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 a 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 to eliminate 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 means in the remanufacturing field such as size restoration, surface modification, and life extension transformation of important parts.

After 8 years of operation, the low-pressure rotor journal of a steam turbine in service in a power plant has multiple wears, which seriously affects the safe operation of the unit. The power plant commissioned Shanghai Steam Turbine Factory to repair the worn parts of the journal.

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 material 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 complete the repair of the rotor journal with less welding heat impact. When Guo Shirui and others conducted a test on the laser cladding of the turbine rotor, they detected the temperature at a position 5 cm away from the cladding area. The temperature was not higher than 70 ℃ during the entire cladding process, which is conducive to controlling the dilution rate and welding deformation. Shanghai Steam Turbine Plant has carried out process tests and process development for rotor laser cladding in the early stage and has the technical capabilities for laser cladding production.

1.2 Introduction to laser cladding equipment
The equipment for repair work is a movable laser cladding workstation. The workstation adopts a container design and can be easily transported to the designated workstation in the 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 uses 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 zero point and the cladding layer side is 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. 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”, the 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 is the weld of the cladding layer. The yield strength Rp0.2 reaches 750 MPa, and the tensile strength reaches 845 MPa. The tensile properties of the cladding layer are basically equivalent to those of the forging body, which can meet the bonding and strength requirements of the repair layer.

(4) Bending test According to GB/T 232-2010 “Metallic Material Bending Test Method”, a bending specimen is cut from the cladding joint and the bending performance test is carried out. The thickness of the bending specimen is 10 mm, the diameter of the bending pressure head is φ40 mm, and the bending angle is 180°. The side bending test surface of the specimen is shown in Figure 6. There is no crack, which meets the specification requirements.

3 Remanufacturing and repair of rotor journal

3.1 Status of rotor shaft diameter before repair
Before repair, the rotor wear area was measured. Wear had occurred in many places on the 340 mm length of the journal of the No. 4 bearing. The most serious place had worn out a groove with a depth of 1.3 mm. The wear status is shown in Figure 7. The rotor surface condition 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. Turning is carried out 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 down until all the worn parts are completely turned clean and the cladding groove is prepared. 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 rotor surface and inside or other defects that may affect the subsequent cladding.
A Leeb 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 matrix.
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
The rotor is measured on a lathe after welding. The dimensional tolerance and surface roughness of the journal after repair have been completely restored to the original design requirements, which can meet the matching requirements between the bearing and the journal. In addition, the end and outer circle of the repaired rotor coupling flange, the circumference on both sides of the journal, and the area near the last stage impeller are measured using a micrometer. The measurement results show that the coaxiality of each measuring position is less than 0.02 mm, and the coaxiality has basically not changed compared to before the repair.

(2) Hardness test
When the rotor journal has a machining allowance of 0.2 mm, a hardness test was performed on 22 points randomly distributed on the cladding layer using a Leeb hardness tester according to GB/T17394.1-2014 “Metallic materials Leeb hardness test Part 1: Test method”. The hardness test results are in the range of 272HV~354HV. The hardness test results of the cladding layer are 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 is not greater than 30HV, which meets the use requirements.

(3) Penetrant test After the rotor journal is finely machined and subsequently ground, the journal is subjected to a penetrant test according to the factory standard for new rotor products. The penetrant test results show that the cladding layer has no defects such as slag inclusions, incomplete penetration, cracks, and pores, and the quality of the cladding layer meets the standard requirements. The state of the journal after completion is shown in Figure 10.

4 Repair effect

4.1 Repair accuracy

The repair of the rotor journal achieved precise local repair and had no adverse effects on other parts of the rotor. After the repair, all areas of the journal could meet the rotor journal diameter tolerance requirements. After the repair, the journal coaxiality was 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 the feedback from the power plant, the low-pressure rotor was put into operation after the repair. 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 complete the repair of the journal wear position of a steam turbine low-pressure rotor and obtained the following conclusions:

(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 mobile laser cladding workstation can realize 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 can ensure that the journal size tolerance, surface roughness, etc. meet the requirements of the design drawings after repair.

(4) After the repair is completed, the rotor is reinstalled back into the unit, and the unit runs stably after commissioning.