US3282743A

US3282743A - Process for relieving residual stresses in metals

Info

Publication number: US3282743A
Application number: US363058A
Authority: US
Inventors: Arnold H Holtzman
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1964-04-22
Filing date: 1964-04-22
Publication date: 1966-11-01
Anticipated expiration: 1983-11-01

Description

Nov. 1, 1966 A. H. HOLTZMAN 3,

PROCESS FOR RELIEVING RESIDUAL STRESSES IN METALS Filed April 22, 1964 2 She ets-Sheet 1 FIGl Nov. 1, 1966 A. H. HOLTZMAN 3,282,743

PROCESS FOR RELIEVING RESIDUAL STRESSES IN METALS Filed April 22, 1964 2 Sheets-Sheet 2 INVENTOR ARNOLD H. HOLTZMAN ATTORNEY United States Patent 3,282,743 PROCESS FOR RELIEVING RESIDUAL STRESSES IN METALS Arnold H. Holtzman, Cherry Hill Township, Gloucester County, N.J., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed Apr. 22, 1964, Ser. No. 363,058 6 Claims. (Cl. 1484) This application is a continuation-in-part of my copending application Serial No. 171,184, filed February 5, 1962, now abandoned.

The present invention relates to a process for relieving residual stresses in metals.

Residual stresses, or internal stresses, are produced in metals by any operation which brings about a nonuniform change in shape or volume throughout a Work-piece. Such a change can be effected by heat treating, quenching, Welding, casting, forming, machining, grinding, plating, and other operations that cause local plastic flow. The stresses may develop directly by expansion or contraction, and may also result from changes in volume and coefiicient of expansion that accompany metallurgical phase transformations.

Residual stresses often have a deleterious effect on the mechanical properties of a workpiece leading to service failure and, moreover, are frequently the cause for the occurrence of undesirable dimensional changes in the piece upon machining. Such stresses can cause metals to crack quickly when exposed to certain atmospheres, to liquids (stress-corrosion cracking), or to liquid metals; or when heated (fire-cracking), aged (stress-precipitation cracking), or cut into. Residual stresses can also cause metals to warp when machined or cut with resultant secondary efiects such as binding and breaking of reamers. Obviously, since residual stresses may affect safety or utility, it is important that the magnitude of these stresses be reduced, i.e., that the metal be stress-relieved. This reduction of stress level ordinarily is accomplished thermally or mechanically, or by combined mechanical and thermal means. In thermal stress-relieving, the temperature of a metal object is increased to a suitable value and that temperature maintained for a suitable time. Since most high residual stresses are produced by thermal gradients caused by too rapid or uneven cooling, with attendant plastic flow, it is apparent that such stresses can be lowered or removed if the structure is properly reheated and then cooled slowly and evenly. However, because of the conditions re quired for thermal stress relief, the procedure cannot be used conveniently or effectively with all types of materials and with pieces of any size and shape. For example, in the thermal stress-relieving of large parts, the need for large furnaces and high heat inputs is an economic disadvantage, as is the complex system of temperature control which is necessary to insure that the thermal input is distributed evenly over the receiving surfaces. The heating of large parts in sections or progressively by passage of the piece through a furnace requires careful control to avoid insuflicient overlapping of successive steps or an excessive rate of movement, and to protect the section adjacent to the heated zone from serious temperature gradients.

In addition to the difiiculties encountered in thermally stress-relieving large parts, this method has limitations also with respect to resulting adverse effects on mechanical properties in certain metals since the thermal cycles used sometimes produce undesirable changes, either microstructural or of other kinds. The adverse effect of thermal stressrelief procedures on mechanical properties is especially pronounced in the case of the precipitation harden- 3,282,743 Patented Nov. 1, 1966 "ice ing alloys, e.g., certain alloys of aluminum, copper, magnesium, or nickel, and the steels. For example, certain aluminum alloys are precipitation hardened, i.e., age-hardened, by heating at about 300 F. after heat treatment, to produce a definite microstructure and definite mechanical properties. Temperatures of the order of 450 to 500 F. are required for producing substantial relief of residual stresses in these alloys in a reasonable length of time, but these temperatures destroy the effects of the previous aging treatment.

