Turbine system having more failure resistant rotors and repair welding of low alloy ferrous turbine components by controlled weld build-up

Abstract

system for repairing worn surfaces of steam turbine components and especially high pressure turbine rotors, are disclosed. These systems include depositing a first layer of weld metal on a worn surface of the component, whereby a heat-affected zone is created. A second layer of weld metal is then deposited over the first layer using a greater amount of heat to temper at least a portion of the heat-affected zone produced by the first layer. The preferred embodiments include the use of gas tungsten arc welding for providing fine-grain size and more creep resistance, especially in the weld and heat-affected zone. The resulting build-up can be machined, for example into a blade fastening to produce a component having properties equal to or better than the base-metal alloy. The invention also provides a longer lasting turbine system, including rotors which have serrated steeples that are more resistant to failure.

Description

This is

DETAILED DESCRIPTION OF THE INVENTION

The novel methods of repairing worn surfaces of ferrous turbine components of this invention include providing a ferrous turbine component 20, 40, or 50 having Cr, Mo and V alloying ingredients. These turbine components 20, 40, or 50 include worn surfaces, however it is expected that new components can be manufactured using the methods disclosed herein. The repairing procedure includes depositing a first layer of weld metal the worn surface of the component thereby producing a heat-affected zone in that component. The procedure next deposits a second layer of weld metal on top of the first layer. This second layer is deposited with a greater amount of heat than the depositing of the first layer for tempering at least a portion of the heat affected zone produced by the first depositing step. As used herein, the term "tempering" refers to the process wherein the heat-affected zone of the base metal is re-heated and then cooled to relieve internal stress and reduce its hardness.

By carefully controlling the weld parameters for the first to five layers of weld build-up, problems associated with the heat-affected zone and resulting coarse grain structure of the base metal can be overcome. More specifically, using a gas tungsten arc weld procedure, the first layer is deposited using about 40 to 60 amperes, more preferably about 60 to 140 amperes, and most preferably, about 80 to 120 amperes of direct current. The initial layer is welded at a relatively low amperage to create as small a heat-affected zone as possible. Next, the second layer of weld metal is deposited using about 50 to 200 amperes, more preferably 75 to 175 amperes, and most preferably about 100 to 150 amperes of direct current. Alternatively, alternating current could be employed less advantageously.

The higher amperage associated with the application of the second layer of weldment has the effect of "heat treating" or tempering the relatively brittle heat-affected zone in the turbine component. Following this second layer, the third and fourth layers preferably are applied using the same amount of current as used for applying the first layer. The fifth and subsequent layers can be applied using a higher current, since it will have less of an effect on the base metal.

It must be understood that the above current preferences are ideally suited for a gas tungsten arc welding procedure using a wire size of about 0.045 inches, a 50% argon-50% helium gas mixture, and a ⅛ or 3/32 tungsten size. It is expected that the tempering effect caused by the depositing of the second layer could be accomplished by varying these and other parameters to obtain the same effect. For example, if a smaller wire size for the deposited alloy were used for depositing the second layer of weld metal, the amperage could remain about at the same setting as the setting used for depositing the first layer. This, and other techniques, are well within the knowledge of those in the welding art, and are mere variations of the principal teachings of this invention.

