some advantages of welding turbine rotors.docx.pdf

WELDING JOURNAL

S O M ADVANTAGm OF T&TLD!mG TURBIAE ROTORS

ACKNOWLEDCMENT This article by Adolph Liithy, reprinted from June 1968 Welding Jour- nal, was presented at the American Welding Society’s 49th Annuul Meeting held in Chicago, Illinois, during April 1-5, 1968.

THE CONTINUOUS ADVANCES that have taken place in the field of power generation, the increasing de- mand for larger units and the constant search for greater economy compel designers to make the best use of every means that engineering places at their disposal. In steam and gas turbines, and also in axial-flow compressors, Sotors are the components subjected to the greatest stresses. Therefore, they demand the greatest care and attention during manufacture.

Since turbines were first built, two distinct sys- tems have been employed with roughly equal suc- cess for the construction of the rotors. On the one hand, there are the one-piece rotors, made from a single forging; on the other, there are the rotors consisting of a long, thin shaft, on to which the wheels carrying the blading are either shrunk or keyed.

In the 1930’s, when welding techniques had made sufficient progress, their advantages were naturally eb:ploited in this field of engineering, too. A sug- gestion made by Dr. Adolf Meyer [l] in 1930 led to tl e first welded turbine rotor. l b o years later the fi st steam turbine with a welded rotor and an out- p i t of 14 Mw was commissioned in the steam power p ant at St. Gilles in Belgium.

CONSTRUCTION OF ROTORS In the construction of low-pressure rotors for

high-output turbines, the design with a long, thin shaft is indeed feasible. However, on account of the enormous centrifugal forces to which such rotors are exposed, especially in the very large machines, it is fraught with constructional problems. Hence, for such units preference is given either to one- piece rotors or to the welded design. As a result of the enormous rise in unit outputs

that has taken place in recent years and the accom- panying tendency to design such units for 1500 and 1800 rev/&, their rotors have become so large that it is almost impossible to construct them in one piece.

One-Piece Rotors

The W e d weight (without blading) of a one- piece rotor is 35.15 tons, which means that the blank from which they are made must weigh at least 100-120 tons. The xnanufacture of such large forgings demands the provision of correspondingly large presses, with the associated lifting and manip ulatirlg gear. Very few steelworks possess such equipment.

For slow-running turbines (1500 and 1800 rev/

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WELDING TURBINE ROTORS WELDING JOURNAL

min) the unit weights of the f i shed rotors may be considerably higher (200-300 tons) so that, with the machinery and facilities available in the steel in- dustry today, it is quite impossible to even consider the manufacture of such one-piece rotors. Further- more, 'it must also be borne in mind that the stipu- lations laid down regarding the quality of the material for such rotors are very strict. It is also a well-known fact that metallurgical segregation tends to increase, the larger such forgings become, a phenomenon which can only be counteracted by improved core forging. Of course, this results in the f i shed product becoming more expensive.

In relation to the inclusions in such large forgings it may be said that vacuum melting or vacuum de- gassing, although it may reduce or even eliminate the risk of gaseous inclusions and their unpleasant repercussions, cannot get rid of nonmetallic inclu- sions. To attain the necessary mechanical strength the

forging must be of a suitable material and subjected to heat treatment. It is particularly important for the properties in the core of the forging to be exactly the same as those at the surface.

Although by suitably selecting the composition of the steel it is possible to influence the subsequent heat treatment to some extent, one can never be absolutely certain that the properties predicted by calculation will be obtained.

Tempering does not in fact depend on the com- position of the steel, on quenching and the succeed- ing annealing process. Instead, it depends to a large extent on the shape and size of the workpiece to be heat treated.

Nondestructive testing of such large forgings is done solely by ultrasonic methods. These very sen- sitive methods can only provide information regard- ing the presence of inhomogeneities inside the object, as it is unable to say much about the shape, nature and position of flaws. In order to obtain in- formation regarding the mechanical strength of the material in the interior, it is impossible to consider using nondestructive methods as things are at pres- ent. Admittedly it is possible to bore single-piece rotors with a hollow drill parallel to their axis (trepanning) and so obtain small specimens of rna- terial. These specimens are by nature very small (microspecimens) ; therefore, they can provide very little information regarding the true strength of the material since they can only be extracted from quite close to the axis of the workpiece. Thus we finally find that the decision regarding the reliability of such single-piece rotors can at best be made in the light of rather doubtful data on the true quality of the material. Acceptance then becomes a matter at the discretion of those responsible. Such a decision is very difficult to reach. This is because, on the one hand, there is the question of reliability in service, the only indication of which is the flaws that appear

on the screen of the ultrasonic detector and which cannot be evaluated with complete accuracy. On the other hand, there are the high costs and long de- livery time for replacing such heavy forgings.

