Microstructure Study of Single-Sided Overlay Welding Heat-Affected Zone in Membrane Water Wall Tubes

Literature Overview and Research Context

This research paper, published in 2021 by researchers at the School of Materials Science and Engineering, Southeast University, investigates the microstructure evolution in the heat-affected zone (HAZ) of single-sided overlay welded membrane water wall tubes used in coal-fired utility boilers. The study is significant because membrane water wall tubes are critical components in modern ultra-supercritical (USC) boilers, where they are exposed to extreme temperatures exceeding 600°C, high steam pressures above 25 MPa, and aggressive corrosion from slag and ash deposition.

The single-sided overlay welding process in this context refers to the application of a corrosion-resistant alloy layer — typically a ferritic stainless steel such as 12Cr1Mo or a superalloy — on the outer surface of the membrane water wall tube, while the inner surface remains as the base carbon-manganese steel. This asymmetric overlay is driven by the need to resist external corrosion from hot flue gas and ash while maintaining the economic viability of the inner surface, which is protected by the steam side.

Core Technical Points and Microstructure Analysis

The research focuses on the HAZ microstructure because this region is often the weakest link in overlay welded components. The HAZ experiences a thermal cycle that does not involve melting but is sufficient to cause significant microstructural transformation, grain growth, and phase precipitation. In membrane water wall tubes, the HAZ is particularly critical because it is subject to combined thermal, mechanical, and corrosion loading during boiler operation.

Base Metal and Overlay Material Specifications

The microstructure of the HAZ is determined by the peak temperature reached during welding and the cooling rate. The research identifies three distinct sub-zones within the HAZ:

1. Coarse-grained HAZ (CGHAZ): Located at the fusion boundary, where the peak temperature exceeds the Ac3 temperature (approximately 850-900°C for 12Cr1MoV steel). In this zone, austenite grains grow significantly, leading to a coarse martensitic or bainitic microstructure upon cooling. The grain size can exceed 100 μm, which reduces toughness and increases susceptibility to brittle fracture.
2. Fine-grained HAZ (FGHAZ): Located at a distance from the fusion boundary where the peak temperature is between Ac1 and Ac3 (approximately 727-850°C). In this zone, the original ferrite grains are partially transformed, and the resulting microstructure is a mixture of fine ferrite and pearlite with a grain size of 20-50 μm. This zone typically exhibits the best combination of strength and toughness.
3. Inter-critical HAZ (ICHAZ): Located further from the fusion boundary where the peak temperature is below Ac1. In this zone, no phase transformation occurs, but precipitation hardening and carbide coarsening may take place. The microstructure remains largely unchanged from the base metal, but the hardness can increase locally due to carbide precipitation at grain boundaries.

Welding Process Parameters Used in the Study

The choice of SAW with a 12Cr1Mo-equivalent consumable is consistent with industry practice for membrane water wall overlay welding. The 150°C preheat is selected to control the cooling rate and prevent hydrogen-induced cracking in the HAZ, while the interpass temperature limit of 250°C ensures that the deposited layers do not experience excessive thermal cycling that could lead to grain growth.

Metallurgical Findings and Implications

The research reveals several important metallurgical findings that have direct implications for the design and inspection of overlay welded membrane water wall tubes.

Grain Size and Microstructural Evolution

The CGHAZ exhibits austenite grain sizes of 80-120 μm, which is significantly coarser than the base metal grain size of 30-50 μm. This grain growth is attributed to the high peak temperature and the relatively slow cooling rate associated with the thick section of the membrane water wall tube. The coarse grain structure results in a local hardness increase of 30-50 HV above the base metal, but a corresponding decrease in Charpy impact energy from 40 J to 15-20 J at the operating temperature of 550°C.

The FGHAZ shows a more favorable microstructure with a mixture of acicular ferrite and fine pearlite, resulting in hardness values of 220-260 HV and Charpy impact energies of 35-45 J. This zone represents the optimal balance of strength and toughness and is the most reliable region for long-term service.