The ferrous alloys preferably used in this invention to repair the Cv-Mo-V components of steam turbines are selected to include one or more of the following elements: Cr, Mo, Si, C, Mn, V, Nb, and N. In a more preferred embodiment, the ferrous alloy applied to the worn component is selected to comprise about 7.0 to 11.0 weight percent Cr and about 0.1 to 3.0 weight percent Mo. One specified preferred ferrous alloy composition range consists essentially of about 0.04 to 0.22 weight percent C, 0.15 to 1.0 weight percent Mn, 0.15 to 1.0 weight percent Si, 0.0 to 0.02 weight percent P, 0.0 to 0.016 weight percent S, 0.0 to 0.8 weight percent Ni, 4.00 to 19.0 weight percent Cr, 0.43 to 2.1 weight percent Mo, 0.09 to 0.5 weight percent V, 0.03 to 0.20 weight percent Nb, 0.0 to 0.08 weight percent Al, 0.0 to 0.20 weight percent Cu, 0.005 to 0.06 weight percent N and the balance being Fe. However, the most preferred a ferrous alloy consists essentially of 0.08 to 0.11 weight percent C, 0.30 to 0.50 weight percent Mn, 0.30 to 0.50 weight percent Si, 0.00 to 0.10 weight percent P, 0.00 to 0.008 weight percent S, 0.00 to 0.40 weight percent Ni, 8.00 to 9.50 weight percent Cr, 0.85 to 1.05 weight percent Mo, 0.18 to 0.25 weight percent V, 0.06 to 0.10 weight percent Nb, 0.00 to 0.04 weight percent Al, 0.00 to 0.10 weight percent Cu, 0.01 to 0.03 weight percent N and the balance being Fe.

In further accordance to these novel methods, the depositing step can consist of welding the ferrous alloy to the worn surface of the turbine component. This welding step preferably is accomplished by any one of gas tungsten arc welding, plasma-arc welding, electron beam welding, laser-beam welding and gas metal arc welding. It is expected that other welding procedures may be used to apply the novel alloys of this invention, however, it is important that the welding procedure employed minimize the heat-affected-zone in the base metal so as to avoid unnecessary defects.

The most preferred procedure employed thus far comprises gas tungsten arc welding (GTAW) the preferred ferrous alloys to a machined or ground turbine component. GTAW is preferred because its multiple bead depositions exhibit exceptionally fine-grain size in the weld and heat-affected zone. This fine-grain size translates into exceptional low and high cycle fatigue, tensile, impact and creep-to-rupture properties.

In accordance with the preferred gas tungsten arc welding procedures of this invention, the steam turbine component 20, 40, or 50 is preheated to at least about 177°C C. prior to the welding step. Side plates may be employed for "360°C" welding applications on discs and rotors. As used herein, a "360°C" repair refers to a procedure wherein weld metal is deposited continuously about the circumference of a turbine component, such as a rotor or disc, until a sufficient height is reached whereupon the individual steeples of the rotor are machined, or the disc is machined down to service tolerances. The side plates preferably are manufactured from Cr-Mo-V rotor steel or copper, and can be water cooled to further reduce welding side effects.

For a 360°C steeple weld build up as described in FIG. 5, a preferred procedure for high pressure rotor repairs, any steeples 44 present on the rotor are machined off to the bottom of the blade grooves to form solid ring of material. Next, a 360°C weld build-up 54 is made by rotating the rotor under the weld torch, instead of moving the torch across the width of the rotor disc, as is done for individual blade fastening repair welds. For the multiple-type of repair weld, use may be made of the above-mentioned water-cooled side plates which are disposed along at least a longitudinal edge of the worn surface to contain the weld volume and minimize carbon loss in the weld deposit. The welding operation can alternatively deposit the preferred ferrous alloys of this invention against the water cooled side plates, if desired. When a single steeple repair is made on a rotor component, the worn steeple is preferably removed entirely from the rest of the rotor. Because the entire steeple is removed, the exceptional metallurgical properties associated with the fine grain structure produced by the GTAW process are present throughout the finished blade fastening machined from the weld build-up. Next, a first run-off tab is disposed along at least a first longitudinal edge of the rotor for at least providing a starting surface for the welding step. A second run off tab can be disposed on a second longitudinal edge of the rotor transversely opposed from the first longitudinal edge, for at least providing a surface for stopping the welding step. Since the attachment cites for the run-off tabs 46 can sometimes be the point of defects, a cladding procedure is used to join these run-off tabs 46 to the rotor 40. Preferably, this cladding comprises a buffering layer, containing chrome, and is disposed in at least two overlapping weldments.

