US3197851A

US3197851A - Method of forming a high tensile stength pressure vessel

Info

Publication number: US3197851A
Application number: US183149A
Authority: US
Inventors: Benjamin J Aleck
Original assignee: ARDE PORTLAND Inc
Current assignee: ARDE PORTLAND Inc
Priority date: 1962-03-28
Filing date: 1962-03-28
Publication date: 1965-08-03
Anticipated expiration: 1982-08-03
Also published as: FR1356306A

Description

3, 1965 B. J. ALECK 3,197,851

METHOD OF FORMING A HIGH TENSILE STRENGTH PRESSURE VESSEL Filed March 28, 1962 3 Sheets-Sheet l INVENTOR BENJAMIN J. ALECK BY HMM+M ATTORNEYS.

3 Sheets-Sheet 2 INVENTOR BENJAMIN J. ALECK ATTORNEYS.

B. J. ALECK METHOD OF FORMING A HIGH TENSILE STRENGTH PRESSURE VESSEL Filed March 28, 1962 m av #33.

Aug. 3, 1965 FIG. 5A

Aug. 3, 1965 Filed March 28, 1962 TRUE STRESS (IN POUNDS PER) SQUARE INCH B. J. ALECK 3,197,851

METHOD OF FORMING A HIGH TENSILE STRENGTH PRESSURE VESSEL 3 Sheets-Sheet 3 FIG. 6.

TRUE STRAIN (PERCENT DEFORMATION) E09 5 log log INVENTOR BENJAMIN J. ALECK iii/M44 I 5%? ATTORNEYS.

United States Patent 0 3,197,851 NET-H61) 0F F012? ENG A HIGH STRENGTH PREESURE VESSEL Benjamin J. Aleelr, .lacisson Heights, N.Y., assignor to Artie-Portland, inc, South Portland, Maine, a corporation of Maine Filed Mar. 23, 1%2, Ser. No. 133,149 15 Gaines. (Cl. 29-421) This invention relates to metallurgical processing and particularly to a metallurgical processing for forming high tensile strength pressure vessels. More particularly, the present invention relates to a novel step in metallurgical rocessing for forming ln'gh tensile strength pressure vemels from metallic materials and particularly from metallic materials having at least two distinct crystal structures, both of which can exist at room temperature.

This application is a continuation-in-part of my earlier filed application Serial No. 834,439 filed by me on August 18, 1959 for Method and Apparatus for Forming Pressure Vessels, and assigned to the assignee hereof, and now abandoned.

There are many uses for high tensile strength pressure vessels and such uses are multiplying over the years. In a nurnber of such uses, the weight of the pressure vessel is of great importance. For instance, if the pressure vessel is to be carried by a man such as, for example, portable oxy-acetylene tanks, it is desirable for the pressure vessels to be as light as possible and still meet the strength requirements.

(me especially critical use for light-weight vessels is in the field of solid fuel rockets, where pressure vessels are employed to store the solid rocket fuel and to contain such fuel during ignition and combustion thereof. During such time, the vessel is subjected to extremely high pressures. In such applications it is obvious that failure of the vessel itself means failure of the vessel itself means failure of the rocket and, accordingly, considerable strength must be built into such pressure vessels. However, the weight of such pressure vessels is a vital factor in rocketry as the weight of the vessel itself will cut into the overall weight of the payload. Accordingly, it is most desirable to create a high strength pressure vessel of very light weight. Naturally, if the material from which the material is made has a very high tensile strength, its overall strength and resistance to pressure will increase for a given weight of material. Thus light weight vessels can be made if extremely high tensile strength material is available.

There are many metallurgical processes available toay for strengthening steels and other metals suitable for pressure vessels and particularly for solid fuel rocket casings. For instance, cold rolled stainless steels are available and these materials have very high tensile strength after having been cold rolled. However, during the fabrication of vessels from such materials, it is generally necesary to weld the cold rolled stainless steel together to form the vessel. During the welding operation the metal in the zone of the welds, of course, is subjected to high heat which heat generally destroys the gain in strength achieved by the cold rolling. Accordingly, the final vessel, though made out of material that generally speaking has high tensile strength, will have distinct low tensile strength zones in the vicinity of the welds which zones will limit the overall strength of the vessel.

When forming pressure vessels from materials that are difficul to weld, such as, for instance, titanium, it is often necessary to thicken the ends of the portions of ice the vessel blank which are to be butt welded to one another in order to get additional strength in the weld. The need for additional strength is due not only to the d-ifiiculty of eifecting a good Weld, but also due to the fact that the welding operation causes a loss in strength previously gained by earlier employed metallurgical processes. However, the technique of thickening the ends is extremely costly because in the present day art, the thickened ends of such vessels are formed by machining out material between the ends whereby to leave a fiaired or flanged end. This is an extremely expensive procedure due in part to the cost of machining and also due in part to the fact that the machining removes (and thus Wastes) costly material, such as titanium.

The main object of the present invention is to provide a novel step in a metallurgical process for forming pres- 1 sure vessels which step will produce an extremely high strength vesel for a given weight.

Still another object of the present invention is the provision of a metallur ical process for working pressure vessels whereby to convert the metallic material forming the pressure vessel into a high tensile strength material.

Still another object of the present invention is the provision of a metallurgical process for forming welded high tensile strength pressure vessels,

Yet a further object of the present invention is the provision of a metallurgical process for working pressure vessels made out of metallic materials having two distinct phases or crystal structures both of which can exist at room temperature.

The above and other objects, and characteristics of the present invention will be understood more fully from the following description taken in connection with the arcompanying drawings.