Dilution and Compositional Gradient

The dilution rate at the fusion boundary was measured to be approximately 18-22%, which is within the acceptable range for maintaining the corrosion resistance of the overlay layer. However, the research also identifies a compositional gradient extending 0.5-1.0 mm into the base metal, where the chromium and molybdenum content decreases gradually from the overlay composition to the base metal composition. This gradient zone is susceptible to intergranular corrosion if exposed to aggressive environments, as the local chromium depletion can fall below the critical threshold of 12% required for passivity.

Phase Constitution and Precipitation Behavior

X-ray diffraction (XRD) analysis reveals that the overlay layer consists primarily of a ferritic matrix with 5-8% retained austenite. The HAZ shows a transition from a fully martensitic structure at the fusion boundary to a ferrite-pearlite structure at the outer edge. In the CGHAZ, the martensite exhibits a lenticular morphology with embedded carbide particles, which contributes to the high hardness but limited ductility.

Comparison of HAZ Properties with Base Metal

Engineering Practice and Inspection Implications

The findings of this research have direct implications for the inspection and quality control of overlay welded membrane water wall tubes. The following recommendations are derived from the study:

1. Hardness mapping: A hardness survey should be conducted on the HAZ region, with measurements taken at 0.25 mm intervals from the fusion boundary outward to a distance of 3 mm. A hardness value exceeding 300 HV in the CGHAZ should trigger a review of the welding procedure and a consideration of post-weld heat treatment (PWHT).
2. Charpy impact testing: Transverse Charpy impact specimens should be prepared from the CGHAZ region to verify that the impact energy at the operating temperature exceeds the minimum requirement of 27 J specified by ASME VIII Div.1 and NB/T 47003.
3. Dilution control: The dilution rate should be verified by optical emission spectroscopy (OES) at the fusion boundary, with a maximum acceptable dilution of 25% for corrosion-resistant overlay applications.
4. PWHT consideration: For the CGHAZ with grain sizes exceeding 100 μm, a PWHT cycle of 750-780°C for 2 hours per 25 mm of thickness, with a controlled cooling rate of 150°C/h, is recommended to refine the grain structure and reduce residual stresses.

Study Insights and Independent Reflection

This research is notable for its systematic approach to characterizing the HAZ microstructure in a specific engineering application. The use of multiple characterization techniques — optical microscopy, scanning electron microscopy, XRD, hardness mapping, and mechanical testing — provides a comprehensive picture of the metallurgical behavior that would be difficult to achieve with any single method.

One insight that stands out is the emphasis on the FGHAZ as the optimal microstructural region. In practical terms, this suggests that the welding procedure should be optimized to maximize the width of the FGHAZ relative to the CGHAZ. This can be achieved by reducing the heat input per unit length, which narrows the CGHAZ and expands the FGHAZ. However, reducing heat input also reduces the penetration depth and may increase the number of passes required, which introduces its own set of challenges.

Another important observation is the identification of the compositional gradient zone as a potential corrosion initiation site. In the field, this zone is often overlooked because it is not visible to the naked eye and is not specifically targeted by conventional NDT methods. Engineers should consider incorporating a microstructural examination protocol into the acceptance criteria for overlay welded membrane water wall tubes, particularly for applications where the tubes are exposed to aggressive flue gas environments.

The research also underscores the importance of welding procedure qualification (WPQ) that goes beyond simple mechanical property testing. A WPQ that includes microstructural evaluation of the HAZ, dilution measurement, and corrosion resistance testing provides a much more reliable basis for predicting long-term performance than one that relies solely on tensile and impact test results.

In conclusion, this study makes a valuable contribution to the understanding of overlay welding metallurgy in membrane water wall applications. The detailed microstructural characterization and the identification of the critical HAZ sub-zones provide engineers with the knowledge needed to design more reliable welding procedures and to develop more effective inspection protocols. The practical recommendations derived from the research are directly applicable to the fabrication and maintenance of USC boiler components, and the methodology can be extended to other overlay welding applications in the power generation and petrochemical industries.