UnitedO States Patent [191 [111 4,354,371 Johnson [45] Oct. 19, 1982

[54] METHOD OF PRESTRESSING THE 4,214,923 7/1980 Price ...... 29/421 R WORKING SURFACES 0F PRESSURE 4,230,426 10/ I980 Avery ...... 72/53 ERS 0R ERS Primary Examiner-Gene Crosby [75] lnventol‘l Daniel E- 30111180111 Dallas, TBX- Attorney, Agent, or Firm—Stephen A. Roen; Arthur L. [73] Assignee: Metal Improvement Company, Inc., Fredenck - Paramus, NJ. [57] ABsTRAcr [21] Appl‘ No" 200,767 A method of prestressing the working surface of pres 2 Filed; Oct, 27 1980 sure chambers or cylinders comprising the steps of ?rst [2 1 ’ sh o t peemng' th eir' wor king sur races t o rorm compres [gé] clci3 ...... "hr/1325161115 sive residual stresses’ and then applying ?uid pressure at [ 1 .' ' ' """""""""""""""""" " ’ a predetermined magnitude to cause tensile stresses in [58] Field of Search ...... 72/53, 40, 61; 148/4, the working surfaces The magnitude of compressive 148/141; 29/421; 51/319 residual stresses established on their outer working sur [55] References Cited faces 'is made substantially equal to the tensile stress U_s_ PATENT DOCUMENTS applied to these surfaces during the pressure phase of 454 592 6/1891 Jackson 72/61 the process‘ 3,4ss1073 12/1969 Burney .. 4,034,585 7/1977 Straub ...... 72/53 6 Claims, 2 Drawing Figures

4,354,371 1 2 the plastic from the elastic zone in said substrate. This .METHOD OF PRESTRESSING THE wouxnwc tension component, unless neutralized, can produce SURFACES or PRESSURE CHAMBERS on microscopic fractures in the internal surface of the CYLINDERS chamber wherein defects exist therein. These cracks act as nucleation sites for failure in a corrosion-fatigue envi BACKGROUND OF THE INVENTION ronment such as may exist in the fluid chambers of This invention relates to a method of increasing the positive displacement pumps. fatigue life and preventing stress corrosion of metal Shot peening is a process in which compressive parts and, more particularly, to a method of increasing stresses are induced in the upper layer or exposed sur the fatigue life‘and preventing stress corrosion of inner, face layers of metallic parts by the impingement of a working surfaces of pressure chambers or cylinders. stream of metal or glass shot, directed at the metal sur Still more speci?cally, the present invention has par face at high velocity and under controlled conditions, ticular application to the ?uid end portions or ?uid whereby increased fatigue strength and resistance to chambers of positive displacement pumps, such as well stress corrosion is imparted to the workpiece. When service or mud pumps, which are used extensively in shot peening is performed properly, there is established the petroleum industry in connection with exploration a predictable amount of residual compressive stress at and drilling for petroleum. In such applications, the the peened upper layer or exposed surface layers of the corrosion-fatigue environment of the ?uid chambers of the positive displacement pumps is of particular con metallic part and which extends for a relatively shallow cern especially when the operating pressures of the depth therein; residual tensile forces are established in pump causes internal stress which approach or may the region below the upper layer in the body of the , exceed the yield strength of the materials used in their substrate which upper layer is in compressive stress. manufacture. . It is, therefore, an object of this invention to provide Shot peening and autofrettage are two recognized a method of increasing the fatigue life and preventing methods of achieving bene?cial residual compressive 25 stress corrosion of inner, working surfaces of pressure stresses on the surface of metal components. The auto , chambers or cylinders. frettage process or method is to be understood to mean Another object of the present invention is to provide for purposes of this application the process of increasing an improved method which utilizes autofrettage but the strength of the walls forming a closed or open which substantially obviates the formation of fractures chamber by the application of ?uid pressure in the 30 during the autofrettage portion of the process. chamber of a magnitude suf?cient to deform the walls A further object of the present invention is to provide beyond‘the yield strength but insufficient to cause frac a method which utilizes autofrettage but substantially, ture and thereafter relieving such ?uid pressure. The effectively neutralizes the deleterious effect of the ap shot peening and the autofrettage methods differ in the plied tensile component on the upper layer of the metal method of application and depth of residual compres lic part during the pressure phase of the autofrettage sive stress. With regard to the depth of residual com process. ‘ pressive stresses achieved on the surface of a wall, shot A still further object of the present invention is to peening establishes a relatively shallow residual com provide an improved method which includes the pro pressive stress, on the outermost surface of the wall of a cess of autofrettage but which lessens, if not eliminates, predetermined depth, whereas autofrettage achieves a the propensity of material subjected thereto to crack residual compressive stress of considerable depth which due to the pressures required to perform said autofret» extends into the substrate approximately an order of tage process thereon. magnitude