Influence of Overlay Welding Method Combinations on Heat-Affected Zone Microstructure of Low Alloy Steel in Nuclear Power Plants

Literature Overview

This study, published in the journal "Welding" in 2014 (Vol. 9, pp. 68-72), was authored by Kuang Yanjun, Huang Tengfei, and Qiu Zhensheng from CGN Engineering Co., Ltd., together with collaborators from Sichuan Atlantic Welding Materials Co., Ltd. and Suzhou Research Institute of Thermal Power Technology. Funded by the Guangdong Provincial Industry-University-Research Cooperation Project (2011A090200096), the research investigates how different combinations of overlay welding processes affect the heat-affected zone (HAZ) microstructure of low alloy steel used in nuclear power plant applications. The significance of this work cannot be overstated, given the stringent safety requirements and long service life demands of nuclear power equipment.

Core Research Objective and Methodology

The primary objective was to evaluate three commonly used overlay welding method combinations for depositing stainless steel corrosion-resistant layers on low alloy steel substrates in nuclear power plant components. The three combinations investigated were:

1. Single-layer electrode-submerged arc overlay welding
2. Double-layer electrode-submerged arc overlay welding
3. Single-layer electrode-submerged arc overlay welding followed by SMAW (shielded metal arc welding) overlay welding

Test plates were prepared under controlled conditions, and metallographic analysis was conducted on the HAZ of each configuration. The researchers examined grain size distribution, the presence and extent of coarse grain zones (CGZ), and the overall microstructural integrity of the base metal near the weld interface.

Key Findings and Technical Analysis

The results revealed a critical distinction among the three process combinations. Both single-layer electrode-submerged arc overlay welding and the single-layer electrode-submerged arc plus SMAW combination produced clearly visible coarse grain zones in the HAZ of the low alloy steel substrate. In contrast, the double-layer electrode-submerged arc overlay welding combination significantly refined the coarse grain zone, effectively reducing the detrimental microstructural changes in the base metal.

The technical explanation for this finding relates to the thermal cycle differences between the processes. Electrode-submerged arc welding (SAW) inherently produces higher heat input and slower cooling rates compared to manual processes. When applied as a single layer, the concentrated thermal energy creates a wide CGZ in the base metal. However, when applied as two successive layers, the first layer acts as a thermal buffer, moderating the peak temperature experienced by the base metal during the second pass. This staged heating reduces the maximum austenite grain growth temperature, thereby limiting coarse grain formation.

The addition of SMAW as a second process after single-layer electrode SAW does not resolve the CGZ problem because the SMAW pass, while having lower heat input, does not retroactively refine the already-formed coarse grains from the first pass. The coarse grain zone is established during the initial high-heat-input pass and persists regardless of subsequent lower-heat-input operations.

Engineering Practice Implications

For nuclear power plant applications, where low alloy steel components such as reactor pressure vessel internals, steam generator tubing, and containment system piping must withstand both mechanical loads and corrosion over decades of service, HAZ integrity is paramount. The coarse grain zone is particularly concerning because it exhibits reduced toughness, increased susceptibility to hydrogen-assisted cracking, and diminished resistance to stress corrosion cracking (SCC).

The finding that double-layer electrode SAW overlay welding effectively refines the HAZ CGZ has direct implications for qualification procedures and welding procedure specifications (WPS) in nuclear fabrication. Engineers should prioritize multi-pass electrode SAW approaches for critical nuclear components requiring stainless steel corrosion-resistant overlay. Post-weld heat treatment (PWHT) remains a standard practice to further refine the HAZ microstructure, but the process selection itself should aim to minimize CGZ formation from the outset.

Study Insights and Reflections

This study provides a valuable data point for process selection in nuclear welding operations. The approach of comparing different process combinations rather than individual processes is particularly instructive, as it reflects real-world scenarios where multiple welding methods are often combined for economic or geometric reasons. The finding that process sequencing matters as much as process selection is a reminder that welding procedure development must consider the cumulative thermal history of the base metal, not just individual pass parameters.

For engineering practice, this reinforces the principle that in nuclear-grade welding, the base metal HAZ quality is as important as the overlay layer quality itself. A corrosion-resistant overlay deposited on a substrate with a severely degraded HAZ may provide limited service life benefit. Future work should investigate the quantitative grain size measurements, mechanical property correlations (toughness, yield strength), and long-term aging behavior of the HAZ under different process combinations.