Claims:WE CLAIM

1. A method for controlling weld distortion and weld cracking in repair welding of casings of steam turbines comprising steps of:-
- grinding the local spot to permit the entry of welding electrode/ filler wire and preheating the spot to be welded so as to reach a temperature of preheat of atleast150° C,
- laying of the weld beads in each spot/ cavity in a direction parallel to the longitudinal centerline of the casing,
- machining of the casing to bring the repaired spots/ cavity/facets of the casing to the required level of dimensional accuracy.
2. A method for controlling weld distortion and weld cracking in repair welding of casings of steam turbines comprising steps of:-
- grinding of the facets required to be repair welded locally to achieve flatness followed by cleaning,
- wrapping of the rear side of the casing fully with heating elements for achieving a uniform preheating of atleast 150°C all along the casing’s surfaces,
- repair welding of the various facets are completed by taking one at a time and proceeding to other facets that are located farther away from the midpoint of longitudinal centerline of the casing,
- selecting the facets for repair welding sequentially according to increasing distance of separation thereof from the midpoint of the longitudinal centerline of the casing,
- dividing each of the facets for welding into multiple sections/ divisions,
- repair welding of each said facet using multiple rounds of weld metal deposition, wherein the first round of weld metal deposition in each facet of the casing, the first weld bead is laid on the centermost division, the next weld bead is laid on the division that lies at the immediate left of the centermost division and the third weld bead is laid on the division that is present at the immediate right of the centermost divisions; and the subsequent weld beads are alternately placed on farther division located on both the left and right sides of the centermost division marked in the facet of the casing,
- within the subsequent round(s) of weld metal deposition, the weld beads are laid using the sequence as followed for the first round of weld metal deposition,
- the direction of laying of each of the weld beads in each round of weld metal deposition in each facet is parallel to the longitudinal centerline of the casing.

3. A method for controlling weld distortion and weld cracking in repair welding of casings of steam turbines as claimed in claim 1 wherein the order of selecting the spots / cavities for repair welding is based on their increasing distance of separation from the midpoint of the longitudinal centerline of the casing.

4. A method for controlling weld distortion and weld cracking in repair welding of casings of steam turbines as claimed in claim 1 or 3 wherein the weld beads that are laid over the previously completed bead(s) are laid in such a way as to completely fuse the previously laid weld bead(s).

5. A method for controlling weld distortion and weld cracking in repair welding of casings of steam turbines as claimed in any of the preceding claims1, 3 and 4 wherein the inter-pass temperature before laying an impending weld bead, at a particular location, is maintained at temperature of 150°C to 250°C.

6. A method for controlling weld distortion and weld cracking in repair welding of casings of steam turbines as claimed in claim 2 wherein the heating elements are preferably the flexible ceramic resistance heating pads that are energized electrically.

7. A method for controlling weld distortion and weld cracking in repair welding of casings of steam turbines as claimed in claim 2 or 6wherein in the subsequent round the weld beads are laid adjacent to the previously laid weld beads and fully fused therewith at the respective locations.

8. A method for controlling weld distortion and weld cracking in repair welding of casings of steam turbines as claimed in any of the preceding claims 2, 6 and 7wherein the inter-pass temperature before laying an impending weld bead at any location within each round of weld metal deposition in each facet is maintained within the range of 150°C to 250°C.

9. A method for controlling weld distortion and weld cracking in repair welding of casings of steam turbines as claimed in any of the preceding claims wherein the height of weld buildup at each of the local spot/cavity/in each facet of the casing forms a ‘distortion allowance’ of at least 5 mm higher than the required dimensional accuracy of the casing.

10. A method for controlling weld distortion and weld cracking in repair welding of casings of steam turbines as claimed in any of the preceding claims wherein checking of the repair weld like ultrasonic examination is carried out prior to said machining.
, Description:A METHOD FOR CONTROLLING WELD DISTORTION AND WELD CRACKING IN REPAIR WELDING OF CASINGS OF STEAM TURBINES

FIELD OF THE INVENTION

[001] The present invention discloses a weld sequencing scheme and a welding procedure to control the issues of welding distortion and preclude the issue of weld cracking commonly encountered during repair welding of steam turbine casings. In particular, the present invention relates to a method for controlling weld distortion and weld cracking in repair welding of casings of steam turbines.

