US3456831A

US3456831A - Austenitic stainless steel pressure vessels

Info

Publication number: US3456831A
Application number: US610713A
Authority: US
Inventors: Johan Ingvar Johansson
Original assignee: Avesta Jernverks AB
Current assignee: Outokumpu Stainless AB
Priority date: 1957-08-16
Filing date: 1967-01-20
Publication date: 1969-07-22
Anticipated expiration: 1986-07-22
Also published as: FI41377B; DE1285489B; CH371095A; FI41377C; JPS5024912B1

Description

July 22, 1969 J. l. JOHANSSON AUSTENITIC STAINLESS STEEL PRESSURE VESSELS Original Filed Sept. 20, 1963 6 Sheets-Sheet 2 o kg/mm AVESTA e32 SK STAINLESS STEEL 60-- as 2101 FERRITIC sTEEL SIS I330 FERRITIC STEEL 0 I0 2 0 so 40 2:0 60 7 0 9. STRAIN d kg/mm d SIS 2|o| FERRITIC STEEL 3O AVESTA e32 SK 0 STAINLESS STEEL s |s EaBTEERTfiE E'FEEL O 5 5 4 l I i 5 i 1 3 2 0 DJ 0.2 0.4 0.6 0.8 L0 L2 L4 STRAIN F/G. 4B INVENTOR.

JOHAN INGVAR JOHANSSON BY Z3 4 AMI/@4440 BW-9 his ATTORNEYS July 22, 1969 J. l. JOHANSSON 3,456,831

AUSTENITIC STAINLESS STEEL PRESSURE VESSELS Original Filed Sept. 20, 1963 6 Sheets-Sheet 3 o kg/mm 7o 832 SK STAINLESS STEEL COLD STRETCHED TO 5% 0 I0 20 3'0 4'0 5'0 6 0 '1. STRAIN FIG: 5A

832 SK STAINLESS STEEL Q a z a r 0 0.: 0.2 0.4 0.6 0.8 L0 STRAIN INVENTOR. JOHAN INGVAR JOHANSSON his A T TORIVEYS TENSILE STRENGTH July 22, 1969 J. l. JOHANSSON 3,456,831

AUSTENITIC STAINLESS STEEL PRESSURE VESSELS Original Filed Sept. 20, 1963 6 sheewsh'eet 4 AVERAGE TIME TO sTREss CORROSION FAILURE IN 40% OuClg, DH 6, IOOC AFTER DIFFERENT DEGREE OF GOLD STRETCHING. uNIAxIAL TENsIoN, APPLIED STRESS= AT IOOC.

TEST sTEEL-AvEsTA e32 SK.

22 21 32 42 6 |00c, kg/mm 2 o 2.5 I0 STRAIN D 2 soc-- III 0! 3 500-- E UJ 400*- E lm L. 9 UNWELDED SPECIMENS o I a; WELDED SPECIMENS g-j FIG. 7 z

0 I I I I I I O 5 IO I5 uNIAxIAL LoAo kq/mm AIsI 3|6 sTAINLEss sTEEL psi I ULTIMATE I TENSILE' STRENGTH so I I 7o-- E l o 60-- l E 0 STRENGT YIEL H I F/G. 6 o l I 9. so l 20-- i l INVENTOR. JOHAN INevAR JOHANSSON o I I I I I I I I I I I I I BY Q Ol23456789l0lll2l3l4l5 COLD STRETCHING WMMDW his ATTORNEYS July 22, 1969 J. JOHANSSON 3,456,831

AUSTENITIC STAINLESS STEEL PRESSURE VESSELS Original Filed Sept- 20, 1963 6 Sheets-Sheet 5 ACCELERATED STRESS CORROSION TESTS IN- 40% COClg,

IOOC USING UNIAXIAL CONSTANT TENSION OF l5 kq/mm TEST STEELCOLD STRETCHED AVESTA 832 MV.

U) m D o I m D: D a E w g IO 0 2 4 e a lo PERMANENT FIG 8 .ELONGATION ACCELERATED STRESS CORROSION TESTS m 40% 0001 I000 USING UNIAXIAL TENs|0N= d TEsT STEEL-AVESTA 832 MV.

c0| 0 STRETCHED STEEL 3 UNSTRETCHED STEEL D o 2 123 am 25.4 29.0 31.1 36.2 d I00 0, kq/mm 0 001.0 3; i STRETCHED E 20-- Lu g :0 d |OOC,kq/mm 001.0 0 +STRETCHED IO I5 20 25 TESTING STRESS kg/mm F/G: 9 INVENTOR.

JO HAN INGVAR JOHANSSON his ATTORNEYS y 1969 J. JOHANSSON 3,456,831

AUSTENITIC STAINLESS STEEL PRESSURE VESSELS Original Filed Sept. 20, 1963 6 Sheets-Sheet 6 FIG /0 MECHANICAL PROPERTIES BETWEEN +20 AND 400C OF AVESTA 832 SK STAINLESS STEEL- COLD STRETCHED DEGREE OF 001.0

STRETOHING E 5 IO g 40-- E g 5 /0 It. 30- 2.5% (h 3 20-- lil 0 /0 TESTING TEMPERATURE c INVENTOR. JOHAN INGVAR JOHANSSON his 47' TOR/VEYS 3,456,831 AUSTENITIC STAINLESS STEEL PRESSURE VESSELS Johan Ingvar Johansson, Avesta, Sweden, assignor to Avesta Jernverks Aktiebolag, Avesta, Sweden, a corporation of Sweden Continuation of application Ser. No. 310,383, Sept. 20, 1963, which is a continuation-in-part of application Ser. No. 754,598, Aug. 12, 1958. This application Jan. 20, 1967, Ser. No. 610,713

Claims priority, application Sweden, Aug. 16, 1957, 7,520/ 57 Int. Cl. B65d 1/00, 7/42 U.S. Cl. 2203 8 Claims ABSTRACT OF THE DISCLOSURE Austenitic stainless steel pressure vessels are made by welding together a plurality of plates of austenitic stainless steel having a normal yield strength with weldments of austenitic stainless steel to form a pressure vessel, selecting a higher than normal value of yield strength within a range of values provided by cold stretching the steel an amount suflicient to elongate it permanently essentially in excess of 0.2% and at most about 10%, and subjecting the vessel internally to a fluid pressure suflicient to stress the weakest portions of the vessel walls to the selected yield strength and thereby increase the yield strength of the said weakest portions to the selected value by permanently elongating the said weakest portions of the walls essentially in excess of 0.2% and at most about 10%, the vessel being subjected to the fluid pressure at a temperature below the recrystallization temperature of the steel and under conditions permitting the vessel walls to expand freely.