Welded Rotors The manufacture of rotors for turbines and com-

pressors from single, smaller forgine which are then welded together (Figure 1) overcomes the difficulties mentioned for single-piece rotors. It is much easier to forge small pieces and it can be done in smaller equipment which is more likely to be available in steelworks. The small pieces, usually in the form of thick disks, can be well forged in a short space of time; they therefore exhibit less tendency toward segregation, while tempering by quenching and annealing is much easier on account of the smaller dimensions. The resultant temper is very uniform all round the object.

Figure 1. seetion through tha intermedm * te pressure rotor of a W-mw stSam turbine. Tbe rotor consists of two ends and three intermediate sectiom, joined togedher by fonr wdda The shaft is about 6 m long, and weighs 23 tons Note the individual cavitks and the holar for introduction of the shielding gas and inspectiom facilitks, shown adjacent to the welds in this section. Length-19 ft. 6.050 in.; diametex-3 ft. 7.m7 ia -- -

Ultrasonic testing of smaller forgings, disks and shaft ends is a simple matter. It is also possible to use gamma rays, betatrons or linear accelerators for testing. The disks can be examined right into the core and, where necessary, specimens of ma- terial can be taken from the blank, for testing in the laboratory. Finally, should there be anything wrong with the material of a piece, this piece can be replaced in a relatively short time and at low cost, without any attempt being made to effect a repair, the success of which is uncertain.

In Figure 2 individual forged sections can be seen, as well as five welds. The finished rotor is a

Figun 2. Section through the low pressure rotor of a 320- mw steam turbine for Karlshnu power station, Sweden. Weight on completion of welding-approx. 48 tons. It con- sists of six forgings welded together. Length-18 &; diame- ter-5 ft. 10.082 in. -

WELDING JOURNAL WELDING TURBINE ROTORS

multicell hollow body, the axis of symmetry of which coincides with the axis of the rotor.

A not insignificant advantage of the welded tur- bine rotor is that, in view of its smaller mass, it absorbs heat uniformly in service. Since it consists of disks welded together, the thermal balance is effected without internal stresses in the axial direc- tion. A two-dimensional state of stress is produced instead of the dangerous three-drm ‘ ensional state. Moreover, the position of the disks relative to one another in the rotor is determined quite by chance, so that there is little likelihood of any stress asym- metry, resulting from forging or tempering, being located in the same plane. Hence there will not be any bending due to heat when running. This risk is naturally far greater for the solid rotor, where it represents the “bogey” of all turbine builders and acceptance engineers. A rotor made of several pieces welded together is inherently less sensitive from the aspect of distortion due to heat.

Individual sections of the rotor (i.e., the disks, possibly drums and shaft ends) are joined together by welding. Since these are parts which rotate in service and are therefore subjected to dynamic stresses, the welds must be of very high quality. They must not contain any weak spots and their strength should match that of the base metal as closely as possible-Figure 3.

Figure 3. Geetion through the high pressure rotor of a 1100-mw steam turbine for 1800 rev/min, which is being built for a nuelear power plant in the U6A. “his rotor weighs 75 tons and collsisds of flve forgings joined torabsr by four welds. hngth-29 ft. 6 in., diao1du-5 ft. 10.082 in.

In the early days of welded rotors these welds were executed by hand. The electrodes, in accord- ance with the state of technical progress reached at that time, were mostly acid-coated and produced by immersion. The alloy of the weld metal could be no more than approximated to that of the base metal, and the strength values were mostly below those prescribed for the base metal.

From the point of execution there were also diffihlties to be overcome because then available sources of current and electrodes exhibited various shortcomings. Control of the current sources was usually poor, and their current/voltage character- istic tended to vary. The electrodes produced by immersion were deposited from one side only, and their melting properties were irregular.

