Pressure Vessels for Oil Cracking ljiveted, hamriier- arid electric-welded, arid pierce-ingot type cracking vessels, with rt7ettiods of fabrication, rnalerials used, strenglh of vessels umder various temperature-pressure conditions, wall fhic.kn,ess,1;fe of the vessels, an,d x-ray method of testing welds are described. (;US'I'AV
EGI.GlT,
J.
c. h f o i ~ ~ AND ; ~ ~E., c. LEONIIARDY
Universal Oil I'roducts Co., Chicago, 111.
Ill? use uf tlrc crackiiig process iii the oil industry is growing a t an ever-increasing rate. It has reached a stage of development in rvhicir w e n ordinary gawliiic from crude oil is being converted under elevated temperaturos and pressures into high antiknock motor fuels. 'L'lie past seven years have shown a decrease in ordinary gasoline production froin crude of F6 to 51 per cent, diereas cracked gasoline increased from 26 to 42 per cent, based upon all the gasoline nrarketed during 1931. This tremendous increase in cracking capacity a t high temperatures and pressures to wvhich petroleum oils are subjected calls for thicker and thicker walled vessels to withstand the increased operating pressures and temperatures. In 1931 the Ciiited States cracked-gasoline production totaled 176,000,000 barrels, and this has been nrnde possihlc lry contiriiied r e s e a r c h which has gone on steadily since cracking first became a significant factor i n gasoline p r o d u c t i o n t w e n t y yeam ago and whiclr continues to this day. There is an estimated expenditure of over $5,000,000 a year lor researcli in cracking, whiclr includes treating, furnace designs, fractionation, corrosion r e s i s t a n c e , a n d metallurgy of tubes and reactio11 chambers. There is a total investment in cracking equipment and accessories e s t i m a t e d a t over $400,000,000. The watchword in cracking operation is "safety, safety, and then more safety." 'Ilie increasing operating temperatures and pressures for gasoline making t h r o u g h o u t t h e years have brought about an increase in manufacturing difficulties to p r o d u c e large vessels to withstand the conditions desired. Figure 1 shows a 10 X 40 foot r e a c t i o n chamber beine nlaced in nosition The industry has passed through the stage of employirig riveted vessels and hammerwelded vessels and is now using electric-welded and seamless forgedsteel vessels. The limitations of the various methods of fabrication namely, riveted, hamnrer-melded, fusion-welded, and seamless forged constructions, are largely governed by the following factors:
T
LI
I
1.
Tornperatareand pmsure conditionshfservice
4.
Material and design requirements.
2. Eficiency of joints lor various services. 3. Plate thickness, size and shnpe of container.
R i v 1 s . r ~VESSELS ~ The cracking conditions employed in early coinmercial practice to produce gasoline from gas oil were about 75 pounds and temperatures of the order of i50' I?. Riveted vessels were good enough under such operating conditions. (The riveted type of construction was adopted because of its success for high-pressure work in the generation of steam, and because it was the only type of vessel generally available.) With the demand for increased yields and larger capacities, making necessary the use of higher temperatures and pressures, it was found that the riveted typo of construction was unsatisfactory owing to l e a k a g e of oil vapors and the consequent fire h a z a r d . E v e n t h e caulking of s u r f a c e s and bare-wire e l e c t r i c welding around the rivek and seams was insufficient to correct these conditions b e c a u s e tlre s t r e s s e s botween the joints were toh great for the weld to resist. The welds cracked, a n d t h e pressure vessels continued to leak and hence were a source of danger. Figure 2 shows one type of riveted r e a c t i o n chamber used for oil cracking. S e v e r a l t h o u s a n d r i v e t e d wsseis have been used in the oil industry. ITAXMER-WELDED VESSELS H a m m e r - w e l d i n g was next adopted for joining steel. This process gave Eairly satisfactory resulk in the f a b r i c a t i o n of lightwalled vessels hut was unsatisfactory for the heavierwalled tvnc. The earlier h a m m e ; ~ w e l d e d vessels were unsatisfactory, mainly because of defective manufacture. With more extensive experience, however, these faults were largely overcome. S m m x s s FORGED VESSELS IZolled seamless cylinder sections have been used in the construction of riveted and welded w.ossels, and by this
1264
Novemb~r,1932
FlGUl$E
RIAL AND
ENGINEEIIING
2. RIve.l.cn REACTIONCIlAXBEll