This is a continuation of Ser. No. 310,383 filed Sept. 20, 1963 and now abandoned, which in turn is a continuation-in-part of Ser. No. 754,598, filed Aug. 12, 1958, and now abandoned.

This invention relates to welded pressure vessels formed of austenitic stainless steels.

Pressure vessels, including sealed vessels such as autoclaves and gas containers, and also long open vessels which are subjected to high static pressures from liquid columns, preferably are formed of materials that are highly stress and corrosion resistant. Those characteristics are possessed by austenitic stainless steels which are, therefore, ideally suited to high pressure vessels.

Due to the high relative cost of austenitic stainless steels, when the nature of corrosive material to be held in a container required their use, it was more economical, prior to the invention, to use stainless clad ferritic steel plates than to use solid stainless steel plates. Various problems associated with the construction and use of vessels formed of clad steel plates made it desirable to use solid stainless steel, if possible. Only the high cost of those steels has prevented their use in many pressure vessels.

One disadvantage of stainless steels, and particularly those stainless steels having a composition including approximately 18% chromium and about 8 to 12% nickel which have austenite as an essential structural constituent, is their lack of a marked yield point and the low value of the yield strength arbitrarily selected relative to the ultimate tensile strength. Various governmental and other safety codes ordinarily have provisions governing States Pate 9 "ice wall thickness of pressure vessels dependent on the yield strength of stainless steels, as well as their other properties, and the relatively low yield strength selected makes necessary thick vessel walls of expensive stainless steel.

In contrast, ferritic type low alloy steels often used in pressure vessels have a marked yield point which enables the yield strength used to calculate Wall thickness to be set almost precisely at the yield point. This value can be defined as the stress which in a tensile test gives a certain elongation without any further increase in stress. However, for materials such as austenitic stainless steels having no marked yield point, a relatively low value for the yield strength has been used in safety codes to calculate wall thickness of pressure vessels.

The present invention overcomes the disadvantages formerly associated with the use of austenitic stainless steels in pressure vessels by providing thinner wall stainless steel ves sels with great savings, for example, up to and even greater than one-half of the amount of stainless steel formerly required for the vessels.

In one embodiment of the invention, sections of stainless steel having austenite as an essential structural constituent, hereinafter referred to as austenitic stainless steel, are welded to form a pressure vessel of the desired shape. The vessel is then subjected internally to fluid pressure, under conditions permitting the vessel walls to expand freely, suflicient to provide a desired yield strength higher than the normal yield strength by cold stretching the vessel walls to cause a permanent elongation in excess of 0.2% and at most 10% in the weakest regions of the walls at a temperature below the recrystallization point of the steel. The cold stretching of the austenitic stainless steel preferably is suflicient to increase its yield strength at room temperature from 15 to the final yield strength being high enough to withstand established working pressures with aprescribed margin of safety. Free expansion of the vessel walls is necessary to effect permanent elongation in all required parts of the vessel.

The resulting inventive pressure vessel, which is formed of a plurality of cold stretched sections welded together, has a markedly higher yield strength for the amount of stainless steel used in its walls. Moreover, stresses at welds between the vessel sections are relieved by the cold stretching, thereby reducing the danger of Weld fracture. It has also been found that, contrary to the general opinion of those in the pressure vessel field, stress corrosion resistance of the vessel is not significantly impaired by cold stretching within the limits of the invention and, in some instances, vessels have shown, after being permanently elongated essentially in excess of 0.2% and at most 10%, improved stress corrosion resistance.

When it is necessary to reduce the amount of distortion of the pressure vessel due to cold stretching, pursuant to another embodiment of the invention, the austenitic stainless steel sections are cold stretched, cold rolled or otherwise cold worked, at a temperature below the recrystallization point of the steel, to raise their yield strength to a selected value prior to being welded together to form the pressure vessel. Subsequently, pressure is applied to the vessel internally to cold stretch the weld joints and adjacent annealed areas essentially in excess of 0.2% and at most 10%, thereby raising the yield strength of the weakest regions of the vessel to the desired value while reducing to a negligible value distortion of the vessel.

If it is necessary to weld pipe sockets, manhole fittings and similar connections to the vessel after the cold stretching, annealing effects at the weldments and adjacent areas can be compensated by suitably reinforcing such connections. Alternatively, those regions where fittings are to be positioned can initially be designed with larger dimensions than the surrounding portions to provide sufiicient strength so that no elongation therein is of standard stainless steels having austenite as an essential structural constituent produced by Avesta Jernverks Aktiebolag of Sweden and their equivalent steels designated by other Swedish, American, British and German code numbers. The table also shows the statistical yield strength of the steels at room temperature (20 C.).

necessary and none will result during the stretching treatment.

These and further advantages of the invention will be more readily understood when the following description is read in conjunction with the accompanying drawings, in which:

FIGURE 1 is a sectional view of a welded cylindrical pressure vessel formed of austenitic stainless steel prior to permanent elongation;

FIGURE 2 illustrates the pressure vessel of FIGURE 1 after it has been cold stretched and fittings added;

FIGURE 3 shows another pressure vessel constructed in accordance with the present invention;

FIGURES 4A and 4B illustrate stress-strain curves for an austenitic stainless steel and ferritic steels;

FIGURES 5A and 5B show stress strain curves for the austenitic stainless steel of FIGURES 4A and 4B after cold stretching;

FIGURE 6 illustrates curves comparing the ultimate tensile strength and yield strength of an austenitic stainless steel as it is cold stretched;

FIGURES 7, 8 and 9 illustrate the stress corrosion characteristics of austenitic stainless steels that have been cold stretched in varying amounts; and

FIGURE 10 shows graphically the mechanical properties of an austenitic stainless steel between C. and 400 C. after cold stretching varying amounts.

Referring to a typical pressure vessel constructed in accordance with the present invention with reference to 0 FIGURES 1 and 2, a pair of domed end walls 10 and '11 are welded to the ends of a cylindrical shell 12 at

circumferential weldments

13 and 14. The shell 12 may be formed of two sections welded together at a weldment 15. One or more longitudinal weldments (not shown) in the shell facilitate construction of the pressure vessel.

Circular apertures in the center of each of the

end walls

10 and 11 receive

cylindrical pipe sockets

16 and 17, respectively, and two

cylindrical pipe sockets

18 and 19 are also welded into circular apertures provided on opposite sides of tthe shell 12. The

pipe sockets

16, 17, 18 and 19 are temporarily sealed by welding domed covers 20, 21, 22 and 23, respectively, over their open ends. Centrally of one of the covers, shown as the cover 20', a smaller connecting pipe 24 is welded to receive a suitable connection from a source of fluid under pressure such as a high pressure water pump (not shown).