Preheating the relatively thick rotor sections p’ior to welding was once done with gas burners. V’elding was carried out on a three-shift basis,

round the clock, but testing of the finished seams could only be performed visually (i.e., with a mag- nifying glass) as there were no other suitable methods. Sometimes a rather primitive penetration test was applied, using parafEn oil as the penetrant liquid and powdered chalk as the indicating layer.

In order to keep at least some check on the execution of the welds it was necessary to employ a second man whose job, in addition to turning the shaft, was to remove as much slag as possible and to keep an eye on craters at the start and finish. Guiding the electrode, keeping the molton pool liquid and ensuring that penetration was uniform on both sides was the responsibility of the welder himself.

When new alloyed electrodes appeared on the market4or example, those containing nickel, chromium and molybdenum in the weld metal- they were immediately tested. If the results were good, these new electrodes were employed in the construction of turbine rotors. Many such rotors manufactured during the 1930’s are sti l l operating and apparently running quite satisfactorily.

Shortly after the end of World War 11, the first really usable mechanized welding systems were marketed and they naturally made their contribu- tion to technical development, including the con- struction of turbines. A modern turbine rotor must meet stringent de-

mands with regard to precision, strength and free- dom from distortion. This means that all factors which have the least effect during manufacture must be carefully checked and selected. As far as materials are concerned for the rotor, it is essential to remember that it must be suitable for welding and that its mechanical properties are appropriate for the task visualized. Various different types of alloy are therefore used, depending on the particu- lar type of rotor, e.g., high or low-pressure rotor, fast or slow-running, compressor or gas-turbine ro- tor. As a rule they are mostly low-alloy, temperable Cr-Mo-V or Cr-Ni-Mo steels. In special cases high- alloy steels may be used, such as 17Cr-13Ni-W or steel containing 12 per cent Cr with Mo, W and V added.

Since the examination of the peculiarities of the numerous kinds of steel used in the construction of turbine rotors is outside the scope of this paper, ref- erence is made to the abundant literature on this subject.

Figure 4 shows a high-pressure turbine rotor of corrosion-resistant steel. The impulse blading, made of the same steel, is also attached to the body of the rotor by welding. The depth of the seam at the foot of the blades is about 120 mm (4% in.).

The composition of the weld metal is very similar to that of the base metal, the welding Wig per- formed by a mechanized submerged-arc process. For reasons connected with vibration, the impulse blades are welded together in groups of four. When

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WELDING TURBINE ROTORS WELDING JOURNAL

F&um 4. High-prsssure rotor of a t a r b h for Beznau nuclau power plant in Switzerland, constructed from 12% Cr-Mo-V alloy stsel.

such steels are used, the welded design allows parts such as a shaft end to be made of a different steel if doubts arise regarding the ability of the high-alloy shaft to run freely in the bearing shells.

Figure 5 shows a disk of a turbine rotor, ready for welding in the workshop. In Figure 6 appear the various stages in preparation of the seam, as prac- ticed in the course of development of welded tur- bine rotor construction. Grooves 1 and 2, as illustrated in Figure 6, were solely employed for manually welded seams. Since these welds are typi- cal peripheral seams on hollow bodies, the problem of welding the seam right through correctly was quite properly of major importance. The standard of welding techniques at that time was not suffi- ciently advanced to solve this problem The execu- tion of a good, uniform root pass, as well as the execution of the entire seam, was in the hands of the welder, himself, and was therefore subject to all human weaknesses.

In groove 1 the weakness was in the root pass as it had a sharp radial notch. With groove 2 an at- tempt was made to reduce this notch by utilizing a freely exposed backing which, for reasons connected with execution, consisted of a tube slit open. This replaced the radial notch by two tangential ones which, from the aspect of guiding into the seam, were estimated to be less harmful (deflection through 90 deg.). As the outputs of the turbines increased, the di-

mensions of their rotors also grew, and their weight too. The welds became deeper in order to stand up to the mechanical stresses. In this way the seam profiles or grooves 3, 4 and 5 were devised. This was at a time when the first mechanized welding processes were introduced, which in turn gave rise to a marked change in the form of the welds. In order to make the best possible use of the numer- ous advantages of mechanized welding for the con- struction of turbine rotors, it was not only necessary

F m 5. In the foreground a disk of an intermdate pressure rotor, showing the projection used for centering.

to change certain features of the rotor design, but to some extent the welding equipment too.