In accordance with the present invention a pressure vessel blank is formed of metal, preferably by Welding. Thereaiiter, the vessel blank is placed in a die of larger size than the blank itself. The vessel blank is then subjected to high internal pressure being sufiicient to stretch jected to high internal pressure as by injecting or compressing a fluid therewithin, the pressure being sufiicient to stretch the metal from which the vessel blank is made. The stretching of the vessel blank will also cause a stretching of the welds and area surrounding the welds whereby to result in an increase in strength of the entire vessel blank including the weld therein.

In some instances, the pressure can be applied to stretch only the weld areas whereby to raise the strength of the weld areas.

The stretch forming technique may be used in combination with various metallurgical processes such as a heat treatment or precipitation hardening to further increase the strength gain in the pressure vessel. For instance, in materials exhibiting a martensitic transformation, if the stretch forming is performed at certain temperatures, it can be employed to induce such martensitic transformation whereby to yield a very high tensile rength pressure vessel made in whole or part of martensitic material, including the weld areas.

In the drawings:

PEG. 1 is a diagrammatic view of an apparatus for rocessing pressure vessel blank at cryogenic temperatures;

FIG. 2 is a sectional View of a pressure vessel formed in accordance with the present invention disposed Within a die;

FIG. 3 is a side elevational view of a pressure vessel which has been formed in the die shown in FIG. 2;

' ing).

FIGS. 4A, 4B and 4C illustrate several steps in the method of forming an open ended pressure vessel having a threaded boss at the end thereof;

FIGS. 5A, 5B and 5C are views of various steps in forming an open ended pressure vessel using a modified form of the novel method disclosed herein;

FIG. 6 is a true stress vs. true strain graph of typical ductile metals; and

PEG. 7 is an elevational view of a cylindrical Welded vessel blank.

Referring first to FIG. 7, a conventional pressure vessel blank 1% is illustrated therein. This blank may be formed of a metal and, as shown, is fabricated by taking a sheet of said material to form a central cylinder E32 by means of a longitudinally extending weld ltld. To enclose the vessel two vessel heads 1% and 1% are welded to the ends of the vessel blank 1% as by circumferential welds 119 and 112, respectively. The vessel heads 1% and 1% may be planar but, preferably, are of convex configuration.

Assuming that the vessel 1% has been fabricated from a very high tensile strength material which was previously treated by some metal working process such as ausforming or cold rolling or the like, the overall strength of the vessel will still be far below that which would be theoretically possible from the tensile strength of the material prior to fabrication thereof. This sharp reduction in the strength of the vessel is due to the fact that when the material is welded as at the Welds 104, ill) and $112, the benefits of the metallurgical process used to raise the tensile strength of the material prior to fabrication of the vessel blank 19%? are generally all or at least partly lost because of the heat of welding. Thus, although most of the material will be extremely strong and of high tensile strength, the welds themselves and the zones immediately adjacent the welds will be sharply weakened whereby to provide zones of weakness which limit the overall strength of the vessel.

in accordance with the present invention the vessel blank 1% is subjected to internal pressure of sufficient magnitude to exceed the elastic limit of at least the weld areas in the material whereby to cause a stretching of the welds and areas adjacent the welds and, preferably, of the overall material itself. Such stretching, as will be described in greater detail hereinafter, can improve the mechanical properties of the welds and of the material, or can effect a change in the crystallographic structure of the welds or material or can be used in combination with known metallurgical process to treat the entire vessel blank after forming whereby to insure high tensile strength throughout the entire vessel including the welds. It will be obvious that upon the application of internal pressure, the areas of minimum tensile strength, that is the areas surrounding and including the welds, will yield first and will continue to yield without any exceeding of the elastic limit of the stronger material unaffected by the welding operation until the strength of the welds has come up to the strength of the overall material. If pressure is continued to be applied after this point then the entire vessel blank including the now strengthened weld areas will continue to stretch to give additional strength to the overall vessel.

PEG. 6 shows a true stress vs. true strain curve for a typical ductile work hardenable metal such as, for example, austenitic stainless steel. It will be noted that true stress is shown in pounds per square inch and true strain is shown in percent of deformation (diametral stretch- As is well known to those skilled in the art, true stress is equal to the tensile force on the material divided by the actual area of the material. True stress is to be contrasted with nominal stress which is equal to the actual tensile force divided by the nominal or original area of the material. True strain is defined as the natural logarithm of unit deformation, i.e. log e, which is equal to ,6 final length (1;) allant length (1.

0 original area (A.,) final areahh) It will be seen that in the portion of the curve designated by the reference character 12% the curve rises almost vertically and substantially linearly. This portion of the curve represents purely elastic deformation and the metal observes l-lookes law, that is, if the stress is relieved anywhere within this portion of the curve the metal will elastically return to substantially its initial shape. Hovever, once the elastic limit has been exceeded at the point 122, t 1e material tends to yield with small increase in true stress, this portion of the curve being observed to be substantially horizontal and running from the point 122 to the point shown by the arrow 3.24. This approximately horizontal portion of the curve is designated by the reference character 126. In this area of the curve there is considerable yielding of the material and Hookes law is not followed. Between the point defined by the arrow 124 and the point defined by the arrow 123 the curve shows a concave upward characteristic and it is in this area where work of a substantial amount commences to be put into the material to sharply increase its tensile strength. This portion of the curve is designated by the reference numeral 13%. Beyond the concave upward portion 13%, the true stress vs. true strain curve commences to follow a sharp upwardly rising part which is substantially linear but slightly concave downward, which part of the curve is designated by the reference character 132. As stress increases this part of the curve becomes more and more horizontal but never bends downwardly due to the fact that as soon as this tendency arises, the material breaks. Generally speaking, it is preferred to stress the material during stretching thereof into the portion of the curve part 132 between the arrows 123 and 134 wherein a substantial strength gain is achieved.

