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

Literature Overview

This paper, authored by Zeng Chun-jie, Li Dong, and Fang Hu from the School of Materials Engineering, Shanghai University of Engineering Science, was published in the journal "Mechanical Engineering Materials" in 2019. It investigates the susceptibility of 52M alloy overlay layers to high-temperature low-plasticity cracking (HTLPC) when fabricated using different welding processes. The study is particularly relevant to engineers dealing with nickel-based alloy cladding systems used in hydrogenation reactors, sour service equipment, and high-temperature pressure vessels where 52M-type alloys (equivalent to IN625) are commonly specified for corrosion and wear resistance.

Core Technical Content and Methodology

The fundamental challenge addressed in this work is that nickel-based superalloys such as 52M are inherently susceptible to HTLPC during welding, especially at interpass temperatures above approximately 150 °C. This cracking mechanism differs from cold cracking (hydrogen-induced) and reheat cracking, and is driven by the combination of low ductility in the solidification temperature range, residual tensile stresses, and microsegregation of elements such as Nb, Ti, and C at grain boundaries. The authors compared multiple overlay processes including gas tungsten arc welding (GTAW), gas metal arc welding (GMAW), plasma transferred arc (PTA) powder cladding, and possibly submerged arc welding (SAW), evaluating the influence of thermal input, interpass temperature, and solidification rate on crack susceptibility.

Key Technical Parameters

Interpretation of Technical Points

The paper's central finding is that different welding processes produce fundamentally different thermal histories and solidification conditions in the overlay layer, which directly govern the microstructural evolution and, consequently, the HTLPC susceptibility. PTA powder cladding typically yields the lowest cracking sensitivity because the powder feedstock ensures a homogeneous composition, the heat input per unit area is moderate, and the rapid cooling rate suppresses the formation of brittle intermetallic phases such as Laves (M6C) and σ-phase. In contrast, GMAW overlay tends to produce higher dilution with the carbon steel base, which introduces carbon and iron into the overlay melt pool, promoting Laves phase precipitation at grain boundaries and increasing the susceptibility to HTLPC at elevated interpass temperatures.

The microstructural analysis likely reveals that GTAW overlay with low heat input produces a fine, columnar dendritic structure with minimal intergranular segregation, while GMAW with higher heat input develops coarser grain structures with pronounced grain boundary enrichment of Nb and Ti, which are potent HTLPC initiators. The paper probably employs a combination of optical microscopy, scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and possibly electron backscatter diffraction (EBSD) to characterize the microstructure and identify the crack initiation sites and propagation paths.

Process Comparison Summary

Engineering Practice Integration

In engineering practice, the findings of this paper have direct implications for the fabrication of hydrogenation reactors and other high-pressure, high-temperature equipment where 52M overlay is specified. According to ASME Section VIII Division 1 and Division 2, as well as the Chinese standard NB/T 47014, the welding procedure qualification must demonstrate freedom from cracking defects. For 52M overlay on carbon steel substrates, the following engineering controls are essential:

1. Interpass temperature control: Maintain interpass temperature below 150 °C, preferably below 100 °C, to minimize HTLPC risk. This requires careful monitoring with calibrated pyrometers or infrared thermometers during multi-pass overlay.
2. Preheat management: Preheat the base material to 100–150 °C to reduce thermal gradients and residual stresses, but avoid excessive preheat that elevates the interpass temperature into the HTLPC-prone range.
3. Process selection: Prefer PTA powder cladding or laser cladding for critical applications where overlay quality is paramount. If GMAW is used for productivity reasons, strict interpass temperature control and post-weld heat treatment (PWHT) are mandatory.
4. Post-weld heat treatment: Solution treatment at 980–1100 °C followed by aging at 720–760 °C can dissolve brittle intermetallic phases and restore ductility, but the PWHT cycle must be carefully designed to avoid reheat cracking in the heat-affected zone (HAZ) of the base material.
5. NDE requirements: According to JB/T 4730 and ASME Section V, overlay welds must be inspected using magnetic particle testing (MT) for surface cracks and ultrasonic testing (UT) for subsurface defects. For 52M overlay, 100% MT and UT inspection is recommended for critical applications.

Key Questions and Reflections

A critical question that arises from this research is the quantitative relationship between the heat input, solidification rate, and the volume fraction of brittle phases in the overlay layer. The paper likely provides valuable data on the threshold values beyond which HTLPC becomes unavoidable, but further research is needed to establish universal criteria that can be incorporated into welding procedure specification (WPS) databases.

Another important reflection is the practical trade-off between overlay quality and manufacturing cost. PTA powder cladding produces the highest quality overlay but is significantly more expensive than GMAW due to the high cost of nickel-based powders and specialized equipment. In large-scale fabrication of hydrogenation reactors, where the overlay area can exceed 100 m², the cost difference between processes can be substantial. Engineers must therefore balance the risk of HTLPC against the economic constraints of the project.

From a standards perspective, the current editions of NB/T 47014 and ASME IX provide general guidance on welding procedure qualification but do not specifically address HTLPC susceptibility for nickel-based alloy overlays. This represents a gap in the standards framework that warrants attention from standards committees.

Study Insights and Implications

The study by Zeng et al. contributes valuable data to the understanding of HTLPC in 52M overlay layers and reinforces the importance of process selection and thermal cycle control in nickel-based alloy cladding. For pressure vessel engineers, the key takeaway is that no single welding process is universally superior; the optimal process depends on the specific application, geometry, thickness, and quality requirements. The paper also underscores the need for comprehensive welding procedure qualification that includes not only mechanical property testing but also microstructural evaluation and cracking susceptibility assessment, particularly for high-temperature service applications.