greater than the depth achieved by shot peening. Furthermore, in the case of autofrettage, such SUMMARY OF THE INVENTION method is limited to a speci?c component con?guration 45 Accordingly, the present invention contemplates a, and only to the inner working surfaces of thick walled method of prestressing the inner working surfaces of pressure chambers or cylinders. This limitation is due to pressure chambers or cylinders comprising the steps of the nature of autofrettage which requires that areas of ?rst shot peening these working surfaces and then auto high stress concentrations on the inner working surfaces frettaging these working surfaces. In a more narrow are initially subject to an applied internal pressure suffi 50 ciently great so that they are stressed by expansion aspect of this invention, these working surfaces are ?rst beyond their yield point but below that required for peened to achieve a desired predetermined magnitude fracture. The other working surfaces, those that were of compressive residual stress on their inner working absent areas of high stress concentrations, are also surfaces and during the next step, autofrettage, these stressed and deformed, but not permanently since the working surfaces are subjected to a predetermined mag loading, due to such applied autofrettage pressure, is nitude of tensile stress on their inner working surfaces, below the point of loading at which the metal ceases to the magnitude of the compressive residual stress being deform in direct proportion to stress. Then the applied substantially equal to the magnitude of this tensile pressure is released resulting in the formation of a resid ' stress. ual compressive stress on the areas of high stress con 60 In a still narrower scope of this invention, these centrations on the inner working surfaces. While such working surfaces are ?rst peened to achieve a desired component part is subject to such a high internal‘pres predetermined magnitude of compressive residual stress sure there is at least a circumferential tension compo on the upper layer of their inner working surfaces and nent and a radial compression component at any radius, during the next step, autofrettage, and during its pres the circumferential tension generally diminishing in the surizing phase these working surfaces are subject to a body of the substrate material from the inner surface of predetermined magnitude of tensile stress, the compres the wall of the chamber to the exterior surface of the sive residual stress on the outer layers effectively, sub part and is at a maximum therein at a radius separating stantially, neutralizing said tensile stress. 4,354,371 3 4 BRIEF DESCRIPTION OF THE DRAWING plied autofrettage pressure is below the point of loading at which the metal ceases to deform in direct proportion The invention will be more fully understood from the to stress. After deformation occurs in tension under the following description when considered in connection applied autofrettage pressure, these other working sur with the accompanying drawings in which: faces are normally in tension; that is, they are at a prede FIG. 1 is a cross sectional and somewhat schematic termined magnitude of tensile stress at their outer layer view of a vane type pump housing being subject to shot and the layer beneath it, referred to as the “inner layer” peening in accordance with the method of this inven which inner layer is approximately an order of magni tion and . tude greater in thickness than the outer layer, which FIG. 2 is a view similar to FIG. 1 showing the same stress decreases as a function of the distance away from vane type pump housing being subjected to the autofret the outer surfaces but which is relatively constant tage process in accordance with the method of this within the outer layer, all in the absence of the previous invention. shot-peening process. Since the previous shot-peening DESCRIPTION OF THE PREFERRED step established a compressive residual stress of a prede 15 EMBODIMENT termined magnitude on only these outer layers of the same inner working surfaces now subject to the applied The method, according to this invention, of prestress ing the inner working surfaces of pressure chambers or autofrettage pressure and the resulting applied tensile cylinders, and in particular the ?uid end portions of stress thereon, it is believed such tensile stresses that would have been established on these outer layers of the well service or mud pumps, which are used extensively 20 in the petroleum industry, to signi?cantly increase their working surfaces during this phase of autofrettage will useful life in ‘fatigue and corrosion-fatigue is as herein be effectively, substantially, neutralized as they will be set forth. negated or at least minimized if the magnitude of the Initially, the inner working surfaces of the ?uid end compressive residual stress established during the previ portions of such a pump are conventionally shot peened ous shot-opening step is made substantially equal to or to achieve a desired predetermined magnitude of com slightly less than the applied tensile stress of antofret pressive residual stress and the desired predetermined tage. However, the inner layers which were not under depth of compressive residual stress. Shot peening is residual compressive stresses of shot peening, because well known in the art and in this regard reference is they were below the depth at which such