Vacuum Brazing Technology of Static Guide Vane Ring

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The fully hydrogen-cooled 300MW steam turbine generator, the stator coil, rotor coil and iron core are all cooled by the hydrogen flow generated by the multi-stage fan. The structure of the multi-stage fan is seven-stage compression. The original production process is complicated. The steel plate needs to be thinned and punched according to the blade shape line. The blade and the blade ring are vacuum brazed after argon arc welding. The production cycle takes about 6 months. The new process is now used for process development again. The new process adopts wire cutting to cut holes, optimizes the hydrogen arc welding and vacuum brazing process, and greatly improves the welding quality of the fan under the premise of greatly shortening the manufacturing cycle.

The blade ring material is 1Cr18Ni9 diamagnetic stainless steel, and the thickness after planing is 4.8mm and 5.3mm. Among them, the blade hole of the smaller-sized stationary blade ring is 2.47mm wide at the narrowest point and only 4.03mm at the widest point. The original punching process belongs to the small hole punching of thin plates, and the punching performance is extremely poor, and due to the serious heat generation of stainless steel during punching, the inner wall of the hole has a tendency of galling and pulling down. At the same time, due to the manufacturing error of the forging die in the forging process, the matching clearance between the blade and the blade hole is difficult to meet the requirements of the drawing. It takes a lot of man-hours to assemble the blade in the later stage. One set of 4068 blades needs to be polished and repaired by hand. two months.

In order to complete the project on time, three hole-cutting processes were considered: wire cutting, laser cutting and water jet cutting. Comparing the three processes, the laser cutting speed is the fastest, but since the laser is a convergent light source, the upper and lower incisions of the hole after passing through the steel plate will produce a slope with a deviation of nearly 0.07mm and an oxide layer with a thickness of 0.01mm, plus the blade itself is forged and formed. The maximum error of the brazing seam can reach 0.24mm, which is not conducive to ensuring the quality of brazing. The water jet uses high-pressure water jet mixed with fine abrasives to cut the steel plate, but the accuracy is between 0.1 and 0.25mm. Although no oxide layer is produced, the accuracy is lower. Compared with the first two, wire cutting has the slowest speed but the highest processing accuracy. Comprehensive comparison, the hole process is determined to be slow-moving wire cutting. In order to improve the efficiency, a small hole for the wire cutting machine to pass through is first used to melt the inside of each blade hole, and the four inner and outer ring steel plates are stacked into a stack. Thread the wire cutting machine molybdenum wire for cutting.

TIG welding

The assembled blade ring is clamped into the tooling for tightening and restraint. The welding sequence is staggered and skipped welding of the inner and outer rings to avoid local overheating and cause serious deformation of the position. About 10 welding seams are welded each time, the welding torch does not swing, and the welding seam is high Not less than 0.4mm. After the argon arc welding is completed, the blade ring is placed on the shape correcting frame before vacuum brazing to correct the shape. In the case of no restraint, the distance between the two ends of the ring and the deviation from the plane of the shape correcting frame shall not exceed 0.8mm to ensure that After the assembly with the wind guide ring in the later stage, the stress will not be too large.

Vacuum Brazing

There is a tip opening at the blade root after argon arc welding, which is prone to stress concentration. During the operation of the generator, the vibration of the unit may have a great influence on the weld and cause crack propagation. In order to eliminate this effect, the inner side of the blade and the blade ring needs to be filled with brazing, and the position of the welding seam is the horizontal overflow of the long welding seam. The uneven temperature causes the deformation of the blade ring and the distortion of the components, which makes it impossible to fit into the blade ring groove of the wind guide ring; and the repeated heating of the stainless steel causes the stainless steel to be in the sensitization range for a long time, resulting in intergranular corrosion and affecting the performance of the components.

Solder Specifications

The brazing material is QNi3 produced by the former Shanghai Iron and Steel Research Institute, which corresponds to the current national brand name BNi82CrSiB.

The nickel-based brazing filler metal can be used in an ambient temperature of 400-600 °C. After a small sample test, the shear strength can reach 180-250 MPa, which is better than the previous product data. Since it is difficult to ensure that the brazing seam can meet the requirements of the brazing gap, the brazing filler metals are powder and amorphous foil. The qualified position of the gap is adhered to the surface of the welding position with powder solder mixed with a specific adhesive, and the larger gap is filled with stainless steel sheet clamped with amorphous solder foil. The solder coating area is 2/3 of the blade length from the blade tip down on both sides.

Welding process

The blade ring is placed flat in the temperature uniform zone in the furnace, and is stacked in multiple layers, and the layers are separated by corundum blocks. Before brazing, in order to ensure the cleanliness of the brazing hole, it is necessary to eliminate the oxide layer generated by the heating of the argon arc welding on the workpiece. By comparing the tensile strengths of the brazed specimens by pickling, machining and bright annealing in the furnace, the comprehensive properties of the bright annealed specimens are better than the other two. In order to ensure the smoothness of the brazing surface, it is necessary to check the smoothness of the hole again after the bright annealing is completed. If the requirements are not met, a second bright annealing is required. Only then can the brazing material be applied in preparation for the subsequent vacuum brazing process. Apply a flow blocking agent to the lower part of each blade ring to prevent the molten droplets from dripping onto the blades below the furnace due to excessive addition of solder, which will affect the air intake effect of the air guide ring. The melting range of solder is 900~1100℃. Since the vacuum degree in the furnace can only reach 2 × 102Pa after the brazing temperature is reached, in order to further improve the brazing quality, special process treatment in the furnace is required to increase the vacuum degree to 10°Pa to meet the requirements of the smoothness of the components after welding. And the quality of the caulking of the brazing seam. The temperature curve of the brazing process is shown in Figure 2, and the bright annealing temperature curve is the same as that of the brazing process. After the welding is completed, the circulating water cooling water pipe is used in the cooling process to quickly pass the temperature of the blade ring through the stainless steel sensitization range of 450~800℃, reducing the degree of intergranular corrosion and providing better performance.

Post weld inspection

A total of 2392 welds of 56 single 1/4 rings were not found with surface defects such as pores and non-brazing. The cross-sectional results of the two stationary vane rings and guide vane ring test pieces showed that the internal brazing material was fully filled, and there was no breakage. . The blade ring and welding seam are silver-white in color, without high temperature oxidation. The deformation has no obvious change compared with that before entering the furnace, and the warpage degree at both ends is still less than 0.8mm, indicating that the temperature in the furnace is uniform, and the residual stress of original machining and argon arc welding has been completely released after solution treatment.

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