The requirements imposed were as follows: pro- duction of a root pass completely free from notch, and a seam of high quality with a weld metal whose composition corresponds as nearly as possible to that of the base metal. The width of the seams had to be as narrow as possible as it usually had to be located between two grooves in which the blade

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WELDING JOURNAL WELDING TURBINE ROTORS

'+- I

Figure 6. Profile of welding grooves for turbm and com- pressors, showing the progress that has been made over the Y-.

roots are held. Moreover, the rotor axis had to be as nearly straight as possible after welding, because the finished rotor had finally to be statically and dynamically balanced.

Weld profiles or grooves 6 and 7 can be used for seams 250.300 mm (10-12 in.) deep.

WELDING PROCEDURE

The Root Pass The parts of the rotor-shaft ends, disks and pos-

sibly drums-when they have been prepared exact- ly as per drawing, have to be carefully cleaned to remove dirt and grease before they can be put to- gether and welded. These parts are placed in the requisite order on the horizontal turntable-Figure 7. It is very important to ensure that the first part to be placed on the table is exactly centered; in most cases this part is a shaft end. The second piece, a disk, is then placed on it, care being taken to in- sert the centering lug exactly in the recess provided for the purpose in the first piece. In this way the first groove is created for the horizontal welding of a vertical wall. The remaining parts can be put in position in much the same way. When the rotor has been assembled, it is checked for true running and any irregularities are corrected.

Each rotor section has three inspection holes (15 mm or 4/8 in. dim) which pass into the hollow interior and are directed at the opposite butt joint. The shielding-gas supply pipes are inserted in these holes so that, during preheating and the subsequent welding, the rear of the root pass can be shielded to prevent oxidation. From the aspect of the shield- ir:g gas, the hollow spaces are in series.

The assembled rotor is preheated by induction. Usually alternating current with an audio frequen- cy is employed. The temperature of each disk is measured by means of a thermocouple and re- ce rded. The preheating temperature is about 50° C h'gher than the martensite temperature of the steeL

Figure 7. Assuubly d an i.pA.p. shaft prior to welding the root psss undm ahidding gfs. The part hanging from the crane hati already been welded, tbem b e i i still four welds to execute on the lower seelion.

Since the parts concerned are relatively solid, it is necessary for the two parts to be joined by welding to be at the same temperature. The maximum difference that can be tolerated is +50 C, if un- pleasant surprises (cracks) during cooling are to be avoided.

Figure 8 illustrates a gas-shielded-arc welding system specially developed for the gas tungsten-arc process, augmented by feeding in a nonconducting filler metal to produce the thick root pass on round hollow objects. This system has three heads at in- tervals of 120 deg for simultaneous welding; they are connected in such a way that they can be started up singly, two at a time or all three at the same time, as required. In addition, each set is capable of moving to and fro at righhngles to the welding direction, so that the full width of the root pass comes under the intluence of the arc. In order to avoid any inadmissible overheating of the head in the deep, narrow groove, the whole electrode holder system, including the filler metal feed, is water-cooled. The filler metal consists of a low-alloy material and has the task of augmenting the volume of the molten disk projections so that the shape of

WELDING TURBINE ROTORS WELDING JOURNAL

Figure 8. The LpJLp. shatt ot the 2SO-mw turbine for

seen ready for execution of the mot pass on tbe gas tung- sten-arc welding mrrhina Note the supporihg p r o w o n at the bottom, bawssn the shaft end and the turntable, which provides a steedy seat for the shaft, despite the end. Also visible arc the inductor cables for heating. In the forwound is the instrument for recording the preheating temperature.

s%snaes power station, Denmark; total weight 53 tons. It is

the root pass is correct, i.e., that it becomes slightly convex over the entire periphery. This patented method of carrying out the root pass ensures that the joint is properly welded through-Figure 9.

To shield the underside of the root pass, any of the usual gases or mixtures of gases may be used. The flow of gas into the cavities must be present during preheating, so as to prevent any oxidation, which could hamper the flow of the molten weld metal. The rate of flow of the shielding gas is very small and ranges from 4 to 10 liters/- at a pres- sure of 1 to 2 cm of water column.

