The valve core of the steam turbine regulating valve is one of the important functional components in the regulating valve of the steam turbine unit. This component mainly uses the operation of the regulating valve core to change the flow area between the valve disc and valve seat, thereby achieving the adjustment of fluid medium parameters. Taking 2Cr12NiMo1W1V valve core forgings as the research object, the smelting method was optimized, and the chemical alloy element composition was optimized. Reasonable hot forming methods were selected, and appropriate austenite and tempering temperatures were adopted to perform performance heat treatment on the forgings. By inspecting the mechanical properties of forged products and observing the microstructure after heat treatment, it can be concluded that the valve core forgings manufactured by this process have good performance and improve the service performance of the regulating valve core products.
0. Introduction
A steam turbine, also known as a steam turbine engine, is a rotating steam power device. High temperature and high-pressure steam passes through a fixed nozzle to form an accelerated airflow and is sprayed onto the blades, causing the rotor equipped with blade rows to rotate while doing external work. Steam turbines are the main equipment in modern thermal power plants and are also used in metallurgical, chemical, and naval power plants.
2Cr12NiMo1W1V is one of the commonly used heat-resistant steels in steam turbines. This steel contains various alloying elements such as Cr, Mo, W, etc. It has high alloy content, high-temperature mechanical strength, stable microstructure at high temperatures, and good heat treatment hardenability. Usually, after quenching and tempering, the microstructure of 2Cr12NiMo1W1V steel is mostly tempered sorbite with excellent comprehensive mechanical properties. Therefore, this steel is widely used in important components of steam turbines, mainly for important components such as valve cores, valve discs, valve stems, bolts, and crossheads used in heat-resistant steel. This article mainly studies the optimization of alloy elements, hot forming process, and quenching and tempering treatment process of 2Cr12NiMo1W1V valve core forging material on the final microstructure and mechanical properties of the steel.
1. Manufacturing process of forgings
1.1 Melting of Steel Ingots
The steel ingot selected for the forging is an 8.5t ingot type refined by VOD. 2Cr12NiMo1W1V steel contains various alloy elements, among which the composition ratio of C, Cr, Ni, Mo and other alloy elements needs to be optimized. Its chemical composition is complex, and the valve core is a device that changes the flow area between the valve disc and the valve seat. It is a valve part that uses its movement to achieve basic functions such as direction control, pressure control, or flow control; ensuring the stability of its microstructure and mechanical properties is crucial. Therefore, during the smelting of steel ingots, the chemical composition should be optimized and the percentage of chemical elements in the ingots should be adjusted reasonably to improve the mechanical properties and usability of the product. Due to the poor operating conditions of the valve core and its high corrosion resistance requirements, the Cr element content should be appropriately increased. At the same time, the content of the Mo element should be increased to improve the material’s stress corrosion resistance performance; Increase the content of the V element appropriately to improve the thermal strength of the material. The detailed chemical composition of 8.5t steel ingots refined by VOD after composition optimization is shown in Table 1. The steel ingot is formed by hot forging using a 5000t hydraulic press, with an initial forging temperature of 1170 ℃± 20 ℃. The billet is then forged using a 5000t hydraulic press, and the forging is then forged at 1000 ℃ × 6h air cooling + 700 ℃ × 12 hours of air cooling and positive tempering for pre-treatment.
Table.1 Chemical Composition Requirements
| Element | ≤(C)/% | ≤(Si)/% | ≤(Mn)/% | ≤(S)/% | ≤(P)/% | ≤(Cr)/% | ≤(W)/% | ≤(Mo)/% | ≤(V)/% | ≤(Ni)/% |
| Composition requirements | 0.20–0.25 | ≤0.50 | 0.50–1.00 | ≤0.025 | ≤0.025 | 11.00–12.50 | 0.90–1.25 | 0.90–1.25 | 0.20–0.30 | 0.50–1.00 |
| Finished product analysis | 0.21 | 0.37 | 0.69 | 0.008 | 0.005 | 11.91 | 1.21 | 1.11 | 0.27 | 0.91 |
1.2 Forging process of valve core forgings
The hot forming of 8.5t steel ingots is carried out on a 5000t hydraulic press. Due to the large tonnage of the forgings, certain process control measures should be taken for the opening, forming, and forging processes of the forgings. The initial forging temperature of the forgings should be controlled at around 1170 ℃, and the final forging temperature should be controlled at around 750 ℃. The forging ratio for each heat is different. To accurately control the heating temperature of the steel ingot, it should be heated to 1170 ℃ in an electric heating furnace, with a holding time of 6-9 hours. Due to the high alloy content of the steel ingot, its alloy diffusion is difficult, and the heat conduction of the steel ingot is poor. Therefore, appropriate gradient heating should be carried out to preheat the steel ingot, and then high-temperature heating should be carried out, followed by furnace forging. The specific heating process is shown in Figure 1.