BACKGROUND OF THE INVENTION

[002] Steam turbines are employed in power plants for generation of electricity. The general design of steam turbines consists of a rotor with carefully designed sets of blades and a complementing stator or a casing which also has multiple sets of blading grooves in which are placed the stationary blades. The clearance between the rotor and the casing is to be precisely maintained in order to avoid dip in efficiency of the turbine. There may be occasions which might warrant a weld repair of the casing so as to bring it to the required level of dimensional accuracy. Generally, these casings have a circular cross section with multiple cast features at various locations within the casing and these casings are invariably produced as two split halves so that it can be easily assembled over the rotor. The material of construction is one among the well-known family of Cr-Mo steels since these are exposed to steam and are required to have extended creep lives.

[003] Doing a weld repair in such casings is likely to result in two major aftereffects namely distortion and weld cracking. The distortion problem is caused because of the localized heat addition in the casing which causes an aberration in the diameter of the casing. Similarly, the use of Cr-Mo steels poses the risk of weld cracking which could be caused by one or more factors like poor baking of welding electrodes, poor surface preparation of the area to be welded, non-maintenance of required levels of preheat and inter-pass temperature between weld passes, welding under excessive restraints etc. Therefore, a carefully devised welding procedure is required in order to carry out repair welding in such steam turbine casings addressing the above issues.

[004] The present invention discloses the welding procedure for repair welding of such steam turbine casings with a view to avoid the problem of weld cracking and control the weld distortion and thereby enabling the successful completion of the repair welding without further rework.

[005] Now, reference may be made to the following prior arts.

PRIOR ART

[006] US patent US 3806693 A describes a method of repairing the inserts in a damaged heat exchanger tube sheet by using an electron beam welding process wherein the beam is moved in a circular path in order to weld an insert in an opening in the inner tube sheet and then the electron beam is used to weld an insert in an opening in the outer tube sheet as well. But, the present invention discloses a method of repair welding of steam turbine casings that will aid in controlling the weld distortion and also in achieving a crack-free weld deposition, by following a special scheme of weld sequence.

[007] US patent US 6916387 B2 details a method of repairing a void on a nickel or cobalt base super-alloy investment casting by vibrating the casting for a time period before repairing the void and during the time of repair the void is filled by repeatedly making incremental weld deposits of a super-alloy filler material using pulsed GTAW process. The present invention is different in the sense that it is concerned with a repair welding procedure of a turbine casing and the said procedure aims at avoiding the weld cracking issues and controlling the weld distortion during repair by way of weld sequencing scheme.

[008] US patent US 4878953 A explains a method of refurbishing a super-alloy casting by generating a plasma arc between an electrode and a section of the casting, forming a shallow pool of molten metal and by adding a powder filler metal in the molten pool so as to produce a highly integral overlay without issues of cracking and distortion. The present invention is markedly different from the above invention in the sense that here, the method of carrying out repair welding in a steam turbine casing is detailed which results in deposition of weld metal without the issues of cracking and weld distortion by using an ad-hoc welding sequence and procedure.

[009] US patent 5591363 is directed to a process of depositing layers of weld metal onto a ferrous NiMoV low alloy steel turbine rotor, where during the deposition of a first layer of weld metal, low levels of amperage are used to prevent a dramatic increase in a level of hardness of the HAZ and during the deposition of a second layer of weld metal, higher levels of amperage are used to temper the heat affected zone. But, the instant invention relates to a repair welding procedure involving the sequential deposition of weld layers during the repair welding of a steam turbine casing so as to avoid the issues of weld cracking and distortion of the said casing.

