The turbine rotor is heavy and has a high speed. The rotor journal may wear due to poor operation of the lubricating oil system or other factors, causing excessive rotor vibration or other safety hazards. For this type of damage to the shaft diameter of the in-service rotor, advanced laser cladding repair and remanufacturing technology can eliminate the damage, and the repair quality meets the long-term safe operation requirements of the rotor. Compared with the traditional welding repair technology, it is more flexible and efficient. The application case involved in this article is a repair technology that uses a semiconductor laser as a heat source and uses a laser cladding method to clad the repair layer on the surface of the damaged journal. The laser cladding layer has high bonding strength with the parent material, low welding heat input, high degree of automation, and intact journal repair weld quality. There is no obvious change in the shape and size of the rotor before and after the repair.
The rotor is the core component of the turbine and needs to withstand harsh working conditions such as high temperature, high pressure, high speed, and high stress. However, due to the quality of the lubricating oil, impurities and other unfavorable operating factors, the rotor journal sometimes wears out, seriously affecting the safe and stable operation of the turbine unit. In recent years, laser remanufacturing and repair technology has been tried in eliminating rotor shaft wear. Compared with traditional technologies such as electroplating, chemical plating, thermal spraying, and arc welding, laser remanufacturing and repair technology has the advantages of high bonding strength, small heat-affected zone, low dilution rate, small deformation, small subsequent processing allowance, strong selection, and high degree of automation. It has become an important tool in the remanufacturing field such as size restoration, surface modification, and life extension of important parts [1-4].
After 8 years of operation, the low-pressure rotor shaft neck of a steam turbine in service in a power plant was worn in many places, seriously affecting the safe operation of the unit. The power plant commissioned Shanghai Steam Turbine Factory to repair the worn parts of the shaft neck.
1 Laser remanufacturing technology and repair equipment
1.1 Laser cladding technology
Laser cladding is an important application of laser processing technology. During the cladding process, a thin layer of base material is melted by irradiating a laser beam with a high energy density (generally 102~104W/mm2), mixed with the same melted filler material, and a certain thickness of heat-affected zone is generated. A cladding layer that is metallurgically bonded to the base surface is formed on the base surface to achieve surface modification, additive manufacturing and other operations. Figure 1 shows the principle of laser cladding.
As a low-heat input remanufacturing method, laser cladding shows its adaptability to additive repair of low-pressure rotors with high dimensional accuracy requirements and difficult welding of the base material. Due to the high energy density of laser cladding, it can repair the rotor journal with less welding heat impact. When Guo Shirui et al. [5] conducted a test on the laser cladding of a turbine rotor, they detected the temperature at a position 5 cm away from the cladding area. The temperature was not higher than 70 °C during the entire cladding process, which is conducive to controlling the dilution rate and welding deformation. Shanghai Steam Turbine Plant has previously conducted process tests and process development for rotor laser cladding [6-7] and has the technical capabilities for laser cladding production.
1.2 Introduction to laser cladding equipment
The equipment for repair work is a mobile laser cladding workstation. The workstation adopts a container design and can be easily transported to a designated workstation in a power plant or factory for laser cladding operations. It is suitable for repairing various parts. The workstation is mainly composed of a high-power laser, a 6-axis robot, a powder feeder, a coaxial powder feeding cladding head, and auxiliary systems such as power supply and cooling to support the operation of the system.
2 Laser cladding layer performance test
In order to ensure the repair effect and the comprehensive mechanical properties of the cladding metal, a special cladding powder with the same material as the low-pressure rotor 30Cr2Ni4MoV steel was developed, and a simulated part cladding test was carried out to verify the repair process and cladding layer performance.
Sample preparation:
The test base material is 30Cr2Ni4MoV steam turbine low-pressure rotor material, and the standard is JB/T 11020-2010 “Technical Conditions for Ultra-pure Steel Low-pressure Rotor Forgings for Supercritical and Ultra-supercritical Unit Steam Turbines”, and its specific chemical composition and mechanical properties are shown in Table 1.
