Discussion on the Heat Treatment Process of 12Cr1MoV Steel for High-Pressure Steam Pipes

 

Abstract:

The high-pressure steam pipes of steam turbines are an essential component of the entire unit, and the selection of tubing material and heat treatment processes directly impact the service life of the entire unit. Our factory uses 12Cr1MoV material for high-pressure steam pipes based on the design performance requirements of the units. The heat treatment parameters of this material have a significant influence on its mechanical properties. Once the chemical composition is determined, the mechanical properties of the tubing mainly depend on the selection of heat treatment parameters, especially for 12Cr1MoV steel. The comprehensive mechanical properties and microstructure of the material meet the technical requirements of GB5310-85 after being tempered at 740°C.

I. Explanation

Our factory produces 12MW and 25MW steam turbine main steam pipes and reheater pipes using 12Cr1MoV steel, with pipe diameters of φ168×14 mm. According to GB5310-85, these pipes require normalization and tempering treatment as shown in Figure 1: Normalization: 980-1020°C, cooled uniformly at a rate of 1 minute per millimeter of thickness. Tempering: 730-750°C, held for 3 hours in the furnace.

After heat treatment with normalization and tempering according to the above technical conditions, the internal and external surfaces of the pipes were severely oxidized, with localized decarburization reaching approximately 1 mm on the exterior surface. We tried various methods, such as acid washing, but the interior surface remained unclean. Heating the pipes to 250°C for 3 hours followed by air cooling did not yield satisfactory results. Another method involved blocking both ends of bent pipes with 6 mm thick steel plates, leaving a φ20 mm vent hole on each end to release gases during high-temperature heating. Although this improved the internal surface oxidation and decarburization, the exterior surface still required acid washing.

Acid washing is labor-intensive, reduces production efficiency, and poses health risks to workers. To solve this issue, we need to conduct a comprehensive analysis of the 12Cr1MoV tubing. Upon receipt, the mechanical properties of the tubing must be verified, ensuring the chemical composition complies with GB5310-85. Since the supplier has already processed the tubing according to GB5310-85 and performed normalization and tempering, meeting the performance requirements before shipment, we only need to perform tempering treatment during usage. Over the years, our factory has consistently performed normalization and tempering on bent high-pressure steam pipes to verify whether secondary treatment of 12Cr1MoV tubing is necessary. Different heat treatment tests were conducted on the tubing.

II. Analysis

According to reports, 12Cr1MoV steel exhibits excellent oxidation resistance, structural stability, thermal strength, creep resistance, and weldability when used below 580°C.

2.1 Oxidation Resistance

12Cr1MoV steel shows good oxidation resistance below 580°C, with a corrosion depth of 0.05 mm/year (based on extrapolated data from 3000-5600 hours of testing). At 600°C, the corrosion depth increases to 0.12-0.22 mm/year, indicating that the oxide layer is prone to flaking off (based on extrapolated data from 1000-1500 hours of testing). In the manufacturing of steam turbines, the steam parameters inside the high-pressure steam pipes are typically 540°C, with wall temperatures below 580°C (see Figure 2).

Figure 2 12Cr1MoV steel oxidation resistance curve

2.2 Structural Stability

12Cr1MoV steel has good structural stability, but prolonged operation can result in globular transformation of pearlite. Although slight to moderate globular transformation does not significantly affect creep strength, complete globular transformation can reduce the thermal strength of the steel. Therefore, for pipes that have undergone globular transformation during operation, metal supervision must be strengthened.

The degree of globular transformation in the pipes serves as a parameter to determine whether the material can continue to be used. The ultimate criterion is whether the expansion of the pipes exceeds the standard limits or if the material’s properties fall below the minimum requirements or design specifications.

2.3 Thermal Strength and Creep Resistance

12Cr1MoV steel exhibits high thermal strength and creep resistance at 500°C due to the presence of dispersed vanadium carbides, which strengthen the ferritic matrix. Vanadium in the steel also reduces the rate at which alloy elements (mainly molybdenum) transfer from ferrite to carbides, thus improving the thermal strength of the steel.

12Cr1MoV steel is sensitive to the cooling rate during normalization. Faster cooling results in higher creep strength. However, temper embrittlement occurs during tempering at 500-700°C. Long-term operation at 570°C can cause globular transformation of carbides, and many 12Cr1MoV steel components exhibit low room temperature impact values. Large-diameter 12Cr1MoV steel steam pipes often show inconsistent local impact values after heat treatment (see Figure 3).

Figure 3 12Cr1MoV steel creep strength at 580°C (Normalization 1000-1020°C, Tempering 740-760°C)

2.4 Weldability

12Cr1MoV steel has good weldability. Manual welding uses E317 electrodes, with preheating to 200-300°C. For small-diameter thin-walled pipes, preheating is generally not required. For superheater pipes, spot welding can be used. For automatic welding, H08 CrMoV welding wire and flux 350 are used, followed by tempering at 710-750°C. Gas welding uses H08 CrMoV welding wire, with subsequent normalization at 100-1020°C and tempering at 720-750°C.

