Method for making an article portion subject to tensile stress and stress relieved article

A wall first portion of an article is provided with reduced tensile stress, of an amount less than that which can result in cracking of the wall, with an elongated opening through the wall first portion. The opening separates areas of a wall first portion and extends generally toward a wall second portion under a lower or different stress.

BACKGROUND OF THE INVENTION
 This invention relates to an article including a member that, during
 operation, includes a portion subject to tensile stress of an amount that
 can result in cracking of the member. More particularly, the invention
 relates to a turbine engine component including a wall having a portion
 subject to low cycle fatigue (LCF) cracking from tensile stress during
 operation.
 Gas turbine engines generally include nozzles, for example turbine nozzles,
 including a plurality of spaced-apart stationary airfoil shaped vanes
 supported between generally circumferentially disposed band type members,
 sometimes referred to as shrouds or shroud segments. One example of such a
 structure is described in U.S. Pat. No. 5,343,694--Toborg et al. (patented
 Sep. 6, 1994). An inner surface of such a band or shroud segment is
 exposed to the flow of fluid under pressure, for example pressurized gas
 including air and products of combustion. Efficiency of the engine is
 based, in part, on the avoidance of leakage of such pressurized fluid out
 of the fluid stream.
 In some gas turbine engine designs, it has been observed that certain
 portions of such a band, particularly on the flowpath side of a turbine
 nozzle vane or vane segment, during operation are under a relatively high
 amount of tensile type of stress. During cyclic operation of the engine,
 such tensile stress results from a cyclic type of bending of the band,
 sometimes referred to as chording. For example, such high tensile stress
 portions have been observed in the band in the vicinity of vane leading
 edges, trailing edges, or both. The relatively high amount of tensile
 stress can be sufficiently high to result, during operation, in the
 formation of a LCF crack in the band. At the same time, other, adjacent
 portions of the same band are in compression or under a tensile stress in
 an amount less than that which would result in LCF cracking. The LCF crack
 in the band extends from the high tensile stress portion of the band
 generally toward, and stops at, the band portion under compressive stress
 or lower tensile stress.
 It has been a practice to repair such cracks by methods including one or a
 combination of welding, diffusion bonding (some forms of which are
 referred to as Activated Diffusion Healing), brazing, etc. However, such
 repair methods can be relatively temporary as well as time consuming and
 expensive. Sometimes attempts to repair such a crack in a component are
 unsuccessful, resulting in scrapping of the component. Preparation of new
 components to avoid such cracking as well as a crack repair of operated
 components, resulting in a restructured article and that extends the
 operating life of an article requiring less frequent repair, is highly
 desirable.
 BRIEF SUMMARY OF THE INVENTION
 The present invention, in one form, provides a method for making a wall of
 an article, the wall including a location in a wall first portion subject
 during operation to tensile stress that can result in cracking of the wall
 first portion, and a wall second portion subject during operation to a
 stress less than that which will result in cracking of the wall second
 portion. The method comprises removing material from the location in the
 wall first portion to provide an elongated opening through the wall first
 portion extending in the wall first portion generally toward the wall
 second portion. In another form, the present invention provides a method
 for repairing a crack in a wall of such an article, comprising removing
 material from the wall first portion about the crack to provide such
 elongated opening through the wall first portion, replacing the crack with
 the opening.
 In still another form, the present invention provides an article comprising
 a wall first portion subject during operation to tensile stress that can
 result in cracking of the wall first portion, and a wall second portion
 subject during operation to a stress less than that which will result in
 cracking of the wall second portion. The wall first portion includes an
 elongated opening there through extending in the wall first portion
 generally toward the wall second portion, the elongated opening reducing
 the tensile stress in the wall first portion.