The shielding gas surrounding the tungsten arc is usually argon, the pressure and rate of flow of which are in line with the values commonly used for this process. The welding data vary with the thickness of the root pass, which itself depends largely on the size and weight of the rotor. For each welding head the source of current is a finely regu- lated rectifier with drooping current/voltage char-

Figure 9. Set of gee tungsten-arc welding eleebode hold- ers, seen from the side, specially designed for the root welds in deep, narrow grooves. Tbe filler metal f a d and tbs tung- sten electrode hdder are both water-cooled. The drive mo- tor and control of the oscillating device can also be seen.

acteristic, giving as soft an arc as possible. Since the voltage used with pure argon has to be 12 to 13 v, the welding current may vary between 150 and 250 amp, according to the thickness of the weld, the degree of preheating and the speed of welding. The latter is quite low and does not normally ex- ceed 60 mm/min.

For simultaneous ignition of the three welding heads a central control i s required, which operates with a superposed high-frequency current. For very large rotors, the root pass is reinforced by two or three additional passes, until the root seam is 12-15 mm thick. This is necessary to enable the rotor to be laid over from its vertical to the horizontal posi- tion, without its straightness being affected.

When the root passes have been carried out, the supply of gas to the cavity is interrupted SO that, when the seams have cooled down, a light and an endoscope can be introduced to examine the inside of the seams for flaws, around the whole periphery. If necessary, a radioactive source such as iridium 192 can be introduced to obtain a gamma radiograph on a narrow film. The fully mechanized method of welding ensures uniform and flawless root welds, the thickness of which is 7 to 9 mm (9/32-3/8 in.) per pass.

The rotor, thus held together by the root welds, is now carefully lifted out of the root-welding ma- chine and kept in its vertical position so that fur- ther nondestructive tests can be performed on the root welds, depending on the circumstances or the acceptance requirements. The inductors attached to the rotor for preheating are left in place as they are needed later for preheating prior to the final weld- ing up of the rotor.

Final Welding The rotor is finally welded up in the horizontal

position by submerged-arc welding, using a nonstop method. The rotor, preheated in the vertical posi- tion, must now be laid over into the welding posi- tion on the turning device for the submerged-arc welding machine and fixed. Since the root welds are relatively weak, this change of position demands

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WELDING JOURNAL WELDING TURBINE ROTORS

great care, especially if the rotor is quite long com- pared with its diameter. It must also be borne in mind that the rotor is laid over in its preheated state, i.e., at 400-4500 C; at this temperature the mechanical strength of the root welds is bound to be somewhat reduced, depending on the type of steel used.

Final Welcling of the Deep Seam "he welding machine is in essence a modified

lathe, on which the available speeds of the face- plate have additionally been adapted to suit the welding speed for the particular diameter of the turbine rotor. Under these circumstances it is pos- sible to machine out a faulty spot in the seam, caused by a disturbance during welding, and then to continue welding as before, without any great loss of time. Moreover, the various facilities for ad- justment on a lathe, the precision and regularity of the rotary movement, as well as the rigidity of the slide on which the welding head is mounted, have proved indispensable for welding deep, narrow grooves.

The fact that, on turbine and compressor rotors, the weldseare nearly always between two rows of blades (or at least in the vicinity of one) has made it necessary for these grooves to be as narrow as possible, regardless of their depth. This need to keep the grooves narrow also has other reasons which must not be ignored. For constructional and ma- terial reasons, the transition zones between weld metal and base metal, which are doubtless weaker than the body, should not have slots for the blade roots milled in them, as the full strength of the tem- pered steel is essential at these points. A further reason, based on economics, is that a narrow groove requires f a r less weld metal than a conventional groove. For instance, a groove 250 mm deep and 22 mm wide requires only 30-40 per cent as much weld metal as a groove of the same depth but of normal width. The effective welding times are also short- ened accordingly. Very often it is possible for rotors having at least two seams with roughly the same diameter to be welded at both points simultaneously with two automatic welding units, this thereby making the process more economical. Of course, welding in two grooves at the same time means that the corresponding fixtures and welding equipment must be avai labldigure 10.