' With continued reference to FIG. 6, it is presently necessary in order to achieve substantial strength gains to cause deformation beyond the arrow 124. That is, it is necessary to stretch the vessel blank sufilciently to move at least into the concave upward portion 136 of the true stress vs. true strain curve. Most preferably, it is desirable to stretch the vessel beyond the concave upward portion of the curve and Well into the concave downward portion 132 of the curve. For stainless steels and particularly for AISI No. 301 stainless steel, the point 124 is-reached at about 5% unit deformation (i.e., e=5%), and this is the minimum deformation above which any substantial tensile strength gain can be achieved. For 301 stainless steel it is desired to stretch the material in the vessel blank somewhere between the 10 and 18 percent unity deformation (i.e., e=1 Q% to 18%). However, some improvement in mechanical properties of the vessel is achievable at deformations below point 124 (i.e., s 5%) and, for certain applications this may be desirable.

A number of metals display what is known as a martensitic tranformation. The mar ensitic tranformation is a rearrangement of the crystallographic structure without any change in the chemical composition of the crystal structure. It is diifusionless. Moreover, such transformations in materials in which they occur are spontaneous at certain temperatures. For instance, as the temperature of the material is dropped, a temperature point will be reached where a martensitic tranformation will cornmence occurring spontaneously. This temperature is known as the M temperature. ihe martensitic transformation will progress further as the temperature of the material continues to be dropped until at a certain temperature, generally known as the M; temperature, there will be maximum spontaneous martensitic transformation, that is as much martensite as can be formed will be formed. It ha been found that the martensitic transformation can be started above M temperature if the material is deformed, that is. if mechanical work is put into the material. However. there is a maximum temperature above which no martensitic transformation will occur even if deformation takes place. This temperature is known as the M temperature. Moreover, it has lmen found that at temperatures below the M, temperature, the martensitic transformation can be made to progress further than it normally would spontaneously, provided the material is mechanically deformed at such a temperature.

ln view of this knowledge and finding. I have discovered that with pressure vessels made of materials which exhibit a martensitic transformation, it is desirable to stretch the vessel blank at a temperature below the the M temperature. and, preferably, close to the M, temperature and most preferably at or slightly below the M, temperature. when the stretching of such vessel blanks is performed at such temperatures. the vessel blank will gain in strength not only due to the stretching of the material, but also due to the crystallographic transformation of the material to the generally stronger martcnsitic phase. While it is preferred to stretch form materials exhibiting the martensitic transformation at about the M, temperature at which temperature the material will be relatively ductile to thereby enable the processor to substantially deform the vessel blank while forming martensite, substantial strength gains can be achieved at temperatures below the M, temperature at which there is a substantial amount of martcnsitc already present and even at or below the M temperature at which all the martcnsite formablc has been formed.

The following table lists the M, temperatures for a number of materials:

Approximate Material: M,temp.(F.) Titanium 1570 Aluminum-copper alloys (percent Al) 13.0 430 m 15.1) 150 Stcel/\1Sl N0.

4330 630 4330 .1792, v, 610 4340 5S0 Iron-nickel alloys (percent Ni)-- In addition to the materials listed in the chart, the ausienitic stainless steels also exhibit a martensitie transforma- .ion and for these steels an equation exists to calculate the approximate M, temperature. This equation is as follows:

Utilizing the above formula. it has been found that with four stainless steels the following occurs: With a stainless steel having the following composition- Percent Cr 16.8 Ni 6.1

Mn 1.33 Si 1.49

Fe, balance.

the calculated M temperature is 71 degrees F. The measured M, temperature is 156 F. The following are other compositions of stainless steel with both measured and calculated M, temperatures:

Percent Cr 17.30 Ni 7.56 Mn 1.33 Si .49 C .031 N .05 Fe. balance.

A material having the above composition has an M temperature calculated by the above presented formula of F. The M, temperatures as measured by standard techniques is 60 F. For stainless steel having the following composition- Percent Cr 16.! Ni 10.2 Mn 1.42 Si 0.46 C .023 N .042

Fe, balance.

the calculated M temperature is --297 F. and the measured M temperature is about 320. F.

Another stainless steel has the following composition-- Percent Cr 11.7 Ni 14.8 Mn 1.25 Si 0.33 C 0.052 N 0.035

Fe, balance.

This material has a calculated M, temperature of -479 F. (obviously impossible) and an actual measured M, temperature of -452 F.

With all materials displaying a martensitic transformation, as already noted, the stretch forming preferably takes place at a temperature below the M temperature and more preferably at a temperature at or below the M, temperature, whereby to effect a martensitic transformation at least due to deformation and, within the preferred temperature range. partially by deformation and partially by the spontaneous transformation due to temperature alone. In any of these events, assuming that the vessel blank to be stretched has been welded, it will be seen that the weld areas will also be stretched and the martensitic transformation will take place therein as well as in the non-weld areas of the Vessel whereby to greatly increase the overall strength of the pressure vessel, including the weld areas.

The step of stretch forming as described hereinbeforc can also be employed in connection with other types of metallurgical treatments than those necessary to effect a martensitic transformation. For instance, with ferrous materials, conventional heat treatment techniques can be employed to effect a pearlitic transformation or a transformation to bainite, preferably a lower bainite, in conjunction with the stretch forming technique. For instance, if it is desired to stretch form a pressure vessel blank and form bainite, then the pressure vessel blank will be heated above the cqttilibrium temperature of the vessel blank material and then quenched to a temperature in the isothermal transformation temperature range of the material to form bainite, with the lower portion of the range being desired, then maintaining the temperature of the vessel substantially constant while stretching it. While data for the determining of isothermal temperatures of various ferrous materials necessary to form bainite are readily available front standard time-temperature transformation curves (T-T-T curves), as an example, and not by way of limitation, the following table of temperature ranges for typical materials is prescntcd.