DETAILED DESCRIPTION OF THE INVENTION
 Examination of gas turbine engine nozzle outer bands, generally of the type
 shown in FIG. 5 and made of high temperature superalloys typically used in
 gas turbine engines, has disclosed the existence of LCF cracks at a
 location in a band first portion under relatively high tensile stress. The
 cracks at such location extended toward and stopped at a band second
 portion under lower tensile stress or under compression. The type of
 engine operation that results in such conditions has been described above.
 The cracks, originating in the band portion under relatively high tension
 and relieving the high tensile stress in that location, extended toward
 but not into the band second portion. According to a form of the present
 invention, repair of a crack in a wall of a member, for example a gas
 turbine engine nozzle outer band, includes providing an opening through
 the wall to replace the crack in a portion subject to cracking under
 tension. In another form, a fluid seal over the opening is disposed on the
 non-flowpath surface of the band to avoid fluid leakage from the fluid
 flowpath through the opening. In another form of the method of the present
 invention, such a wall is prepared, prior to cracking during use, by
 providing such an opening.
 The present invention will be more fully understood by reference to the
 drawings. In the diagrammatic sectional view of FIG. 1, a member or wall,
 shown generally at 10, represents a wall of an article or a replacement
 article substantially the same as the article. For example, the wall has
 been made of a high temperature superalloy band or band segment of a gas
 turbine engine nozzle. The wall includes a wall first portion 12 including
 a location generally at 13, subject during operation to tensile stress in
 an amount that can result in cracking of wall first portion 12, for
 example LCF failure or cracking. Wall 10 includes a wall second portion 14
 that is not subject during operation to a stress that will result in
 cracking of wall second portion 14. For example, such lower stress can be
 a smaller tensile stress less than that which would result in LCF
 cracking. In some article designs, the second portion is under compression
 during operation. Evaluation of operated article or member 10 after
 operation has disclosed a crack 16 at location 13 in wall first portion
 12, crack 16 resulting from excessive tensile stress in wall first portion
 12.
 According to a form of the present invention shown in FIG. 2, an elongated
 opening 18, for example a slot, is provided through wall 10 at location
 13, about crack 16. Opening 18 replaces crack 16 with elongated opening
 18, separating areas of the wall first portion 12 and selected to provide
 relief of tensile stress developed in first wall portion 12 during
 operation. Elongated opening 18 at location 13 extends through wall 10
 from wall first portion 12 toward wall second portion 14 which, during
 operation is not under a stress which will result in the formation of a
 crack, for example a LCF failure crack. In one preferred form of the
 invention, elongated opening 18 extends through wall 10 at least up to
 second wall portion 14, for example which is under compression.
 Elongated opening 18 can be generated through wall 10 by a variety of known
 material removal methods including one or a combination of
 electrodischarge machining, mechanical drilling or machining, laser
 cutting, electrochemical machining, grinding, etc. In one embodiment, it
 is preferred to remove any potential stress concentrations, frequently
 called stress risers, at an opening end 20 of elongated opening 18 by
 relieving any relatively sharp corners. For example, such relief can be
 provided by drilling a hole at end 20 as shown in FIG. 2.
 In FIG. 3, the member of FIG. 2 represents a wall of a gas turbine engine
 component, for example the outer band of a turbine nozzle, having a
 flowpath surface 22 and a non-flowpath surface 24. In some examples,
 elongated opening 18 is sufficiently large to allow excessive leakage of
 fluid, such as gas from the flowpath surface or cooling air from the
 non-flowpath surface, or both, between flowpath surface 22 and
 non-flowpath surface 24. Then, a fluid seal shown diagrammatically
 generally at 26 in FIG. 3 is provided in one form of the present
 invention. In the embodiment of FIG. 3, fluid seal 26 includes
 substantially rigid spaced-apart seal retainers 28, secured with
 non-flowpath surface 24. Held between seal retainers 28 is a seal 30,
 shown to be in the form of a flexible or spring-like member to accommodate
 flexing of the wall during operation. For example, such a flexible seal
 member has been made of a high temperature Ni base alloy commercially
 available as IN 718 alloy.
 The diagrammatic fragmentary sectional view of FIG. 4 shows a form of fluid
 seal 26 in the shape of a C-channel 32 secured with surface 24 of wall 10.
 Seal retainers 28 in FIG. 3 and C-channel 32 in FIG. 4 can be secured with
 surface 24 by a variety of convenient bonding means capable of
 withstanding the conditions of operation. For operation in a gas turbine
 engine, bonding includes fusion welding, tack welding, brazing, etc.
 The fragmentary sectional view of FIG. 5 shows wall 10 as one embodiment of
 a gas turbine engine turbine nozzle outer band segment, made of a high
 temperature Ni base superalloy commercially available as Rene' 80 alloy.
 Wall or band 10 assists other nozzle portions in supporting a plurality of
 nozzle vanes, a portion of one of which is shown at 34. Vane 34 includes a
 leading edge 36 and a trailing edge 38, within a gas flowpath represented
 by arrow 40. An elongated opening 18 is provided at the above identified
 location 13 in wall first portion 12 in which, during engine operation,
 tensile stresses were generated at a level sufficient to result in
 cracking, for example crack 16 in FIG. 1, of wall first portion 12.
 Opening 18 extends through the band toward wall second portion 14 which,
 in this design during engine operation, is in compression and resists the
 formation and the propagation of cracks, for example of the type shown in
 FIG. 1. Frequently, a crack will extend through edge 42 of wall 10. In
 that example, opening 18 extends through edge 42, constituting a
 separation in wall first portion 12. To avoid leakage of gas from flowpath
 40 through opening 18, flexible gas seal 26 generally of the type
 described in connection with FIG. 3, is secured over opening 18 on
 non-flowpath surface 24.
 One form of the present invention provides a replacement article,
 represented by wall 10 in the drawings, substantially the same as an
 operated article in which it has been observed that a crack developed in a
 wall during operation. Elongated opening 18 is provided in the replacement
 article 10 at location 13 of crack 16 in the operated article to avoid
 cracking in the replacement article at location 13 during operation of the
 replacement article. Such a location 13 is identified by evaluation of the
 operated article after operation.
 The present invention, in its various forms as methods and articles, has
 been described in connection with specific examples, embodiments and
 materials. However, it should be understood that they are intended to be
 representative of, rather than in any way limiting on, its scope. Those
 skilled in the various arts involved will understand that the invention is
 capable of variations and modifications without departing from the scope
 of the appended claims.