Abstract:
A method and device for thermal creep-sizing an annular-shaped structure. The creep-sizing device includes a ring member with through-holes present between the inner and outer diametrical boundaries of the ring member, and with through-slots alternatingly extending from each through-hole to either the inner or outer diametrical boundary. In use, the ring member is placed within the annular-shaped structure, and pins are installed in the through-holes in the ring member to cause the outer diametrical boundary of the ring member to diametrically expand. The structure and creep-sizing device are then heated so that the mechanically expanded ring member causes the structure to undergo thermal creep-sizing.

Description:
FIELD OF THE INVENTION 
     The present invention relates to processes and apparatuses for creep-sizing annular-shaped structures. More particularly, this invention relates to a thermal creep-sizing process that employs a mandrel configured to be installed and then expanded within a hoop structure, so that a subsequent thermal treatment diametrically expands the hoop structure as a result of thermal creep. 
     BACKGROUND OF THE INVENTION 
     Hoop structures and various other components with annular-shaped sections at times must be diametrically expanded to attain or restore desired diametrical conditions, such as during the manufacturing or reconditioning of shrouds and nozzle supports of gas turbine engines. For relatively ductile materials, sizing can be accomplished by hydraulic expansion methods while the component is at or near room temperature (“cold sizing”). However, a component can be susceptible to tensile fractures during cold-sizing if formed from certain materials, including superalloys commonly employed in gas turbine engines. For these materials, sizing must be performed at an elevated temperature. One such method is generally referred to as hot creep sizing, and involves a high mass fixture with a coefficient of thermal expansion (α) that is relatively constant for the temperatures used and equal to or higher than the structure being sized. A difficulty with hot creep sizing is the requirement for slow and tightly controlled heating and cooling rates in order to match the growth of the fixture with the component being sized, which slows processing throughput. Another thermal creep-sizing method is known as warm sizing, and involves expanding a preheated component on a mandrel that is maintained at a lower temperature throughout the sizing operation. With many materials including superalloys, the component must be heated to very high temperatures, e.g., 1800° F. (about 980° C.) or more, which may pose a hazard to the operator. 
     In view of the above, it would be desirable if an improved method were available for sizing a hoop structure that avoided the disadvantages of prior art methods. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a method and device for thermal creep-sizing an annular-shaped structure, including hoop structures and components with annular sections. The invention provides a method by which controlled sizing of an annular-shaped structure is achieved with a creep-sizing device that is mechanically expanded in a manner that provides controlled and accurate sizing of the structure during a thermal treatment. 
     According to this invention, the creep-sizing device includes a ring member having inner and outer diametrical boundaries relative to the axis of the ring member. Through-holes are present in the ring member between the inner and outer diametrical boundaries, with through-slots extending from each through-hole through the ring member to either the inner or outer diametrical boundary. Finally, pins are provided that are sized to be received in the through-holes. Each pin has a diameter approximately equal to one of the through-holes when the ring member is in a free-state, i.e., not deformed by any force applied externally to the ring member. 
     The method of this invention made possible by the above-described creep-sizing device generally entails placing the ring member (without pins) within an annular-shaped structure so that the outer diametrical boundary of the ring member is adjacent an inner surface of the annular-shaped structure. The pins are then inserted into the through-holes in the ring member, which causes the outer diametrical boundary of the ring member to diametrically expand. The annular-shaped structure and the device of this invention can then be heated so that the mechanically expanded ring member causes the structure to undergo thermal creep-sizing at a temperature at which the material of the structure is ductile and therefore less likely to fracture. 
     In view of the above, it can be seen that a significant advantage of this invention is that it entails sizing an annular-shaped structure at an elevated temperature, thereby significantly reducing the risk of tensile fractures as compared to cold sizing methods. Furthermore, the ring member employed by this invention can be of relatively low mass, and its desired sizing effect is mechanically induced instead of relying on a high coefficient of thermal expansion. As a result, the ring member can be heated relatively rapidly without stringent control of the heating and cooling rates as compared to hot creep-sizing methods. Finally, and in contrast to warm creep-sizing methods, the thermal creep-sizing method of this invention can be performed without any requirement to handle the creep-sizing device or the structure being sized while at an elevated temperature. 
     Other objects and advantages of this invention will be better appreciated from the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a fragmentary plan view of a mandrel for a creep-sizing device in accordance with a preferred embodiment of this invention. 
     FIG. 2 represents a cross-sectional view of the mandrel of FIG. 1 with a pin installed in a through-hole within the mandrel. 
     FIG. 3 is a fragmentary cross-sectional view of the mandrel of FIGS. 1 and 2 installed within a nozzle support of a gas turbine engine for expanding the nozzle support in accordance with a thermal creep-sizing method of this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides a method for sizing hoop structures and various other components with annular-shaped structures, by which the structures are diametrically expanded to attain or restore desired diametrical conditions. While the invention will be described in reference to the manufacturing or reconditioning of a nozzle support of a gas turbine engine, those skilled in the art will appreciate that the invention is applicable to various other applications and components. 
     FIG. 3 represents a section of a gas turbine engine nozzle support  10  undergoing thermal creep-sizing in accordance with the present invention. The diameter of the nozzle support  10  is critical for its function, and therefore must be precisely sized when manufactured, as well as during reconditioning if returned from service. According to this invention, sizing of the nozzle support  10  is performed with an annular-shaped mandrel  12  having through-holes  14  in which pins  16  are installed, one of which is shown in FIG.  3 . The mandrel  12  is shown as being installed entirely within the annular-shaped section of the nozzle support  10 , with a shoulder  18  at an outer perimeter  22  of the mandrel  12  shown engaging a flange  24  on the interior wall  26  of the nozzle support  10 . 