method of fabrication the longitudiiial joints of the cylinder, which norma]lv are subiectcd to much stress than the girth join&, are eliminated. Figure 3 is an example of rolled seamless cylinder sections with riveted oonstmction. Seamless forged vessels are made from large ingots cast from open-hearth furnaces. For a large pressure vessel it is necessary to cast extremely large ingots to take care of tlie discard and tlie loss of metal during the process of manufacture. Ingots weighing over 200 tons have been cast for t h i s p u r p o s e . To pour ingots of such size, it is necessary to utilize s e v e r a l open-hearth furnaces to supply the metal. h great deal of care is e x e r c i s e d to pour the ingots in this manner. The several furnaces must be carefully regulated to be in readiness for tapping as the pouring progresses. It is the best practice to pour tlie ingots from the bottom of the mold in order to restrict oxidation and to give a cleaner surface. The mold is u s u a l l y set wit.11 the large end up to east l a r g e -ingots. The natural FrGUnE cooling and solidification from the bottom to the top proceeds at a slower rate than when the large end is down, thus affording greater opportunity for the nonmetallic imparities to gravitate upward ttirougli the molten metal. These particles will become entrapped within the steel if solidification proceeds too rapidly. Hot top molds having a refractory-lined bottlc-neck seetion a t the top, known as sinkhead, are used to keep the metal o”lll0r MrACs. of cILIxoLI1-
upper section in a molten state until the ingot proper has solidified. The pipe in the top of the ingot caused by the contraction of the metal on solidifying is confined to this section. This method decreases the size of the section discarded or cropped to remove the pipe and insure soundness. iii the
CHEMI.
1265
The inold is stripped from tile ingot as soon as it Ius cooled sufiiciently to solidify throughout. Very large ingots are subjected to great iuternal stresses during tho cooling, owing t,o tlie fact that the metal in tlie central portion of the ingot is considerably hotter than at the surface. To avoid this condition, the ingots, while still hot, are placed in furnaces and “soakcd” so tliat tlie mass is heated unifornily throughout the entire cruss section. Tlie aunenling temperature, which has heen brought up slowly to the forging heat, is Iield approxirnately one hour for each inch of tliickness of the ingot diameter. The temperature for forging lov-carbon steels ranges from 2200’ to 2250’ F. Following this, tlie ingot is tiikeu out of tlie lienting furnace, and tlie sinkliead contaioing the pipe at the top is sliced oiT witli a cutter in a powerful hydraulic press. The ingot block is then reheated to forging temperature and upset in a hydraulic press for tlie purpose of reclining its diameter to the desired dinrensions. The ingot block is again rclieated, and a cure 24 inclies in diameter uuslied out of its center. This hole is usually enlarged to about 27 iiielies by drifting, so as to nerrnit the insertion of tlie first mandrel for the expanding Operation foliowing. The bore of the hollow ingot is then drawn and enlarged by rewated lieating to the forging temperature and tlie applicatioii of successively larger mandrels. This is con-
3. Ror.i.su SEAM~.ESS CYLINDER Scmmxs
\VIT~I
RIVETED
CONSTRUC’I’ION
tinned until the tube is roughly drawn out and expanded to the desired diameter. A great deal of skill is required for these operations. It is essential that the work be done ~vit,liin the forging temperature range, preferably near the lower limit. It is difficult to control tlie temperature for such a large mass of steel. The expanding and drawing operations tend to enlarge surface defects. These defects must he coinpletely chipped out before they devclop too far; otlrerwise they may beeome serious in the later forging opcration. Internal defects cannot be observed or corrected. The roughly shaped cylinder is macliined inside and out if it is necessary to have a uniform wall tliickness and diameter. The cylinder is then reheated and tlie ends closed in to form heads. Manways and nozzlos are forined into 8x1 intepal piece with the rest of the cylinder. Several heats are required to form these parts. Owing to upsetting, there is considerable excess metal in the lieads and manways. Tlie nianivays and nuzzles are machined and the vessel given a hydrostatic test. Heavy equipment, such as large molds, hydraulic presses, mandrels, and lifting facilities, are required to liandie the massive forging during the various operations. Seamless forged vessels arc suitable for high-pressure and
1266
1 h' D
U S T K I A L A X D E N G I N E 13 I( I N G C I-I E M I S T 1%Y
