High-Temperature Low-Plasticity Cracking Sensitivity of 52M Alloy Clad Layers Prepared by Different Processes

Literature Overview and Research Background

This study addresses a critical and often underappreciated failure mechanism in weld overlay applications: high-temperature low-plasticity cracking (HTLPC). The 52M alloy, a cobalt-chromium-tungsten-based hardfacing alloy widely used in oil and gas well components, petrochemical equipment, and high-temperature wear applications, is particularly susceptible to this cracking mode. The research systematically compares the HTLPC susceptibility of 52M overlay layers produced by different cladding processes, providing engineers with process selection guidance for high-temperature service applications.

HTLPC is distinct from cold cracking and hot cracking. It occurs during cooling through a temperature range of approximately 400-800°C, where the alloy exhibits a minimum in ductility due to the coexistence of solid and liquid phases in the interdendritic regions. In pressure vessel and piping applications, this cracking mode can be particularly insidious because it may not be detected during standard post-weld inspection and can propagate under cyclic thermal loading.

Comparative Process Analysis

The study evaluated three primary cladding processes for depositing 52M alloy: submerged arc welding (SAW) overlay, plasma transferred arc (PTA) powder cladding, and gas tungsten arc welding (GTAW) overlay. Each process was optimized to produce overlay layers of comparable thickness (3-5 mm) on low-alloy steel substrates.

Process Parameters and Thermal Cycle Comparison

The thermal cycle analysis reveals that SAW overlay produces the highest heat input and slowest cooling rates, which paradoxically increases HTLPC susceptibility. The extended time spent in the critical temperature range (400-800°C) allows sufficient time for strain-induced cracking to initiate and propagate in the interdendritic regions. In contrast, PTA cladding produces intermediate thermal cycles, while GTAW overlay generates the highest cooling rates, which tend to suppress HTLPC by rapidly traversing the critical temperature window.

Cracking Mechanism and Microstructural Evidence

Metallographic examination of cross-sections revealed that HTLPC in 52M overlay layers follows interdendritic boundaries, with crack lengths typically ranging from 50 to 500 micrometers. The cracks are characteristically straight or slightly tortuous, perpendicular to the weld travel direction, and are most prevalent in the upper portion of multi-pass overlays where the thermal cycle is most severe.

The mechanism involves the following sequence: during solidification, the alloy forms a dendritic microstructure with liquid films remaining in interdendritic regions. As the temperature drops through the 400-800°C range, the alloy enters a two-phase region where the solid matrix has very low ductility. Simultaneously, thermal contraction and residual stress generate tensile strain in the interdendritic regions. When the combined effect of low ductility and high tensile stress exceeds the cohesive strength of the interdendritic boundaries, cracking initiates.

Cracking Susceptibility Rating

An important finding is that preheating, which is commonly employed to reduce hydrogen-induced cold cracking, can actually exacerbate HTLPC in 52M alloy overlays. Preheating to 200-300°C increases the time spent in the critical temperature range during cooling, providing more opportunity for cracking. This finding has direct implications for welding procedure development and challenges conventional wisdom about the benefits of preheating for all cracking modes.

Countermeasures and Engineering Recommendations

The study proposes several effective countermeasures for minimizing HTLPC in 52M alloy overlays:

1. Process selection: PTA powder cladding offers the best compromise between deposition rate and cracking resistance, making it the preferred process for high-temperature service applications.
2. Interpass temperature control: Maintaining interpass temperatures below 150°C for multi-pass overlays reduces the cumulative time in the critical temperature range.
3. Micro-alloying: Adding small amounts of boron (0.05-0.1%) or titanium to the 52M alloy composition can modify the solidification behavior and reduce interdendritic cracking susceptibility.
4. Post-weld treatment: Low-temperature stress relief at 350-400°C for 2 hours can reduce residual stresses without inducing additional cracking, as this temperature is below the HTLPC critical range.

For pressure vessel engineers, this research reinforces the importance of process qualification testing that specifically addresses HTLPC susceptibility. Standard bond strength tests and macrograph examination per NB/T 47014 may not detect HTLPC, which requires specific testing protocols such as the arc crack test or dilution test combined with microstructural examination of the overlay layer.

Study Insights and Reflections

The systematic comparison of cladding processes for HTLPC resistance provides a clear decision-making framework for engineers selecting overlay processes for high-temperature applications. The finding that GTAW overlay produces the lowest cracking susceptibility, despite its low deposition rate, suggests a hybrid approach: using GTAW for the first one or two passes to establish a crack-free foundation, followed by PTA cladding for building up the required overlay thickness at a more economical rate. This hybrid strategy aligns with practical experience in the field and provides a practical solution that balances quality with productivity.

The research also highlights an important gap in current standards: NB/T 47014 and ASME IX do not specifically address HTLPC susceptibility in their qualification requirements. Engineers must supplement standard qualification procedures with additional testing, particularly for cobalt-based and high-alloy overlays intended for high-temperature service. This underscores the need for continued development of qualification standards that encompass all relevant cracking mechanisms, not just cold cracking and hot cracking.