The

end walls

10 and 11, and the shell 12, are formed of an austenitic stainless steel including about l8% chromium and 10% nickel. Eight to fourteen percent nickel has also been used and the particular proportions depend on the exact end use of the vessel. Such steels are normally referred to as 18-8 stainless steels. The highly acid resistant steels also contain from 1.5 to 4.5% molybdenum. The following Table 1 sets forth a number In addition to possessing the characteristics of high resistance to corrosion, the steels listed in Table I possess excellent properties from a constructional point of view. Their tensile strength is good, elongation extraordinarily good and impact strength excellent. Their relatively low yield strength in the as-delivered condition is, however, disadvantageous. Thus, when calculating the thickness of walls necessary for a pressure vessel, the calculations must be based on a standardized yield strength value prescribed for the grade steel that is to be used in the pressure vessels. The low yield strengths designated for these stainless steels has made necessary the use of walls uneconomically thick.

Using steels of the ferritic type; i.e., ordinary low alloy pressure vessel steels which have a more or less marked yield point, the yield stress in accordance with the generally adopted code definitions is set equal to the lower yield point. This value can be defined as the stress in which a tensile test gives a certain elongation without any increase in stress.

Since the austenitic stainless steels have no marked yield point, the so-called e value, the stress that gives 0.2%. permanent elongation of the material, is used as the basis for calculating a code yield strength.

The foregoing is made evident by considering FIG- URES 4A and 4B which illustrate stress-strain curves of two typical pressure vessel ferritic steels SIS 1330 and SIS 2101 and an austenitic stainless steel grade Avesta 832 SK. Since the scale of FIGURE 4A is so small that the differences in the initial portions of the curves cannot be observed, the strain scale has been greatly enlarged in FIGURE 4B to show in detail the yield point of the ferritic steels and the absence of any marked yield point for the austenitic stainless steel. Thus, the SIS 1330 and SIS 2101 steels show marked yield points at 26 and 34 kg./mm. respectively, while the curve for the Avesta 832 SK steel shows no marked yield point. Note that the a value for the specimen plotted is approximately 28 kg./mm.

Beyond the yield points of the low alloy ferritic steels in FIGURE 4B, the stress-strain curves for the ferritic and austenitic steels differ greatly in shape. The low alloy ferritic steels first undergo a significant elongation without increase in stress (dashed portions of curves) and then their curves turn sharply up, leveling out soon thereafter. The highest stress calculated from the original cross-section which these steels can withstand; i.e., their ultimate tensile strength, is reached after only about 20% strain. Following this maximum point, a local contraction occurs and the specimens fracture.

Examining the curve of the austenitic steel, the material shows a marked strain hardening rate from the very beginning and the maximum stress; i.e., the ultimate tensile strength, is reached at a strain of about 50%. Note that the ultimate tensile strength of this steel is substantially higher than that of the low alloy ferritic steels.

The curve of FIGURE 48 also shows that austenitic stainless steel tends to harden very rapidly when deformed at room temperature; i.e., at a temperature below its recrystallization point. Thus, permanent elongation of only about 5% results in a yield strength increase of about 15 kg./mm. This increase does not appreciably reduce the elongation characteristics or the impact strength value of the steel. Referring to FIGURES 5A and 5B, which show on two scales stress-strain curves for the Avesta 832 SK steel after having been cold stretched 5%, note that the value is approximately 17 kg./mm. higher than for the unstretched material shown in FIGURES 4A and 4B.

Prior to the present invention, the wall thickness of the pressure vessel shown in FIGURE 1 had been determined by using a calculating value for the yield strength (a found in the Swedish Pressure Vessel Code. The following table lists the calculating values for pressure vessels formed of various steels, including a low alloy ferritic steel 1430 when the wall thickness is 30 mm. or less, and four austenitic stainless steels designated both by the Avesta grade and the Swedish SIS number.

TABLE 2.CALCULATING VALUES FOR THE YIELD STRENGTH (um- ACCORDING TO THE SWEDISH PRES- SURE VESSEL CODE Steel Grade Temp 1430 832 MV 832 MVI 832 SK 832 SKT C. S S30 mm. 2333 2333 2343 2343 The calculating value d' for each grade of steel is lower than the statistical value for the yield strength 0' to insure that it encompasses all melts of the particular steels. For example, while the yield strength of the Avesta 832 SK specimen shown in FIGURES 4A and 4B is representative of the statistical normal yield strength a of this grade, other specimens may have yield strengths as low as 20 or 21 kg./mm. Safety codes must allow for such variances when concerned with high pressure vessels even though the low calculating values (T make necessary the use of excessive amounts, in certain instances, of expensive 18-8 stainless steel.

According to the invention, the wall thickness of the pressure vessel shown in FIGURE 1 is calculated using a selected higher yield strength a' for example 30 kg./mm. which is substantially higher than the calculating value for yield strength (o' specified by the Pressure Vessel Code, and which is also higher than the statistical normal yield strength (a of the steel. This results in a calculated requirement of substantially thinner walls in the vessel with a resultant significant saving of expensive stainless steel. The pressure necessary to cold stretch the vessel walls to obtain the selected yield strength is then calculated and the vessel subsequently stretched. A typical calculation to satisfy the Swedish Pressure Vessel Code for a vessel formed of Avesta 832 MV (equivalent to American AISI 304) is set forth in the following example.

Example I Assuming the requirement of a pressure vessel in the category of a thin-walled vessel with an inner diameter of 2000 mm. that is to be subjected to a 12 atmospheres overpressure at a temperature of 20 C., and formed of Avesta 832 MV austenitic stainless steel, according to the Code formula min.-

iXI

bar 200 USFXZ where s minimum wall thickness of the vessel shell in mm.

D =inner diameter of the vessel in mm.

p=working pressure in atmospheres overpressure a =calculating value for the yield strength of the material at its use temperature in kg./mm. (see Table 2) s safety factor: 1.5

z joint efliciency=0.9 for X-ray controlled joints According to the Code, a =20 kg. per mm. (from Table 2; note that the yield strength 1 =24 kg./mm. from Table 1). Therefore min.

And rounding off s 7 mm.

Constructing the pressure vessel with a wall thickness of 7 mm. uses 30% less steel than would be required for a vessel constructed with a wall thickness of 10 mm.