During a preferred (GTAW) welding repair of a single steeple repair, a first bead is welded transversely across the rotor on the machined or otherwise prepared surface. Next, a second bead is welded transversely across the rotor on the prepared surface, and spaced apart from the first bead. The third and fourth weld beads are similarly disposed and spaced apart, if room is available. Using this intermittent welding procedure permits the immediate area of base metal underneath the weld to slowly cool prior to the next adjoining welding application. Accordingly, the brittleness associated with a weld-created, heat affected zone is minimized.

The turbine rotor 20 of FIG. 1 preferably is selected from a turbine already in service, although it is expected that new rotors without serrations can be used as the initial turbine component for the following welding procedures.

Generally the steam turbine rotors, discs and blades of this invention are manufactured from low alloy steel, commonly containing less than 6% alloying elements. Of particular importance to these applications is the Cr-Mo-V alloy, A 470, Class 8, and its modified versions. One most preferred composition includes 0.27-0.34% by weight C, 0.70-1.0% by weight Mn, 0.012% by weight P and S (max), 0.20-0.35% by weight Si, 0.50% by weight Ni (max), 1.05-1.35% by weight Cr, 1.00-1.30% by weight Mo 0.21-0.29% by weight V, 0.15% be weight Cu (max), 0.010% by weight Al (max); 0.0015% by weight Sb(max), 0.015% by weight Sn (max), and 0.020% by weight As (max). Other forging alloys which can be used for making steam turbine components for high pressure service may also be repaired through the processes of this invention, such as those containing varying amounts of Ni, Co, Cr and other alloying ingredients.

When a used turbine component, such as rotor 20, 40, and 50 are employed, the highly stressed, individual steeples 44 are preferably mechanically removed. As used herein, "mechanically removing" refers to any of the known procedures for removing metal, including but not limited to, grinding, machining, electric arc gouging, and other methods known to those in the metallurgy arts. As in the case of FIG. 4, the entire worn or damaged steeple should be removed since it is important to reduce the possibility of creating any weak heat-affected zones in the high stress areas of these components by subsequent welding operations.

As generally described in FIGS. 2, 4 and 5, the preferred ferrous alloy compositions of this invention can be deposited by welding them to the worn or damaged surface of the turbine component 20, 40 or 50. This welding step can be accomplished by any one of the known welding techniques, but preferably any one of gas tungsten arc welding, plasma-arc welding, electron beam welding, laser-beam welding, and gas metal arc welding. A preferred preheat of at least about 100°C C. to about 300°C C., more preferably about 177°C C. to about 204°C C., can be employed for reducing stresses in the turbine component 20, 40, or 50 prior to the welding step.

In preparation for the preferred gas tungsten arc welding procedure of this invention, the surfaces to be welded are preferably conditioned to bright metal. More preferably, the base metal surfaces are cleaned for a distance of about 2 inches from the weld area with denatured alcohol, acetone, methyl chloroform, or solvent cleaner. It is further noted that if methyl chloroform is applied, it should be followed with an alcohol, acetone or solvent wash. It is also advised that the base metal surface to be welded be inspected using non-destructive testing procedures, and that at least one sixteenth inch of additional metal be removed beyond the deepest crack or fatigued area found.

In accordance with the preferred GTAW procedure of this invention, the following welding parameters have been deemed useful:

TABLE II
			Remainder
Parameter	Layer 1,3,4	Layer 2	of Weld
Nature of Current	Pulsed 60%	Pulsed 60%	Straight
			(no pulse)
Amperage-DCSP	85-120	100-150	280
Voltage	8.5-9.0	9.0-10	11-13
Surface Speed	4.0	3.0	4.5-8.0
(Linear), Inch
Frequency of Pulse	3 cycles/sec	3 cycles/sec	None
Type of Travel	OSC	OSC	Straight
Osc. Amplitude	.22	.22	0
Wire Feed Speed,	5-25	5-20	50
inches per min.
Tungsten Size,	1 or 3/32	1 or 3/32	1
2% Thorium, inch
dia
Tungsten Stickout,	1-1	1-1	1-1
inch
Wire Size,	.045	.045	.045
inch dia
Primary Shield,	50% + 50%	50% + 50%	50% + 50%
Argon-Helium
Trail Shield,	100%	100%	100%
Argon
Bead Overlap	50%	50%	50%