Isothermal transformation .\faterial: temperature range Nickel steel (lI'I'E- Ni. ll C.) 700" F. to 800 F. AISA 4H) hardenable stainless steel 800 F. to 1300' F. AISA 43-10 steel (100 F. [0 ll00 F. AlSA i030 steel 500 F.lO1l00 F.

During the isothermal period of the process, the material will transform into bainite. Moreover. if the temperature tends to be towards the bottom of the range presented, the bainite will be what is commonly known as lower bainite. which is a very strong tough material. Naturally, with respect to transformation to bainite, this is only applicable to ferrous materials.

From the foregoing. it will be seen that the stretch forming of pressure vessel blanks in combination with various tuetallttrgical processes will give rise to very strong high tensile strength pressure vessels. Stretch forming is particularly useful with respect to welded vessels in view of the fact that the welds of the vessel are treated along with the rest of the material of the vessel after formation of the vessel blank whereby to give high strength throughout the ve el. which is not ordinarily achievable by conventional techniques in which the vessel material is treated prior to formation into a pressure vessel and the benefits of the treatment are lost in the vinicity of welds when welding is performed. The gain from stretch forming may be due to an improvement in grain structure of the vessel blanks throughout. or the increase in yield point due to working. or any of these in combination with gains coming from known metallurgical processes.

Various techniques may be employed in stretch form ing pressure vessel blanks into high tensile strength vesscls, and these will now be described in connection with the drawings and particularly in connection with FIGS. 1 through 5C thereof. Moreover, the method will be described in connection with a pressure vessel blank formed of stainless steel designated as AlSI stainless steel numher 302 which has the following composition:

Percent Carbon --ma. t- .l5 Manganese --max- 2 Silicon 1 Chromium 17 to 19 Nickel 8-10 Phosphorus max .041 Sulfur ...max.. .03

Austenitic iron, balance.

Certain of the stainless steels coming within the ranges of tlte AlSl No. 302 stainless steel have an M, temperature ln the vlelnily of -320' F. Accordlngly. lf It is desired during stretch forming to produce a martcnsltlc transformation of such stainless steel. it will be necessary to stretch form the vessel blank preferably at the M, tempcralurc of 320 F. Of course, a certain amount of martcnsitic transformation can be achieved during stretch :all

(ill

8 forming at higher temperatures up to the M temperature of the steel. However, it is preferred to work at or below the M temperature in order to maximize the amount of martensitic transformation.

To effect stretch forming at the hi temperature of the 302 stainless steel, it is, of course, necessary to em ploy cooling apparatus in connection with the stretching apparatus. which apparatus is illustrated in FIG. 1 and is generally designated by the reference character 12. The vessel blanks which are made of 302 stainless steel are generally designated by the reference character 10.

The apparatus 12 includes a die 14 the internal surface of which is substantially identical to the final surface to be achieved for the finished pressure vessel to be formed from the vessel blank 10. Die 14 is disposed within a cold chamber 16 of an insulating chest 13 having a relatively thick thermal insulating wall 20 surrounding the chamber 16. Wall 20 is provided with a door or closure 21 to provide access to chamber 16. Door 21 may be hinged as at 23 and have a handle 25. Also disposed within chamber 16 are cooling coils or trays 22 here shown to be in the form of two flat coils although a helical coil may be employed. The cooling trays 22 may be formed by taking a piece of tubing and bending it to follow a tortuous path. The cooling trays 22 are employed to cool the die 14 to the desired working temperature which is preferably about 320 F. in order to produce a martensitic transformation. It should be understood that cooling of the die, although advantageous, is not absolutely necessary to working the present invcntion as the important factor is the cooling of the vessel blank which may be achieved without a pre-cooling of the die.

To achieve the very low tetnpcratures of the order of -320 F., a coolant or refrigerant such as liquid nitrogen is preferably employed. As shown herein the liquid nitrogen coolant is contained in a tank 24 having an outlet 26 which is connected to a conduit 28 for conveying the liquid nitrogen out of the tank 24. A valve 30 may be interposed in the conduit 28 to control. the flow of the liquid nitrogen. As shown herein the liquid nitrogen is supplied to the cooling trays 22 by a branch pipe 32 having a control valve 34 interposed therein. The conduit 28 is provided with a second branch pipe 36 which goes to the intake of a pump 38 having an outlet 40 the flow through which is controlled by a valve 42. The outlet pipe 40 extends through the insulating wall 20 of the cooling box 18 and into the chamber 16. in the chamber 16 the pipe 40 passes through an opening 44 in the die H and is threadedly connected to the pressure vessel blank 10 at a threaded inlet 46 therein. The die 14 has an opening 48 at its other end to which is connected an outlet pipe 50 having a valve 52 interposed therewithin to control the flow of fluids therethrought. If desired, the pipes 32. 36 and 40 may be provided with check valves 54 to limit the pressure therewithin. Furthermore, temperature measuring instruments 56 may be included'to monitor the temperature of various parts such as the die, the trays and the chamber. Moreover. if desired. a bleed valve 58 may be included in communication with the pipe 40 to relieve the pressure in the pressure vessel 10 at the end of the pressure step as will be described hereinafter. To prevent the refrigerant from picking up heat, pipes or

conduits

28, 32, 36 and 40 may be thermally insulated as by insulation 59.

' To form a pressure vessel in accordance with the presu nltt'ogen from the latter two plpes.

