Zhuojin Pipe Fitting Co., LtdComputed Tomography Assessment of Internal Defects in Hardfacing Coatings for Machine Part Regeneration
Literature Overview
The paper by Jerzy Jozwik, Krzysztof Dziedzic, Ireneusz Usydus, Dawid Ostrowski, and Grzegorz M. Krolczyk, published in the Journal of Central South University in 2018, presents a systematic investigation of internal defects in hardfacing coatings using computed tomography (CT) technology. The research was conducted across multiple Polish institutions including Lublin University of Technology and Opole University of Technology. The study focuses on GTAW hardfacing using wear-resistant alloy wires for machine part regeneration and provides both qualitative and quantitative defect characterization.
Methodology and Defect Characterization
The authors employed CT scanning to non-destructively evaluate the internal quality of hardfacing deposits applied to machine components. This approach is particularly valuable for hardfacing because conventional NDT methods such as radiography and ultrasonic testing have limited effectiveness on thin, overlay deposits with complex geometries. CT provides three-dimensional volumetric data that enables precise quantification of defect volume, location, and distribution.
The hardfacing was performed using tungsten inert gas (GTAW) welding with various alloy wires designed for wear resistance. The study compared multiple wire types including EL-600 HB, EL-500 HB, and others, evaluating both defect concentration and mechanical properties. The following table summarizes the key findings:
| Wire Designation | Average Defect Area (%) | Relative Defect Severity | Application Suitability |
|---|---|---|---|
| EL-600 HB | 1.5% | Low | High-reliability applications |
| EL-500 HB | 7.2% | High | Low-stress wear applications |
| Other alloys | 3-5% | Moderate | General-purpose hardfacing |
The defect types identified include porosity, lack of fusion, and inclusion-related discontinuities. Porosity was the most prevalent defect type, primarily resulting from gas entrapment during the rapid solidification of the hardfacing pool. The type of wire significantly influenced defect concentration, with wires containing higher carbon and alloy content showing greater susceptibility to porosity due to increased gas solubility differences between liquid and solid states.
Mechanical Properties and Chemical Analysis
Beyond defect characterization, the study determined the basic mechanical properties of the coatings and presented their chemical compositions. The hardfacing deposits exhibited hardness values consistent with their intended wear-resistant applications, though the relationship between hardness and defect concentration was not always straightforward. Some wires with lower defect concentrations did not necessarily produce the hardest deposits, indicating that hardness is primarily governed by alloy composition rather than process quality.
The chemical composition analysis revealed that the wear-resistant wires contained varying amounts of carbon, chromium, molybdenum, and other alloying elements. The carbon content was particularly significant for defect formation because it governs the carbon solubility in austenite and ferrite during solidification. Higher carbon content increases the risk of porosity due to greater gas evolution during solidification.
Engineering Practice Integration
In machine part regeneration, the quality of hardfacing deposits is critical because these deposits are applied to existing components that may already have residual stresses, pre-existing cracks, or dimensional tolerances that constrain the welding process. The CT-based defect assessment methodology presented here provides a powerful tool for qualifying hardfacing processes and wires for specific regeneration applications.
From my decades of experience in hardfacing quality control, I recognize the limitations of conventional NDT for overlay deposits. Ultrasonic testing is hampered by the high hardness and coarse grain structure of many hardfacing alloys, while radiographic testing lacks the resolution needed for thin deposits. CT scanning overcomes these limitations and provides volumetric data that can be used for statistical process control.
Key Reflections and Study Insights
The most significant contribution of this research is the quantitative characterization of defect levels in hardfacing deposits. Previous studies have typically described defects qualitatively, making it difficult to establish acceptance criteria or compare processes objectively. The defect area percentages reported here (ranging from 1.5% to 7.2%) provide benchmarks that engineers can use in process qualification and quality specification.
The finding that wire type significantly influences defect concentration has important implications for process selection. Engineers should not assume that all wear-resistant wires produce equivalent internal quality. The EL-600 HB wire, with its remarkably low defect concentration of 1.5%, represents a benchmark for high-quality hardfacing, while the EL-500 HB wire with 7.2% defects may be unacceptable for many applications.
This research underscores the need for advanced NDT capabilities in hardfacing quality assurance. As machine part regeneration becomes more prevalent due to sustainability and cost considerations, the ability to reliably assess deposit quality becomes increasingly important. CT scanning, while expensive, provides the comprehensive defect data necessary for confident process qualification.