III. Concept

Based on the analysis of the material’s multiple properties, to solve the severe oxidation and decarburization on the internal and external surfaces of the pipes, we propose modifying the heat treatment process by eliminating the normalization step at 980-1020°C and directly tempering at 730-750°C. This would reduce oxidation and decarburization, extend the service life of the pipes, and eliminate the need for labor-intensive acid washing, benefiting the health of the workers. Since the cooling rate of bent pipes is relatively fast, the degree of oxidation and decarburization is reduced. Direct tempering at 730-750°C proved effective, with the mechanical properties and microstructure meeting national standards.

IV. Process Testing

  1. The testing was conducted in a 45kW box-type resistance furnace. Three batches of six furnaces were tested, with an instrument temperature deviation of ±5°C, unlimited heating rate, holding for 3 hours, and air cooling in the furnace until ≤250°C (see Figure 1, tempering part).
  2. Technical Requirements (see Table 1 and Table 2).

Table 1 Chemical Composition (%)

Technical Condition C Si Mn Cr Mo V S P
GB5310-85 0.08～0.15 0.17～0.37 0.40～0.70 0.90～1.20 0.25～0.35 0.15～0.30 ≤0.040 ≤0.040

Table 2 Mechanical Properties

Technical Condition Sampling Position σs (MPa) σb (MPa) δ5 (%) aku (J/cm²)
GB5310-85 Longitudinal 254.80 470.4 21 58.8
Transverse 254.80 441 19 49
  1. Test Results (see Table 3, Table 4, Table 5)

(1) Performance

Table 3 First Test Performance

Serial No. σs (MPa) σb (MPa) δ5 (%) ψ (%) aku (J/cm²) HB Notes
1 473.5 598.5 24.5 71.5 195.0 177.0 Raw Material Retest
2 382.0 511.5 32.0 78.5 284.0 166-179 Normalization + Tempering
3 418.0 547.0 29.5 75.0 210.0 152-156 Tempering After Bending
4 453.0 581.5 26.0 74.5 216.5 174.1 Raw Material Tempering

Table 4 Second Test Performance

Serial No. σs (MPa) σb (MPa) δ5 (%) ψ (%) aku (J/cm²) HB Notes
1 441.5 569.5 26.5 75.0 253.1 174-175 Raw Material Retest, Flattened H=93.3, No Cracks
2 489.9 600.0 26.0 73.0 212.5 180 Raw Material Retest, Flattened H=93.3, No Cracks
3 477.5 570.1 28.8 78.0 171.5 158-162 Tempering After Bending
4 592.5 680.4 25.5 71.1 182.5 152-152 Raw Material Tempering
5 675.5 753.0 20.5 71.8 147.1 217-229 Normalization + Tempering After Bending

Table 5 Third Test Performance

Serial No. σs (MPa) σb (MPa) δ5 (%) ψ (%) aku (J/cm²) HB Notes
1 425.7 523.2 31.0 78.6 308.8 152-153 Sample Taken from Straight Pipe, Tempered
2 476.5 476.5 35.0 80.9 318.8 143 Sample Taken from Elbow, Tempered

(2) Microstructure (see Table 6, Figures 4, 5)

Table 6

Serial No. Microstructure Notes
1 Ferrite + (20-30%) fine-grained pearlite Tempering After Bending
2 Ferrite + (20-30%) fine-grained pearlite Tempering of Raw Material
3 Ferrite + (20-30%) fine-grained pearlite Normalization + Tempering After Bending
4 Ferrite + (20-30%) fine-grained pearlite Raw Material Retest

Note: In Tables 3, 4, and 6, “normalization + tempering” refers to samples taken from the elbow of the pipe.

Figures 4 and 5: Etched with 4% HNO₃ alcohol solution at 100x magnification.

Microstructure of raw material after tempering at 730°C: Ferrite + granular pearlite + carbide particles. Microstructure of pipe after bending and normalization + tempering at 730°C: Spheroidite.

V. Conclusion

Through analysis and process testing, the following conclusions can be drawn:

  1. After bending 12Cr1MoV steel pipes through medium-frequency heating and air cooling, eliminating the normalization step and performing tempering at 730-750°C, the mechanical properties meet the requirements of GB5310-85.
  2. Compared to the method of (980-1020°C normalization) + (730-750°C tempering), the microstructural changes of 12Cr1MoV steel pipes tempered at 730-750°C are negligible.
  3. Based on multiple heat treatment tests, the data confirm that tempering at 730-750°C instead of normalization + tempering is feasible. This reduces oxidation and decarburization, extends service life, and lowers costs.