high-ternperature service. The diameter is limited to about 6 fcet, and the size to the weight of tho ingot possible to be cast. The weight of the finished vessel after passing t,hrriugh tlie various operations is less than 50 per cent of the original ingot. If the construction of the vessel requires uumerous nozzles on the side of the cylinder, it is the usual practice to provide rnacliincd bosses for the installation of .iiiin?. . Er'Bo~l~lisC-'?I'~:l.l,2-:U VESfiELS
One iinportamt advance in the manitfacture of high-pressure vessels was tlic developsuesit of electric fusion welding for thick-walled and
Vol. 24, No. 11
thickuess of tlic plate aud the dianieter of the cylinder to be rolled. Tire forming of the cylinder is done either by heavy duty rolls or hydraulic presses. Rolls are best suited for this work, inasmuch as they can form the plate with one seeup and a single heat. Great care is taken to form the cylinder to the true dimeusions required in order that the various srctions can lie matched UD and assembled. Loose scale, which trn",rlE t.n n a t t l l l at tha
thls
not done the
i u l ~aervice
Elect.ric fusion F I G ~ J R E5 . G~s-FtnwiA X x ~ . a l . l u a ~ R N C I S temperature. welded iilatcs iii)t.o ti Tho l o n a i t u d i n a l inches t h i c k Iiavt: e d g e s of t h e XTeld permitted the utilization of mure suitnble rterlr fc,r iiigli- groove are tlieri tack-weliled tugether, and the section removed tenipenture sen& and the building of larger Y~ESCIS of t,o the rvelding station. Tire wlding is done in a horizontal mort: complicated design and shape. The liighly successfril position either by manually operatcd welding inachines or resiilts obtained by this nlcthod iliilile it ~miircessaryto hand operators. Four or five layers of weld arc deposited 011 the outside resort solely to seamless orged vessels which n m ~ h otlservise l he demanded for the increasingly high temperatures and groove, and the section is then rolled over 180" and the t,ack welds removed from the inside. The inside groove is then pressures required for oil cracking. Approximately fifteen hundred electric-welded pressure chipped out by air hammem to such a depth as to remore the first bead applied in the outside groove of the weld. vessels have been installed in the oil industry since 19%. Extensive equipment is required to fabricate iargc heavy- The inside groove is then welded coinpletely and the vessel walled vessels by electric welding, wliich in soulc cases weigh revolved back to the original position for completing tlie as 1nuc11as 200 tons, and the method used and work involved outside groove. About eight or ten weld leyers or beads are deposited per in constructioii varies greatly, depending upon iL$ shape, weight, thickness, and design. The first phase of the work one inch thickness of metal. Each bead is thoroughly after tlie steel has been tested and approved for coi~struction cleaned and chipped as the weld progresses. The welding operation usually sets up some distortion by the laboratory is to plane the edges of the plates preparatory to welding. Figure 4 shows a typical joint as in the area of the joint. This condition is corrected hy again forming the section to the true diameter. This is an essential planed for welding. The plates are clamped don7si by hydraulic or air pressure operation inasmuch as the sections must match up almost devices on a large plate &mer, ~rsuallyof sufficient size to perfectly for the girth or circumferential welding. The aceonmodate a plate of 30 or 40 Eect iii Icnat,li. The cdgrs sections are then stress-reliered and preparation made for their assembly. The of the plate are planed work of assembling by a t r a v e l i n g die the cylinder sections liead which operates and heads is done by hack and fort11 froin tack-welding these e a c h e n d u n t i l the units together as they metal is removed to rest on rolls. the desired dirnenLiftsfrom overhead sions. The plate is c r a n e s are u s u a l l y s t r e s s - r e l i e v e d or necessary to o b t a i n normalized in a furthe proper alignment. nace to reniove the A great deal of care residual s t r a i n s left. must be exercised to from the mill finish. keep the alignment of This leaves the matethe various units as rialin a dead soft c w straight as possible. dition for forming. If these units are not Plates 1.25 inches matched up properly, or more in thickness difficultieswill be etiare iisually fornied countered in applying hot a t temperaturei the first layer of the ranging from 1000" circumferential weld. t o 1200" F. Thca A f t e r completion temperature r a n g e of the work just defor rolling depends F~cmre6 . ISSIIB VIEW.or ELECTRIC FU~NACE scribed, nozzles and largely upon tlie Furnsee aeeommodntcs v o i d 11 feet i n diameter and 88 fect in ieeztb
~ " \ e m h c r ,193%
I N D U S T 1 5 1 A I.
