Zhuojin Pipe Fitting Co., LtdA New Method for Life Assessment of 15CrMoG Steel Pipes
Literature Overview
This paper by Pan Jinping et al., published in 2012 in Metal Heat Treatment (金属热处理), presents a novel life assessment method for 15CrMoG steel pipes used in power plant boilers. The research was supported by the Zhejiang Provincial Quality and Technical Supervision Bureau (Grant No. 20100229). The study combines accelerated aging simulation, hardness testing, and computational modeling to develop a practical methodology for evaluating the remaining life of 15CrMoG steel pipes based on the degree of pearlite spheroidization.
Technical Background and Methodology
15CrMoG is a low-alloy heat-resistant steel widely used in power plant boiler tubes, superheater tubes, and other high-temperature pressure components. The material is characterized by a pearlitic microstructure that undergoes progressive spheroidization during long-term service at elevated temperatures. This microstructural evolution leads to a reduction in mechanical properties and ultimately to failure.
Accelerated Aging Simulation
The authors used a high-temperature accelerated aging method to simulate the pearlite spheroidization evolution process of 15CrMoG steel pipes. This approach involves exposing steel specimens to elevated temperatures for extended periods to accelerate the natural aging process that occurs during service.
| Parameter | Description |
|---|---|
| Material | 15CrMoG low-alloy heat-resistant steel |
| Application | Power plant boiler tubes |
| Key degradation mechanism | Pearlite spheroidization |
| Assessment parameter | Brinell hardness (HB) |
| Modeling tool | Matlab simulation |
| Aging method | High-temperature accelerated aging |
Hardness-Spheroidization Relationship Model
A relationship model between Brinell hardness and pearlite spheroidization degree was established using Matlab software. The model captures the progressive softening of the material as the pearlite structure transitions from a lamellar to a spheroidal morphology.
Key Technical Findings
Temperature-Time Relationship for Spheroidization
The study proposed a formula describing the relationship between temperature and time for the spheroidization process. This formula quantifies how the degree of spheroidization evolves as a function of service temperature and exposure time. The relationship is non-linear, with the spheroidization rate accelerating at higher temperatures.
Life Assessment Methodology
The proposed life assessment method involves the following steps:
- Field hardness measurement: Measure the Brinell hardness of the in-service 15CrMoG steel pipe using a portable hardness tester.
- Spheroidization degree determination: Use the hardness-spheroidization relationship model to determine the current degree of pearlite spheroidization from the measured hardness value.
- Temperature-time extrapolation: Apply the temperature-time relationship formula to estimate the accumulated spheroidization based on the actual service temperature history.
- Remaining life prediction: Compare the current spheroidization state with the critical spheroidization threshold to estimate the remaining useful life of the steel pipe.
Engineering Practice Integration
Quality Control and Inspection Protocol
| Inspection Method | Purpose | Frequency |
|---|---|---|
| Brinell hardness testing | Determine current spheroidization degree | Periodic (every 1-2 years) |
| Metallographic examination | Verify microstructure and spheroidization | At overhaul intervals |
| Ultrasonic testing (UT) | Detect wall thinning and internal defects | Periodic |
| Radiographic testing (RT) | Detect weld defects and internal cracks | At fabrication and overhaul |
| Chemical analysis | Verify material composition | At fabrication and when material source is uncertain |
Welding Considerations for 15CrMoG Steel Pipes
15CrMoG steel pipes in power plant boilers are typically connected by welding. The welding process must be carefully controlled to minimize additional microstructural degradation:
- Preheating: A preheat temperature of 200-250°C is typically required to prevent cold cracking and control the cooling rate.
- Heat input control: The welding heat input should be kept within the range of 1.0-2.0 kJ/mm to avoid excessive grain growth in the HAZ.
- Post-weld heat treatment (PWHT): A PWHT cycle of 620-680°C for 2-4 hours is required to relieve welding residual stresses and restore the microstructure.
- Cooling rate control: The cooling rate from the welding temperature should be controlled to prevent the formation of brittle martensitic structures.
FMEA Analysis for 15CrMoG Steel Pipe Failure
| Failure Mode | Root Cause | Detection Method | Preventive Measure |
|---|---|---|---|
| Creep rupture | Excessive temperature and stress | UT wall thickness measurement | Temperature monitoring, stress reduction |
| Spheroidization degradation | Long-term high-temperature exposure | Hardness testing, metallography | Periodic life assessment, replacement planning |
| Stress corrosion cracking | Chloride contamination, tensile stress | PT, MT | Environmental control, stress relief |
| Weld fatigue failure | Cyclic thermal loading | MT, UT | Weld quality control, fatigue analysis |
Key Questions and Reflections
The proposed life assessment method provides a practical approach for evaluating the remaining life of in-service 15CrMoG steel pipes. However, several limitations should be acknowledged. The hardness-spheroidization relationship model is based on accelerated aging data, which may not perfectly replicate the natural aging process under actual service conditions. Factors such as cyclic thermal loading, mechanical stress, and environmental exposure during service can accelerate or modify the spheroidization process in ways that are not captured by simple isothermal aging.
Furthermore, the method relies on the assumption that hardness is a reliable indicator of the overall mechanical property degradation. While hardness is correlated with tensile strength, it may not fully capture the degradation of ductility, toughness, and creep resistance, which are critical for the structural integrity of boiler tubes. A more comprehensive assessment should include additional tests such as tensile testing, impact testing, and creep testing on coupon samples extracted from the in-service pipe.
Despite these limitations, the method represents a significant advancement in the practical life assessment of 15CrMoG steel pipes. The combination of non-destructive hardness measurement, microstructural analysis, and computational modeling provides a cost-effective and reliable approach for remaining life estimation. Power plant operators can use this method to prioritize pipe replacement activities, optimize maintenance schedules, and ensure the safe and reliable operation of boiler systems.
This research demonstrates the value of integrating materials science, metallurgical analysis, and computational modeling in the development of practical engineering assessment methods. The proposed methodology bridges the gap between fundamental microstructural evolution research and the practical needs of power plant maintenance and safety management. Engineers and inspectors working with 15CrMoG steel pipes should adopt this approach as a supplementary tool in their life assessment practices, complementing traditional methods such as wall thickness measurement and visual inspection.