Welding 600 MPa High-Strength Steel for Pressure Pipes in Pumped Storage Stations

Abstract: Pressure steel penstocks are the core load-bearing components of water conveyance systems in pumped storage power stations, playing a critical role in ensuring operational safety, functional performance, and the timely progress of construction projects. Thanks to their high strength and excellent toughness, 600 MPa-class and higher-strength steels have become widely used in fabricating these penstocks. Drawing on the branch penstock fabrication project at the Huizhou Zhongdong Pumped Storage Power Station, this study investigates welding consumables, methods, and parameters for high-strength steels. Based on the experimental findings, an optimized welding scheme is proposed, providing a valuable reference for advancing the application and technological development of high-strength steels in pressure steel penstock fabrication for pumped storage power stations.

The Huizhou Zhongdong Pumped Storage Power Station is located in Zhongdong Village, Gaotan Town, Huidong County, Huizhou City, Guangdong Province. The station has a planned installed capacity of 1,200 MW, consisting of three reversible pump-turbine-generator units, each with a capacity of 400 MW. Its main facilities include an upper reservoir, a lower reservoir, a water conveyance system, an underground powerhouse cavern complex, a surface switchyard, and permanent internal access roads. Within the water conveyance system, the branch steel penstocks have diameters ranging from DN3000 to DN2270 mm and steel plate thicknesses of 44–95 mm, using Q490S, a 600 MPa-class high-strength steel. Drawing on the fabrication project for these branch pressure steel penstocks, this paper investigates the key welding parameters for 600 MPa-class high-strength steel, identifying and summarizing the optimal welding methods, consumables, and parameters.

1. Welding Experiments

1. The tensile, bending, and impact properties of the Q490S steel plates were tested strictly in accordance with Clause 6.4 of GB/T 31946-2015, Steel Plates for Pressure Steel Penstocks of Hydropower Stations.
2. The steel plates were welded using two methods: pure gas-shielded welding and a hybrid approach combining gas-shielded welding with submerged arc welding. Welding consumables were sourced from manufacturers and brands such as Jinertai, Mudan, and Lincoln. Details of the experimental comparisons are presented in Table 1. The primary difference between Test Plate 1 and Test Plate 2 lay in the choice of welding consumables. In contrast, the distinction between Test Plate 3 and Test Plates 1 and 2 was mainly due to differences in welding methods and parameters. After completing all three experimental sets, the test specimens were comprehensively evaluated for tensile, impact, and bending properties.
3. Welding of the test plates was conducted under strict conditions:
   • Prior to welding, electrodes and fluxes were baked and maintained at the temperatures specified by the electrode manufacturers. During welding, electrodes awaiting use were stored in an electrically heated electrode holding oven, retrieved only as needed, with the oven lid kept closed at all times. Electrodes were not permitted to remain in the oven for more than four hours; if this limit was exceeded, re-baking was required, with a maximum of two re-baking cycles allowed.
   • Before welding, all mill scale, rust, oil, and other contaminants were thoroughly removed from the weld zone and the groove flanks within a range of 15–20 mm. Following each weld pass, the surface was immediately cleaned, and the subsequent pass was initiated only after a self-inspection confirmed the absence of defects.
   • For automatic submerged arc welding, run-on (arc-starting) and run-off (arc-extinguishing) plates were affixed at both ends of the weld seam. The material, groove configuration, and dimensions of these auxiliary plates matched those of the workpiece; typically, the dimensions were no less than 50 mm × 100 mm.
   • In multi-pass welding, inter-layer joints (tie-ins) were staggered by at least 30 mm. Each weld seam was executed continuously in a single pass. If welding was interrupted, the weld surface was thoroughly cleaned before resuming, and welding proceeded according to the original procedure only after verifying the absence of cracks or other defects.

2. Experimental Results

2.1 Weld Quality Inspection

2.1.1 Visual Inspection of Welds

Visual inspection of the weld seams on the three test plates was conducted strictly in accordance with GB/T 50766, Code for Fabrication, Installation and Acceptance of Pressure Steel Pipes for Hydropower and Water Conservancy Projects. A meticulous visual examination of the entire weld surface was performed. The weld seams on all three test plates exhibited a uniform and smooth profile, with a natural and fluid transition between the weld and the base metal, and no discernible height discrepancies or misalignment. No crack-type defects—including linear or crater cracks—were observed on the weld surfaces or within the heat-affected zones. At the interface between the weld edges and the base metal, no undercut exceeding 0.5 mm in depth or 10 mm in length was detected. Furthermore, the weld surfaces were free from visually perceptible pores, whether circular, elliptical, or elongated, as well as other surface defects such as slag inclusions, incomplete penetration, or weld overlap. Cross-sectional analysis of the weld seams showed a full, convex profile, with weld reinforcement maintained within a reasonable range of 2–4 mm. The weld width was uniform and consistent, fully covering the entire groove area. These results demonstrate excellent welding process stability and strict adherence to proper operational procedures.