As an alternative to thermal stress relief, mechanical methods employing moderate temperatures have been used to reduce residual stresses. Mechanical methods, e.g., hand peening, shot peening, pressing, rolling, and stretching, usually are applied to machines or structures which either are too large to be placed in a furnace, or of such a shape that thermal stress relief would cause excessive distortion. Although mechanical methods involve less expensive equipment, they are seldom as effective as thermal methods in that they ordinarily produce only superficial stress relief, and also they generally require extensive deformation of the piece. Cold compression treatment, for example, results in considerable compressive deformation and, when applied to complex shapes, requires an investment in dies.

The use of hydrostatic pressure to reduce stresses in metal objects also has been proposed but the procedure is cumbersome and particularly diflicult when large parts are to be treated, requiring large containers of high wallstrength to resist the hydrostatic pressures used. More importantly, the gradually applied pressure is incapable of effecting a deep permanent stress relief.

Since the relieving of residual stresses in metals is of vital importance in the metallurgical art owing to the danger involved in the use of metal structures containing such stresses, a method which effectively relieves stress and, at the same time, does not possess the disadvantageous features of the heretofore-known methods is extremely desirable and would advance the art considerably.

It now has been found that residual stresses in metals can be relieved by subjecting the metal to the action of an explosively induced low-intensity transient pressure wave, for example, a shock wave.

In accordance with the process of the present invention, a metal object to be stress relieved is positioned in spaced relationship to a detonating explosive; a shock wave transfer medium is placed between the explosive and the metal object to provide a buffer zone therebetween; and the explosive is detonated, the pressure wave resulting from detonation of the explosive and traversing the metal object, or at least that portion of the object to be stress relieved, having an intensity which exceeds, but is less than five times, the Hugoniot elastic limit of the metal object. Preferably, the metal object to be treated is spaced apart from a layer of detonating explosive, preferably conforming to the general contour of the object, and the space therebetween is filled with a liquid, preferably water.

The term shock wave transfer medium is used herein to denote a material or substance which occupies the buffer zone, that is, the space between the explosive and the metal object being treated, which material or substance is capable of attenuating or partially absorbing the pressure wave, e.g., shock Wave, produced by detonation of the explosive and directed toward the metal object to be stress relieved.

The term Hugoniot elastic limit is used herein in the conventional sense to denote the intensity of a pressure wave in a material at the dynamic elastic limit of the material. The elastic limit of a material can be defined as the maximum stress that can be applied to a body of the material after the removal of which the body is able to regain its original dimensions. When the stress is applied by shock wave techniques, the elastic limit becomes the dynamic elastic limit, which is a point at which a discontinuity occurs in the pressure vs. specific volume curve for the materal (commonly known as the Hugoniot curve), this discontinuity indicating that a marked change has occurred in the compressibility of the material. As a consequence, the dynamic elastic limit is referred to more specifically as the Hugoniot elastic limit. This property is described in detail and values are given for a variety of materials in Response of Metals to High Velocity Deformation, edited by P. G. Shewmon and V. F. Zackay, New York, Intcrscience, 1961; pp. 165-203, G. E. Duvall, Some Properties and Applications of Shock Waves; and pp. 249-274, F. S. Minshall, The Dynamic Response of Iron and Iron Alloys to Shock Waves.

In the present process, detonation of the explosive causes a shock wave to enter the shock-wave-transfer medium. Under some conditions, i.e., when the detonation velocity of the explosive is sufliciently high relative to the speed of sound in the metal object to be stress-relieved, the pressure wave traversing the metal object also is a shock wave, i.e., travels faster than sound in the metal. In such cases, the pressure of the shock wave in the metal object under a specific set of conditions, i.e., type and amount of explosive charge, distance between the charge and the metal object, and density of the shockwave-transfer medium, can be determined by measuring the shock wave velocity in the object and the velocity with which the free surface of the object moves when the shock wave reaches that surface (free-surface or particle velocity). A number of techniques are known for measuring shock wave velocity and free-surface velocity, any one of which can be used. Such techniques are described in detail in Modern Very High Pressure Techniques, R. H. Wentorf, Jr., editor, Washington, Butterworths, 1962; chapter 11, by W. E. Deal, Jr.: Dynamic High-Pressure Techniques. These include, for example, optical methods using a smear or streak camera, or a framing camera, and electrical methods such as the pin or condenser method. Multiple-flash X-ray photographs also can be employed. The pressure, P, of the shock wave in the metal object is calculated from the equation:

P P U U when p is the initial density of the metal, and U and U are the shock velocity therein and the particle velocity, respectively. In the above equation, density, shock velocity, and particle velocity in e.g.s. units give the pressure in dynes/cm. Pressure in kilobars is obtained by dividing the pressure in dynes/cm. by

When the sonic velocity of the metal object is high relative to the detonation velocity of the explosive used, the transient pressure disturbance effected in the metal object may not be a shock wave. In such a case, the pressure of the wave in the metal object can be determined by finding the pressure of the shock wave in the shockwavetransfer medium adjacent the metal surface by methods described above, and multiplying this value by 2, the pressure in the metal object being approximately double that in the adjacent medium, which has a lower sonic impedance than the metal object.