.3 t0 the central cylindrical section 62 thereof by threaded ecuring elements (not shown) which may be removed to permit the detachment of the hemispherical section 60 from the remainder of the die 14. Thereupon the pressure vessel blank 10 may be disposed within the interior of the die 14. The method of forming the pressure vessel blank 10 will be described hereinafter in greater detail. Suffice it to say at this time that the outer shape of pressure vessel blank 10 is of smaller size than the interior surface of the die 14. With the pressure vessel blank 10 disposed within the die the pipe 40 may be connected to the inlet 46 and the removable hemispherical section 60 may be secured to the remainder of the die 14 to thus close the die. Thereafter, the door 21 may be closed.

After the completeion of the insertion of the blank 10 in the die 14, the valve 30 may be opened and the valve 34 may be opened to permit liquid nitrogen in the tank 24 to flow under its own pressure into the cooling trays 22 to thus lower the temperature of the die 14. At the same time or shortly thereafter the pump 38 may be energized and the valve 42 may be opened to permit liquid nitrogen under a pressure of the order of 2000 to 2500 p.s.i. to be fed through the pipe 4%) into the interior of the pressure vessel blank 10. With the liquid nitrogen being supplied to the interior of the pres sure vessel blank as well as the cooling trays the temperature of the die and the pressure vessel blank 10 will rapidly drop to approximately the temperature of the liquid nitrogen, that is 320 F. Moreover, with the liquid nitrogen being supplied to the interior of the pressure vessel blank 10 at high pressure the liquid nitrogen will force the pressure vessel blank to be stretched outwardly to conform the outer surface thereof to the inner surface of the die 14. This stretching may be of the order of 10% to 20% which is well beyond the elastic limit of the vessel blank material thereby resulting in a plastic deformation of the vessel blank. This degree of deformation at the working temperature of -320 P. will cause a substantial amount of transformation to the martensitic phase to produce a substantial gain in stength over and above that produced by work hardening. Accordingly, when the pressure on the inside of the pressure vessel blank 10 is relieved the pressure vessel blank will not return to its original shape but will, save for a slight elastic shrinkage, substantially conform to the shape of the interior of the die 14.

Accordingly, after the application of the pressure on the interior of the vessel by introducing liquid nitrogen therein under pressure, the pressure may be relieved by shutting off the pump 38, closing the valve 42 and opening the relief valve 48 which will vent the liquid nitrogen in the tank to atmosphere, At the same time the valve 39 may be closed to discontinue supplying liquid nitrogen to the cooling trays 22. Thereafter, the door 21 in the wall 2t? may be opened, the hemispherical section 6% of die 14 may be removed from the remainder of the die and the pressure vessel, now stretched fully to size, may be removed from the remainder of the die and there after the method may be repeated.

It will be understood that the pressure vessel, after being stretched to size, may be readily removed from the die as after the pressure is relieved from the interior of the vessel, the vessel will shrink slightly within the elastic range and thus pull away from the Wall of the die.

The purpose of the vent 5% should be explained. As the die 14 is relatively well sealed, and as there is air between the outer surface of the pressure vessel blank and the inner surface of the die, when pressure is applied to the interior of the vessel blank It? to cause a stretching thereof the air in the space between the blank and the die will be compressed. Unless this air is permitted to flow out between the space it will cause some malshaping of the final pressure vessel. Accordingly, the

l0 vent Si is provided to permit the air to flow from the space between the vesssel and the die to the atmosphere through the valve 52.

The shape of the initial vessel blank 10 is of some importance in achieving a pressure vessel of the desired qualities. A shown in FIGS, 1 and 2 the vessel blank 10 has a uniform Wall thickness and is made of two hemispherical end sections which are Welded or otherwise connected to a cylindrical center section. While this particular shape will work satisfactorily for the formation of a closed ended high strength pressure vessel it can be demonstrated that hemispherical sections and cylindrical sections are stretched at difierent rates under uniform pressure. Specifically, the hemispherical sections cannot be stretched linearly as much as the cylindrical section for the same percent increase in area. Accordingly, in lieu of the shape of the blank 19 shown in fragmentary view in FIG. 1 a blank perhaps of the shape of a dog bone would be preferable in order to end up with a vessel having uniform strength throughout. Other shapes of vessel blanks may be calculated depending upon the desired shape of the end product.

While the method described hereinbefore is eminently suited to forming high tensile strength pressure vessels having closed ends and uniform wall thicknesses, it cannot be used readily as such to form an open ended vessel. If an attempt were made to form an open ended vessel by making a vessel blank similar in configuration to but smaller in size than the open ended vessel desired, when pressure is applied to the interior of the blank the open end would stretch and upon stretching would break any seal theretofore made to contain the pressure. With the seal broken, pressure would be lost and there would be relatively little stretching accomplished.

Accordingly, additional steps must be performed in order to manufacture an open ended high tensile strength vessel in accordance with the present invention. The simplest method of accomplishing this is to form a ves sel blank having closed ends as has been described hereinbefore. The closed ended blank is then stretch formed in accordance with the aforedescribed method and the final vessel coming out of the die will be of a shape equivalent to the shape of two open ended vessels with their open ends joined together. Thereafter, the stretch formed vessel (which is really two connected open ended stretch formed vessels) may be cut along the line of connection of the two open ended vessels. After cutting, there will be two vessels of the desired shape. This is illustrated in FIG. 3 of the drawings, wherein a finished stretch formed closed ended vessel 10' is shown. The dotted line 64 in FIG. 3 is the line of juncture of two identical open ended vessels. Accordingly, when the stretch formed vessel it? is cut along the line 64 two open ended vessels will be produced.

In lieu of simultaneously forming two open ended vessels as described in the preceding paragraph, a single open ended vessel can be so formed by fabricating a close ended vessel blank and stretching it as described above to the desired form except for the inclusion of the unwanted closed end. After stretching at sub-zero temperatures, the unwanted end can be removed as by cutting or otherwise to yield the desired open ended vessel.