[0010] European patent EP 2189238 A1 provides the method of performing a weld repair by a robot programming method wherein the robot calculates the spatial coordinates of the layer that is required to be laid and the software of the robot decides the weld metal volume and the sequence of laying of welds. Whereas in the proposed invention, the procedure for repair welding of steam turbine casings has been disclosed. This procedure uses a welding sequence that is applicable for all types of casings and is not calculated with programming devices like robot etc, which is meant for controlling the weld distortion and avoiding weld cracking issues.

[0011] US patent US 20090274553 A1 states a repair method for casings of gas turbine engines meant for repairing the damaged areas of the internal holding structures in the casing using the Cold Metal transfer Gas Metal Arc Welding process by keeping the casing on a rotating table. But, the invention talks about a method of repair welding the casing of steam turbine engines by employing a welding procedure incorporating a weld sequencing scheme which would solve the issues of both weld distortion and weld cracking with no need for any special rotatable worktables.

[0012] Thus, none of the aforementioned prior arts fulfill the requirements of the present invention, for which it is designed.

OBJECTS OF THE INVENTION

[0013] An object of the present invention is to provide a method for controlling weld distortion and weld cracking in repair welding of casings of steam turbines.

[0014] Another object of the present invention is to provide a method for controlling weld distortion and weld cracking in repair welding of casings of steam turbines which obviates shortcomings of prior arts.
[0015] Yet another object of the present invention is to provide a method for controlling weld distortion and weld cracking in repair welding of casings of steam turbines which is simple.

SUMMARY OF THE INVENTION

[0016] According to the invention, a method of repair welding a steam turbine casing is disclosed. This repair welding procedure involves the deposition of weld layers in a sequential manner and under controlled welding environment so as to prevent the cracking of welds and to result in good control of distortion of the casing during the course of the repair welding process.

[0017] According to this method, the areas that are required to be repaired are to be identified. These areas are to be ground to make sufficient access for the welding electrode to reach the spots intended for repair. Subsequent to grinding, these areas are to be inspected with a Dye penetrant test (DPT) to ensure absence of cracks. After this inspection, the spots to be repaired are to be cleaned. Then, the whole of the outer side of the casing has to be wound with resistance heating elements for preheating the casing to the required temperature which is generally higher than 150°C. After this, the midpoint of the longitudinal axis (i.e. the length direction) present in the parting plane of the casing, is located. The parting plane can be defined as the plane that separates the casing into two split halves.

[0018] The first weld bead is laid at the spot which is nearest to this mid-point of the longitudinal axis. The weld bead is laid parallel to the longitudinal axis of the casing and not in a circumferential manner. The, next weld bead is laid in the next repair location that is next nearest to the midpoint on the longitudinal axis aforementioned. This weld bead is also laid parallel to the longitudinal axis of the casing. Similarly, the subsequent spots are chosen based on their increasing distance of the separation from the midpoint of the longitudinal axis aforementioned. Each of the weld beads are to be laid in a direction parallel to the longitudinal axis of the casing and none of these weld beads are deposited in circumferential manner. In case a weld bead is required to be laid very adjacent to a previously laid weld bead, then care is to be taken to properly fuse the boundary of the previously laid weld bead with the current weld bead. The total height of deposition of the repaired weld is always higher than the required dimensions by a margin of atleast5 mm so as to permit the machining of the casing after the weld repair activity.

[0019] In case, the repair is to be performed fully across the circumference in one or more facets of the casing, the rear side of the casing is to be wrapped with heating elements for preheating the casing as stated earlier to a minimum temperature of 150° C. The facet that is closest to the mid-point of the longitudinal axis of the casing aforementioned is chosen first. The weld beads are laid sequentially starting from the center and alternating to both the left and right sides of the casing at regular pitch spacing. The next round of welding is also similarly done until the required height of weld build-up is reached in this facet. The direction of laying all these weld beads is as mentioned in the preceding paragraph. Likewise, one facet is completely laid with weld metal. Subsequently, the facet that is next closest to the midpoint of the longitudinal axis of the casing is selected and repair welding is carried out as stated above. Similarly, as many facets as required are repair welded.