In the preferred embodiment of the present process wherein the space between the metal object and the explosive is filled with water, an alternative method can be used to determine the pressure of the wave in the metal. The velocity of the shock wave can be measured at the water-metal interface, e.g., by use of a framing camera (such as the Beckman and Whitley model 189), and the pressure of the shock wave can be found by referring to Underwater Explosives, by Robert H. Cole, Princeton University Press, 1948, page 40, FIGURE 2.3, wherein velocities at a shock front in water are plotted as a function of pressure for high pressures. The pressure of the wave in the metal object is approximately twice the shock pressure in the water at the interface.

The Hugoniot elastic limit of a metal can be determined by a study such as that carried out by Minshall, above cited. The Hugoniot equation of state of the metal is studied by using the pin or capacitor technique (electrical methods) to measure the velocities of elastic and plastic waves in the metal and the velocities with which the free surface moves when these waves reach that surface and are successively reflected from it. The pressure at the Hugoniot elastic limit, p is given by:

wherein p =1/V is the initial density of the metal, U is the velocity of the Hugoniot elastic wave, and u is the particle velocity.

In order to describe the invention more fully, reference is now made to the accompanying drawings, which illustrate suitable embodiments.

FIGURE 1 shows a perspective view of an assembly for stress-relieving a flat metal object by the process of the present invention;

FIGURE 2 depicts an assembly for stress-relieving a cylindrical metal object by the present process;

FIGURE 3 depicts an assembly for stress-relieving a metal object of irregular contour;

FIGURE 4 shows a top view in detail of the metal object used in the assembly of FIGURE 3, said object having both planar and curved surfaces and nonuniform thickness, i.e., having flange and web portions;

FIGURE 5 shows a cross-section of the object shown in FIGURE 4 together with superposed buifer, e.g., water, and explosive layers; and

FIGURE 6 shows a different cross-section of the same object together with superposed buffer, e.g., water, and explosive layers.

In all figures, identical elements are indicated by identical symbols.

In FIGURE 1, the metal object 1 to be treated, in the form of a plate, rests at the bottom of vessel 4. A layer of detonating explosive 2 is positioned above metal plate 1 in such a way that the horizontal surfaces of the explosive layer and the metal plate lie in parallel planes and the vertical surfaces are essentially aligned. The explosive layer 2 is held in its desired position in vessel 4 by any convenient means (not shown), e.g., supported on a thin piece of wood which fits snugly in place in vessel 4. A shock-wave-transfer medium 3, in this case water, is present in vessel 4 to a depth suflicient to fill the space between metal plate 1 and explosive layer 2 completely, forming a buffer layer therebetween. Explosive layer 2 is initiated by line-wave generator 8, which is actuated by any standard initiator e.g., an electric blasting cap 9, the lead wires 7 of which extend to a source of electricity (not shown).

Detonation of the line-wave generator 8 produces a detonation front which arrives simultaneously at a number of points along the adjoining edge of explosive layer 2, producing therein a straight-line detonation traveling through layer 2. Detonation of layer 2 induces a shock Wave in the water layer 3, which traverses the water 3 and causes a transient pressure disturbance in the metal 1 having an intensity greater than, but less than about five times, the Hugoniot elastic limit of the plate metal measured as previously described.

FIGURE 2 illustrates in cross-sectional view an assembly for stress-relieving a metal object 1, which in this case is a solid steel cylinder. The explosive layer 2 and sh0ck-wavetransfer medium 3 in this embodiment completely surround the longitudinal surface of cylindrical object 1 in a substantially uniform relationship to provide essentially consistent relief of internal stress throughout the cylinder. Two line-

wave generators

8 and 8 are attached to explosive layer 2 and again any standard initiator, e.g., electric blasting caps such as shown in FIG- URE 1 may be attached to the line-

wave generators

8 and 8 to initiate them. The buffer layer of a shockwave-transfer medium 3, i.e., water, extends laterally the length of the cylindrical object 1 and is of substantially uniform width throughout. The cylindrical object 1 in FIGURE 2 is seen to be positioned in vessel 4 in such a way that it rests on one of its planar surfaces; however, any suitable holding means could be used to affix the cylinder and the explosive layer in a desired position.