     FIGS. 2 and 3 show sections of the mandrel  12  in greater detail. The inner and outer perimeters  20  and  22  of the mandrel  12  are preferably circular and concentric, so that the mandrel  12  has a uniform section. In FIG. 2, the through-holes  14  can be seen as being equally spaced in the mandrel  12  midway between the inner and outer perimeters  20  and  22  of the mandrel  12 . While spacing of the holes  14  may vary according to the application, a suitable spacing is roughly every five degrees over the entire circumference of the mandrel  12 . The holes  14  are shown as being aligned parallel to the axis of the mandrel  12 . The diameter of each hole  14  is depicted as being about one-half of the radial width of the mandrel  12 , and countersunk to facilitate insertion of the pins  16 . While the mandrel  12  is in a free state, i.e., undeformed by any externally-applied force, the holes  14  and pins  16  preferably have roughly the same diameter for a reason that will be explained in the discussion below. 
     A particularly important feature of the invention is the presence of slots  28  and  30  that intersect the holes  14 . The slots  28  extend radially through the mandrel  12  to either the inner or outer perimeter  20  or  22 , in an alternating manner as shown in FIG.  1 . Similarly, the slots  30  alternatingly extend toward either the inner or outer perimeter  20  or  22  of the mandrel  12 , in the opposite direction of its associated slot  28 . However, the slots  30  do not penetrate completely through the mandrel  12  in the radial direction. Instead, each slot  30  is shown as extending slightly more than halfway through that portion of the mandrel  12  between its corresponding hole  14  and perimeter  20  or  22 . As a result, the mandrel  12  is made up of arcuate sectors  32  held together by bridges  34  that enable the mandrel  12  to flex and diametrically contract and expand relative to its free state prior to the pins  16  being installed. However, installation of the pins  16  causes the mandrel  12  to rigidly and precisely assume an annular shape with a predetermined outer diameter. 
     The particular dimensions of the mandrel  12  can be varied to adapt the invention to a wide variety of uses. For sizing the superalloy nozzle support  10  represented in FIG. 1, the mandrel  12  and pins  16  are preferably formed of a wrought corrosion and heat resistant alloy, such as Inconel  718 , in order to withstand the high temperatures necessary to size the support  10 . To further illustrate the invention, a nozzle support of the type shown in FIG.  3  and formed of Inconel  718  might have an undersized diameter of about 75.5 cm that must be expanded to a diameter of about 75.687 cm. Such a nozzle support can be sized with a mandrel  12  having a “fixtured” outer diameter of about 75.687 to about 75.692 cm with the pins  16  installed (and having a smaller “free-state” diameter before the pins  16  are installed). A suitable radial width (the distance between the inner and outer perimeters  20  and  22 ) for such a mandrel  12  is about 2.54 cm, and a suitable thickness (transverse to the radial width) is about 1.3 cm. In addition, suitable diameters for the through-holes  14  and pins  16  are about 11.10 mm if the holes  14  are placed midway between the inner and outer perimeters  20  and  22  and spaced about five degrees over the entire circumference of the mandrel  12 . The slots  28  and  30  preferably have widths of about 3 mm. The slots  30  preferably form bridges  34  that have a radial dimension of about 2.5 mm. 
     In use, the mandrel  12  (without the pins  16 ) is preferably coated with a suitable high-temperature anti-seize compound, and then positioned within the nozzle support  10  (or another annular-shaped structure to be sized) so that the shoulder  18  of the mandrel  12  engages the flange  24  on the inner wall  26  of the nozzle  10 . Without the pins  16  in place, the mandrel  12  is able to be diametrically collapsed about 10%, more or less, and therefore can be accommodated within a cross-sectional area whose diameter is less than the free-state diameter of the mandrel  12 . Consequently, the mandrel  12  can be accommodated within the nozzle support  10  even if the diameter of the support  10  is undersized by about 10%. Once the mandrel  12  is installed, pins  16  (also preferably coated with an anti-seize compound) are preferably installed in every other hole  14 , and then tapped into place until the shoulder of each pin  16  contacts the mandrel  12 . The remaining pins  16  are then installed in the remaining holes  14  in the same manner. As the pins  16  are inserted, the mandrel  12  tries to expand to its “fixtured” diameter as defined above, but is constrained to some degree by the nozzle support  10 . If necessary, additional mandrels may be installed at different axial locations within the support  10  using the same processed outlined above to simultaneously resize other diameters of the nozzle support  10 . The support and mandrel assembly is then processed through a conventional heat treat cycle, e.g., about 0.5 to about 1.5 hours at about 1800° F. (about 980° C.) or more, during which time the nozzle  10  becomes sufficiently ductile to be diametrically expanded without sustaining physical or microstructural damage. The heating and cooling rates are not as critical to the sizing operation of this invention as compared to prior art techniques. For example, a temperature difference of up to 100° F. (about 55° C.) between the mandrel  12  and nozzle support  10  is acceptable during heating and cooling to achieve the tight dimensional tolerance described above for the nozzle support  10 . After cooling, the mandrel  12  can be disassembled from the resized nozzle support  10  by removing the pins  16 . 
     While the invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art. For example, the physical configuration of the mandrel  12  and pins  16  could differ from that shown. Therefore, the scope of the invention is to be limited only by the following claims.