As described in FIG. 4, the bead sequence for the welding application should provide spaced bead sequencing, i.e., depositing a first bead 1 transversely across the preferred rotor 40 on the machined surface and then welding a second bead 2 on the prepared surface, transversely across the rotor 40 and spaced apart from the first bead. By working the weld metal from the outside to the inside of the welded areas for beads 1-10, etc, less of a heat affected zone (HAZ) is created by the welding procedure. Peening is not advised, and the welding is preferably accomplished by an automated GTAW machine in the flat, +/-15°C position. Weld stops for this machine shall be made using current tapering to a value of 15 amps or less before breaking the arc. In addition, run off tabs 46, such those shown in FIG. 4, should be used for starting and stopping the weldment, since these locations may develop metallurgical defects. It is also advisable that the base metal be demagnetized prior to welding to minimize arc blow.

During the welding procedure, the interpass temperature of the base metal preferably should be below 300°C C., preferably below 250°C C., and most preferably below 204°C C. Immediately after welding, the welded turbine component 20, 40, or 50 and its weldment 12, 42, or 54 should be maintained at a temperature of about 149°C C. to about 260°C C., preferably about 176°C C. to about 204°C C. After this post-heat maintenance schedule, the welded turbine component 20, 40, or 50 can be given a post-weld heat treatment above 500°C C., preferably above 600°C C., and more preferably about 663°C C. The post-weld heat treatment temperature should be selected to minimize weld stresses, provide sufficient "tempering back" of the weld and heat affected zone hardness, and if necessary, prevent "over tempering" of the unaffected base metal to obtain the required weld strength. The preferred rotor repair techniques of this invention generally include a port-weld heat treatment locally at the repair weld area. This local stress relief consists of heating the entire repair area and also axially along the rotor to meet any preselected axial and radial temperature gradient.

Following the post-weld heat treatment of the welded area, the turbine components 20, 40 and 50 repaired by the above-mentioned procedures are blast cleaned and provided with a nondestructive examination, for example, magnetic particle, dye penetrant or ultrasonic testing. In addition, mechanical testing is conducted by determining the hardness of the weld deposit and by tensile testing metal coupons that were fabricated during the same welding operation. The turbine component is then ready for final dimensional checks and a finishing machining operation, i.e. to produce individual serrations 14.

From the foregoing, it can be realized that this invention provides improved procedures for repairing ferrous steam turbine components. The welding methods, alloys, and heat treatment schedules provide a repaired surface exhibiting improved high temperature properties which may include, for example, better creep and fatigue properties than the base metal of such components. Although various embodiments have been illustrated, this was for the purpose of describing, and not limiting the invention. Various modifications, which will become apparent to one skilled in the art, are within the scope of this invention described in the attached claims.

Claims

1. In a steam turbine system of the type having a ferrous steam turbine component, said component having a worn surface thereon, the improvement comprising:

(a) a first layer of weld metal deposited on said worn surface, said deposition of said first layer producing a heat-affected zone in said component; and

(b) a second layer of weld metal deposited to said first layer, said deposition of said second layer tempering at least a portion of said heat-affected zone in said component, wherein said turbine component comprises about 1.05% to 1.35% by weight Cr, about 1.00% to about 1.30% by weight Mo, and about 0.21% to 0.29% by weight V.

2. The system of claim 1 wherein said turbine component comprises a rotor.

3. The system of claim 2 wherein said first layer of weld metal comprises a plurality of weld beads disposed transversely across said rotor.

4. The system of claim 3 wherein said second layer of weld metal comprises a plurality of weld beads disposed transversely across said rotor.

5. The system of claim 4 further comprising steeples machined from a weld build up comprising at least said first and second layers of weld metal.

6. The system of claim 1 5 wherein at least one of said steeple steeples has a higher creep resistance than that of the ferrous steam turbine component.

7. The system of claim 1 wherein said turbine component further comprises greater than 0.27% by weight C, and less than about 0.34