Study Notes on Microstructural Analysis of SMAW Overlay Welded Metals

Introduction and Technical Context

Microstructural analysis of shielded metal arc welding (SMAW) overlay welded metals is a fundamental aspect of quality assurance in bimetal product manufacturing and pressure vessel fabrication. The microstructure of an overlay weld determines its mechanical properties, corrosion resistance, and service life. This study provides a detailed examination of the microstructural evolution in SMAW overlay welds, including the effects of welding parameters, filler metal composition, and interpass temperature on phase formation, grain morphology, and dilution behavior.

Microstructural Zones and Phase Evolution

SMAW overlay welds exhibit three distinct microstructural zones, each with unique characteristics that influence overall performance:

Overlay Metal Microstructure

The overlay metal in SMAW welding typically exhibits a columnar dendritic structure that grows perpendicular to the fusion boundary. The dendrite arm spacing (DAS) is a critical parameter that influences mechanical properties:

• Primary dendrite arm spacing (PDAS): 50–150 μm for typical SMAW parameters
• Secondary dendrite arm spacing (SDAS): 5–20 μm, influenced by cooling rate and composition
• Dendrite morphology: Columnar in the first pass, transitioning to equiaxed in subsequent passes due to constitutional undercooling

The phase composition of the overlay metal depends on the filler metal alloy system. For austenitic stainless steel overlays (e.g., E309L, E316L), the microstructure consists of austenite (γ) with varying amounts of ferrite (δ) determined by the Schaeffler diagram. The target ferrite content is typically 5–25% FN to prevent hot cracking and maintain corrosion resistance.

Heat-Affected Zone Microstructure

The HAZ is the most critical zone from a service performance perspective, as it is subject to thermal cycling without melting. Key microstructural features include:

1. Grain growth: The HAZ grains can grow by 2–5× their original size, reducing toughness. The maximum grain size in the HAZ should be limited to <100 μm for pressure vessel applications per NB/T 47002.
2. Phase transformation: In low-alloy steel base metals, the HAZ may exhibit a mixture of bainite, martensite, and retained austenite depending on the cooling rate and base metal composition.
3. Carbide precipitation: Chromium carbides (M23C6, M7C3) may precipitate along grain boundaries in the HAZ of stainless steel overlays, leading to chromium depletion and intergranular corrosion susceptibility.

Dilution Analysis and Its Microstructural Consequences

Dilution—the mixing of base metal into the overlay weld—is a critical parameter that directly affects overlay composition and microstructure. The following table presents typical dilution rates for different SMAW overlay configurations:

The dilution effect is particularly significant for corrosion-resistant overlays. For example, in a 316L overlay on carbon steel:

• At 15% dilution, the Cr content drops from 18.5% to approximately 16.8%, and the Mo content drops from 2.5% to approximately 2.1%.
• At 20% dilution, the Cr content drops to approximately 16.2%, below the 18% threshold required for passive film stability.
• The carbon content increases from <0.03% (filler) to 0.10–0.15% (diluted), promoting carbide precipitation and sensitization.

Mechanical Properties and Microstructure Correlation

The mechanical properties of SMAW overlay welds are directly related to their microstructure. The following table presents typical property values and their microstructural basis:

Key Microstructural Factors Affecting Mechanical Properties

1. Grain size: Finer grains improve both strength and toughness (Hall-Petch relationship). The grain size in the overlay can be controlled by adjusting welding parameters—higher travel speed and lower current produce finer grains.
2. Phase fraction: The ratio of austenite to ferrite in stainless steel overlays significantly affects ductility and toughness. A balanced microstructure (10–20% FN) provides optimal mechanical properties.
3. Precipitation: Carbide and intermetallic precipitates can either strengthen or embrittle the overlay, depending on their size, distribution, and coherency with the matrix.
4. Retained austenite: Retained austenite in martensitic overlays can improve toughness but may transform to martensite during service at elevated temperatures, causing dimensional instability and cracking.

Engineering Practice and Quality Assurance

The microstructural analysis of SMAW overlay welds is essential for the following quality assurance activities in pressure vessel fabrication:

1. Procedure qualification (NB/T 47014 / ASME IX): Microstructural examination of procedure qualification welds verifies that the welding procedure produces the intended microstructure and mechanical properties. The Schaeffler diagram is used to predict and verify the ferrite content in austenitic overlays.
2. Production weld inspection: Random microstructural examinations of production overlay welds ensure that the welding procedure is being followed correctly and that the resulting microstructure meets specifications.
3. Failure analysis: When overlay welds fail in service, microstructural analysis identifies the root cause—whether it is related to excessive dilution, improper heat treatment, or welding defects.
4. Post-weld heat treatment verification: PWHT effectiveness is verified by examining the microstructure for signs of stress relief, such as spheroidized carbides, reduced dislocation density, and tempered martensite.

Study Insights and Practical Implications

This study reinforces the fundamental principle that microstructure is the bridge between welding process parameters and service performance. For engineers involved in bimetal product manufacturing, the following practical implications are most significant:

• Dilution control is paramount: The first pass of an SMAW overlay is the most dilution-prone, and its microstructure may not meet specification requirements. Multi-pass overlay designs should be used for critical applications, with the first pass serving as a transition layer and subsequent passes achieving the target composition.
• Microstructural heterogeneity is inherent: SMAW overlay welds exhibit inherent microstructural variation across the weld width and thickness due to the asymmetric thermal profile and variable cooling rates. This heterogeneity should be accounted for in performance predictions and quality acceptance criteria.
• Post-weld heat treatment is essential for martensitic overlays: Martensitic overlay welds require stress relief or tempering to improve toughness and reduce residual stresses. The PWHT temperature and duration should be optimized to balance toughness improvement with minimal hardness reduction.
• Microstructural examination should be part of routine quality control: While NDT methods (RT, UT, MT, PT) detect macroscopic defects, they cannot assess microstructural quality. Periodic metallographic examination of production welds provides valuable feedback on procedure adherence and weld quality trends.

In summary, the microstructural analysis of SMAW overlay welds is not merely an academic exercise but a practical necessity for ensuring the reliability of bimetal products and pressure vessels. The microstructure determines whether an overlay will resist corrosion, withstand fretting, maintain bond integrity, and perform reliably over its design life. Engineers who understand and control the microstructural evolution in overlay welding are better equipped to design specifications, qualify procedures, and ensure product quality. This study provides a solid foundation for integrating microstructural considerations into the day-to-day engineering practice of bimetal product manufacturing and pressure vessel fabrication.