The process parameters of laser cladding of the test piece are: scanning power 2400W~3500W, scanning rate 10mm/s~30mm/s, powder feeding rate 1.0r/min~1.8r/min, no post-weld heat treatment, and after cladding, samples were taken according to Figure 2 for tensile, bending, metallographic and hardness specimens.
(1) Metallographic structure analysis
The macroscopic morphology of the cross section of the laser cladding joint is shown in Figure 3. The cladding layer is well bonded to the substrate, and there are no macroscopic defects such as cracks, pores, incomplete fusion, and incomplete penetration in the cross section of the joint.
The microstructures of the parent material area, cladding layer area, and heat-affected zone of the cross section of the cladding layer joint are analyzed. The cross-sectional microstructure of the laser cladding joint is shown in Figure 4. The parent material, cladding layer, and heat-affected zone are all bainite structures, but their organizational morphologies are different due to differences in their cooling processes, and the heat-affected zone has the finest structure.
(2) Hardness test
The Vickers hardness test was carried out on the cladding joint. The test method was in accordance with GB/T4340.1. The hardness test results are shown in Figure 5. The fusion line is taken as the 0 point and the cladding layer side is taken as the positive direction. It can be seen from the figure that the substrate hardness is in the range of 250~350 HV; the substrate hardness near the fusion line is affected by the heat of laser cladding, and the hardness value is in the range of 360~410 HV, and the width of the heat-affected zone is about 1.2 mm; the hardness of the cladding layer tends to be stable beyond 1 mm from the fusion line, and the hardness is in the range of 270~350 HV, which is close to the substrate hardness.
(3) Tensile test
According to GB/T 228.1-2010 “Metallic materials tensile test Part 1: Room temperature test method”, a plate tensile specimen was cut from the cladding joint for tensile performance test. The tensile rate was carried out according to GB/T 228.1 A224. The fracture position of the tensile specimen was the weld of the cladding layer. The yield strength Rp0.2 reached 750 MPa and the tensile strength reached 845 MPa. The tensile performance of the cladding layer was basically the same as that of the forging body, which could meet the bonding force and strength performance requirements of the repair layer.
(4) Bending test
According to GB/T 232-2010 “Metallic materials bending test method”, a bending specimen was cut from the cladding joint for bending performance test. The thickness of the bending specimen was 10 mm, the diameter of the bending pressure head was φ40 mm, and the bending angle was 180°. The side bending test surface of the sample is shown in Figure 6. There is no crack and it meets the specification requirements.
3 Remanufacturing and repair of rotor journal
3.1 State of rotor shaft diameter before repair
Before repair, the rotor wear area was measured. Wear occurred in many places on the 340 mm length of the journal of the No. 4 bearing. The most serious place has worn out a groove with a depth of 1.3 mm. The wear state is shown in Figure 7. The surface state of the rotor can no longer meet the safe operation of the unit. It is urgent to complete the repair through safe and effective remanufacturing means to avoid replacing the entire rotor.
3.2 Machining before repair
First, the damaged part of the rotor shaft diameter needs to be cleaned. The main purpose is to cut off the wear groove and clean the adjacent materials affected by wear. On this basis, the cladding groove is prepared to ensure the subsequent cladding quality.
Pre-welding processing is carried out on a CNC lathe, and turning is performed with the minimum processing amount, that is, the damaged area that has been completely turned clean will not be processed further, and the area that has not been completely turned will continue to turn the diameter until all the worn parts are completely turned clean and the preparation of the cladding groove is completed. The journal before cladding is shown in Figure 8.
3.3 Inspection before repair
Before cladding, the damaged journal neck is subjected to penetration testing and ultrasonic testing to detect whether there are cracks on the surface and inside of the rotor or other defects that may affect the subsequent cladding.
