This invention relates to a forging method for generally axisymmetric articles and to a forging press wherein generally axisymmetric articles are forged in an incremental fashion.
In forging, a workpiece is compressed between two or more forging dies by a machine termed a forging press. The workpiece plastically deforms to a new shape determined by the shapes of the forging dies and the amount of compression. The forging may be accomplished in a single press stroke, or there may be multiple press strokes to gradually deform the workpiece to the required final shape of the article.
The forging operation thins the workpiece in the direction of force application and causes it to enlarge in the perpendicular plane. The workpiece is thereby deformed to the final forged shape. The final forged shape must be distinguished from the final article shape, because in general it is not possible or desirable to forge the workpiece to precisely the final desired article shape. The degree to which the final forged shape approximates that of the final desired article determines the difficulty of the forging operation to some degree. It is relatively easy to uniformly forge the workpiece over its entire plan view area, termed pancake forging. However, in a typical situation involving a complexly shaped final desired article, pancake forging leaves large amounts of material to be machined away to reach the details of the shape of the final desired article. In a more-advanced approach to forging, the workpiece is forged to a near-net-shape (NNS) configuration that closely approximates the shape of the final article but is intentionally slightly oversize to permit ultrasonic inspection, removal of sufficient material to account for distortion experienced during heat treatment, and final machining of the details. In this NNS forging approach, the amount of metal machined away is relatively small. NNS forging requires considerably more ingenuity in designing the forging process than does pancake forging.
Forging is used in a wide variety of operations to produce both small and large articles. To deform the workpiece, a forging press must apply the required force. The production of large articles is particularly challenging because the larger the article, the larger is the required forging force. Consequently, a larger and more expensive forging press is needed to accomplish the forging. As noted, NNS forging usually requires greater forging forces, and thence a larger forging press, than pancake forging.
In some cases, it is desired to produce an article whose size and material of construction are such that the force capacity of the available forging press is exceeded. To forge such articles, it is known to incrementally forge the workpiece using an open-die forging operation. In incremental open-die forging, the design of the forging dies and the operation of the forging press are such that only a portion of the workpiece is forged at any one time. The workpiece is moved incrementally relative to the forging dies after each region is forged, eventually leading to complete forging of the entire workpiece. Unfortunately, open-die forging and incremental open-die forging cannot achieve near-net-shape configurations for most articles, because the unconstrained portion of the workpiece is allowed to expand to whatever size and configuration results, rather than to a near net shape.
In one application, a workpiece is forged into an axisymmetric turbine disk for use in a large land-based gas turbine. Such turbine disks are as much as 70-96 inches in diameter or larger. They are made of nickel-base or iron-base superalloys and cannot be forged to a desired near-net-shape geometry even on a press having a capacity of 50,000 tons. The axisymmetric near-net-shape, dimensional, and mechanical property requirements of the final turbine disks are quite stringent. The existing incremental forging techniques for such disks cannot meet these requirements.
Accordingly, there is a need for an improved approach to the forging of large axisymmetric articles. The present invention fulfills this need, and further provides related advantages.