Abstract:
A forging method including extruding a billet to form a ring shaped hollow workpiece; reducing a cross section of the workpiece; and forging the workpiece in a closed die having a first split die including a plurality of first die segments and a second split die including a plurality of second die segments by sequentially advancing pairs of opposing die segments from the first split die and second split die towards each other.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a divisional of U.S. application Ser. No. 11/968,684, filed Jan. 3, 2008, which is allowed, the entire contents of which is hereby incorporated by reference. 
         [0002]    The invention relates to a near net shape forging process for compressor and turbine wheels and turbine spacer wheels. In particular, the invention relates to a near net shape forging process for compressor and turbine wheels and turbine spacer wheels formed of NiCrMoV and CrMoV. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    Existing forging processes for the manufacture of compressor and turbine wheels rely on open die forging. Open die forging processes require additional input material tonnage and more heat treatment, and more forging processing steps. 
         [0004]    Current closed die forging processes involve higher press tonnages. The use of closed die forging requires investments of higher capacity presses. However, there are currently no high capacity presses suitable for economical closed die forging of turbine and compressor wheels and turbine spacer wheels formed, for example, of CrMoV and NiCrMoV. 
         [0005]    U.S. Pat. No. 6,240,765 discloses a closed die forging process including a die set having a stationary die and a movable die in facing-but-spaced-apart relation to the stationary die along a press access and defining a work piece volume therebetween. U.S. Pat. No. 6,240,765 starts with a workpiece geometry which covers the entire plan view area of the dies. As the workpiece covers the entire plan view area of the dies, the strain rates to be used are much lower which results in frequent heat treatment steps between the various incremental forging steps. The process of U.S. Pat. No. 6,240,765 therefore requires greater input material tonnage. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0006]    According to an embodiment of the invention, a method of forging a workpiece comprises (a) incrementally advancing the workpiece in a closed die forge, the closed die forge comprising a stationary, flat die and a first split die comprising a plurality of first die segments, each die segment being incrementally advanced in sequence to contact the incrementally advancing workpiece; (b) replacing the stationary, flat die with a second split die comprising a plurality of second die segments; and (c) forging the workpiece forged in (a) between the first split die and the second split die, wherein the first die segments are stationary and at least some of the plurality of second die segments are incrementally advanced in sequence. 
         [0007]    According to another embodiment of the invention, a forging method comprises extruding a billet to form a ring shaped hollow workpiece; reducing a cross section of the workpiece; and forging the workpiece in a closed die comprising a first split die comprising a plurality of first die segments and a second split die comprising a plurality of second die segments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  schematically illustrates the input material for a compressor or turbine wheel according to the prior art and the invention; 
           [0009]      FIG. 2  depicts a first stage preforming process using a flat bottom die and a split top incremental die; 
           [0010]      FIG. 3  discloses an incremental advance of a die segment from  FIG. 2 ; 
           [0011]      FIG. 4  discloses another advance of an incremental die segment of the first stage preforming; 
           [0012]      FIGS. 5 and 6  show third stage preforming using the flat bottom die and incremental split top die; 
           [0013]      FIGS. 7 and 8  schematically depict the fourth stage preforming with the flat bottom die and incremental split top die; 
           [0014]      FIGS. 9 and 10  schematically depict the fifth stage preforming with a split bottom die and a stationary top die; 
           [0015]      FIG. 11  schematically depicts sixth stage preforming with a split bottom die and a stationary top die; 
           [0016]      FIG. 12  schematically depicts a turbine spacer wheel; and 
           [0017]      FIG. 13  schematically depicts a forging process according to another embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    Referring to  FIG. 1 , a turbine or compressor wheel  2  may be forged from a starting workpiece. According to current forging processes, the starting workpiece  4  comprises approximately 30% more material than a starting workpiece  6  according to an embodiment of the invention. 
         [0019]    Referring to  FIGS. 2 and 3 , the first stage preforming may be performed with a closed flat bottom die  14  and a closed top incremental split die  16 . The closed top incremental split die  16  includes a closed top die first segment  18 , second segment  20 , third segment  22 , fourth segment  24 , and a fifth segment  26 . It should be appreciated, however, that the closed top incremental split die may be formed of any number of segments. A stop  40  is provided after the closed top die fifth segment  26  to close the die. 
         [0020]    As shown in  FIG. 2 , the workpiece  6  is initially contacted by the first segment  18  and the second segment  20 . The first segment  18  is incrementally advanced as shown in  FIG. 3 . The first stage preforming shown in  FIGS. 4 and 5  is done at a high strain rate such as to minimize die chilling to avoid intermediate reheats of the workpiece  6 . 