The actual welding head is an ordinary head used for submerged-arc welding, with high-frequency ignition and employing alternating current. It is very important that its attachment to the welding machine be very stable and the feed of filler metal and flux into the deep, narrow groove be extremely accurate.

The source of welding current used in this case i:. a normal welding transfomer having a current/ vdtage characteristic appropriate to submerged-arc v"e1ding.

Figure 10. In order to improve tbe emnomien et welding -provided the &~IU&M are roughly equal-two welding machines may be cmployed simultaneody on separate grooves.

Various advantages are gained by using alternat- ing current; among them are the avoidance of the very unpleasant magnetic "blow" on the arc and the ability to obtain better and more regular me- chanical properties in the weld metal. Submerged- arc welding in a deep, narrow groove imposes cer- tain requirements on the fdler metal and flux feed to the welding head. In this respect, the filler metal feed device (Figure 11) must be so designed that it guides the filler metal reliably and accurately into the molten pool. The welding current should be applied, if possible, just before the filler metal emerges from the guide nozzle. Before the filler metal enters the nozzle it should be straightened, the current being applied to it by sprung contacts. To prevent inadmissible overheating due to radia- tion from the sides of the groove, remembering that the rotor disks are preheated to 300-450 C (de- pending on the tyhe of steel used) and due to the flow of welding current itself, the contacts feeding the current to the filler metal are water-cooled.

Unless current contacts are prevented from

WELDING TURBINE ROTORS WELDING JOURNAL

touching the sides of the groove while welding is in progress, a short circuit may result and cause a g preciable damage to the rotor disk. To prevent this, all parts are coated with a layer of insulating, heat- resistant material (Al,08, SOz, or the like).

It is extremely important for the flux to possess certain definite properties which are essential in deep, narrow grooves. The most important of these is that the molten slag should break away of its own accord after solidifying, so that it simply falls down as the rotor revolves during welding and does not have to be chipped off. This property of self- detachment, or rather, loosening from the surface of the weld, must be complete; no splinters or crumbs must be allowed to remain adhering to the seam. It is quite impossible to remove such traces of slag during welding, especially when they are firmly lodged, without disturbing the smooth execu- tion of the welding operation and hazarding the success of the process. The latter also applies to the removal of any other irregularities in the seam, such as pipes, cracks on the surface, incompletely fused spots, inclusions and craters at the start or end of the seam, but also residue and traces of fused current contacts, and the like. The lack of access to the flaw (especially when the groove is still deep), the heat radiated by the adjoining disks, and the narrowness of the groove make it extremely dif- ficult to intervene, so that local rectification of such flaws is practically impossible. Such flaws can only be eliminated by machining on a lathe.

The flux feed does not present any great difficul- ties. The feed pipe should, of course, be insulated from the welding current, and the rate of feed should be continuous and regular. "his task is best left to a flux recovery and treatment system.

The composition of the filler metal is chosen so that it matches the alloy of the rotor disks, the loss or gain of certain elements being made good as far as possible by the flux. The flux may be of the mol- ten or agglomerated type and should be intended for use with alternating current. It is most im- portant that the molten slag should be self-detach- ing, but that it should nevertheless protect the bead while still hot. As f a r as the grain size of the flux is concerned, this should be chosen so that under set welding conditions (Le., at the set current, voltage and welding speed) degassing is assured, although a sficiently thick protective coat of slag is produced. The welding conditions are determined by prior

tests, being mainly governed by the type of steel, the shape of the weld groove, the height of the pre- heating temperature, the diameter of the filler wire and its composition, and the welding speed.

When starting to weld at the bottom of the groove, mall diameter filler metal is chosen to pm vent penetration through the root weld. When this danger has ceased to exist, the rest of the seam is

welded with larger diameter filler metal. The values of current and voltage used for welding are much the same as used for general submerged-arc weld- ing of such diameter. Admittedly, it may be that one or the other welding parameter has to be slightly adjusted to suit the prevailing conditions, but experience has shown that this adjustment is rarely more than 210 percent of the preset values.