[0020] After completion of repair welding, all these repair welds are to be subjected to Ultrasonic inspection and subsequently need to be machined in milling machines to bring the casing to the required level of dimensional accuracy.

[0021] This procedure can be used for effectively controlling the cracking of the welds and the distortion caused owing to welding in repair welding of steam turbine casings.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

[0022] Further objects and advantages of this invention will be more apparent from the ensuing description when read in conjunction with the accompanying drawings of the exemplary embodiments and wherein:
Figure 1 shows: The steam turbine casing.
Figure 2 shows: The steam turbine casing with longitudinal axis LL marked by dashed line and repair spots marked with filled ovals.
Figure 3shows: The Rear side of the steam turbine casing shown in figure 1.
Figure 4 shows: The Schematic representation of the weld sequence followed in first round of weld deposition in the facet that is first closest to the midpoint M of the longitudinal axis LL (figure 2) of the casing.
Figure 5 shows: The Schematic representation of the weld sequence followed in second round of weld deposition in the facet that is first closest to the midpoint M of the longitudinal axis LL (figure 2) of the casing.
Figure 6 shows: The Schematic representation of weld sequence that is followed in first round of weld deposition in the facet that is second closest to the midpoint M of longitudinal axis LL (figure 2) of the casing.
Figure 7 shows: The Schematic representation of weld sequence that is followed in second round of weld deposition in the facet that is second closest to the midpoint M of longitudinal axis LL (figure 2) of the casing.
Figure 8 shows: Schematic representation of weld sequence that is followed in first round of weld deposition in the facet that is third closest to the midpoint M of longitudinal axis LL (figure 2) of the casing.
Figure 9 shows: Schematic representation of weld sequence that is followed in second round of weld deposition in the facet that is third closest to the midpoint M of longitudinal axis LL (figure 2) of the casing.

DETAIL DESCRIPTION OF THE PRESENT INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS

[0023] The invention imparts teaching in respect of a method for controlling weld distortion and weld cracking in repair welding of casings of steam turbines.

[0024] Thus, this invention describes the method of repair welding of a steam turbine casing under controlled welding environment including a weld sequencing scheme so as to prevent the weld cracking and to minimize to the best possible extent, the welding induced distortion caused due to such repair welding.

[0025] Steam turbine casings are generally made as two split halves and are assembled at site at the time of commissioning of the turbine.

Figure 1 shows the three dimensional view of one half of such a steam turbine casing. It can be observed that there are multiple machined facets (A1) in the steam turbine casing that are of varying radii. There may be requirement for repair of such casings that have undergone long service or due to site troubles etc. For accomplishing such a repair, weld metal has to be built-up in the locations requiring the repair. The nature of a repair welding can be divided into two cases. The first case (hereinafter referred to as ‘Case 1’) refers to a situation where the weld repair has to be done only in local spots that are present across the various facets of the casing. The second case (hereinafter referred to as ‘Case 2’) refers to a situation where the weld repair is to be carried out on the whole surface of one or more machined facets (1) of the casing. The procedure that is to be followed for each of these two cases are detailed hereunder.

[0026] Case 1:
1. The location(s) on the casing requiring repair are marked. These areas are ground well so as to produce a local cavity of sufficient width and depth that would allow for welding using a welding electrode. These local spots / cavities are referred as S1, S2, S3, and S4 in Figure 1.

2. After grinding, the local cavities are inspected for presence of any residual cracks or discontinuities by using a dye penetrant inspection. If there are no defect indications, the surface quality in the local spots/ cavities can be construed to be satisfactory. The areas identified for repair welding are thoroughly cleaned by using cloths to remove all sorts of oil and grease settlements and dirt before commencing welding.

3. Thereafter, the spot that is closest to the midpoint M of the longitudinal axis LL (Figure 2) of the casing is first chosen for welding. This longitudinal axis LL coincides with the centre line of the casing.

4. Prior to commencing welding, this area is preheated for sufficient duration using any known method preferably using an oxy-acetylene flame to a minimum temperature of 150° C. An infrared thermometer is used to assess whether the spot has reached the required preheat temperature as mentioned above.