FIGURE 3 illustrates an assembly for stress-relieving a metal object 1 of irregular contour. Water 3 is present as the shock-wave transfer medium and explosive layer 2 is positioned over the water 3 and metal object 1 so that the horizontal plane of the explosive layer 2 is parallel to the horizontal axis of the metal object 1. The vertical surfaces of the explosive layer and the metal object are not ali ned at all points, i.e., the explosive layer does not have exactly the same curved contour as the metal object, although the explosive layer 2 overlies all points on the horizontal surface of the metal object 1.

The metal object 1 shown in the assembly of FIGURE 3 is shown in greater detail in top view in FIGURE 4. The metal object is a cavity die forging having a combination of curved surfaces and planar surfaces. As used in the assembly of FIGURE 3, however, the object shown in FIGURE 4 may be considered as essentially a fiat object of uneven surf-ace. The forging has flange portions in and web portion 11); i.e., the outermost portion 1a is of greater thickness than the innermost portion 1b.

The cross-sections shown in FIGURES 5 and 6 show the metal object of FIGURE 4 together with superposed water and explosive layer. Zia and 1b again indicate the flange portions and web portion, respectively, 2 the explosive layer, and 3 the water buffer layer. The area between flange portions la on the side opposite to the explosive is filled with a support material 10, preferably a metal, to prevent distortion of web portion 112 during treatment, and the water 3 is seen to completely occupy the space between explosive layer 2 and metal object 1.

The process of the invention provides a means of achieving a substantial reduction in residual stresses in metals without adverse effects on the mechanical properties or shape of the workpiece and without the need for expensive equipment such as large furnaces, large tanks with pressure-resistant walls, temperature-control devices, or dies. As is shown in the accompanying drawings and in the following examples, the process can be used to relieve stresses in metal objects of widely different shapes and degree of uniformity. In addition to the fiat metal object, the forging, and the cylinder shown (FIGURES l, 3, and 2, respectively), the metal to be stress-relieved may be in the form of a rod, disk, or the like, such as any of the possible configurations formed by the conventional metallurgical operations of casting, extrusion, sheeting, forging, etc.; and unsymmetrical as well as symmetrical samples can be treated since the exact configuration of the sample is not critical.

A critical feature of the process of this invention is the passage through the stress-containing metal object of a transient pressure disturbance of an intensity which exceeds the Hugoniot elastic limit of the metal but is less than about five times this limit. Below this limit the metal behaves elastically and a permanent relief of stresses does not occur. When the transient pressure is excessive, i.e., when this pressure is about five or more times the Hugoniot elastic limit for the metal, useful stress relief is not produced, e.g., stresses may be increased or the piece may be distorted. Values of the Hugoniot elastic limit for various metals can be found in Response of Metals to High Velocity Deformation, supra, particularly on pages 193, 259, 270, and 271. As is seen therein, the Hugoniot elastic limit of an unannealed aluminum is 5.4 kilobars and that of the same aluminum annealed is 0.9 kilobar. Thus, for relieving stresses in aluminum and aluminum alloys the present process employs a shock pressure in the metal of about 1-30 kilobars. Most of the steels are seen to have a Hugoniot elastic limit of l0- l2 kilobars, with those having received special treatment having higher limits. Thus, for steels the present process can employ a shock pressure of about 10-60 kilobars, although pressures greater than about 50 kilobars generally are not preferred to assure that additional stresses are not introduced into the metal object.