A N D E N G 1N E E R I N G
C 13 E M I S T R Y
1267
pipe coimectioris are welded on the ridcs of tlie ~ c s ~ c l s .gas ooritainiiig carbon monoxide and hydrogen. The miiieral These connections are usually built up by weid deposits so or iiiorgauic conrponents produce a protective slag and permit as tu become an integral part of the vessel. The connections controlled combustion of the wood flour. :ire then faced and drilled with threaded holes so as to permit The amount of heat concentrated in the weld, tlie degree of secupe bolting with stud bolts to the companion flange fusion, arid tire extent of oxida,tion vary greatly if the arc pipes leading to the vessel. After completion of tlie con- is not stable. An arc of uniformly short length must be nections. the vessel is subjected to annealiiie. Tlre ternnerature of the annealing furnace is brought up slowly for about 5 Iiours and held at 1100" to 1200" F. for approximately 1 lioiir per inch of maximum weld thickness and is thcn alloived to cool uniformly at a low rate, either in tbe furnace or in a specially designed cooling compartment. Figure 5 slroms a gas- or &fired fiirmcc aiid Figure 0 an electric-heated furnace, for mnealiiig and etrcss-rdiesiog welded vessels. .\ftcr being annealed, the vessel is iiicjreoted and finislied off. Tlie finishing usually consists of cliipping ot7 rough welds and correcting any noticeai)le defects. The vessel is then flanged up tightly for test. Specificatioiis require that the vessel he subjected to alterriatirig stress prcssilres with m r m oil. The hammer test may also he e.iiiployed in addition to the pressure test. The initial pressure test is based on a fiber stress of 11,250 pounds per square inch. The lianirner test is inade a t 15,000 pounds per square inch internal jiressure, and the final test (lasting one hour) a t 18,750 pounds. Tho latter figures represent approximately 75 pcr cent of the yield point of tlie steel, this test being much more severe than that of a riveted or hammer-welded vessel. During the test the welds are carefully inspected for defects or distortion. After repairs, the welded surfaces (both inside FlGUAE 7. EDGP: SPECIMEN OF 3-Iurx PLATE REJECTED BEand out) are sand-blasted for a final inspection. CAITSE OF SMALL FrssunEs A N D GAS POCKETS PI~IKCIPLES OF ELECTXIC FUSION WELDING maintained and tlie current regulated for the type and &e Tlrerc are several electric fusion welding niethods wtiicli of the eleetrodc used. This insurcs good fusion and helps are siiirilar, inasmuch as welding results froin an electric the reducing gas set off by the coating to exclude the air. It is essential that the weld be kept clean; otherwise it sliort circuit of sufficient intensity to fuse tlru parts to be joined. Various types of electrodes and means of protecting will contain impurities, such as scale and slag imclusions, and tlre arc and metal being deposited are employed. The cur- gas formations, and will sliow evidence of poor fusion. These rent aniperage and the polarity uf the electrode when direct are highly objectionable. They can be reduced greatly by the careflil use of stiff Iruslies and air hammers. currmt is employcd may vary. The value of propRest results are ohwly peening and chiptaiiied for pre ssii r ? ping each layer canvessel work under riot be emphasized too cqmlitions which give s t r o n g l y . Peening the greatest p r o t e c does much to relieve tion to the arc and residual stresses and inetal heingdeposited, to reduce distortion. particularly with reT l i c density of t h e spect to the oxygen weld i s also m a d e and nitrogen of the slightly greater. Tlre atmospiiere. A suit,. object of chipping is ahle arrangement emi o give each layer a ploys a covered elect r o d e with a liighclean s u r f a c e free from slag, blow holes, amperage direct curcracks, and unfused rent, using the base metal. Ron& Dord a t e as the negative t i o n s of t h i weld pole a n d t h e electrode as the nositive vole. This metirod is known as re- should also be chipped off in order to make the surface quite verse polarit;, because of the fact that the earlier and regular. It is interesting to note that alloy welds may be made more common