Zhuojin Pipe Fitting Co., LtdEffect of Overlay Welding Method Combinations on HAZ Microstructure of Low-Alloy Steel in Nuclear Power Plants
Literature Overview
This paper by Kuang Yanjun, Huang Tengfei, Qiu Zhensheng, Wang Ruomeng, and Zhu Ping (2014), published in the journal Welding, investigates the influence of different stainless steel overlay welding method combinations on the heat-affected zone (HAZ) microstructure of low-alloy steel used in nuclear power plants. Funded by the Guangdong Province Industry-Academia-Research Integration Project (2011A090200096), the study was conducted collaboratively by CGN Engineering Co., Ltd., Sichuan Atlantic Welding Materials Co., Ltd., and Suzhou Research Institute of Thermal Power Technology. The research is particularly relevant to nuclear-grade piping and pressure vessel components where corrosion-resistant overlay cladding is mandatory for service in aggressive coolant environments.
Core Technical Findings
The authors prepared test plates using three overlay welding method combinations on low-alloy steel substrates and examined the resulting HAZ microstructure through metallographic analysis:
| Method Combination | Description | HAZ Coarse Grain Zone Observation |
|---|---|---|
| Single-layer bipolar submerged arc overlay welding (S-BSAW) | One pass of bipolar submerged arc welding | Obvious coarse grain zone present |
| Double-layer bipolar submerged arc overlay welding (D-BSAW) | Two sequential passes of bipolar submerged arc welding | Coarse grain zone significantly refined |
| Single-layer bipolar submerged arc + SMAW overlay welding (S-BSAW+SMAW) | First pass bipolar SAW followed by SMAW cap pass | Obvious coarse grain zone present |
The key finding is that the double-layer bipolar submerged arc welding (D-BSAW) combination effectively refines the coarse grain zone (CGZ) in the HAZ of the low-alloy steel substrate, whereas the single-layer approaches leave a pronounced coarse grain region. This result has direct implications for the mechanical integrity and fracture toughness of overlay-cladded nuclear components.
Technical Interpretation
The Coarse Grain Zone Problem
In low-alloy steels such as those specified under ASTM A387 or ASME SA-387 grades commonly used for nuclear pressure vessels, the HAZ during overlay welding is subjected to rapid heating and cooling cycles. The CGZ forms in the temperature range of approximately 900–1200 °C, where grain growth occurs before the austenite begins to transform during cooling. This region is particularly susceptible to brittle fracture and has reduced toughness compared to the base metal or weld metal.
Why Double-Layer BSAW Refines the CGZ
The double-layer bipolar submerged arc welding process introduces a beneficial re-heating cycle. When the second layer is deposited, the previously formed CGZ from the first pass is re-heated to a temperature that promotes grain boundary nucleation and limits further grain coarsening. This thermal cycling effect effectively acts as an in-situ tempering and grain refinement mechanism. The bipolar electrode configuration, which provides higher deposition rates and deeper penetration than conventional single-electrode SAW, ensures sufficient thermal input for this refinement effect while maintaining good metallurgical bonding.
The SMAW Cap Pass Limitation
The combination of single-layer BSAW followed by SMAW cap welding does not achieve the same CGZ refinement because the SMAW process typically uses lower heat input and shallower penetration. The thermal profile of the SMAW pass does not adequately re-heat the CGZ region to the temperature range required for grain refinement, leaving the coarse grains from the initial BSAW pass largely unchanged.
Standards and Engineering Practice Context
For nuclear power plant applications, the overlay welding of corrosion-resistant stainless steel onto low-alloy carbon steel substrates is governed by stringent codes including ASME BPV Code Section III (for nuclear components) and NB/T standards in China. The HAZ microstructure directly affects the component's ability to withstand:
- Thermal cycling during reactor start-up and shutdown
- Fatigue loading from pressure transients
- Neutron irradiation embrittlement in reactor vessel applications
- Stress corrosion cracking resistance in high-temperature water environments
Process Recommendations for Nuclear Overlay Welding
Based on the findings of this paper and general nuclear welding practice, the following process considerations are recommended:
| Parameter | Recommended Range | Rationale |
|---|---|---|
| Number of overlay layers | Minimum 2 layers | Ensures CGZ refinement through thermal cycling |
| Interpass temperature | 150–250 °C | Controls cooling rate and minimizes hydrogen-induced cracking |
| Post-weld heat treatment (PWHT) | 550–620 °C per NB/T 20251 | Required for residual stress relief and further grain refinement |
| Weld metal composition | ASTM A213 TP321 or equivalent | Ensures adequate Cr-Mo-Ni content for corrosion resistance |
| Dilution control | ≤ 20% base metal dilution | Maintains corrosion resistance of the overlay |
Key Questions and Reflections
The study raises several important questions for engineering practice. First, the paper does not quantify the degree of grain refinement in terms of ASTM grain size number or provide quantitative toughness data (such as Charpy V-notch impact energy) for the different HAZ conditions. From a code compliance perspective, nuclear authorities would require such quantitative data to approve a specific overlay welding procedure.
Second, the paper does not address the effect of the number of layers beyond two. It is reasonable to expect that three or more layers of bipolar submerged arc welding could provide even finer grain structures, but the diminishing returns in terms of production efficiency must be considered.
Third, the interaction between the overlay welding process and the subsequent PWHT cycle deserves further investigation. If PWHT is performed at the standard nuclear temperature range of 550–620 °C, the residual CGZ from single-layer processes might be partially refined during PWHT, potentially negating the advantage of the double-layer approach. However, PWHT alone cannot replicate the microstructural benefits of in-situ grain refinement during welding.
Study Insights and Implications
This paper provides valuable guidance for the selection of overlay welding methods in nuclear applications. The finding that double-layer bipolar submerged arc welding effectively refines the HAZ coarse grain zone offers a practical solution to a well-known metallurgical challenge. For engineers designing overlay welding procedures for nuclear piping systems and pressure vessel components, this research supports the recommendation to use at least two layers of bipolar submerged arc welding when HAZ toughness is a critical concern.
The work also highlights the importance of considering the entire welding sequence—not just the final weld metal properties—when evaluating overlay welding procedures. The HAZ, which is part of the base metal, often becomes the weakest link in the joint, and its microstructure must be actively managed through process design. This holistic approach to overlay welding procedure qualification is essential for ensuring the long-term reliability of nuclear components operating under demanding service conditions.