5. On reaching the required preheat temperature, the welding is commenced in the spot. The weld bead is laid in a direction that is parallel to the longitudinal axis LL of the casing (Figure 2). Further weld beads are also laid in the same spot/cavity so as to completely fill the cavity with each of the weld bead being laid in the same direction as mentioned earlier and by properly fusing the previously laid weld beads. Prior to laying each of such subsequent weld bead, the inter-pass temperature at the location of welding is brought to a range of 150° C to 250° C by allowing sufficient duration of time between the weld passes. The spot/ cavity is filled by welding to a sufficient height so as to permit the machining of this spot at a later stage for obtaining the required dimensional accuracy.

6. After completion of the first spot S1 (Figure 2), then the next nearest or next closest spot of repair to the point M (figure 2) exempting the already repaired spot(s) is selected. For the present condition portrayed in Figure 2, spot S1 has been repaired first as described in steps (4) and (5) mentioned above and then the spot S2 is to be selected next. This is because spot S2 is next closest to the point M compared to remaining spots S3 and S4 (Figure 2).

7. Welding is completed in the second spot S2 as per the steps (4) and (5) and the next nearest spot to the midpoint M of the longitudinal axis of the casing (Figure 2) is subsequently chosen leaving behind the already repair welded spots. Since spots S1 and S2 (Figure 2) are already welded, the spot that is next nearest to the point M is S3 and this is taken up for welding next.

8. Similarly, the welding of each local spots/cavity is completed as per the steps (3) to (7) and the repair welding of each and every spot is completed. The direction of laying of all the weld beads is always parallel to the longitudinal axis LL (Figure 2) of the casing.

9. Subsequent to completion of welding in all these spots, the quality of repair weld in each of the spots is assessed by employing an ultrasonic examination method. After this, the casing is machined at the repair welded spots until the required dimensional accuracy is achieved in the casing.

[0027] Case 2:

1. Prior to commencing welding, the areas identified for repair welding are thoroughly ground or milled locally to bring about flatness and after such machining, the surfaces are cleaned by using cloths to remove all sorts of oil and grease settlements and dirt.

2. The preheating of the casing to a minimum temperature of 150° C is achieved by the method as stated hereinafter. The whole of the rear side of the casing as shown in figure 3 is fully wrapped with heating elements / coils preferably by the flexible ceramic pads which work by the principle of resistance heating. After wrapping the heating elements, the entire casing is fully blanketed by a flexible refractory material preferably the ceramic wool/ glass wool. The glass wool is firmly placed on the casing’s rear surface above the heating elements and it is loosely placed on the other side of the casing where the facets are to be repair welded. The heating elements are energised by electrical means and the temperature of preheat is maintained at a minimum level of 150° C by using appropriate instrumentation incorporating a feedback signal of the current temperature of the casing as sensed by a thermocouple placed near the proposed location(s) of repair welding. The achievement of the required level of preheat for commencement of welding is checked by using an infrared thermometer.

3. The facet that is nearest to the midpoint M of the longitudinal axis of the casing is chosen for welding first ahead of the remaining facets.

4. The circumference of this facet is divided for example into ten equal divisions. If the radius of this facet is R, then circumference of this facet ispR where p = 3.1416. This circumference of the facet is then divided by 10 to get 10 equal divisions. All of these ten divisions are marked.

5. Next, the welding is commenced from the centremost mark that is closest to the midpoint M (figure 2) of the longitudinal axis LL of the casing. The weld bead is laid in a direction parallel to the longitudinal axis LL of the casing. Then, the second weld bead is laid on the first mark present to the left of the firstly laid weld bead on the centremost mark as stated earlier. Then, the third weld bead is laid on the first division present to the right of the centremost mark where the very first weld bead was deposited. Similarly, the subsequent weld beads are laid alternately both on the left side and right side of the centremost mark and by gradually progressing towards both the extremities of this facet of the casing. This sequencing scheme is schematically represented in figure 4. All these weld beads are laid in a direction parallel to the longitudinal axis LL (figure 2) of the casing. Between the laying of one weld bead and the subsequent weld bead as defined in the manner stated earlier and as shown in the sequential order presented in Figure 4, the inter-pass temperature is maintained within the range of 150° C to 250° C at the location of the subsequently impending weld bead by allowing sufficient length of time to pass. An infrared thermometer can be implemented to measure the inter-pass temperature of the location where the weld is intended to be laid. After checking and confirming that the inter-pass temperature at the location of the impending weld is falling within the range of150° C to 250° C, the laying of the weld bead at that location is commenced.