forms of electric fusion welding generally g., nickel, chromium, ete., have the poles oppositely connected from that descrihpd. direct or by fusing on wires-. The function of the covered electrode is to liberate a highly when making welds with the regular covered welding rod. reducing gas around the arc and cover the metal being deAUTOMATIC WELDING posited with a slag protection. One type of covering is made Automatic welding machines have been designed and used from a mixture of wood flour, kaolin, ferromanganese with sodium silicate to form a paste. The electrode is coated to some extent. I n these, electric motor-driven devices, with the mixture and then dried. The combustion of the regulated by elements particularly sensitive to arc conditions, nood flour under the existing conditions produces a reducing feed the electrode at the right rate to maintain a stable and I
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I N D U S T 1%I A L A N D E N G I N E E 1%I N G C H E M I S T R Y
1268
A
B
D A. Ercellent type oi weld B . Porous weld C. Defective weld showing slag inclusions and porosity
Vol. 24, No 11
C
E' D . Jamtion of longitudinal and girth +Is, iadicsting cor+idersble ~ u r o s i t ~, , &. Defective weld. showing slag inolusions and lack of iusion slung walls of joint3 I'. Pcrieot weld
constant arc. The electrode is also oscillated back and forth across the seam as the weld progresses. Occasionally the operator has to make minor adjustments in the speed of the machine to compensate for irregularities in the seam or uneven burning of the electrode. Much of the physical hardship generally connected with welding has, by the use of automatic means, been removed. It would not be feasible or practical to veld a large vessel solely by automatic or manually operated welding machines. Many of the parts or units of a vessel are too inaccessible or small to make this possihle.
MATERIAL REQUIXWMENTS The quahty of the steel entering into the fabrication of an electric-welded pressure vessel should be carefully selected for the service for which it is intended. It should be quite free from impurities or defects which affect the welding process. The sensitive arc becomes unstable if the plate or electrode has these present. Carbon segregations, excessive sulfur, oxides, gas pockets, small IaminatioIis or fissures arc especially objectionable. A weld under these conditions may lack sufficient penetration and show poor fusion. Residual stresses set up during the welding operation will increase the size of defects in the plate. These cannot bo relieved by annealing. The stress concentration around these defects will progress and consequently develop still farther during the operation of the vessel. This is accelerated also by corrosion, which attacks such defects rapidly. Small cracks which have heen found adjacent to the weld in reaction chambers may be attributed partly to these causes. Open-hearth steel gives good results. For reaction cham-
bers, dephlegmators, or henvy-ivalled vessels, the following specification is satisfactory: MANWFACTUHE 1.4. S c o ~ a . These specifications cover grade A quality steel suitable for pressure vessels fabricated by the electric furion weldina oroeess. B. Pnoc&. The steel shall be made by the open-hearth process. CKEMICAL PROPEETIES AND ' h w s 2. CnnMmAl. COMPOSITION. The steel shall conform to the following requiroments 5 s t o chemiciil composition: Carbon. Far plates $14 inch or under in thickness, not uvcr 0.25 eroent. {or plates over a/, inch in thicknosc, not over 0.30 per cent. Manoenme. For dates 31, inch or under in thickness. 0.30
3. .LADLEANALYSIS. An analysis of each melt of steel shell be made by the stccl mill t o determine the percentages of the elements specified in seciion 2. This analysis sliall he made from a test incot, takrn durine the Dourine of the melt. The chemioal comGsition thus de(erminid shar be reported to the manufaoturer or his representative, and shall conform to the requirements specified in soction 2. 4. CIIECK ANALYSIS. A. An analysis shall be made by the iiianilfacturer from a biuken tension test smcimen of each Dhte