In Figure 12, welding is carried out without inter- ruption, except when some disturbance occurs. The various passes are positioned so that a complete pass round the whole circumference of the groove is placed alternately on the left and the right. In the case of rotors of small diameter, the bead is transferred to the opposite side of the groove after a quarter or third of a revolution. This is necessary to maintain the axis of the rotor in a perfectly straight line, when the shaft is long and thin. It is also important to make sure that the molten slag is always made to form toward one side of the groove and is not trapped. Unless this is done, its

Figure It. Fhrrrl weldiug of a deep groove on a low-pns- mare tmMne mbr. Note the thermal inSntptiOn on both sides of the seam. Beneath it are the inductor cables for preheat- ine.

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'#ELDING JOURNAL WELDING TURBINE ROTORS

removal will prove rather difficult and there is a risk of the regularity of welding being seriously disturbed.

It is a matter of experience with the welding equipment to know what are the best settings of the welding parameters. Such information cannot be found in any book or obtained during any course of instruction.

Referring to Figure 13 and as already mentioned, the welded turbine or compressor rotor is a hollow, multicell body which is rotated and therefore has to be balanced very accurately. This means that when the individual disks are being put together, and later when they are being welded, great care must be taken to keep the axis of the whole in a perfectly straight line.

F h m 14. An 4A.p. rotor on the latha Aftex atresa re- fiw and p m c h i d n g the rotor is checked for true run- h. w m t 45 tons, length 7.83 m, 7 welda This rotor is for a 140-mw turbine supplied to Narcre, Spain,

Figure 13. Macrograph of a weld 180 mm deep, showbig and the side penetration. the build-up of the various

The dimensional accuracy of the finished rotor, especially its length, depends on the number of welds and is affected by the shrinkage that occurs during welding, this being already taken into ac- count in the preparation of the individual parts. Since this shrinkage is affected by a variety of factors, it has to be determined by prior tests on an exact replica. Having once determined it, it remains practically unchanged for all similar welds, pro- vided the parameters specified for them are strictly adhered to.

In general, it may be stated that when the neces- sary care is applied to the welding of turbine and compressor rotors, distortion of the axis, even of the largest objects, can be kept within tolerable llmits.

"he distortion in the axis of the i.p. rotor, seen in Figure 14 on the lathe, was less than 0.3 mm, al- though the rotor contained seven seams and is considerably longer than the 1.p. rotor in Figure 15, the weight of the two being roughly the same.

When welding is finished .and before stress re- lieving is carried out, the welds must be very care- fdly tested. Regarding these tests, it may be said

Figure 15. Low-pressure rotor of a 320-mw turbine for

During premachining after stress relieving and check for true running.

Karlshamn, Sweden. weight 48 toae, 5 welds, depth 250 mm.

that the nondestructive methods of testing available nowadays (especially ultrasonic testing) are capa- ble of providing sutEcient information about flaws that occur during welding, so that the decision as to whether a weld is sound or not can be reached with a good conscience. In the most unfavorable case, i.e., when there is any doubt about how a flaw indication is to be interpreted, the flaw can be 10- cated with ultrqsonic testing quite accurately. The flaw is cut out of the weld and the hole is

welded up again. In this way flaws can be located, their nature determined and, what is usually more important, their cause can be established.

CONCLUSION The use of welding in the construction of turbine

and compressor rotors permits increasingly large units to be built, regardless of their size and weight. The sole condition is that the material from which they are made must possess the requisite strength and be easy to weld.

WELDINGTURBIIWROTORS WELDING JOURNAL

Welding is the modern, progressive and eco- MOTNOTE namical method of producing rotors for compressors and It improves their sde and refi-

the time for manufacture and de- Every, and can be employed almost without regard to the size and weight of the rotor.

*At that time Dr. Meyer was Technical Director of Brown Boveri dc Cie in Baden, Switzerland. In 1935 he was award- ed an Honorary Doctorate by the Stevens Institute of Tech- nology, Hobuken, NJ., and in 1965 he was honored by the American Society of Mechanical Engineers, which presented him with a golden gas turbine wheel.

EELPING HAND-- Quincy division shipyard for SMOOTH SUDE-Gracefully entering water for tirst h e initiai sea trials, nuclear attack submarine U S Sunfish March 16 was US. Navy replenishment fleet oilcr USS (SSN-649) gets helping hand from Navy tug. Wichita (AOR-I), built on Quincy division’s Slipways 11.

DESIGNERS PLANNERS, INC. NAVAL ARCHITECTS - MARINE ENGINEERS

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78 Naval Engineers Journal, February I9b9