Next the pressure P necessary to cold stretch the sealed pressure vessel to increase its yield strength to 30 kg./mm. is calculated. The formula for calculating P which provides wall stresses in the vessel equal to the desired yield strength, is in principle the same as for calculating wall thickness. However, the safety factor and joint efficiency may be disregarded and 1 subsituted therefor. Therefore, the basic formula for thin-walled vessels is where s=actual wall thickness of the vessel shell in mm.

o =the desired yield strength of the steel in kg./mm. D =the inside diameter of the shell According 21 kg./cm. is the fluid pressure required to cold stretch the vessel sufiiciently, about 2% in the weakest regions of the vessel in this instance, to provide a yield strength of 30 kg./mm. Of course, thicker portions of the vessel will be less afiected, and in some instances unafiected, by the cold stretching. However, because the vessel is permitted to expand freely, the weakest regions will be stretched the greatest amount and will, therefore, increase more in yield strength than other portions of the vessel.

Applying these calculations to the vessel of FIGURE 1, after it has been welded together with relatively thin walls, a high pressure water source is connected to the pipe 24 and the pressure in the vessel is increased in stages to the pressure of 21 kg./cm. to cold stretch the walls of the vessel sufficiently to obtain a minimum yield strength of 30 kg./mm. Even though the vessel expands freely, it walls will be shaped substantially the same as the original unstretched vessel. Thus, the yield strength of the austenitic stainless steel in its stretched area increases rapidly and the initially weakest regions first subjected to stretching increase in yield strength to such an extent that comparatively quickly the stretching spreads to other portions of the vessel.

Control measurements may also be carried out as the pressure is increased on the change of circumference of the vessel. Note that the end portions of the shell 12 will be stretched slightly less than other regions since they are lent support by the domed walls and 1 1. Also reinforced and overdimensioned portions and those portions which because of their location in the vessel will not be subjected to sufficiently high stresses, will be stretched little or not at all. The same also holds true for those portions which already have the necessary yield strength. Therefore the circumference will. not change precisely the same percentage as the weakest regions of the vessel.

When the cold stretching is completed, the temporarily welded covers 20, 21, 22 and 23 are cut ofi and, in the example shown in FIGURE 2,

ribbed pipes

25 and 26 are welded to the

pipe sockets

16 and 17, a straight pipe 27 is welded to the socket 18, and a flanged elbow pipe 28 is welded to the socket 19.

Since the

sockets

16 and 18 are formed of relatively thin material, the adjacent vessel walls are reinforced by extra welded

sheet metal plates

16a and 18a. On the other hand, the

pipe sockets

17 and 19 are sufliciently strong to stiffen the adjacent portions of the vessel walls and compensate for weakening due to the socket apertures.

When it is desirable to weld a pipe socket to a pressure vessel subsequent to cold stretching, a reinforcement with suflicient thickness to compensate for the heating and annealing effects during the subsequent welding may form a portion of the vessel. Thus, the shell 12 of FIG- URES 1 and 2 includes a plate 29 that is considerably thicker than the remaining plates. Subsequent to cold stretching of the vessel, an opening is cut in the plate 29 and a manhole fitting 30 welded into the opening, as shown in FIGURE 2. The manhole fitting is dimensioned to compensate for weakening due to the aperture, and the thickness of the plate 29 is chosen to compensate for the decrease in yield strength caused by the welding operation. A manhole cover 31 is held in place by a crossbar 32 joined to the cover by a nut and bolt 33.

EXAMPLE II Considering another exemplary pressure vessel constructed in accordance with the invention, assuming a thin-walled shell having an inner diameter of 6000 mm. to be subjected to 4 atmospheres overpressure at a temperature of 100 C., and formed of Avesta '832 SK steel (equivalent to American A181 316), according to the Swedish Pressure Vessel Code, a ,,=16.2 (from Table 2). Substituting in Formula 1 s,,,,,,,: 12.3 mm.

Thus a conventional pressure vessel formed of Avesta 832 SK steel requires walls 12.3 mm. thick.

A higher yield strength value U of 35 kg./mm. is then selected. However, since the vessel will be cold stretched at 20 C., this yield strength value must be converted to a value that may be used in Formula 1. Table 2 shows that at 20 C. the calculating value a is 21 kg./mm. and at C. the value is 16.2 kg./mm. Referring to FIGURE 10, which shows the mechanical properties of Avesta 832 SK steel at temperatures ranging from 20 C. to 400 C. after cold stretching varying amounts, it is apparent that a direct ratio cannot be made to obtain the higher yield strength necessary at 20 C. to provide the selected yield strength at 100 C. Because the curves for 0%, 2.5%, etc. have generally the same contours, the value can be obtained by calculating the difference between the selected yield strength at 100 C. and the code yield strength at 100 C., which is 35 minus 21:14 kg./mm. and adding this amount to the code yield strength at 100 0; thus 16.2 plus 14:30.2 kg./mm. Substituting a calculating value of 30.2 kg./mm. in Formula 1 And rounding off S =7 mm.

Constructing the vessel with a wall thickness of 7 mm. results in a saving of about 43% of the steel used in a conventional vessel having a wall thickness of 12.3 mm.

Next the pressure P necessary to cold stretch the sealed pressure vessel to increase its yeld strength to 35 kg./mm. at 100 C. (30.2 kg./mm. at 20 C.) is calculated by substituting in Formula 2 Therefore 7 kg./crr1. is the fluid pressure required to cold stretch the vessel sufficiently, about 2%, in the weakest regions of the vessel, to provide a yield strength (v0.2) at 100 C. of 35 kg./mm.

It is apparent from Example II and FIGURE 10 that austenitic stainless steels provide, when cold stretched, higher yield strengths at elevated temperatures as well as at room temperature. Tests show that prolonged heating does not impair the higher yield strengths obtained at elevated temperature. Note also that the real yield strength increase remains practically unchanged at different temperatures and, therefore, the relative yield strength increase is higher at elevated temperatures. This enables great savings in the amount of stainless steel used as is illustrated by Example II.

Examples I and II demonstrate in detail how the principles of the present invention are applied to the construction of austenitic stainless steel pressure vessels under the applicable Swedish Code. The following Examples III, IV and V compare the manner of calculating the wall thickness of the inventive pressure vessels pursuant to the Swedish Pressure Vessel Code, 1959, Berechnung von Druckbehaltern, AD-Merkblatt Bl, August 1959 (West Germany) and ASME Boiler and Pressure Vessel Code, Section VIII, 1962. The following Table 3 showsthe designations used in the different Codes as well as some basic formulas of calculation.

TABLE 3 Designations and formulas Swedish Pressure German AD- Definition Vessel Code Merkblatt B1 ASME Minimum thickness of shell s im, mm. S, nun. t, inches. Design pressure p kg./cn1. p, kgJcm. P, p.s.1. I ii e Shen' 13/2, mm. 131/2, m. R, inches. I Da/2, mm.

