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United States Patent [191 [111 4,354,371 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 ’ s h 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
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