Finite Element Analysis of Cracked Surface Effects on Wear-Resistant Overlay Weld Layer Performance

Literature Overview

Published in 2000 in the Journal of Shenyang University of Technology, this study by Ma Hongyan and colleagues from Shenyang University of Technology investigates a counterintuitive phenomenon in wear-resistant overlay welding: the potential beneficial effect of geometric surface cracking on the wear resistance of the overlay layer. The research was supported by the Liaoning Provincial Science and Technology Commission Natural Science Foundation.

The authors employed finite element analysis (FEA) to calculate stress distributions in overlay weld layers with and without surface cracking. The FEA program was written in Fortran 77 and executed under the Fortran Power-Station 4.0 environment. This early computational study represents one of the pioneering applications of numerical methods to overlay welding problems, predating the widespread availability of commercial FEA software for welding applications.

The central finding—that geometric surface cracking can relax welding residual stresses and, under certain conditions, improve the wear resistance of the overlay surface—challenges conventional quality control paradigms that universally classify surface cracks as unacceptable defects.

Core Technical Analysis

The Role of Residual Stress in Overlay Welding

Welding residual stresses in overlay layers are a well-documented phenomenon arising from the thermal cycle of welding. As the molten weld metal solidifies and cools, it contracts, but this contraction is constrained by the surrounding cooler base material and previously deposited layers. This constraint generates tensile residual stresses in the weld metal, which can reach magnitudes approaching the material's yield strength.

The significance of residual stress for wear resistance is multifaceted:

• Crack initiation: Tensile residual stresses promote crack initiation at surface defects, inclusions, or microstructural heterogeneities, reducing the effective wear life of the overlay
• Spalling: High tensile stresses can cause delamination of the overlay from the base material or between successive passes, leading to catastrophic wear failure
• Contact stress interaction: In sliding contact applications, residual tensile stresses superimpose with contact stresses, accelerating subsurface crack formation and material removal

The conventional approach to managing residual stresses includes post-weld heat treatment (PWHT), peening, and controlled welding sequences. However, this study suggests that surface cracking may serve as a natural stress relief mechanism.

Finite Element Model and Findings

The FEA model developed by the authors considered the stress state in the overlay layer under various surface conditions: intact surface, surface with geometric cracks of varying dimensions, and surface with different crack geometries. The analysis revealed several important insights:

The key finding is that shallow geometric surface cracks can act as stress relief mechanisms, reducing the effective tensile residual stress in the overlay layer. This stress relaxation can improve wear resistance by:

1. Reducing the driving force for crack initiation during wear
2. Decreasing the likelihood of spalling under contact loading
3. Allowing the overlay to accommodate plastic deformation without catastrophic failure

However, the study also implicitly acknowledges that this beneficial effect is conditional. If cracks are too deep, too numerous, or oriented unfavorably, they become initiation sites for wear-related failures rather than stress relief mechanisms.

The Crack Wear Paradox

The relationship between surface cracking and wear resistance is not straightforward and depends on several interacting factors:

• Crack depth: Shallow surface microcracks (less than 50–100 μm) may relax near-surface stresses without compromising structural integrity, while deeper cracks (>200 μm) reduce the effective load-bearing area and accelerate wear
• Crack density: A moderate crack density may provide adequate stress relief, but excessive cracking leads to material loss through crack propagation and spalling
• Crack orientation: Cracks oriented parallel to the wear direction may be less detrimental than those oriented perpendicular to it
• Service conditions: Under low contact stress conditions, stress relief from cracking may dominate; under high contact stress, the structural weakening effect may prevail

Engineering Practice Implications

Quality Control Philosophy Reconsideration

This study challenges the traditional quality control approach that categorically rejects all surface cracks in overlay welds. In practice, many overlay weld specifications (such as those in AWS D10.9 or ISO 2553) classify surface cracks as rejectable defects. However, the findings suggest a more nuanced approach may be warranted:

1. Distinguish between macroscopic and microscopic cracking: Macroscopic cracks visible to the naked eye or with low-magnification magnification are generally unacceptable, as they indicate significant process or material problems. Microscopic surface cracking (hairline cracks visible only under magnification) may be tolerable in certain applications.
2. Consider the service environment: In applications with low contact stress and high cyclic loading (such as pump impellers in low-pressure service), the stress relief benefit of microcracking may outweigh the structural concern. In high-pressure or high-impact applications, even microcracking should be avoided.
3. Implement FEA-based acceptance criteria: Rather than applying blanket crack rejection criteria, engineers can use FEA to evaluate whether a specific crack pattern is acceptable for a given application, considering the expected service stresses and loading conditions.