stress could made to, for example, the sixth edition of “Shot Peening be established, will be fully in tension. After local yield Applications” published by Metal Improvement Com ing occurs in the material where there are areas of high pany, the assignee, in 1980, and the method disclosed in stress concentrations, then the applied pressure is re US. Pat. No. 3,485,073 to Burney, dated Dec. 23, 1969, leased. This results in the creation of a residual com the relevant portions of which are incorporated herein pressive stress of considerable depth on the high stress by reference. FIG. 6, in the aforementioned publication 35 concentration areas of the inner working surfaces of the illustrates qualitatively the distribution of stress in a chambers and also a residual compressive stress on the member which has been shot-peened on its outer sur other working surfaces but only on their outer layer. On face which member is not under any external load. FIG. 2 is shown how the autofrettage process can be Since the member is in equilibrium with no external practiced on a vane type pump housing and wherein forces, the area under the stress distribution curve in the 40 sealing of the working chamber is accomplished by regions of compressive stress must be equal to the corre plugs secured in the inlet and outlet ports of the pump sponding area under the curve in the region of tensile housing. stress. Furthermore, the peened surface or working Accordingly, it is believed that the present invention surface of the member has its maximum, predetermined, provides a method which utilizes autofrettage but compressive residual stress, located at or near its outer 45 which neutralizes the deleterious effect of the applied surface, and this magnitude of stress and stresses of tensile component during the pressure phase of the comparable intensity extend a predetermined depth in autofrettage process on the outer layers of the other the body of the substrate material before the magnitude working surfaces, those surfaces which did not have of the stress signi?cantly decreases. This predetermined areas of high stress concentration. depth wherein such a gradient of residual compressive Although but one embodiment of the invention has stress is established extends within what is referred to as been described in detail, it is to be expressly understood the “outer layer” of said working surface and its bound that the invention is not limited thereto. Various aries are coextensive therewith; that is the thickness of changes may be made in the method steps without de the outer layer and the predetermined depth are equal. parting from the scope and spirit of the invention as the After this shot peening process is performed on the 55 same will now be understood by those skilled in the art. inner working surfaces of the ?uid end chambers, they What is claimed is: are then subjected to the autofrettage process. This l. A method of prestressing the inner working sur process initially requires subjecting the inner working faces of the walls forming a pressure chamber compris surfaces of the chambers to a pressure suf?ci‘ently great ing the steps of: so that only areas of high stress concentration on the 60 (a) shot peening the inner working surfaces of said working surfaces are stressed by expansion beyond their walls of said pressure chamber; material yield strength but at an internal pressure which (b) sealing said pressure chamber and applying a ?uid is insufficient to fracture or break any of these highly pressure in the chamber of a magnitude suf?cient stressed areas particularly those that exist at the bore to deform the said walls de?ning said chamber intersections of the ?uid end chambers. The other 65 beyond the yield strength but not suf?cient to working surfaces, those that were absent areas of high cause fracture of said walls, and stress concentrations, are also stressed and deformed, (c) releasing such applied ?uid pressure and unsealing but not permanently since the loading due to such ap said pressure chamber. 4,354,371 5 6 2. The method of claim 1 wherein the outer layers of pressure chamber a predetermined magnitude of tensile said inner working surfaces of the pressure chamber is stress of a value substantially equal to the magnitude of shot peened to a desired predetermined intensity. the compressive residual stress. 3. The method of claim 1 the outer layers of said inner 5. The method of claim 4 wherein said tensilestress is working surfaces of siad pressure chamber are shot circumferential. peened to achieve a desired predetermined magnitude and predetermined depth of compressive residual stress. 6. The method of claim 1 wherein said shot peening 4. The method of claim 1 wherein said outer layers of intensity and said ?uid pressure are selected such that said inner working surfaces are subjected to said ‘shot the tensile stress in the outer layer of working the sur peening of an intensity to provide a compressive resid faces of the walls of said pressure chamber is substan ual stress in said outer layers, and. wherein the outer tially neutralized by the tensile stress on the outer layers layers of said inner working surfaces of said pressure of the working surfaces of the walls achieved by the chamber are subjected to a ?uid pressure of a predeter ?uid pressure. mined magnitude to apply in the walls de?ning the t t * t i 15

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