2.1.2 Non-Destructive Testing of Internal Weld Quality

To evaluate the internal quality of the weld seams on the three test plates, Ultrasonic Testing (UT) was conducted in strict accordance with GB/T 11345, Manual Ultrasonic Testing of Steel Welds and Classification of Testing Results, as well as the project-specific testing plan. The aim was to comprehensively detect potential internal defects such as slag inclusions, incomplete penetration, lack of fusion, or internal cracks. Prior to testing, the inspection equipment was calibrated to ensure high-precision performance. The surfaces of the weld inspection zones were pre-treated to remove spatter, mill scale, and oil stains, with surface roughness controlled to a maximum Ra of 6.3 µm to guarantee effective acoustic coupling and prevent interference with the ultrasonic signals. The inspection employed a combined zigzag scanning and oblique parallel scanning approach, ensuring 100% coverage of the weld seam cross-sections. Ultrasonic couplant was applied uniformly, the scanning speed was limited to 150 mm/s, and an overlap of at least 15% was maintained between adjacent passes. Any suspicious signals were re-examined multiple times for precise localization. Systematic inspection and data analysis revealed no defect signals exceeding the prescribed standards. Specifically, there were no point-like slag inclusions larger than 3 mm, no linear slag inclusions exceeding one-third of the weld thickness, and no instances of incomplete penetration or lack of fusion along the fusion lines or groove roots. Additionally, no internal cracks or other hazardous defects were detected. All inspection results meet the quality requirements for Class I welds as defined in GB/T 11345, demonstrating that the internal weld structure is sound, dense, and possesses reliable mechanical properties and operational safety.

2.2 Performance Testing

2.2.1 Test Items

Mechanical performance testing was conducted on the base metal and the three welded test plates in strict accordance with the current national standards system. The testing was based on GB/T 228.1, Metallic Materials—Tensile Testing—Part 1: Method of Test at Room Temperature, GB/T 232, Metallic Materials—Bend Test, and GB/T 229, Metallic Materials—Charpy Pendulum Impact Test. A systematic experimental design was implemented to comprehensively evaluate the fundamental mechanical properties of the base metal and the quality of the welded joints.

• Tensile Testing
  Representative specimens were taken from the base metal as well as the weld zones and heat-affected zones of the three test plates. The specimens were machined to standard dimensions in accordance with the relevant standards and tested using a microcomputer-controlled electronic universal testing machine. The primary objective was to assess the mechanical behavior of the base material and welded joints under axial tensile loading. By analyzing the stress-strain relationships, the study aimed to determine whether the materials possess adequate load-bearing capacity and to evaluate the impact of the welding process on material strength, ensuring that welded joints would not compromise structural integrity.
• Bending Testing
  Using the standard three-point bending method, base metal and test plate specimens were accurately mounted onto the support fixture of a bending test machine. Load was applied gradually according to the specified mandrel diameter and loading rate until the specimens reached the predetermined bending angle. The test aimed to assess the plastic deformation capacity and overall integrity of both the base material and the weld seam, with particular attention to detecting any hidden brittle defects or poor fusion within the welded joints. Post-test visual inspection of the specimen surfaces provided a means to evaluate resistance to cracking under bending loads.
• Impact Testing
  Impact testing was performed to evaluate the toughness of the materials under dynamic loading conditions. Charpy V-notch specimens were tested at a low temperature of -40 °C using a pendulum impact testing machine. The primary objective was to assess the impact resistance of the base metal and welded joints under harsh, low-temperature conditions. By measuring the energy absorbed during impact, the test determined the material’s and welds’ resistance to fracture under sudden loading, thereby mitigating the risk of brittle failure in service due to low-temperature embrittlement.

Table 1. Experimental Specimens for Each Group

Using the standard three-point bending method, the base metal and test plate specimens were securely mounted on the support fixture of a bending test machine. Load was applied gradually—following the specified mandrel diameter and loading rate—until the specimens reached a predetermined bending angle. This test primarily aims to evaluate the plastic deformation capacity and overall integrity of both the base material and the weld seam, with particular attention to detecting any hidden brittle defects or poor fusion within the welded joint. Post-test visual inspection of the specimen surfaces allows assessment of the material’s resistance to cracking under bending stresses.

Impact Testing was performed to assess the toughness of the materials under dynamic loading conditions. Charpy V-notch specimens were tested at a low temperature of -40 °C using a pendulum impact testing machine, delivering instantaneous impact loads. The main objective was to evaluate the impact resistance of both the base metal and welded joints under harsh, low-temperature conditions, thereby verifying that the material possesses sufficient toughness to absorb impact energy effectively. By measuring the absorbed impact energy, the test determines the material’s and welds’ resistance to fracture under sudden loading, mitigating the risk of brittle failure in structural components during actual service due to low-temperature embrittlement.

2.2.2 Comparative Analysis

Based on the experimental results described above, the tensile strength and bending performance of the three test plates were largely consistent; however, the impact energy values of Test Plates 2 and 3 were notably lower than those of Test Plate 1.