1.3 Forging Forming
We are forming the forging on a 5000t hydraulic press, pressing the pliers along the mouth line to the size of the forging Φ 480mm × 600mm. The thickness of the inner pier in the leakage plate is about 750mm high. The forging ratio is 1.9, as shown in Figure 2 and subsequently elongated to Φ 545mm × 2950mm, sawing machine cutting to forging size Φ 545mm × 750mm, with a forging ratio of 4.2, resulting in precision forming, as shown in Figure 3. After forging, perform positive tempering treatment and then roughen the forging, as shown in Figure 4.
Figure.1 Steel Ingot Heating Process
Figure.2 Forging Drawing (Unit: mm)
Figure.3 Forging Forming Diagram (Unit: mm)
Figure.4 Rough Turning Drawing of Forgings (Unit: mm)
1.4 Quenching and tempering heat treatment of valve core forgings
Due to the heat resistance and high temperature resistance of valve core forgings, a higher austenitizing temperature should be chosen for their heat treatment temperature in order to achieve more alloy elements dissolved in the austenitic matrix and increase the stability of the matrix performance. The quenching and tempering heat treatment adopts the heat treatment process shown in Figure 5.
Figure.5 Quenching and tempering heat treatment process for valve core forgings
2. Inspection related to forging products
After quenching and tempering heat treatment, the sample is intercepted from the forging, and the sampling position and specific size are shown in Figure 6. The metallographic specimen was 15 mm × 15 mm × 15 mm, the tensile specimen was a standard tensile specimen with a gauge diameter of 10 mm and 50 mm, and the impact specimen was a Charles V-notch specimen. A Nikon LV200 N optical microscope observed the microstructure. The mechanical tensile test and impact toughness test were carried out by WDW-100 D biaxial tensile testing machine and JB-350 B pendulum impact testing machine, and the experimental results were analyzed.
Figure 7 shows the microstructure photo of the forging after quenching and tempering heat treatment. After quenching and tempering heat treatment, the microstructure matrix of the forging is uniform martensitic structure, and no strip or sheet shape is found δ Ferrite and carbide are uniformly distributed, and this microstructure usually has high high-temperature strength and toughness, significantly improving the performance of valve core components.
Figure.6 Sampling Location of Forging Specimens (Unit: mm)
Figure.7 Organizational photo of valve core forging after quenching and tempering ( × 400x)
Table 2 shows the mechanical tensile and impact properties of valve core forgings measured after heat treatment. The results show that the mechanical tensile properties and impact toughness of the forging fully meet the technical requirements of the product, and the mechanical properties of the mechanical performance data are good, with high impact toughness, large tensile and impact margins, and uniform impact values of the material, indicating that the mechanical properties of the material matrix are relatively uniform and stable.
Table.2 Tensile Performance Results of Output Shaft Forgings
| Tensile property | Rm/MPa | Rp0.2/MPa | A/% | Z/% | Akv/J |
| Test value | 1004 | 840 | 19 | 60 | 24、25、23 |
| Mechanical requirement value | ≥930 ≥930 |
≥760 | ≥14 | ≥35 | ≥14 |
Note:
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3. Conclusion
The mechanical properties of 2Cr12NiMo1W1V steel forgings manufactured by adjusting the composition ratio, controlling the forging ratio and forming process, and adopting a reasonable heat treatment system fully meet the technical requirements of the forgings. The microstructure of the forgings is relatively uniform, extending the material’s service life, improving the material’s safety in use, and reducing equipment maintenance costs; this further reduces the difficulty and cost of repairing the components in the steam turbine.
Author: Lang Xueqin
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