:1.5 Included 111 the allowable stress. Maximum allowable stress us r/SF, kgJmm. K/s, kg./mm. p.s.1. Joint efiiciency Z V At temp. l C. DXp DaXD P R Basic formula for tl1in-walled cyllndrical shells smin. m -m -SXE+0 6P At temp. 120 C. Da p 200K/sXv+p The three basic formulas of calculation in Table 3 are 20 TABLE 4 identical in principle, although there are some dltferent Avesta 832W factors. In Sweden the maximum jolnt efiic1ency 1s 0.9 steelType $182335 Werkstofi 4301 M81304 but in West Germany and the United States a o1nt etfi- Swedish ciency of 1 may be used. Since the matenal costs of T prevssuri ASMEC d (I t emper- 8558 l 0 e 11 crstamless steel vessels are comparat1vely h 1gh, all calcu mums Code ADMerkblattW2 p a e values) lat1ons have been made w1th maxlmum Joint efficlc c um/3, Smckgreme In Sweden and Germany the pressure vessel d1mene/ m. /i s./mm. KgJmmJ P.S.l. sions are based on the yield strength of the material. By 20 1 3 3, 2 50 120 1.0 12. 1,20 us1ng a safety factor wh1ch 1n both cases 1s 1.5, the 100 212 m3 113 1L8 17,850 allowable stress is limlted to /3 of the stated calculatlng 150 302 0.3 10.3 11.2 10, 000 h ASME 0d 200 302 8.6 0.3 10.9 15,500 value for the y1eld strength. However, t e c e 250 482 8.7 10' 7 157150 indicates directly the allowable stress wh1ch has been 2% 2g 8.0 18.2 12,328 based on the ultimate tensile strength of the material in 400 752 IIII I 141650 such a way that the allowable stress 1s 25% of the ult1- Avesta 832 SK mate tensile strength. Steel Type SIS 2343 Werkstofi 4430 nrsr 310 When calculations are based on the gigldbstrength, thle lsawedish v rovi e usln t e ressure ncreased y1eld strength alue p y g Tempw Vessel ASME Coded 1nvent1ve cold stretchlng method 1s used 1n the formula. aml-es 2: gi ll terpolated values) he X2 rec grenze W The cold stretchmg pressure (P required to provide the 40 a C. OE kg r/m 2 X2l3kHmm-2 KgJmmj PM higher y1eld strength 1s so chosen that the resultlng wall 2 4 4 7 stress in the vessel is ecual1 to bsuch y1eld strizngth. ITh; g 22 2 13 328 for alcu atin as een 1ven as ormu a 100 21 10.3 11.3 13.1 18,600 fiormula c d 1 S d 150 302 0.7 10.3 12.6 17,900 1n Example I for thln-wa e vesse s 1n we en. g 2 g 3 12.3 17,500 i 50 82 .5 s. 12.1 17,250 In a th1ck Walled vessel the str ss (1 strlbutron 1s some 300 572 80 8'0 120 00 what more compllcated; for that reason speclal formulas 350 662 7'7 120 17,100 are used for calculation of the shell thickness. The 1- 400 752 11.9 10, 900

culation of P is also based on those special formulas in The following three examples compare resulting wall a similar way as shown above in Example I. thicknesses for calculations according to the Swedish,

Diflerent codes designate various allowable stresses German and United States codes.

for austenitic stainless steel. Table 2 shows in detail the E l 111 Swedish Code calculating values and the following Table 4 Thin-wall d h ll; compares, for Avcsta 832 MV and 832 SK steels, the Swedish, German and interpolated ASME Code values. Specifically, the Swedish and German values below correspond to the calculating values in the Swedish Pressure Vessel Code and AD-Mcrkblatt W2 1956, respectively. AISI 304,Werkst0ff 4301 However, the code values have been reduced to /3 (safety factor) in order to correspond to the allowable stress and thus be comparable with the ASME values.

Design pressure12 kg./cm. =171 p.s.i. Inside diameter-000 mm.=7-8% inches Working temperature-20 C.=68 F.

The required wall thicknesses of the steel according to the different codes in unstretched and cold stretched state will be as follows:

Sweden Germany United States Maximum allowable stress according to code- ;=13.3 kg./mm. %=14. 7 kg./mn1. 18,750 p.s.i., 13.2 kg./rn.m.

Minimum thickness of unstretched shell 9.2 mm. 8.2 mm. 0.36 inch, 9.1 mm. Yield strength of cold stretched material. 35.0 kg./mm. 35.0 kgJmm. 35.0 kg./rnIn. 49,780 p.s.i. Minimum thickness of cold stretched shell 5.7 mm. 5.1 mm. 0.20 inch, 5.1 mm. Reduction in wall thickness when using 38% 38% 44% the cold stretching method. Cold stretching pressure 20.0 kg./em. 18.6 kg./cm. 18.2 kg./cm. 260 p.s.i.

MaterialAvesta 832 MV corresponding to SIS 2333,

1 1 Example IV Thin-walled shell:

Design pressure4 kg./cm. =57 p.s.i.

Inside clian1eter6000 mm.==236.22 inches Working temperature100 C.=212 F.

Material-Avesta 832 SK corresponding to SIS 2343,

A181 316, Werkstotf 4436.

In this case the working temperature is 100 C.=2l2 F. However, for practical reasons cold stretching is carried out at room temperature. This means that the material has to be cold stretched to a higher yield strength than the one applicable at the working temperature. As mentioned in connection with Example II, the increase in yield strength obtained at room temperature remains substantially unchanged at higher temperatures. Therefore the required yield strength value at room temperature is calculated by adding the desired increase in yield strength at working temperature to the value applicable at room temperature according to the code.

Based on the values applicable in Sweden the calculation would consequently be as follows:

Yield strength at room temperature:

(32.0-l6.2) +21.0=36.8 kg./rnm.

Based on the German code it would be:

Yield strength at room temperature:

(32.0- 17.0) +22.0=37.0 l g./mrn.

The ASME code does not state any yield strength values and for this reason we have used the Swedish values in the calculations for the United States. These values also correspond fairly well to the German values.

The required wall thicknesses of the steel according 10 is thinner as compared to the ASME calculation than is the case in the other two examples.

In this example special formulas for thick-walled shells have to be used according to Swedish and German codes. However, when using the ASME code the basic formula 15 is still applicable.

The Swedish formula (chapter 8 in the code) 8 pl: 100 a... z

2 l 1 v... z\ p SF For a simplified calculation of s the code offers special tables.

The German formula (AD-Merkblatt B 10 October 1954 The yield strength which the material must attain during the cold stretching process, carried out at room temperature, is calculated in the same way as shown in EX- ample IV. At room temperature the calculating values are 22.0 kg./mm. (Table 2) and 27.0 kg./mm. according to Swedish and German codes respectively.

