduced so that only the bore is reduced at the site.

Ring Forging Ring forging produces rings from pierced blanks by open die forging over a mandrel. The process is shown in Figure 5-8. Slight rotation of the ring on each press stroke reduces the ring wall uniformly and increases both the inside and outside diameters. The height of the ring remains nearly constant, but may require edging.

Combined Processes The above operations are usually combined to produce shapes ranging from simple to complex in a wide range of sizes. The process is specially suited for very large, and sometimes very complex forgings, which are well beyond the range of impression die processes, such as the 60,000 pound as-forged crankshaft shaft shown in Figure 5-9 and the large valve body shown in Figure 5-10.


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