(1) Tensile Strength: Overall, the tensile test results met the required standards and demonstrated excellent consistency. The measured tensile strengths of the three test plates were highly uniform: Test Plate 1 reached 630 MPa, Test Plate 2 reached 640 MPa, and Test Plate 3 reached 628 MPa. The variation among the samples was only 12 MPa, with all values falling within the 620–650 MPa range. This indicates that the welding process exhibits strong stability with respect to “strength control,” as tensile strength did not fluctuate significantly due to differences in heat input during welding. Compared to the base metal’s tensile strength of 708 MPa, the test plates’ tensile strength was slightly lower but still satisfied common engineering requirements. This minor reduction is likely attributable to microstructural changes in the heat-affected zone—such as slight grain coarsening—without compromising the overall load-bearing capacity of the joints.

(2) Bending Performance: All specimens successfully passed the bending test, demonstrating sufficient plastic reserve. The bending results for the base metal and all three test plates indicated “no cracks,” satisfying the Level 1 acceptance criteria of GB/T 232, which state that “no visible cracks or fractures shall appear on the specimen surface after bending.” This confirms that the base metal possesses excellent plastic deformation capacity and resistance to brittle fracture under bending loads. Additionally, the welded joints—including the weld center, fusion line, and heat-affected zone—were free from hidden brittle defects, such as lack of fusion or microcracks. The welding process did not significantly degrade the material’s plasticity, and the joints are capable of withstanding bending-induced deformation, meeting the structural requirements for installation and in-service conditions.

(3) Impact Energy: Significant differences were observed in the impact performance, particularly at the weld centers. Impact energy serves as a critical indicator of material toughness, reflecting resistance to fracture under sudden loads. In this study, notable disparities were evident between the weld centers and heat-affected zones, as well as among the weld centers of the three test plates.

First—impact energy of the heat-affected zones: Test Plates 1 and 2 performed excellently, whereas Test Plate 3 showed substantially lower values. The average impact energies of the heat-affected zones were 233 J and 232 J for Test Plates 1 and 2, respectively, demonstrating high toughness and indicating that these zones did not develop embrittling microstructures (such as martensite or coarse ferrite) during welding. In contrast, the heat-affected zone of Test Plate 3 exhibited an average impact energy of only 59 J—approximately 25.3% of Test Plate 1’s value—highlighting a significant reduction in toughness.

Second—impact energy at the weld center: Test Plates 1 and 3 exhibited normal performance, while Test Plate 2 showed an anomalously low value. The average impact energies at the weld centers of Test Plates 1 and 3 were 179 J and 176 J, respectively, indicating uniform fine-grained ferrite and pearlite microstructures and no serious solidification defects such as segregation, porosity, or slag inclusions. Conversely, the weld center of Test Plate 2 registered an average impact energy of only 35.3 J—roughly 19.7% of Test Plate 1’s value—constituting an abnormally low result.

3. Conclusion

Based on the welding experiments conducted for the fabrication of branch pressure steel pipes at the Huizhou Zhongdong Pumped Storage Power Station, a systematic investigation into the welding process characteristics of Q490S—a 600 MPa-class high-strength steel—was carried out. The key findings are as follows:

• Influence of Welding Consumables and Methods: Different combinations of welding consumables and welding methods have a significant impact on weld quality. Certain welding schemes demonstrated superior process stability and joint performance, highlighting the importance of selecting appropriate consumables and procedures.
• Weld Quality Assurance: Results from weld inspections indicate that the established welding processes produce uniform welds with smooth transitions, free from internal defects exceeding permissible limits. These outcomes fully satisfy the quality requirements for Class I welds.
• Mechanical Properties: Mechanical testing shows that proper welding parameters ensure that the tensile strength and bending performance of the joints meet the required standards while providing adequate plasticity reserves. However, impact toughness at the weld center and within the heat-affected zone (HAZ) is highly sensitive to the choice of welding consumables and variations in welding methods. Some welding schemes exhibited reduced toughness, emphasizing the need for careful process optimization.
• Optimal Welding Scheme: The use of a specific brand of welding consumables, combined with a hybrid approach—gas-shielded welding for the root pass and submerged arc welding for the fill passes—effectively balances the strength and toughness of high-strength steel welded joints. This represents an optimal welding scheme for the fabrication of 600 MPa-class pressure steel pipes and provides reliable technical guidance for similar engineering projects.

This study establishes a solid foundation for the application of 600 MPa-class high-strength steel in welded pressure pipes for pumped storage power stations. Nevertheless, as these facilities continue to operate under increasingly demanding conditions, welding technology for high-strength steels will require further refinement. Future research could focus on welding processes for even higher-strength steel grades, exploring more suitable welding consumables and advanced heat-input control methods to meet stricter operational requirements. Additionally, integrating intelligent welding technologies could enable automated control of welding parameters, thereby improving both weld quality stability and production efficiency.