The pressure wave of an intensity within the abovespecified range is introduced into the metal object containing residual stresses by detonating an explosive adjacent, and preferably in contact with, a shock-wave-transfer medium which in turn is adjacent, and preferably in contact with, the metal object and which separates the metal object from the explosive. The transient pressure in the metal is controlled, inter alia, by the amount and detonation velocity of the explosive used, the density of the shock-wave-transfer medium, and the distance between the explosive and the metal object. To provide suificient mass between the explosive layer and the metal object for the necessary shock attenuation without the need for an excessively large volume of the shock-wave-transfer medium, the latter preferably will have a density of at least about one gram per cubic centimeter. While the density of the shock wave-transfer medium can vary greatly, the sonic impedance of the medium should be less than that of the metal object to assure the required pressure in the object. Gases are not suitable as the sole buffer medium since they do not provide sufficient mass and also because the high temperatures resulting from shocks in gases may have a deleterious effect on the properties of the metal object. Thus, the shock-wave-transfer medium can be a liquid or a somewhat compressible solid. A certain degree of compressibility is required in order for the shock pressure to be reduced as a result of compressive work. On the basis of economy, ease of use, and property requirements, liquids are the preferred shock-wave-transfer media, especially aqueous media. Solids which can be used are, for example, rubber, foamed polymers, e.g., polystyrene foam, and metals, e.g., lead.

The shock-wave-transfer medium has a surface adjacent the explosive and an opposite surface adjacent the metal object containing stresses and separates the metal object from the explosive. Upon initiation of the explosive, a shock Wave of a specific pressure enters the buffer layer, travels through this layer, and causes a transient pressure disturbance of less intensity in the metal object. Usually, the buffer layer substantially completely fills the space between the explosive and the metal object so that it is not thrown against the metal object by the action of the shock wave, a condition which usually is to be avoided in order to assure shock attenuation.

Greater shock attenuation is achieved with denser and thickerbuifer layers. Therefore for a specific combination of explosive composition and loading, and metal of a specific Hugoniot elastic limit, a thinner buffer layer can be employed with a denser shock-wave-transfer medium than with a lower-density material to achieve a desired pressure. The particular thickness of buffer layer which will be used in any specific case will depend on how much attenuation is needed (i.e., on the pressure required in the metal with respect to the pressure of the shock wave produced at the explosive-buffer interface) and the density of the attenuating material. As is illustrated in Example 1, buffer layer thicknesses of one-quarter, one-half, and one inch are equally satisfactory when Water is used as the shock-wave-transfer medium with an explosive layer detonating at about 7000 meters per second and at a loading of 2 grams per square inch. As

a general rule, the buffer layer should be at least one-six teenth of an inch thick. The maximum thickness of the buffer layer cannot be readily fixed on the basis of operability inasmuch as an extremely large spacing between the explosive and the metal object can generally be compensated for by a higher explosive loading.

In the process of the invention the pressure wave introduced into the stress-containing metal object is produced as a result of the detonation of an explosive. To obtain the desired stress relief, the explosive used must be a high or detonating explosive, that is one whose reaction rate exceeds the velocity of sound therein, or one having a reaction rate of at least about 1200 meters per second. The shape of the explosive is not critical, e.g., it can be spherical or in the form of a layer of any desired size and configuration, provided that the intensity of the pressure wave introduced into the metal object is relatively uniform along the entire surface at which the wave enters the object. Uniform intensity is desirable to prevent distortion of the metal object. For better uniformity, the explosive preferably is in the form of a uniform layer which can be positioned substantially parallel to the metal object. However, a point explosive charge can be used if the charge is sufficiently separated from the metal object so that the pressure along the wave front is relatively uniform 'when the wave reaches the metal object. With point charges the explosive will be separated from the metal object by a distance of several feet or more; with an explosive layer, there is no advantage to having a separation of more than about a foot.

The exemplified compositions, which detonate at high velocity, i.e., of the order of 5000-7000 meters per second, are especially suitable for use in the method of the present invention because they are readily formed into easily handled, tough, flexible sheets having a uniform quantity of explosive per unit area. The tough, flexible sheet-like nature of these compositions is advantageous because they handle easily. Moreover, the uniform distribution of explosive therein results in a uniform detonation velocity and insures consistency of detonation, both of these features being desirable in the present method. However, the use of other detonating explosives is equally feasible; for example, those cohesive, gelatinous detonating explosives based upon nitroglycerin, for example, blasting gelatin, can be formed into sheet-like uniformly dense explosive layers. Noncohesive solid and liquid high explosives may also be used by maintaining them in a suitable container. Castable explosives, for example those like amatol (TNT-ammonium nitrate mixture) or cy-clotol (a TNT-RDX mixture), naturally may be readily cast into plate-like or annular charges for use in the instant process. The quantity of explosive used will depend upon the Hugoniot elastic limit of the stress-containing metal object, the level of residual stresses in the object, the amount of reduction of stresses desired, the thickness of the object, the particular explosive composition used, and the composition and thickness of the buffer layer, among other factors. With a particular set of conditions, increasing the quantity of explosive increases the pressure of the shock entering the shock-wave-transfer medium. This may be desired for treating objects of higher stress levels or thicker objects, and can be used provided the Hugoniot elastic limit is not exceeded by five times or more. Explosive loadings which are too high for a specific set of conditions can be compensated for by a thicker and/or denser buffer layer or use of a lower-velocity explosive. In general, for obtaining the pressures required in the common metals with the preferred shock-wave-transfer media and explosives in the form of a layer and in the velocity range of 5000-7000 meters per second, explosive loadings of about 2 to 4 grams per square inch are used for metal pieces about 3 inches thick. The explosive loading may be increased for thicker metal pieces and reduced for thin metal pieces, and generally falls within the range of 1 to grams per square inch.

