Zhuojin Pipe Fitting Co., LtdEffect of Quenching Process on Microstructure and Residual Stress in Large Thick-Walled Steel Pipe
Literature Overview
The paper by Hu Zhiyu, Han Jinfeng, Meng Jiahong, Bi Xiangrong, Sun Wenbin, Wang Haiyan, and Gao Xueyun, published in Transactions of Materials and Heat Treatment (2023, Vol. 44, No. 6, pp. 167–172), investigates the effect of quenching process parameters on the microstructure and residual stress distribution in large thick-walled steel pipe made of 30Cr2NiMo alloy steel. The research was funded by the Inner Mongolia Natural Science Foundation (No. 2020BS05006) and the Inner Mongolia University of Science and Technology Innovation Fund (No. 2019QDL-B09). The study employs numerical simulation to analyze temperature, microstructure, and residual stress distributions under different quenching conditions.
Core Technical Content
Material and Geometry Characteristics
30Cr2NiMo is a medium carbon alloy steel widely used for high-strength applications such as large-diameter thick-walled steel pipes, pressure vessels, and heavy machinery components. The chemical composition and mechanical properties are as follows:
| Property | Specification |
|---|---|
| Carbon (C) | 0.28–0.34% |
| Chromium (Cr) | 1.80–2.20% |
| Nickel (Ni) | 0.50–0.80% |
| Molybdenum (Mo) | 0.20–0.30% |
| Hardness (after quenching and tempering) | 28–32 HRC |
| Tensile strength | ≥ 800 MPa |
| Yield strength | ≥ 620 MPa |
| Impact toughness (KV2) | ≥ 47 J at 20°C |
The large thick-walled steel pipe geometry includes a step section where the pipe diameter changes, creating a geometric discontinuity. This step is a critical location for stress concentration and potential quench cracking.
Quenching Process Simulation
The numerical simulation models the heat transfer during quenching, considering the following physical phenomena:
- Convection heat transfer: Heat transfer from the steel surface to the quenching medium (oil or air).
- Phase transformation kinetics: Transformation of austenite to martensite, bainite, and retained austenite, governed by the JMA (Johnson-Mehl-Avrami) equation.
- Thermal expansion and contraction: Differential cooling rates across the wall thickness cause thermal stresses.
- Transformation-induced plasticity (TRIP): The volume expansion during austenite-to-martensite transformation generates additional residual stresses.
The simulation results show that the residual stress distribution is highly sensitive to the quenching medium and cooling rate. The step section of the pipe experiences the highest residual stress due to the geometric stress concentration and the differential cooling between the thick and thin wall sections.
Comparison of Quenching Strategies
The authors compare three quenching strategies:
| Quenching Strategy | Maximum Residual Stress at Step | Microstructure at Thin Section | Microstructure at Thick Section Surface | Microstructure at Thick Section Core |
|---|---|---|---|---|
| Conventional oil quenching (continuous) | ~600 MPa | Martensite | Martensite + retained austenite | Martensite + bainite + retained austenite |
| Oil quenching + air cooling + oil quenching (optimized) | < 500 MPa | Martensite | Martensite + retained austenite | Martensite + bainite + retained austenite |
| Air cooling only (no oil) | ~400 MPa | Bainite + martensite | Bainite + martensite | Bainite + ferrite + pearlite |
The optimized quenching process involves an initial oil quenching period of 420 seconds, followed by an air cooling period of 480 seconds, and then a final oil quenching period. This strategy achieves the following benefits:
- The initial oil quenching rapidly cools the surface to initiate martensite transformation.
- The air cooling period allows the temperature gradient across the wall thickness to equalize, reducing thermal stresses.
- The final oil quenching ensures complete martensite transformation in the remaining austenite.
Residual Stress Analysis
The residual stress distribution reveals several important patterns:
- The maximum residual stress occurs at the step section, where the geometric discontinuity creates a stress concentration factor of approximately 1.5–2.0.