Process Controls to Manage Surface Cracking

While the study acknowledges potential benefits of controlled surface cracking, the practical approach should focus on minimizing uncontrolled cracking through proper process design:

• Powder/wire selection: Use surfacing alloys with lower crack sensitivity, such as those with reduced carbon content or with additions of Ni, Mo, or Mn that promote crack-free solidification
• Heat input control: Optimize heat input to achieve adequate stress relief through thermal cycling without generating excessive residual stress. Lower heat input generally reduces residual stress but may increase crack susceptibility in some alloys
• Interpass temperature management: Maintaining appropriate interpass temperatures (typically 100–200 °C for many overlay alloys) allows stress relief between passes while preventing excessive grain growth
• Post-weld treatment: Induction tempering or localized heat treatment can relax residual stresses without affecting the overlay's microstructure and hardness
• Peening: Light peening of the overlay surface can introduce beneficial compressive residual stresses, counteracting the tensile stresses from welding

Application to Pipe and Fitting Surface Engineering

In the context of pipe and fitting manufacturing, surface cracking in overlay welds is particularly relevant for:

• Valve seat overlay: Hardfacing of valve seats with crack-sensitive alloys (such as high-Cr cast irons or cobalt-based alloys) where surface integrity is critical for sealing performance
• Wear ring overlay: Hardfacing of wear rings in pumps and compressors where cyclic loading and erosion-corrosion combine to challenge overlay integrity
• Pipe end hardfacing: Hardfacing of pipe ends for erosion resistance in slurry service, where the overlay must withstand both impact and sliding wear

For these applications, the FEA approach demonstrated in this study can be adapted to predict the service life of overlay welds with specific crack patterns, enabling more rational acceptance criteria and potentially extending component life.

Study Insights and Reflections

This 2000 study is notable for its early application of FEA to overlay welding problems, at a time when computational resources were limited and commercial welding FEA software was not widely available. The Fortran-based approach, while computationally intensive by modern standards, demonstrates the authors' commitment to rigorous numerical analysis. The finding that surface cracking can improve wear resistance under certain conditions is both scientifically interesting and practically significant.

However, several limitations of the study should be acknowledged:

• The FEA model appears to be relatively simple, likely using linear elastic assumptions that may not fully capture the plastic deformation and contact mechanics of wear processes
• The study does not provide experimental validation of the FEA predictions through actual wear testing of cracked versus uncracked overlays
• The geometric crack models used may not accurately represent the complex crack networks observed in real overlay welds
• The study does not address the role of environmental factors (such as corrosion or erosion) in the interaction between cracking and wear

The most important practical takeaway is that surface cracking should not be viewed as an absolute rejection criterion without consideration of the specific service conditions and crack characteristics. A more nuanced approach, informed by FEA and experimental validation, can lead to better engineering decisions.

Summary

Ma et al.'s finite element analysis reveals that geometric surface cracking in wear-resistant overlay weld layers can relax welding residual stresses and, under certain conditions, improve surface wear resistance. This finding challenges conventional quality control practices that universally reject surface cracks and suggests that a more nuanced, application-specific approach to crack acceptance criteria may be warranted. Engineers working with overlay welds should consider the crack depth, density, and orientation in conjunction with expected service stresses when evaluating overlay weld quality. The FEA methodology demonstrated here provides a valuable framework for making informed decisions about acceptable crack patterns in specific applications, particularly for wear-critical components in piping systems and process equipment.