The explosive can be initiated by any conventional initiating device, e.g., blasting cap, exploding wires, detonating cord, line-wave generator, surface-wave generator or any suitable combination thereof. The location of initiation on the explosive charge can be at a point, e.g., at a point along an edge, a corner, or in the center of a layer, along a line such as an edge of a layer, or simultaneously over the entire surface of a layer opposite the surface adjacent the layer of a shock-wave-transfer medium. Pointor line-initiation produces an oblique shock wave at lower pressures and for this reason is preferred in the present process. For example, as is shown in the case of the plate-like layer shown in FIGURE 1, a linewave generator can be attached at one linear boundary of the layer and the line-wave generator initiated, e.g., with a blasting'cap. The line-Wave generator shown in FIG- URES l, 2, and 3 as well as others which can be used to initiate an explosive layer simultaneously at a plurality of points along a line are described in detail in U.S. Patent 2,943,571. When the metal object to be stressrelieved is in the form of a cylinder or a rod, the explosive layer around the cylinder or rod can be initiated by afiixing one or more line-wave generators along one circular edge of the explosive layer, or a conical explosive charge filled with an inert material along one circular edge, and initiating with one or more blasting caps.

The particular surface of the metal object at which the pressure wave enters the object will depend on the configuration of the metal object and generally will be decided on the basis of convenience and economy. For example, in the case of a metal plate, this surface will be the surface of greatest area, i.e., the horizontal surface. Such an arrangement is desirable for the following reasons: (1) as large an area as possible can be treated in one operation; and (2) the pressure wave is caused to pass through the region of minimum thickness in the object. In the case of metal objects having irregular surfaces, generally it also will be desirable to have the wave enter at the surface which will allow treatment of as large an area as possible and which will present a minimum thickness through which the wave must travel. However, in the case of irregularly shaped objects, convenience of maintaining the object in a stable position becomes a determining factor. For example, the cavity die forging shown in FIGURE 4 was more conveniently maintained in a stable position when placed flat as shown in FIGURE 3, than if it were allowed to stand on its curved surface. In the case of metal objects whose surface is completely or largely curved, e.g., spheres or rods, the buffer and explosive layers will surround the curved surface so that the largest possible area can be treated in one operation.

In most cases an explosive layer will be positioned essentially parallel to the metal object. This means that the explosive layer may be parallel to the horizontal surface of a flat even-surfaced object or to the horizontal axis .of a flat uneven-surfaced object, e.g., the object of FIGURE 4. However, in some cases, e.g., those in which the metal object is so shaped that parallel positioning would locate some regions of its surface considerably closer to the explosive than others, it may be desirable to position the explosive layer at an angle to the horizontal surface or axis of the metal object. Such positioning is within the scope of the present process provided that the layer of explosive is of suflicient dimensions to induce a pressure wave which will pass through the object in all of the regions where stress relief is desired.

As is illustrated in the drawings and examples, when the metal object to be subjected to the explosively induced pressure wave has portions of significantly less thickness than other portions, it may be desirable to support the thinner portions on that side of the object which is away from the explosive, i.e., the side which the wave reaches last. For example, in the object shown in FIGURE 4, the web portion, or reduced-thickness portion, of the forging is supported by filling the area between flanges on the side opposite the explosive with a material having strength sufficient to prevent deformation. Preferably, the material will be a solid, e.g., a metal such as steel or lead, or concrete. For convenience it is preferred to use Woods alloy since this metal, because of its low melting point, can easily be applied in the molten form.