- The stress distribution through the wall thickness shows a compressive stress near the surface and tensile stress in the core, a pattern typical of quenching-induced residual stresses.
- The optimized quenching process reduces the maximum residual stress from approximately 600 MPa to below 500 MPa, a reduction of more than 15%.
| Location | Conventional Quenching (MPa) | Optimized Quenching (MPa) | Reduction |
|---|---|---|---|
| Step section (surface) | 600 | 480 | 20% |
| Step section (mid-thickness) | 450 | 360 | 20% |
| Thin wall section (surface) | 500 | 420 | 16% |
| Thick wall section (core) | 300 | 250 | 17% |
Engineering Practice Integration
In manufacturing large thick-walled steel pipes, quench cracking is a major quality concern. The residual stress at the step section can exceed the material's yield strength, leading to plastic deformation and potential cracking during or after quenching. The optimized quenching process presented in this paper provides a practical solution to reduce the risk of quench cracking while maintaining the required microstructure and mechanical properties.
The quenching process parameters must be carefully controlled to ensure consistent results. The following process control points are critical:
| Control Point | Parameter | Acceptance Criteria |
|---|---|---|
| Austenitizing temperature | 850–880°C | Uniform temperature across charge |
| Oil temperature | 40–60°C | Maintained during quenching |
| Oil quenching time (first stage) | 420 ± 30 s | Timed from immersion |
| Air cooling time | 480 ± 60 s | Timed from oil removal |
| Oil quenching time (second stage) | Until temperature < 100°C | Continuous monitoring |
| Tempering temperature | 550–600°C | 2 hours minimum |
The residual stress level after quenching and tempering is typically reduced by 30–50% compared to the as-quenched condition. However, the stress concentration at the step section remains a critical concern, and additional stress relief procedures may be required for high-integrity applications.
Key Reflections and Study Insights
The numerical simulation approach used in this paper provides a powerful tool for optimizing quenching processes without the need for extensive trial-and-error testing. The ability to predict residual stress distributions under different quenching conditions is particularly valuable for large components where experimental testing is impractical. However, the accuracy of the simulation depends on the correct representation of the material's phase transformation kinetics and the heat transfer boundary conditions.
The finding that a two-stage quenching process (oil-air-oil) can significantly reduce residual stress while maintaining the desired microstructure is practically significant. This approach leverages the principle of differential cooling to control the transformation kinetics and minimize thermal stresses. In my experience with large component heat treatment, the key to successful quenching is not simply to quench as fast as possible, but to manage the cooling rate to achieve the desired balance between transformation and stress.
The residual stress of 600 MPa at the step section in conventional quenching is concerning, as it approaches or exceeds the yield strength of the material. This level of residual stress can lead to dimensional instability during subsequent machining operations and increase the susceptibility to stress corrosion cracking in service. The optimized quenching process that reduces this to below 500 MPa is a meaningful improvement, but further reduction may be necessary for critical applications.
The microstructure analysis reveals that the thick wall section core contains a mixture of martensite, bainite, and retained austenite. This mixed microstructure is typical of thick sections where the cooling rate is insufficient for complete martensite transformation. The presence of bainite and retained austenite in the core can affect the mechanical properties, particularly the hardness and toughness. The tempered microstructure should be verified by metallographic examination to ensure it meets the specified requirements.
Reference Value and Outlook
This paper provides valuable insights into the optimization of quenching processes for large thick-walled steel pipes. The numerical simulation approach offers a cost-effective alternative to experimental trial-and-error, enabling the development of optimized process parameters before production. Future research should focus on extending the simulation to three-dimensional models that account for the full geometry of the pipe, incorporating the effects of quenching medium flow and agitation, and developing automated process control systems based on real-time temperature monitoring. The integration of residual stress measurement techniques such as X-ray diffraction and neutron diffraction with numerical models would enhance the predictive accuracy and provide validation data for simulation calibration.