The required wall thickness according to the different codes in unstretched and in code stretched design will be as shown below:

Sweden Germany United States Maximum allowable stress according to code... -%=10. 4 kgn/mm. ':=11. 7 kglmrn. 17,040 p.s.i., 24.2 kgJmm.

Minimum thickness of unstretched shell 67.2 mm. 51.7 mm. 1.88 inches 47.7 Yieldtstreggh 1 1% stretched material: k I 2 mm 35 g. nun. 32.0 kg. mm. 38,000 p.s.i., 27.3 k mm.

At 20 C. 68 F 38.4 kg-ilnm. 41.5 kgJmm. 54,500 p.s.i., 38.4 kgjmmfi Minimum thickness of cold stretchedshelL- 27.2 min. 24.1 mm. 1.18 inches, 30.0 mm. Reduction in wall thickness when using 58% 53% 37% the cold stretching method. c ld stretching pressure 500 kg-lcm. 485 kgJcm. 7,700 p.s.i., 540 kgu/cmfi to the different codes in unstretched and in cold stretched state will be as follows:

If it is desirable to produce pressure vessels that will retain their original shape to a greater extent than the Sweden Germany United States 16. 2 17. 0 1 Maximum allowable stress according to code =10. 8 kg-l =1L 3 kgJem. 18,600 p.s.i., 13.1 kg./mm.

Minimum thickness of unstretched shell 12. 3 mm. 10. 6 mm. 0.36 inch, 9.2 mm. Yield strength of cold stretched material:

At, 100 0., 212 F 32.0 kit-Imm- 32.0 kg/mm. 32.0 kgJmmfi, 45,514 p.s.i.

At 20 0., 68 F 6-8 g-lmm- 37.0 kg-lmm. 36.8/kg mmfl, 52,200 p.s.i. Minimum thickness of cold stretched shcll 36.3 mm. 5.6 mm. 0.22 inch, 5.6 mm. Reduction in wall thickness when using 49% 47% 39% the cold stretching method. Cold stretching pressure 7-8 kg-Icm- 6-9 kg./cm.-'* 6.9 kg.]cm. 9s p.s.i.

Example V vessel of FIGURES 1 and 2, the austenitic stainless steel Thick-walled shell:

Design pressure250 kg./cm. =3550 p.s.i. Inside diameter--200 mm.=7.9 inches Working temperature-650 0:662 F.

Material-Avesta 832 SKT corresponding to SIS 2343 (stabilized), Werkstofi 4573 In the ASME code there are no figures specified for this grade. Because of its high yield strength it seems correct to use the ASME values for 'a grade with an allowable 65 plates may be cold stretched, cold rolled or otherwise cold worked prior to being welded together. Referring to FIGURE 3, a pressure vessel includes a cylindrical shell 34 enclosed by

domed ends

35 and 36. The shell is formed by a number of arcuately bent austenitic stainless steel plates 37 of Avesta 832 MV steel which have been permanently elongated about 5%. This increases the yield strength 0- to about 35 kg./mm. Since this value can be substituted for 20 kg./mm. the calculating value a' for yield strength given in the Code (see Table 2), the

stress which is as high as possible. In this case A181 316 vessel wall scan be made substantially thinner with great savings in stainless steel, as shown by the calculations in Examples I to V.

The plates 37 are then welded together along longitudinal joints 38 and circumferential joints 39 using welding electrodes of the same or similar grade steel as the plates 37. The welding heat causes annealing of the welded joints and narrow zones of the plates adjacent to them which must be assigned, therefore, a calculating value (T of yield strength of only 20 kg./mm. even though they have a statistical yield strength of 24 kg./mm.

To impart to the circumferential and longitudinal welded

joints

38 and 39, and the adjacent annealed zones, the same strength characteristics as those of the cold worked plates 37, the finished vessel is subjected to an internal fluid pressure calculated to raise the yield strength of the weldments and annealed zones to 35 kg./mm. Since such pressure will cold stretch and elongate permanently only the weld joints and the narrow adjacent annealed zones, the overall circumferential change in the vessel will be extremely small and will have a negligible influence on the shape of the pressure vessel.

The yield strength selected to cold stretch the welded vessel may also be higher than that of the prestretched plates, if desired, to insure a vessel of greater uniformity and still provide a reduction in distortion.

Still a further reduction in distortion of the vessel may be obtained if the

joints

38 and 39 are welded with electrodes adapted to impart to the weldments the yield strength of the cold worked plates. If the welded joints are produced in this manner, then only the material in the narrow annealed zones of the plates adjacent to the welded joints will be permanently elongated when the calculated pressure is applied to the completed vessel. Although the width of those zones is small, it may be reduced still further by welding in a cold state; i.e., by cooling the welded joints with a stream of air or the like after each welding run. In this manner only an extremely small change in the shape of the vessel is effected by the cold stretching.

It has been found that the selected yield strength should be sufiiciently high after cold stretching to provide a permanent elongation of the austenitic stainless steel in the weakest regions of the pressure vessel essentially in excess of 0.2%. Experiments have shown that which the presently used austenitic stainless steels shown in Tables 1 and 2, the yield strengths selected have most often provided permanent elongations of the weakest re gions of the pressure vessels on the order of 1 to 4%. However, higher yield strengths resulting in greater percentages of permanent elongations may also be selected so long as the permanent elongation is at most 10%, for reasons which will be explained in greater detail hereinafter.

With respect to the extent of permanent elongation of the austenitic stainless steel, considering the curves of FIGURES 4A and 4B, note the initial extremely high rate of increase in yield strength of Avesta 832 SK steel up to about 0.365% strain and then the less steep but still high rate of increase. In other words, a permanent elongation of 0.2% requires a strain of about 0.365%, as shown in FIGURE 4B, and encompasses the steepest rise of the curve which must be used to obtain the benefits of the invention. The rate of increase following 0.365% is also high, see FIGURE 4A, and may be used but only up to at most about 10% elongation.

The upper limit of cold stretching austenitic stainless steels is critical because, if exceeded, the ratio of yield strength to ultimate tensile strength will increase beyond about 0.8, the upper limit of that ratio required for safety. Referring to the curves shown in FIGURE 6, the ultimate tensile strength and yield strength of grade AISI 316 stainless steel are shown when cold stretched with reference to the cross-section of the original unstretched material. Note that the yield strength (a of the austenitic stainless steel increases at a high rate during the first 10% elongation, this also being obvious from the curves of FIGURES 4A and 4B. This high rate of yield strength increase enables the margin between the normal working stress to which the vessel is subjected and the yield strength to remain at a safe level even though the pressure vessels have walls substantially thinner than those previously thought acceptable.