The present process is applicable to any metal which contains unfavorable residual stresses. Metals, both ferrous and nonferrous, which commonly require stress relief in commercial practice and to which the present process can advantageously be applied include, for example, the steels, e.g., carbon steel, carbon-molybdenum steel, chromium-molybdenum steel, chromium stainless steel, chromium-nickel stainless steel, and weldments of dissimilar steels; aluminum alloys; copper alloys; lead alloys; magnesium alloys; nickel alloys; tin alloys; and zinc alloys. While applicable to all metals, both alloyed and unalloyed, the process finds particular advantage when applied to those metals which cannot be thermally stress-relieved without accompanying adverse effects on mechanical properties, i.e., metals which undergo a deterioration in mechanical properties when subjected to the temperaturetime conditions required for thermal stress relief. Included among such metals are the precipitation hardening alloys, i.e., alloys which harden by precipitation at room temperature or above from a supersaturated solid solution of the alloy obtained by rapid cooling of a hot solution thereof. This phenomenon occurs, for example, in the case of certain alloys of aluminum, copper, magnesium, or nickel, and the steels. The process of this invention is particularly useful with precipitation-hardened alloys of aluminum with minor (e.g., 20% or less) amounts of at least one of copper, magnesium, zinc, manganese, iron, chromium, nickel, titanium, and silicon. As is shown in the subsequent examples, the precipitation-hardening aluminum alloys acquire no undesirable properties as a result of stress-relief treatment according to the process of this invention.

The following examples serve to illustrate specific embodiments of the process of the present invention. However, they will be understood to be illustrative only and not as limiting the invention in any manner. I

The residual stresses reported in the examples are measured by the method of Siebel and Pfender as described in Metals Handbook, 1955 Supplement, American Society for Metals, Cleveland, Ohio, page 96. Blocks are cut out of a section of the metal object and are measured before and after sectioning. The lengths are introduced into standard elasticity equations which are used to calculate the stresses.

Example 1 A stress-relief assembly as depicted in FIGURE 1 is erected. The metal object 1 is a Z-inch-thick plate of aluminum alloy, 6 inches wide and 12 inches long. The alloy, which is of the precipitation hardening type, has the following composition: 93.6% aluminum, 4.4% copper, 0.8% silicon, 0.8% manganese, and 0.4% magnesium. The residual stresses in the plate amount to 15,000 p.s.i. The explosive layer is an explosive sheet, 6 inches wide and 12 inches long, comprising a blend of PETN in a 50/50 mixture of butyl rubber and a thermoplastic terpene resin (mixture of polymers of fl-pinene having the formula (C H the explosive load being 2 grams per square inch. This explosive sheet is described in detail in U.S. 2,999,743. The explosive sheet is characterized as strong, flexible, waterproof, uniform in density, and nonresilient, the composition detonating at a uniform velocity of about 7000 meters per second. A line-wave generator (6-inch equilateral triangle) of the type shown in FIGURE 2B of US. Patent 2,943,571 is taped to the shorter edge of the explosive sheet in such a way that there is uninterrupted contact between the line-wave generator and the explosive sheet. A commercial detonator (a No. 8 electric blasing cap) is fastened to the generator at the angle opposite the base contacting the explosive sheet. The metal plate is immersed in a wooden box containing sufficient water so that a 1-inch layer of water covers the plate. A A-inch-thick piece of wood of such dimensions as to fit snugly in the box, parallel to the bottom of the box, is placed over the water layer to P ovide a means of supporting the explosive sheet. The explosive sheet and affixed initiating means are afiixed to the bottom surface of the wood support so that the explosive contacts the water layer and so that the edges of the sheet are in alignment wih the edges of the metal plate. The blasting cap is initiated by application of an electric current, causing detonation of the line-wave generator and the explosive sheet, and introducing into the metal plate a transient disturbance at a pressure of about 10-20 kilobars determined as described hereinbefore (from shock velocity measurements in water). Residual stress measurements made on the aluminum alloy plate after this stress-relief treatment show that the stress has been reduced to psi. The reduction in thickness of the plate after such treatment amounts to only 0.7%.

Similar results are obtained when the experiment is repeated under the same conditions except that the depth of the water layer is 0.25 or 0.5 inch.