taken from aiy md&dual plaee'on the plate and the man;fncturer reserves the right to reject all material nhich shows B check analysis of over 0.35 per cent carbon. C. An andlysk shall be made by the manufacturer from driliines taken at the center of both too and bottom edws of each plite and the results reported to theburchaser
November, I ' J J
I N D IJ S T I1 I A 1,
A N D E N G 1 N li: I2 H I N G C t i E 21 I
~ ' H l l b l C ~hoPr.nrIoa L AUD I'EiTS
5. Ta+iluu TESTS A he m a t e d sllzll corlforni to the following requirements as to tensile propertieh: 55 nno-~n.oon
1 ensile airenqtii. lb /as I D Yield pmnt, lb /sq I D IXuii4ation in 8 inohcs, % (inin )
>o 5 teaaile atrerigtb i ti5o.ono tensile strength
Tena,lc st,ongtfl ,,otld be drternnned On the tevt and ,t iijOclmentaken i o n g ~ t u d ~ m from y the tal, of the ihnll not exceed 70,000 poundr per square inch C Should Jbove rule for minimum elongation Rive a value stlikn 25 per cent, the minimum allowable elongation shall be 25 pci cent, mbiect to modifications of section 6. 1) The yield iroirit shall be dctermined in the drop of the bonm of the testing ma.cinne 6 Moornrc~rro\sr v E ~ o v o n ~ m For~ material over =I4 inch i i i thickness, a dcduetion iionr the percentwe uf elonphiua specihe& in wrtions &A and C of 0 125 per cent shall he nude for e,rh increme of '/a inch of the specified thicknes3 above a/, inch to a m i n m u m of 20 per cent.
1269
brokeir tcbt specimen, do iiiucli to assure that the steel in each plate 15 liomogenouus and smtable for uelding and service conditions Usually it is necessary to discard or "crop" at least 43 per cent of the plate to meet these specifimtioia Opeii-hearth steel is specfied because it has less impurities present (especially oxides and nonmetallic inclusions) than steel made by otlicr processes. Figiire 7 sliows a inicrograph of a 3-iiicli plate which was rejected a t laboratory due to the emknee of porosity and small defects. The porosity and small fissures weie developed in the manufacture of the plate by rolling out gas pockets el,trapped in tile steel, Tlro mlcrograpli showiiig the cross section of tlie weld in Figure 12 (see also section oii Structure) sliows some indication of gas pockets and small defee& wliicli opened up during the welding. .\\NEALI?6
01% SIllhSb ftbLIE\I.\I4
In welding, tho uiieveii ekpansiun aiid wrrtraction ultimately dewlops a lionmade. uniform stress distribution. (The stresses are cau~edby tlie progressnt: heating and In order to detoct high-carbon segregacooling of the joint as the metal 1s beiiig tions, the plates are spot-drilled a t the top deposited.) The amount of this depends and bottom edges of the plate or in other largely upon the thickness of the plate beriuestionable places. Iiejection of the ing welded, size of the electrode used, plate is permitted in the event that the method of application, and the heat charanalysis of these drillings sliows a carbon :I( tmstics of the welding process. content in excess of 0.33 per cent. Carbon segregations show The concentration of up readdy as hard spots slirinkagc strains plus when the plate is being those of t h e normal machined. L a m i n a operating stresses mal tions and high-carbon be of sufficient intens e g r e g a t i o n s in the sity to stress the joint plate are r e a d i l y exbeyond the 3GId point posed when the edges Furthermore, t h e r e are times when a vessel of the plate are being machined; the inspecneeds to be given an tor watches this work intermediate treatment closely t o dcoide before it is completely whether such defects welded. This is especially t r u e of h e a v y w a r r a n t rejection of walled vesaelsand those tlie plate. These precautions, i n a d d i t i o n nure-irire weld Protected-aro weld having complicated art o the a n a l y s e s , the r a n g e r n e n t s of maiiI U L ~O.R~ Sovnm U ~Fouvo O UI US DraamLur tellsion, telislle, and Frouns 11 NOYIII:TAI.LIF~ ways or nosz,es and I Y P C WELDS ( X 100) bend t e s t s , and the t h e i r reinforcements. Inelosioiis %"pearRI dark s y o t ~on surfnoe of v e i L ~ C I L ,? i > ~ ~ i i n enot r i ~ eliltrd Without this treatment checJr Rns,?.srsirom a (4Ugii.t isdo) Bend and homogeneity tests arc albu
A
1270
I N D U S T 11 I A I.