Often times it is desirable to form a high tensile strength pressure vessel which has a thickened Wall portion. When such a vessel is desired the method described in connection with FIG. 1 must be modified, as with a portion of the wall of the vessel blank thicker than the remainder of the wall there may not be uniform stretching when pressure is applied. To overcome this ditdculty supplementary means for forcing out or stretching the thickened Wall portion can be employed. Such a means may be, for instance, a multi-armed jack 74 as shown in FIGS. 4A and 4B. in lieu of the jack 74, having a multiplicity of angularly related distendable arms, a plurality of angularly related jacks may be employed. Re-

pad

.5. ferring now to FIGS. 4A and 4B the vessel blank 19" is shown having two hemispherical end sections 65 and 68 of substantially uniform thickness and a central cylindrical section '70 having an inwardly directed flange 72. If such a vessel blank were placed in a die 14" and if pressure were applied to the interior of the vessel blank to stretch it to conform to the inner surface of the die 14" the relatively thin walled sections of the vessel blank ill" would stretch out to conform to the die. But, the thickened wall portion represented by the flange 72 would resist such deformation. Hence, the end product would not be of the desired shape.

To overcome this, while the vessel blank ill?" is being formed the multi-armed jack 74 is disposed within the blank in operative engagement with the flange 72. After completion of the vessel blank 1d" the blank may be disposed within the die 14" with the inlet for the pressure fluid connected to the pipe 49'. The vessel blank may then be cooled either by cooling the die by such means as the cooling trays 22 or by introducing refrigerant into the interior of the vessel blank. After the cool-' ing of the vessel blank to the temperature as hereinbefore described, the jack 74 is operated to distend its arms and thus stretch the thick portion of the wall i.e. flange 72, of the vessel blank 16''. Thereafter, pressure can be applied to the interiaor of the vessel blank to stretch the thin wall portions as hereinbefore described. In this way a pressure vessel having a portlon of its wall of thicker dimension than the remainder can be formed.

If it is desired to form an open ended pressure vessel having a thick wall portion as shown in FIG. 4C then the finished vessel shown in FIG. 43 may, after removal from the die, be cut through its center section to yield two finished open ended pressure vessels as shown in FIG. 4C. Often, the internal flange is utilized in open ended vessels to provide a threaded connection. Accordingly, in FIG. 4C, the flange 72 is shown provided with a thread 73.

It is possible to form a single open ended pressure vessel in accordance with the present invention. The open ended vessel may have a thickened wall portion or it may be provided with a uniform wall thickness throughout. The method of forming a single vessel will be substantially the same in either event. Such a method is illustrated in FIGS. 5A to SC wherein a vessel blank 143" having a flange 7a is shown. The flange 75 is adjacent the open end '77 of the vessel. In forming an open ended pressure vessel from an open ended pressure vessel blank 10", the vessel blank, after formation, is first placed in a die 14"", the interior of which is formed to the shape desired for the final pressure vessel. The vessel blank is then cooled, preferably by cooling the die although direct cooling may be employed. After cooling the open end 77 is stretched by mechanical means to cause it to assume the shape designed for it in the final stretched vessel. The remainder of the vessel blank 16" at this point remains unstretched. However, with the open end 77 now stretched to its final shape, a seal may be effected with the open end to subject the remainder of the vessel blank to stretching pressures, which seal will not be lost as the end 77 cannot stretch any further to break the seal. Accordingly, when'pressure is applied, the remainder of the vessel blank will be stretched to cause it to conform to the shape of the die and thus form a finished open ended vessel.

Referring now particularly to FIGS. 5A, 5B and 5C a vessel blank 19" having an open end 77 is shown. As shown herein the vessel blank ld has'a flange 76 adjacent the open end although, as indicated hereinbefore, this method is applicable to open ended blanks having uniform wall thicknesses as well. The vessel blank 16" as described is placed in die 1 5' the internal surface of which conforms to the final shape of the vessel. The vessel blank 10 is then cooled. Preferably, al-

though not necessarily, cooling is accomplished by cooling the die as by introducing refrigerant into cooling trays such as the cooling trays 22 as shown in FIG. 1. When the vessel blank has been cooled to the desired sub-zero temperature the open end 77 is stretched by mechanical means. Such a means may be a multi-armed jack like the jack 74 shown in FIGS. 4A and 4B. In lieu thereof a tapered mandrel may be inserted into the open end '77 of the cooled vessel blank and a force applied to the shaft 89 connected to the mandril 78 to force the mandrel inwardly of the blank and thus stretch out the open end '77. With the outer wall of the open end of the vessel blank stretched to engage the die a seal may be effected as by a plate 84 and an O-ring 86. Plate 84 is preferably provided with an inlet opening 88 to which the pipe 40" may be connected for conveying the pressure medium such as, for instance, liquid nitrogen from a pump to the interior of the vessel blank. After connection, the vessel blank may be stretched by introducing the pressure medium into the interior of the blank as hereinbefore described. After application of pressure to stretch the remainder of the vessel blank the pressure may be cut OE and the stretched vessel may be removed from the die and it will be in the proper form and of extremely high tensile strength.

In connection with the modification described above with regard 'to FIGS. 5A to SC, the mandrel 7 3 may itself be employed to effect the seal of the open end. In such an event the mandrel should be provided with an opening to permit the pressure medium, preferably the refrigerant under pressure, to be introduced into the interior of the vessel blank after stretching of the open end. Moreover, if a thickened wall portion is located at other than the open end, a suitable stretching means, such as multiarmed jack 74, may be employed to stretch such thickcued wall portion separately from the stretching of the open end of the vessel and the thin wall portion thereof.