FIGURE 6 also shows that if the austenitic stainless steel is stretched no more than about 10%, the material will still yield sufficiently when subjected to an overload to meet safety standards because the ratio of yield point to ultimate tensile strength will not exceed 0.8 (68,000 p.s.i./ 85,000 p.s.i.). This is the maximum ratio permitted by safety-standards and represents a critical value insofar as consequences are concerned, as does the permanent elongation of about 10% upon which it depends. Of course different melts of AISI 316 steels, and other austenitic stainless steels, have slightly different properties; thus a permanent elongation of 10% might for some provide a ratio of .77, and for others .82, and those differing properties must be considered when selecting the maximum amount of cold stretching to which the vessel will be subjected. I

The upper limit of 10% is also important as regards the stress corrosion resistance of austenitic steels. Originally the inventive cold stretched vessels were considered unacceptable by the regulatory authorities due to the deep-rooted conviction that the susceptibility of austenitic stainless steels to stress corrosion increases if such steels are subjected to cold-working. Sections of previous pressure vessel codes, which do not permit testing of stainless steel vessels at pressures 50% greater than their working pressures (ferritic vessels are so tested) to avoid any possibility of cold working the stainless steel, reflect this conviction as well as the prior literature and attitudes in the art of stainless steel. However, extensive tests carried out in connection with the present invention show that cold-working essentially in excess of 0.2% and at most about 10% does not impair significantly the stress corrosion resistance of austenitic stainless steels and, in fact, in certain instances improves their stress corrosion characteristics.

In one test, the results of which are shown in FIG- URE 7, from 15 mm. thick plates of Avesta 832 MV and Avesta 832 SK steels that had been cold stretched 0, 2.5, 5 and 10%, specimens were taken partly for corrosion tests and partly for determining the yield strength of the cold stretched steel. Comparative tests were carried out in 40% CaCl pH 6 and C. under uniaxial tensile stress. The tensile stress was chosen in relation to the yield strength at 100 C. so that in all cases it amounted to /3 of the yield stress (0 A similar test was carried out with specimens from the same plates that had been welded and then cold stretched and cut so that the central part consisted of weld metal.

One curve in FIGURE 7 shows the characteristics of Avesta 832 SK steel when cold stretched 0, 2.5, 5 and 10%. Also shown is another curve for a welded specimen. The similar results shown are obtained when the weld filler metal is alloyed at least equally or preferably alloyed somewhat higher than the base metal.

It is important to note that the specimens of FIGURE 7, cold stretched to different percentages, should not be tested under the same load but under a load which is proportional to the yield strength at 100 C. An experiment using the same loading stress would not give results as useful as these since the intention is to use the effects of raising the yield strength by cold stretching to decrease the wall thickness of the vessel and increase the stresses in the wall as compared to an unstretched vessel.

FIGURES 8 and 9 show the results of another stress corrosion test series conducted on grade Avesta 832 MV; i.e., non-molybdenum alloyed steel. In this test the corrosion has been accelerated and it can be seen from FIG- URE 8 that the time to fracture increases with increasing 15 degyree of cold stretching at identical loads up to about 10 o.

The lower curve of FIGURE 9 shows the time to fracture for unstretched specimens while the upper curve shows the time to fracture for stretched specimens. It is apparent that the unstretched steel is inferior to the cold stretched steel with regard to stress corrosion resistance.

In still another stress corrosion test series conducted with welded vessels cold stretched at dilferent internal pressures to different degrees of elongation, the results showed that the unstretched vessels have gone to fracture in considerably shorter time than the cold stretched ones, which is in agreement with the above tests. The failure always occurred near the longitudinal weld of the shell in the unstretched vessels whereas in the cold stretched vessels, the failure always occurred in material that was not influenced by welding. Most probably this can be explained by the fact that the high stresses normally resulting at the weldment have an unfavorable influence on resistance to stress corrosion. These stresses are relieved during the cold stretching; consequently, this is still another advantage derived from the present invention.

The pressure vessels formed pursuant to the present invention have the same resistance to general and intergranular corrosion as do unstretched vessels. Experiments using the Huey test (boiling 65% HNOg) have shown that the austenitic stainless steels can be cold stretched 10% without impairing their resistance to such corrosion. In non-oxidizing corrosive solutions (boiling 2% H SO +'3% Na SO cold stretching to 10% does not affect the rate of general and intergranular corrosion.

Moreover, the susceptibility to pitting of the inventive pressure vessels have not been impaired by cold stretching up to 10%; this was proved by tests using acid (10% FeCl 50 C.) and neutral chloride solutions (4% NaCl+0.2% K Fe(CN) 25 C.).

Extensive tests have also shown that austenitic stainless steel pressure vessel embodying the principle of the invention have excellent fatigue trength. Six test vessels of Avesta 832 SK stainless steel having a diameter of 400 mm. and a wall thickness of 4 mm. were filled with salt solution and subjected to pulsating inner pressure at a temperature of 150 C. The solution contained 15.5 grams NaCl per liter of distilled water; the solution was aerated prior to testing. Two of the six vessels were constructed of normal stainless steel, two were cold stretched 2% and two were cold stretched 5%. The maximum number of load changes used in this test series was 50,000.

One vessel in each group was subjected to a maximum stress equal to the stress permissible in calculations according to the Swedish Pressure Vessel Code. The other vessels were subjected to higher maximum pressures so chosen that the resulting stress conformed to the calculating value of the Code. In other words, in this case the stress was equal to the value to be used when dimensioning a pressure vessel with a safety factor and a joint efliciency equal to one.

After the above test series were concluded Without any material difliculties resulting, the 2% stretched vessel that had been subjected to the highest maximum pressure was then subjected to 55,000 more load changes under the same conditions. Thereafter the pressure was increased so that the maximum stress corresponded approximately to the original yield strength of the material, in this case about 29 kg./mrn. i.e., 85% of the yield strength of the 2% stretched material. Following another 44,000 load changes the test series was interrupted due to failure of the testing equipment.

Thorough laboratory investigations of the vessels thus tested were made and no change could be found. In these investigations special attention was paid to the nozzles and weld points in the vessels.

From the foregoing it is apparent that the invention provides austenitic stainless steel pressure vessels having walls greatly reduced in thickness in comparison to the wall thickness previously required, thereby affording great savings in the amount of stainless steel used. It will be apparent that the above-described embodiments of the invention are illustrative only and modifications thereof will occur to those skilled in the art.