A N D :k N G I N E E R I N G C H E kl 1 S T R Y
V"l. 24, 3-0.11
0.30 per cent carbon steel is a h o u t 1530° F. D u r i n g a r e c e n t investigation it was f o u n d t h a t t h e plate and weld m e t a l l o s t a p p r o x i m a t e l y 10 per cent of its o r i g i n a l s t r e u g t l i and increased over 15 per cent in e l o n g a t i o u by a thorough stress-relief t r e a t m e n t a t 1200° F. Greater ductility at the slight loss of t e n s i l e strength and yield point is not ohjectionalile, ~ O X D E S T l i L C T I \ ~ ETEaTs OF
WELDS Sondestructive m e t hod 5 for testing a n d e x p l o r i n g electric velded joints are 1 1 0 ~ being used. During the past year and a half at least six inanufacturers have equipped their shops with x-ray Sacilities. The x-ray apparatus employed in connection witli this work is similar to that used in medical work, escept that the machines are designed t.o operate a t much higher voltages in order to penetrate steel up to 5 iuches in thickness. The x-rays pass through t h e joimt a u d ruake a PhotomiaroEraphs ( x 100) tnkeu from bend test epecimen at lpoationa ahown i n sketoh: this is conaidered 8 shadow picture of the deweld even though there la a m e evidcnee ol dag and oxide ii~clusions. A . Weld metal. E . Junction of arid plate. c. Weld r n ~ i i i . D . IJndirturbed plaie metnl. fects. For the Dnruase of identification a i d r&rence such pressure vessels lest standing overuiglit iit a ccild slitip the seam is stamped numerically from one end to the other, have been knowii to crark the moment the welding was spaced in such a manner that a nuniher u d be shown on each exposure. These stations are identified on the xresumed the next morning. ray negative by means of stenciled lead markers hearing the number of the station to which they are pasted. Similar h N E A 1 , i N u PRACTICES markers to identify the welder are also placed between these The aimealiii~ or stress-relieving methods of various dtatiolls. rriarrufacturerfi do not vary much a t the present time. It. is The x-ray eyuipinei,t (Figure X), housed in a lead wall conimon practice in most shops to anueal heavy plates and euclosure, as a protection against the x-rays, is mouuted heads before any fabrication is done to Iiormalize stresses on a car set on rails. The vessel to be x-rayed is mounted on set up during thcir manufacture. A heat treatmeut US 1200" rolls parallel to tho tracks of the x-ray cap. In investigating to 1500" F. held over a "soaking" period of one hour. for each longitudinal seams, the cas is moved ahead from one station inch of inetal thickness is the procedure. Welded nozzles. to another in such a manner that the successive exposures manways, and details are given a similar trcatment before slightly overlap each other. The latest x-ray machines Ibcing installed into the heads or cylinder of tlie vessel. This are equipped to take two longitudinal exposures a t one time. also applies to completed units, such as heads with nozzles Tlic exposures on the longitudinal seams are from 12 to 16 and manwxys installed, and the d d e d sections of the cylinches long. It is considerably more difficult and slower to inder before being assembled Sor the eircumSereiitia1 welding. At the conqrletioii of t.he longitudinal and circumferential x-ray the circumferential or girth seams. This is due to the welding, or also in some cases when considered necessary fact that only one exposure can be made a t a time and also during its progress, the vessel is stress-relieved by aiinealing that the exposures must he reduced in length, oiving to the in accordance with the above procedure. Tiwe precautions diffraction and dispcrsion of x-rays throiigli a curved surfare. prevent, t,he accumulation of residual strains xlriclr vonld The cylinder is turned from station to station in front of the stationary x-ray machiiie in taking girth seam exposures. othenvise progress during tlie stages of Eahricatioir. Radiographs taken from x-ray negatives may Ire mi.leading; moreorcr, scratches and blemishes on the film of t,be ncgative are reproduced on the positive radiograph in The grain structure of tlie d i l arid plate is nut changed such a nramier as to indicate that, the d d might have appreciably by stress-relieviiig a t a kinperatwe under that cracks or other serious defects. X-ray nrgatives are, thereof the critical point. Tiiii rritical tnnprmture of 0.20 to fore, used ahnost excliisirelg for obseminp: vclilr i n the shop.
Figure 'J shows riidiograi)li*