When the same explosive layer described in this example is placed in direct contact with an aluminum alloy plate of the same composition and size as that used in this example, the plate having a compressive stress level of 17,000 p.s.i., and the explosive is initiated in the same way, so as to introduce into the aluminum plate a shock wave at a pressure of about kilobars, the plate after shocking has a tension stress level of 40,000 p.s.i. Thus, explosive hardening techniques, which employ pressures of about 100 kilobars or more, have a completely opposite etfect when contrasted with the present process, which markedly reduces stresses in metals.

Example 2 An assembly as depicted in FIGURE 3 is erected. In this case, the metal object to be stress-relieved is a cavity die forging, such as that depicted in FIGURE 4, and has the following composition: 90.2% aluminum, 5.5% zinc, 2.5% magnesium, 1.5% copper, and 0.3% chromium. The sample has been solution heat-treated and is at an unstable temper. The flange portion of the forging varies in thickness from 2.215 inches to 2.845 inches, depending on the location of measurement. The explosive layer in this instance is an explosive sheet comprising a blend of PETN (35%) and red lead (50%) in a binder (15%) consisting of 50% butyl rubber and 50% of a thermoplastic terpene resin (mixture of polymers of fl-pinene having the formula (C H and described in detail in US. Patent 3,093,521. This composition has a detonation velocity of about 5000 meters per second. The explosive load of the sheet is 2 grams per square inch. The thickness of the Water layer is one inch. Initiation of the explosive layer is by means of a line-wave generator and blasting cap as described in the preceding example. Before the assembly is completed, the web portion (see 1b of FIGURE 4) of the forging is supported on the side of the forging which is away from the explosive by filling the area between flange portions (see 1a of FIGURE 4) on the side opposite the explosive with Woods alloy, in a manner similar to that shown in FIGURES 5 and 6. To facilitate removal of the Woods alloy subsequent to the stress-relief treatment, the area to be filled is lined with aluminum foil prior to the filling. After the explosive has been initiated and the shock wave has passed through the forging at an estimated pressure of 10-20 kilobars, the Woods alloy sections are removed by simply lifting them out manually. After treatment, the flange portion of the forging varies in thickness from 2.206 inches to 2.837 inches. The average compressive deformation is only 0.26% (less than the deformation required by the cold compression method of stress relief). The forging is then artificially aged to the T6 temper and cut up to determine tensile properties and residual stresses. The residual stress range (dilference between the maximum and minimum residual stress as seen in the stress distribution plot for the explosively shocked forging) is 5-6 k.s.i. (kips per square inch; 1 kip=l000 p.s.i.) as sampled at various locations in the flange portions. In contrast, a similar forging (as quenched) which has not been subjected to stress relief treatment has a residual stress range of 25.6-

Kips per square inch Cold compression 2.5-7.5 Thermo-mechanical 4-9.5

The tensile properties of the forging are not adversely affected by the explosive treatment. The explosively stress-relieved forging has a tensile strength of 78,900- 87,400 p.s.i., yield strength of 68,00077,600 p.s.i., and elongation in 4D of 11-14% (all above minimum values for die forgings of the composition used).

I claim:

1. A process for relieving stresses in a metal object which comprises positioning said object in spaced relationship to a detonating explosive, placing a shock-wave-transfer medium therebetween, and detonating said explosive, the pressure wave resulting from said detonation of said explosive and traversing said metal object having an intensity which exceeds, but is less than about five times, the Hugoniot elastic limit of said object.

2. A process of claim 1 wherein said shock wave transfer medium is liquid.

3. A process of claim 2 wherein said explosive is a layer of explosive of the general contour of the juxtaposed surface of said object.

4. A process of claim 3 wherein said metal object is a precipitation-hardened alloy.

' 5. A process for relieving stresses in a precipitation hardened aluminum alloy object which comprises positioning said object in spaced relationship to a layer of detonating explosive, the space between said object and said explosive being filled with water, and detonating said explosive, the pressure wave resulting from detonation of said explosive and traversing said object having an intensity of about from 1 to 30 kilobars.

6. A process of claim 5 wherein said explosive has a detonation velocity of about from 5,000 to 7,000 meters per second and the thickness of the layer of water between said explosive and object is about from A to 1 inch.

References Cited by the Examiner UNITED STATES PATENTS 1,891,234 12/1932 Langenberg 1484 2,703,297 3/1955 Ma'cLeod 148-4 FOREIGN PATENTS 879,933 10/ 1961 Great Britain.

OTHER REFERENCES DAVID L. RECK, Primary Examiner.

C. N. LOVELL, Assistant Examiner.