While it is presently preferred to apply the internal pressure to the vessel blank by means of the refrigerant under pressure as has been described in detail hereinbefore, the present method can be worked by utilizing other means of applying pressure to the interior of the vessel blank. For instance, explosions within a vessel blank may be employed to supply'the necessary pressure for stretching the vessel blank to conform to the interior of the wall of the die. However, the advantages of utilizing the refrigerant itself are great. By utilizing the refrigerant itself as the vehicle or means for applying pressure to the interior of the vessel blank a very rapid cooling and an assurance of a maintenance of the vessel blank at very low temperatures is achieved. As a matter of fact, if the refrigerant serves as the pressure vehicle, it may be possible to eliminate die cooling means such as trays 22 and still cool the vessel blank to the desired temperature. Accordingly, it may be possible to reduce the cost of the equipment for working the present invention. Moreover, there is no danger of any corrosive condensations as migl possibly occur if other fluids are injected into the interior or" the vessel blank under pressure to effect the stretching. Furthermore, it will be apparent that some fluids, such as, for instance, water, are inconvenient to use as they might freeze and thus prevent the application of the in ternal pressure. For these reasons the use of the refrigerant as the pressure means is the preferred embodiment.

In the example given above, both the weld area and the remainder of the vessel blank were stretched upon the application of pressure to the interior thereof. However, it will be understood that a vessel blank can be fabricated from very highly tensile strength metal, the method of fabrication of the blank being as by welding. However, upon the welding being employed to form the vessel blank, the benefits of the prior metallurgical processing for yielding the high strength material will be lost in the weld areas due to annealing in such areas, whereby to render a vessel blank made generally of high tensile strength material but with weakened areas in and around the welds. With such a vessel, all that is necessary to do is to stretch the weld areas to increase their strength with no need for stretching the remainder of the material making the vessel blank. Naturally, in view of the fact that the welds and the area surrounding them are substantially weaker than the remainder of the vessel blank, the welds will stretch first whereby to give the result desired.

One of the advantages of stretching only weld areas is realized when extremely large vessel blanks are to be formed. Generally speaking, the cost of building dies to house huge pressure vessels as might be employed as solid fuel rocket casing for an intercontinental ballistic missile will be so great as to render the process not readily useable. Accordingly, when fabricating such a vessel blank by using stretch forming techniques, what can be done is that a plurality of individual sections can be formed of very high tensile strength material such as cryogenically rolled stainless steel. Each of the sections is essentially cylindrical. The sections can then be joined together as by welding whereby to weaken the material in the area of the welds but to render a very large vessel rimarily made of extremely strong material. Thereafter, without the use of a die, pressure can be applied to the interior of the vessel so fabricated to stretch the weld areas and thereby gain strength in the weld areas. If it is desired to stretch the weld areas to effect a martensitic transformation of the material in the weld zones, then local temperature control in the area of the welds can be effected to bring the weld areas close to the M temperature of the material prior to stretching whereby to induce a substantial degree of martensitic transformation upon stretching the weld areas. Accordingly, if the material from which the Vessel was made is an A181 302 stainless steel, it will be necessary to cool the weld areas by liquid nitrogen or similar cryogenic material in order to bring the stainless steel in the weld areas down to its iv temperature.

However, it will be seen that this type of stretching will not require a die and can be done out in the open with relatively simple equipment.

Furthermore, it is possible to have high tensile strength pressure vessels to which additional hardware must be secured after formation. Ofttimes it is necessary to secure the hardware by welding or other process employing great heat, which process will tend to anneal the vessel material in the area in connection with the additional hardwar Accordingly, in order to bring the vessel back up to strength after securing the hardware, it will be necessary only to stretch form the vessel blank in the area where hardware has been secured. Thus, when working on very large vessels, or it may be necessary only to isolate certain portions of them for stretching whereby to cut down the amount of work to be done, the size of equipment, and so forth.

While I have herein shown and described the preferred form of this invention and have suggested several modifications therein, other changes and modification may be made therein within the scope of the appended claims without departing from the spirit and scope of this invention.

Having thus described my invention, what I desire to secure and claim by Letters Patent is:

l. The method of forming a high tensile strength pressure vessel from a metallic material having a true stress v. true strain curve with a concave upward portion therein, comprising the steps of forming a vessel blank from said metallic material, and then stretching said vessel blank at least to strain said material within the concave upward region of said curve.

2. The method of forming a high tensile strength pressure vessel from a metallic material having a true stress v. true strain curve of such type that as true strain increases true stress first rises substantially rapidly and linearly, then remains substantially constant, then inid creases in accordance with a concave upwardly curved relationship and then increases in accordance with a concave downwardly curved relationship, comprising the steps of forming a vessel blank from said metallic material, and then stretching said vessel blank to strain said material into the concave downward portion of said curve.

3. The method of forming a high tensile strength pres sure vessel from a metallic material having a true stress v. true strain curve with a concave upward portion therein, comprising the steps of forming a vessel blank from said metallic material, at least in part by welding, and then stretching said vessel blank including the weld therein at least to strain said material within the concave upward region of said curve.

4. The method of forming a pressure vessel, comprising the steps of forming a vessel blank from metallic material which exhibits a martensitic transformation, and then stretching said vessel blank at a temperature below the M temperature of said metallic material, whereby to effect a martensitic transformation of at least a portion of said material.

5. The method of forming a pressure vessel, comprising the steps of forming a vessel blank from metallic material which exhibits a martensitic transformation, and then stretching said vessel blank at a temperature about the M temperature of said metallic material, whereby to effect a martensitic transformation of at least a portion of said material.