The hardness tester is used to test the hardness of 16 points randomly distributed on the journal. The hardness of the entire area to be clad is in the range of 260~300 HV, which is consistent with the hardness of the rotor substrate.
The test results show that the damaged part of the journal neck has been completely cleaned, and there are no defects on the surface and inside of the area to be clad, which meets the requirements of laser cladding repair work.
3.4 Laser cladding repair
During the laser cladding process, the robot clamps the cladding head to achieve axial feeding, and the rotation of the rotor is driven by the lathe.
Before cladding, the area to be clad needs to be carefully cleaned, and the journal is scrubbed with acetone. Subsequent cladding operations can only be performed after confirming that there is no oil, water or other stains remaining.
As shown in Figure 9, the cladding operation is performed layer by layer. During the cladding process, the cladding quality needs to be monitored and the surface of the cladding layer needs to be cleaned between layers until the thickness of the cladding layer meets the dimensional requirements. During the cladding process, the temperature between layers is monitored to avoid overheating. After the cladding is completed, the rotor journal is slowly cooled to room temperature under the cover of the insulation material before machining.
3.5 Inspection after repair
(1) Dimension inspection After welding, the rotor is measured on a lathe. After the repair, the dimensional tolerance and surface roughness of the journal have been completely restored to the original design requirements, which can meet the matching requirements between the bearing and the journal. The micrometer was used to measure the dimensions of the end and outer circle of the repaired rotor coupling flange, the circumference of both sides of the shaft neck, and the area near the final impeller. The measurement results showed that the coaxiality of each measuring position was less than 0.02 mm, and the coaxiality was basically unchanged compared to before the repair.
(2) Hardness test When there was a 0.2 mm machining allowance left on the rotor shaft neck, a Leeb hardness test was performed on 22 points randomly distributed on the cladding layer using a Leeb hardness tester in accordance with GB/T 17394.1-2014 “Metallic materials Leeb hardness test Part 1: Test method”. The hardness test results were in the range of 272 HV~354 HV. The hardness test results of the cladding layer were similar to the hardness values measured in the previous cladding test, and the deviation between the hardness values of each point on the same circumference was no more than 30 HV, which met the use requirements.
(3) Penetrant testing After finishing the rotor journal and subsequent grinding, the journal was subjected to penetrant testing according to the factory standard for new rotor products. The penetrant testing results showed that the cladding layer had no defects such as slag inclusions, incomplete penetration, cracks, and pores, and the quality of the cladding layer met the standard requirements. The status of the journal after completion is shown in Figure 10.
4 Repair effect
4.1 Repair accuracy
The journal repair of the rotor achieved precise local repair without adverse effects on other parts of the rotor; after the repair, the journal diameter tolerance requirements of the rotor can be met in all areas, the coaxiality of the journal after the repair is less than 0.02 mm, and the roughness Ra≤0.4 μm, achieving the repair of parts with high dimensional accuracy requirements.
4.2 Rotor operation
According to feedback from the power plant, the low-pressure rotor was put into operation after the repair, and the lubricating oil temperature, bearing temperature and shaft vibration of the unit all met the requirements, and the operation was good.
5 Conclusion and Prospect
The laser cladding remanufacturing technology was used to successfully repair the worn position of the journal of a steam turbine low-pressure rotor, and the following conclusions were obtained:
(1) The bonding strength between the cladding layer and the substrate is high, and the strength and chemical composition of the cladding layer meet the use requirements of the shaft diameter position.
(2) The use of a movable laser cladding workstation can realize the online repair of the rotor, and the repair period can meet the needs of the power plant maintenance cycle.
(3) The rotor journal repaired by laser cladding remanufacturing can ensure that the journal size tolerance, surface roughness, etc. meet the requirements of the design drawings after the repair.
(4) After the repair is completed, the rotor is reinstalled back into the unit, and the unit runs stably after commissioning.
