High-Temperature Low-Plasticity Crack Sensitivity of 52M Alloy Overlay Layers Prepared by Different Processes

Literature Overview

This paper, published in Materials for Mechanical Engineering (2019, Vol. 43, No. 5, pp. 30-33) by Zeng Chunjie, Li Dong, and Fang Hu from Shanghai University of Engineering Science, investigates the high-temperature low-plasticity cracking (HLLPC) sensitivity of 52M alloy overlay layers prepared by laser overlay welding and TIG (GTAW) overlay welding on S304 stainless steel substrates. The study examines microstructure, phase composition, and HLLPC behavior under varying temperatures and strain levels.

Core Technical Content

52M Alloy and Its Applications

52M is a nickel-chromium-cobalt-based superalloy (approximately 52% Ni, 20% Cr, 13% Co, with Mo, Al, Ti, and other elements) widely used for high-temperature corrosion and oxidation resistance. It is commonly applied as an overlay coating on stainless steel and nickel alloy substrates in:

• Gas turbine hot sections
• Heat exchanger tubes in aggressive environments
• Chemical processing equipment at elevated temperatures
• Marine and offshore components exposed to high-temperature salt spray

The 52M alloy is characterized by a single-phase austenitic structure at service temperatures, which provides excellent ductility and resistance to cracking under thermal cycling conditions. However, the formation of secondary phases such as Cr₂₃C₆ during welding can significantly affect the alloy's performance.

Comparison of Laser and TIG Overlay Welding Processes

Microstructure and Phase Analysis

Both laser and TIG overlay layers exhibit a single-phase austenitic structure, which is the desired microstructure for 52M alloy applications. However, there are significant differences:

1. Grain size: The laser overlay layer has significantly finer and more regular grain morphology compared to the TIG overlay layer. The rapid solidification rate in laser welding promotes high nucleation density and limited grain growth.
2. Phase composition: Both overlay layers contain Cr₂₃C₆ precipitates, which form at grain boundaries and intragranular locations. The laser overlay layer has fewer Cr₂₃C₆ precipitates due to the rapid cooling rate that limits carbon diffusion and carbide formation.
3. Grain boundary characteristics: The finer grains in the laser overlay layer result in more grain boundaries per unit area, which can act as both crack initiation sites and crack arrest points depending on the boundary characteristics.

High-Temperature Low-Plasticity Cracking (HLLPC) Behavior

HLLPC is a cracking phenomenon that occurs in materials subjected to plastic deformation at elevated temperatures where the material's plasticity is reduced. This is particularly relevant for overlay coatings that experience thermal cycling and mechanical loading during service.

The study evaluates HLLPC sensitivity by subjecting the overlay layers to controlled strain at elevated temperatures and examining the resulting crack populations. Key findings include:

The laser overlay layer demonstrates lower HLLPC sensitivity despite having a finer grain structure, which might intuitively suggest more crack initiation sites. The explanation lies in the reduced number of Cr₂₃C₆ precipitates. In the TIG overlay layer, the Cr₂₃C₆ precipitates at grain boundaries create stress concentration sites that promote crack initiation under thermal-mechanical loading. The laser overlay layer, with fewer precipitates, has cleaner grain boundaries that are more resistant to crack initiation.

The Paradox of Strain Sensitivity

An interesting finding is that while the laser overlay layer has lower overall HLLPC sensitivity, the rate of crack increase with strain is higher. This suggests that once cracks are initiated in the laser overlay layer (which requires higher strain due to cleaner grain boundaries), they propagate more readily through the fine-grained structure. In contrast, the TIG overlay layer has more cracks at lower strains (due to precipitate-assisted initiation) but the cracks are more evenly distributed and less likely to coalesce into catastrophic failure.

Engineering Practice Integration

The findings of this study have direct implications for the selection of overlay welding processes for high-temperature applications:

1. Process selection criteria: For applications where HLLPC is a primary concern (thermal cycling with mechanical loading), laser overlay welding is preferred despite its higher equipment cost. For applications where deposition rate and cost are primary concerns, TIG overlay welding may be acceptable with appropriate process optimization.
2. Welding parameter optimization: For TIG overlay welding, reducing the heat input and interpass temperature can minimize Cr₂₃C₆ precipitation and improve HLLPC resistance. This may require multi-pass welding with lower current and faster travel speed.
3. Post-weld heat treatment: Solution treatment of the overlay layer can dissolve Cr₂₃C₆ precipitates and restore the single-phase austenitic structure. However, this must be carefully controlled to avoid grain growth and substrate damage.
4. Quality assurance: Non-destructive testing methods for detecting HLLPC susceptibility include:

• Thermomechanical fatigue testing of coupon specimens
• Microstructural examination for Cr₂₃C₆ precipitation
• Grain size measurement and distribution analysis
• Grain boundary character distribution (GBCD) analysis using EBSD

For pipe fitting and piping applications, the HLLPC sensitivity of overlay layers is particularly relevant for:

• High-temperature flanges and fittings in power generation
• Piping spools in chemical processing with thermal cycling
• Valve components in steam systems
• Heat exchanger tubesheets with dissimilar metal overlays

Key Reflections

The most significant contribution of this paper is the systematic comparison of HLLPC behavior between two fundamentally different overlay welding processes. The finding that laser overlay welding produces overlay layers with lower HLLPC sensitivity, despite the potentially disadvantageous fine grain structure, challenges the conventional understanding of grain size effects on cracking. The reduced Cr₂₃C₆ precipitation in laser overlay layers is the dominant factor, highlighting the importance of phase composition control in high-temperature alloy overlay welding.

The study also highlights the complexity of HLLPC behavior, where multiple factors (grain size, precipitate distribution, strain level, temperature) interact in non-trivial ways. This complexity underscores the need for application-specific testing rather than relying on generalized material property data.

Study Insights and Implications

This paper provides valuable insights into the microstructural mechanisms governing HLLPC in 52M alloy overlay layers. The clear correlation between Cr₂₃C₆ precipitation and HLLPC sensitivity provides a actionable quality criterion: minimizing Cr₂₃C₆ formation through process optimization is the most effective strategy for improving HLLPC resistance. For engineers specifying overlay welding processes for high-temperature applications, this work provides the technical basis for selecting laser overlay welding over conventional arc welding when HLLPC resistance is a critical requirement. The economic trade-off between process cost and service life must be evaluated on a case-by-case basis, considering the consequences of HLLPC failure in the specific application context.