         [0021]    The closed top die second segment  20  is then incrementally advanced, as shown in  FIG. 4 . The process details may be designed to eliminate the requirement for an intermediate reheat of the workpiece  6 . 
         [0022]    Referring to  FIGS. 5 and 6 , in a third stage of the preforming using the flat bottom die  14  and the top incremental split die  16 , the third segment  22  is advanced to contact the workpiece  6 . The third stage preforming process shown in  FIGS. 5 and 6  is done at a high strain rate so as to minimize die chilling and avoid intermediate reheat processing of the workpiece  6 . 
         [0023]    As shown in  FIGS. 7 and 8 , the fourth stage preforming using the flat bottom die  14  and the top incremental split die  16  is shown. The fourth segment  24  is advanced to contact the workpiece  6  as shown in  FIGS. 7 and 8 . 
         [0024]    As shown in  FIGS. 2-8 , the four preforming stages include incremental advancements of the first segment  18 , the second segment  20 , the third segment  22 , and the fourth segment  24  of the closed top incremental split die  16  against the flat bottom die  14 . The four preforming stages are carried out at high strain rates to eliminate the intermediate heat treatment of the workpiece  6 . 
         [0025]    As shown in  FIGS. 9 and 10 , in a fifth stage of the preforming of the workpiece  6 , the stationary flat bottom die  14  is replaced by a closed bottom incremental split die  28 . The closed bottom incremental split die  28  includes a closed bottom die first segment  30 , a second segment  32 , a third segment  34 , a fourth segment  36 , and a fifth segment  38 . The stop  40  is provided after the closed top incremental split die fifth segment  26  and the closed bottom die fifth segment  38  to provide a closed die. 
         [0026]    As shown in  FIGS. 9 and 10 , the segments  18 ,  20 ,  22 ,  24 ,  26  of the closed top split die  16  remain stationary and the segments  30 ,  32 ,  34 ,  36 ,  38  of the closed bottom incremental split die  28  are advanced to further shape the workpiece  6 . As shown in  FIG. 15 , the second segment  32  of the bottom die  28  is incrementally advanced. A heat treatment may be done on the workpiece  6  after the fifth stage preforming to bring the workpiece  6  back to temperature. 
         [0027]    In the sixth stage preforming, shown in  FIG. 11 , the fourth segment  36  of the closed bottom incremental split die  28  is advanced. The sixth stage preforming is done at a very slow strain rate to minimize the load requirements. 
         [0028]    As shown in  FIGS. 2-11 , the incremental forging of the compressor and/or turbine wheels may be performed in six preforming stages. The process shown in  FIGS. 2-11  may be implemented across all of the frames and stages of the compressor and/or turbine. As the rotors have a significant length in the plan view area, the multistage and multi-preforming forging schedule shown in  FIGS. 2-11  may be employed. In the first four preforming stages, the material of the workpiece  6  first flows primarily in the plan view direction using the bottom flat stationary die  14  and the top incremental split die  16  at high strain rates. The incremental forging is then completed in the fifth and sixth preforming stages using the bottom incremental split die  28  and the top incremental split die  16  at slower strain rates. 
         [0029]    The top and bottom incremental split dies may be designed so that they are modular. For example, the bore-web sections are similar for stages 2-16 of compressor wheels, rim-web sections are similar between stages 10-16 and stages 2-5. Variable rim-web sections for stages 6-9 can be represented by minimum web geometry. This permits the same basic die set to be used for various stages of wheels with minimum modifications without having the need to invest in a new die set for each stage of compressor/turbine wheels. The split die design enables a modular die design across various stages of GT wheels. 
         [0030]    As the dies are at a lower temperature compared to the workpiece, having a thin plate made of a low thermal conductivity material at the interface between the dies and the workpiece is beneficial. This is more beneficial at the last stages of forging which are done at lower strain rates. This is desirable because die-chilling effect at the last stage could be high (due to the lower strain rates) leading to higher heat loss in the workpiece. Thus, having a thin lower conductivity material plate helps to reduce die chilling and thereby reduce the load requirements substantially. 
         [0031]      FIG. 12  schematically depicts a turbine spacer wheel  42  which may be forged to near net shape according to an embodiment of the invention. 
         [0032]    A billet having an initial diameter may be extruded in a die and mandrel arrangement with a container. The billet is forced through a mandrel and a punch and is shaped by an outer die including a container. 
         [0033]    The geometry of the starting workpiece for the turbine spacer wheel may be a ring-shaped hollow profile. Such a workpiece reduces the load requirements of the incremental forging process. To achieve the ring-shaped profile, the mandrel extrusion process described above may be used. The use of mandrel extrusion for forming the workpiece starting geometry also permits subsequent drilling of the solid workpiece at the end of forging. 