6. Subsequent to the deposition of first pass of weld beads in each of the ten divisions identified by the marks and as shown in figure 4, the second round of weld deposition is commenced. This is also done similar to the sequencing scheme employed earlier for the first round of deposition of the ten weld beads at each of the marks made earlier (figure 4). In other words, the first weld bead is laid adjacent to the already laid weld bead at the centremost mark. The second weld bead is laid adjacent to the first weld bead present to the left of the centremost weld bead. Then, the third weld bead is laid adjacent to the first weld bead that is present to the right of the centremost weld bead. Similarly, the subsequent weld beads are laid alternately both on the left side and right side of the centremost weld bead and by gradually moving towards both the extremities of this facet of the casing. This sequencing scheme followed in the second round of weld deposition is schematically represented in figure 5. It is to be ensured that the each of the weld beads laid in the second round of weld deposition properly and sufficiently fuses with the already laid weld bead made in the first round of weld deposition and the direction of laying of each of the weld beads is always parallel to the longitudinal axis LL (figure 2) of the casing and the inter-pass temperature before commencing any of the weld bead is maintained and checked as stated in the previous step (5).

7. Similarly, the subsequent rounds of weld deposition is carried out until the required height of the weld build-up is reached in this facet of the casing as stated in the previous step (6). The total height of weld build-up achieved after all the rounds of the weld deposition is made in such a way that at least a minimum of 5 mm of extra weld metal is provided as ‘distortion allowance’ in addition to the required level of dimensional accuracy so that any radial aberration of this facet of the casing due to repair welding can be accommodated within this extra weld material.

8. After completion of weld build-up in the facet closest to the midpoint M (figure 2) of the longitudinal axis LL of the casing as stated in steps (5) to (7), the facet that is next closest to the midpoint M of the longitudinal axis LL of the casing is chosen for repair welding subsequently. This facet is divided in to ten divisions circumferentially as per the procedure stated in step (3).

9. Then, deposition of weld beads is carried out following the procedure stated in steps (5) to (7) in the facet as identified in step (8). The schematic representations of the first round and second round of deposition of weld beads in the facet that is second closest to the midpoint M of the longitudinal axis LL of the casing are shown respectively in figure 6 and figure 7. Similarly, the schematic representations of the first round and second round of deposition of weld beads in the facet that is third closest to the midpoint M of the longitudinal axis LL of the casing are shown respectively in figure 8 and figure 9.

10. Similarly, subsequent facets based on their increasing distance of separation from the midpoint M of the longitudinal axis (figure 2) of the casing are selected sequentially for the repair welding. The deposition of weld beads in each of the facets chosen as stated above is made as per the procedure mentioned in steps (4) to (7). As an example, the schematic representations of the first round and second round of deposition of weld beads in the facet that is third closest to the midpoint M of the longitudinal axis LL of the casing are shown respectively in figure 7 and figure 8.

11. The repair welding in as many facets as required are completed fully as per the procedures stated in steps (4) to (8). After the completion of repair welding in as many facets as required, the quality of repair weld in each of the facet so laid is assessed by employing an ultrasonic examination method. After this, the casing is machined at the repair welded facets until the required dimensional accuracy is achieved in the casing.

[0028] It is to be noted that the present invention is susceptible to modifications, adaptations and changes by those skilled in the art. Such variant embodiments employing the concepts and features of this invention are intended to be within the scope of the present invention, which is further set forth under the following claims:-