I claim:

1. In the manufacture of welded pressure vessels from plates of austenitic stainless steel, the method comprising the steps of welding together a plurality of plates of austenitic stainless steel having a normal yield strength with weldments of austenitic stainless steel to form a pressure vessel, selecting a higher than normal value of yield strength within a range of values provided by cold stretching the steel an amount sufficient to elongate it permanently essentially in excess of 0.2% and at most about 10%, and subjecting the vessel internally to a fluid pressure suificient to stress the weakest portions of the vessel walls to the selected yield strength and thereby increase the yield strength of the said weakest portions to the selected value by permanently elongating the said weakest portions of the walls essentially in excess of 0.2% and at most about 10%, the vessel being subjected to the fluid pressure at a temperature below the recrystallization temperature of the steel and under conditions permitting the vessel walls to expand freely.

2. In the manufacture of welded pressure vessels from plates of austenitic stainless steel, the method comprising the steps of selecting a higher than normal value of yield strength within a range of values provided by cold stretching the steel an amount sufiicient to elongate it permanently essentially in excess of 0.2% and at most to a percentage at which its yield strength is increased to a value that provides a ratio of yield strength to ultimate tensile strength of .8, welding together a plurality of plates of austenitic stainless steel with weldments of austenitic stainless steel to form a pressure vessel, and subjecting the vessel internally to a fluid pressure suflicient to stress the weakest portions of the vessel walls to the selected yield strength and thereby increase the yield strength of the said Weakest portions to the selected value by permanently elongating the said weakest portions of the vessel walls an amount essentially in excess of 0.2% and at most to a percentage at which the yield strength of the said weakest portions is increased to a value that provides a ratio of yield strength to ultimate tensile strength of .8, the vessel being subjected to the fluid pressure at a temperature below the recrystallization temperature of the steel and under conditions permitting the vessel walls to expand freely.

3. In the manufacture of welded pressure vessels from plates 'of austenitic stainless steel, the method comprising the steps of selecting a higher than normal value of yield strength within a range of values provided by cold stretching the steel an amount sufficient to elongate it permanently essentially in excess of 0.2% and at most about 10%, welding together a plurality of plates of austenitic stainless steel with weldments of austenitic stainless steel to form a pressure vessel, and subjecting the vessel internally to a fluid pressure sufficient to stress the weakest portions of the vessel wall to the selected yield strength and thereby increase the yield strength of the said weakest portions to the selected value by permanently elongating the said weakest portions of the vessel walls an amount essentially in excess of 0.2% and at most about 10%, the vessel being subjected to the fluid pressure at a temperature below the recrystallization temperature of the steel and under conditions permitting the vessel walls to expand freely.

4. In the manufacture of welded pressure vessels from austenitic stainless steel plates, the method comprising the steps of selecting a higher than normal value of yield strength within a range of values provided by cold stretching the steel an amount suflicient to elongate it permanently essentially in excess of 0.2% and at most about 10%, cold working the steel plates to stress their weakest portions to the selected yield strength and thereby increase the yield strength of the said weakest portions of the plates to the selected value by permanently elongating the said weakest portions of the plates essentially in excess of 0.2% and at most 10%, welding the cold worked-plates together with weldments of austenitic stainless steel to form the pressure vessel, and subjecting the vessel internally to a fluid pressure suflicient to stress the Weakest portions of the vessel walls to the selected yield strength and thereby increase the yield strength of said weakest portions to the selected value by permanently elongating the said weakest portions essentially in excess of 0.2% and at most 10%, the vessel being subjected to the fluid pressure at a temperature below the recrystallization temperature of the steel and under conditions permitting the vessel walls to expand freely.

5. In the manufacture of welded pressure vessels from austenitic stainless steel plates, the method comprising the steps of selecting a first higher than normal value of yield strength within a range of values provided by cold stretching the steel an amount suflicient to elongate it permanently essentially in excess of 0.2% and at most about 10%, cold working the plates to stress their weakest portions to the selected first value of yield strength and thereby increase the yield strength of the said weakest portions of the plates to the selected first value by permanently elongating the said weakest portions of the plates essentially in excess of 0.2% and at most 10%, welding the cold-worked plates together with weldments of austenitic stainless steel to form the pressure vessel, selecting a second higher than normal value of yield strength within said range of values, and subjecting the vessel internally to a fluid pressure suflicient to stress the weakest portions of the vessel Walls to the selected second value of yield strength and thereby increase the yield strength of said weakest portions of the vessel walls to the selected second value by permanently elongating the said Weakest portions essentially in excess of 0.2% and at most 10%, the vessel being subjected to the fluid pressure at a temperature below the recrystallization temperature of the steel and under conditions permitting the vessel walls to expand freely.

6. The method defined in claim wherein the selected second value of yield strength is higher than the selected first value of yield strength.

7. An austenitic stainless steel pressure vessel comprising a plurality of stainless steel sections, austenitic stainless steel Weldments joining the steel sections to form the vessel, the steel sections and the weldments having been stressed to a selected value of yield strength higher than the normal yield strength of the steel by subjecting the vessel internally to fluid pressure suflicient to cause a permanent elongation in the weakest portions of the sections and weldments essentially in excess of 0.2% and at most about 10%, the vessel having been subjected to the fluid pressure at a temperature below the recrystallization temperature of the steel and under conditions permitting the vessel walls to expand freely.

8. An austenitic stainless steel pressure vessel comprising a plurality of stainless steel sections that have been cold worked to elongate them permanent essentially in excess of 0.2% and at most 10%, austenitic steel weldments joining the cold worked steel sections to form the vessel, the steel sections and the weldments having been stressed to a selected value of yield strength higher than the normal yield strength of the steel by subjecting the vessel internally to fluid pressure suflicient to cause a permanent elongation in the weakest portions of the sections and weldments essentially in excess of 0.2% and at most about 10%, the vessel having been subjected to the fluid pressure at a temperature below the recrystallization temperature of the steel and under conditions permitting the vessel walls to expand freely.

References Cited UNITED STATES PATENTS 2,337,247 12/1943 Kepler. 2,579,646 12/1951 Branson 29421 2,652,943 9/1953 Williams 2203 2,961,530 11/1958 Macha. 2,914,346 11/1959 Ryder 29482 X RAPHAEL H. SCHWARTZ, Primary Examiner US. Cl. X.R.

UNITED STATES PATENT OFFICE Patent No.

Inventor(s) July 22, 1969 Dated .Tnhan Tnguar Tnha'nccnn It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

golumn 7, line 1, "According should read Accordingly-;

Column 11, fifth line in the last table, "6. 8 kg./mm.

should read --36. 8. kg./mm.

sixth line in the last table, "36.3 mm." should read -6 3 mm.-

ColumnlS, line 33, "have" should read has-;

line 39, "trcngh" should read -strength; line 70, "change" should read -damage;

Column 18, line 18, "permanent" should read permanently.

SIGNED IND SEALED MA! 5 1970 Attcst: mm 1:. W. JR.

Col-1.01am 0! Monte Edward M. mach, h.

Attcsting Officer