         [0034]    At the end of the extrusion process, a portion of the billet between the mandrel and the outer die and punch may be machined off to form the starting workpiece. The starting workpiece may be then used for the subsequent forging steps previously described. 
         [0035]    Referring to  FIG. 13 , the turbine spacer wheel  42  may be forged by the closed top incremental split die  16  and the closed bottom incremental split die  28 . A shrink ring  52  may be shrink fitted onto the stop  40 . The first segments  18 ,  30  are incrementally advanced toward one another. The second segments  20 ,  32  are then incrementally advanced toward one another. After incremental advancement of the first segments  18 ,  30 , a reheat may be performed on the workpiece  6  to raise the temperature of the workpiece. 
         [0036]    In the third and fourth preforming stages, the third die segments  22 ,  34  are incrementally advanced towards one another and the fourth die segments  24 ,  36  are incrementally advanced toward one another. 
         [0037]    The forging of the turbine spacer wheel as described requires only one reheat cycle in the incremental forging process which may be performed after the incremental advancement of the first segments  18 ,  30 . 
         [0038]    Die stress analysis may be carried out after the forging process to estimate the die life. The maximum forces and stresses from the forged spacer wheel may be mapped onto the individual dies. During the forging of the turbine spacer wheel, the stop  40 , which is used to control the flow of the workpiece  6 , was subjected to high bursting stresses. It was also observed that a region  56  in each of the second segments  20 ,  32  was subjected to a very high tensile stress. The region is near the a fillet at the top region of the second segments  20 ,  32 . The remainder of the second segments  20 ,  32 , for example 95%, were in a safe compressive stress zone. 
         [0039]    The forging process for forming the turbine spacer wheel may also be performed using the shrink ring  51  in place of the stop  40 . In that case, the fillet region of the second segments  20 ,  32  were subject to less tensile stress than in the forging process using only the stop  40 . As the remainder, e.g., 95% of the die regions remain in a state of compressive stress, the life of the dies is improved. 
         [0040]    The closed die forging processes described above have been developed to permit the load requirements to be within the existing press requirement of 6 kton. The closed die forging processes described herein thus may be used with existing presses, which may have a capacity of 7 kton. 
         [0041]    As there are currently no available closed die forgers for CrMoV and NiCrMoV compressor and turbine wheels, the use of the closed die forging processes described herein will allow use of existing forgers and provide better material properties and fracture appearance transition temperature (FATT) values. It should be appreciated, however, that other alloys may be used. 
         [0042]    The closed die forging processes described herein also eliminate much of the required subsequent machining after the forging and thus provide a material savings of approximately 30%. 
         [0043]    The use of the stop for restricting the material flow at the exit end of the dies permits the compressor and/or turbine wheels to be manufactured with a very high shaped difficultly factor. In addition, the use of die stress analysis to design and optimize the stop provides for a suitable shrink ring which increases the life of the stop. 
         [0044]    The use of the incremental split dies described herein also permits the use of a modular die design across all of the stages of the compressor and/or turbine wheels. This permits the same setup of dies to be extended across all of the stages without the need for providing a new die set for each stage. This permits the same basic modular die set to be used for all of the stages and frames of the compressor and turbine wheels. 
         [0045]    Closed die forgings are carried out both in open air and under protective atmosphere. The closed die forging processes described herein permits the forging and heat treatment processes to be performed in air due to the die and/or workpiece geometry. This permits the use of less expensive die materials. 
         [0046]    The preform shapes at the intermediate stages are also chosen such that the flow of the material of the workpiece  6  is primarily in one direction. The advantages of an open die configuration are thus available within the closed die described herein. This allows a lowering of the press requirements for use of a closed die. 
         [0047]    The strain rates may also be chosen such that cooling of the workpiece is minimal. The strain rates may also be chosen so as not to increase the press requirements. 
         [0048]    The geometry of the starting workpiece for the compressor and turbine wheels does not cover the entire plan view area of the dies. This permits the advantages of open die geometry to be obtained using a closed die. The closed die forging processes described herein may thus be thought of as a form of hybrid forging. 
         [0049]    The geometry of the starting workpiece of the turbine spacer wheel may be a ring-shaped hollow profile. The geometry of the starting workpiece may be obtained by extrusion with a mandrel and container as described herein. The use of the hollow billet for forming the starting workpiece has at least two advantages, including, but not limited to, reducing the input material tonnage and eliminating subsequent machining. The use of the hollow billet to form the starting workpiece also reduces the load requirement during the near net shape forging. 
         [0050]    Although the embodiments have been described in the context of forging compressor and turbine wheels and turbine spacer wheels, it should be appreciated that the process described herein may be used to forge other components, for example steam turbine rotors. 
         [0051]    While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.