6. The method of forming a pressure vessel, comprising the steps of forming at least in part by welding a vessel blank from metallic material which exhibits a martensitic transformation, and then stretching at least the weld in said vessel blank at a temperature below the M temperature of said metallic material, whereby to effect a martensitic transformation of at least a portion of said material.

7. The method of forming a pressure vessel, comprising the steps of forming at least in part by welding a vessel blank from metallic material which exhibits a martensitic transformation, and then stretching at least the weld in said vessel blank at a temperature about the M temperature of said metallic material, whereby to effect a martensitic transformation of at least a portion of said material.

8. The method of forming a pressure vessel, comprising the steps of forming at least in part by welding a vessel blank from metallic material which exhibits a martensitic transformation, and then stretching at least the weld in said vessel blank at a temperature below the M temperature of said metallic material, whereby to effect a martensitic transformation of at least a portion of said material.

9. The method of forming a high tensile strength metallic pressure vessel by stretching at a temperature at which a metallurgical transformation occurs in the metallic material of said vessel, comprising the steps of forming a pressure vessel from said material and injecting a fluid at said temperature and under pressure to bring said vessel blank to said temperature and to stretch said vessel blank.

19. The method of forming a high tensile strength hollow article of predetermined size and shape and having at least one open end in a die having a portion of said predetermined size and shape, comprising the steps of forming from metallic material which exhibits a martensitic transformation a closed ended hollow article blank having a portion of smaller size than said portion of said die, placing said article blank within said die with said portion of said article blank within said portion of said die, adjusting the temperature of said article blank to below the M temperature thereof, applying fluid pressure to the interior of said article blank to stretch said portion of said article blank to substantially conform to said size and shape of said portion of said die while said blank is below said M temperature, removing said stretched article blank from said die, and removing from said stretched article blank the material other than that forming said portion thereof.

11. The method of forming a high tensile strength vesselof predetermined size and shape and having at least one open end in a die having an internal size and shape substantially equal to the size and shape of two of said vessels disposed in open end to open end relation, comprising .the steps of forming from metallic material which exhibits a martensitic transformation a closed ended vessel blank of smaller size than the interior of said die, placing said vessel blank within said die, adjusting the temperature of said article blank to below the M temperature thereof, applying internal fiuid pressure to the interior of said vessel blank to stretch said blank to substantially conform to said interior of said die while said blank is below said M temperature, removing said sretched vessel blank from said die, and dividing said resulting closed ended vessel along its central transverse plane to thereby form two open ended vessels of said predetermined size and shape.

12. The method of forming a high tensile strength vessel of predetermined size and shape and having at least one'open end, in a die having an internal size and shape substantially equal to the size and shape of said vessel comprising the steps of forming from metallic material which exhibits a martensitic transformation a single open ended vessel blank of smaller size than the interior of said die, placing said vessel blank within said die, adjusting the temperature of said article blank to below the M temperature thereof, by mechanical means stre ching the part of said vessel blank adjacent said open end to cause said part to substantially conform to the size and shape of the part of the die confronting said part of said vessel blank while said blank is below said M temperature, sealing said open end of said vessel blank, and applying fluid pressure to the interior of said vessel blank to stretch the remainder of said vessel blank to substantially conform to the remainder of said die while said blank is below said M temperature.

' 13. The method of forming a high tensile strength ve sel of predetermined size and shape and having a wall with a relatively thick portion and a relatively thin portion in a die having a size and shape substantially equal to said predetermined size and shape of said vessel, comprising the steps of forming from metallic material which exhibits a martensitic transformation a vessel blank having a size smaller than the interior of said die and having thick and thin Wall portions, placing said vessel blank in said die, adjusting the temperature of said article blank to below the M temperature thereof, by mechanical means stretching said thick wall portion of said blank to substantially conform to the portion of said die confronting said thick wall portion wtnie said blank is below said M temperature, applying fluid pressure to the interior of said vessel to stretch said thin wall portion of said vessel blank to conform to the portion of said die confronting said thin wall portion while said blank is below said M temperature.

14. The method of forming a high tensile strength pressure vessel from a metallic material having a true stress v. true strain curve with a concave upward portion therein, comprising the steps of forming a vessel blank from said metallic material, and then stretching said vessel blank by applying a fluid pressure to the interior of said vessel lank at least to strain said material within the concave upward region of said curve.

15. The method of forming a pressure vessel, comprising the steps of forming a vessel blank from metallic material which exhibits a martensitic transformation, and then stretching said vessel blank at a temperature below the M temperature of said metallic material, by comprising a fluid at said temperature inside said vessel blank, whereby to effect a martensitic transformation of at least a portion of said material.

References Qited by the Examiner UNITED STATES PATENTS 2,337,247 12/43 Kepler 29-446 X 2,503,191 4/50 Branson 29-421 2,579,646 12/51 Branson 29-421 2,861,530 11/58 Macha 113-44 2,866,429 12/58 Staples 113-44 3,024,938 3/62 Watter 29-4711 3,064,344 11/62 Arne 29-421 3,068,562 12/62 Long 29-421 waits roan A. WlLTZ, Primary Examiner.

THOMAS H. EAGER, Examiner.

Claims

1. THE METHOD OF FORMING A HIGH TENSILE STRENGTH PRESSURE VESSEL FROM A METALLIC MATERIAL HAVING A TRUE STRESS V. TRUE STRAIN CURVE WITH A CONCAVE UPWARD PORTION THEREIN, COMPRISING THE STEPS OF FORMING A VESSEL BLANK FROM SAID METALLIC MATERIAL, AND THEN STRETCHING SAID VESSEL BLANK AT LEAST TO STRAIN SAID MATERIAL WITHIN THE CONCAVE UPWARD REGION OF SAID CURVE.