CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Post-Weld Heat Treatment on Microstructure and Properties of 10Cr9Mo1VNb Boiler Steel Welded by Hot-Wire TIG
Literature Overview
This paper published in Metal Heat Treatment in 2024 by Lai Chunming, Li Qin, Zhou Jialin, Wu Xinghuan, and Huang Yan from Hunan Chemical Vocational and Technical College, Wuhan University of Science and Technology, and Hunan Chemical Design Institute Co., Ltd., investigates the effects of post-weld heat treatment (PWHT) on the microstructure and mechanical properties of 10Cr9Mo1VNb boiler steel welded using the hot-wire TIG (HW-TIG) process. This research is particularly relevant to the fabrication of high-temperature pressure vessels and boiler components where the combination of advanced welding processes and proper heat treatment is essential for achieving the required performance.
Core Technical Content
Material and Process Characteristics
10Cr9Mo1VNb is a high-temperature creep-resistant steel used in boiler components, superheaters, and high-temperature pressure vessels operating above 550°C. The alloy contains chromium for oxidation resistance, molybdenum for creep strength, and vanadium-niobium for grain refinement and precipitation strengthening. The hot-wire TIG process is a variant of GTAW where an additional hot wire is fed into the arc, providing supplementary heat input and filler metal deposition.
| Parameter | Value |
|---|---|
| Base material | 10Cr9Mo1VNb |
| Welding process | Hot-wire TIG (HW-TIG) |
| Base material thickness | 12–25 mm |
| Preheat temperature | 200–300°C |
| Interpass temperature | 200–300°C |
| PWHT temperature | 760–790°C |
| PWHT duration | 1.5–3.0 h |
| Cooling rate (after PWHT) | Controlled |
Microstructural Evolution
The study examines the microstructural changes in the weld zone, heat-affected zone (HAZ), and base metal following PWHT. Key observations include:
- Weld zone: The as-welded microstructure consists of martensite and bainite with retained austenite. After PWHT, the martensite transforms to tempered martensite with fine precipitates of Mo₂C, Cr₇C₃, and MX-type carbides.
- HAZ: The coarse-grained HAZ (CGHAZ) exhibits the most severe microstructural changes. Without PWHT, this region contains untempered martensite with high hardness. After PWHT, the martensite is tempered, and precipitation strengthening occurs.
- Base metal: The base metal microstructure is relatively stable, but precipitation coarsening occurs during PWHT.
Mechanical Property Evaluation
| Region | As-Welded Hardness (HV) | After PWHT Hardness (HV) | Tensile Strength (MPa) | Impact Energy (J) |
|---|---|---|---|---|
| Weld zone | 380–420 | 280–320 | 520–560 | 45–55 |
| CGHAZ | 400–450 | 290–330 | 540–580 | 50–60 |
| Base metal | 250–280 | 240–270 | 530–560 | 60–70 |
PWHT Process Optimization
The researchers investigated the effects of PWHT temperature and duration on the microstructure and properties. Key findings include:
- Temperature effects: Increasing PWHT temperature from 760°C to 790°C promotes tempering of martensite and precipitation of fine carbides. However, temperatures above 790°C may lead to excessive grain growth and precipitation coarsening.
- Duration effects: Increasing PWHT duration from 1.5 h to 3.0 h promotes more complete tempering and precipitation. However, excessive duration may lead to over-tempering and loss of strength.
- Cooling rate effects: The cooling rate after PWHT significantly affects the final microstructure. Slow cooling (furnace cooling) minimizes residual stresses but may promote precipitation coarsening. Air cooling provides a better balance between residual stress relief and microstructural stability.
Defect Analysis and Countermeasures
| Defect | Cause | PWHT Effect | Countermeasure |
|---|---|---|---|
| Hardness mismatch | Uneven tempering | Equalizes hardness | Optimize PWHT temperature |
| Residual stress | Thermal gradients | Relieves stress | Adequate PWHT duration |
| Precipitation coarsening | Excessive temperature/time | May worsen | Control PWHT parameters |
| Grain growth | Excessive temperature | May worsen | Limit PWHT temperature |
| Cracking | Residual stress | Prevents cracking | Appropriate PWHT |
Engineering Practice Integration
For engineers fabricating high-temperature pressure vessels and boiler components, this research provides critical guidance on the PWHT process for hot-wire TIG welds. The key engineering considerations include:
- PWHT temperature selection: The temperature must be high enough to promote complete tempering and precipitation but low enough to avoid grain growth and precipitation coarsening. For 10Cr9Mo1VNb, the optimal range is 760–790°C.
- PWHT duration: The duration must be sufficient to achieve uniform tempering throughout the weld cross-section. For thick sections (above 25 mm), longer durations may be required.
- Cooling strategy: The cooling rate after PWHT must be controlled to prevent the formation of new residual stresses and to maintain the desired microstructure. Furnace cooling is generally preferred for thick sections.
- Quality verification: Post-PWHT inspection should include hardness mapping, microstructural examination, and mechanical property testing to verify that the PWHT has achieved the desired results.
Key Reflections and Insights
The most significant finding from this research is that the PWHT process is not merely a stress relief operation but a critical step in developing the final microstructure and properties of the weld joint. For 10Cr9Mo1VNb welded by hot-wire TIG, the PWHT must be carefully optimized to achieve the desired balance between toughness and strength while ensuring long-term creep resistance.
The research also highlights the importance of understanding the interaction between the welding process and the PWHT process. The hot-wire TIG process produces a specific as-welded microstructure that responds differently to PWHT than conventional TIG welds. Engineers must consider this interaction when developing welding procedures for critical applications.
Study Value and Outlook
This research provides valuable process guidance for the fabrication of high-temperature pressure vessels and boiler components. Future work should investigate the long-term creep properties of hot-wire TIG welds following PWHT, as well as the effects of service conditions on the microstructural stability. The development of accelerated aging tests to predict long-term performance would further enhance the practical value of this research.