Assessment of Internal Defects in Hardfacing Coatings Using Computed Tomography

Literature Overview

The paper by Jozwik et al. (2018), published in the Journal of Central South University (Vol. 25, No. 5, pp. 1144-1153), presents a systematic investigation into the internal quality of hardfacing coatings produced by TIG welding using wear-resistant alloy wires. The study employs X-ray computed tomography (CT) as the primary non-destructive evaluation (NDE) tool for characterizing volumetric defect distribution within the deposited layers. This work is particularly relevant to engineers involved in the regeneration and refurbishment of critical machine parts, where coating integrity directly determines service life and operational safety.

Core Technical Approach

The researchers applied tungsten inert gas (TIG) hardfacing using multiple alloy wire consumables, including EL-600 HB and EL-500 HB grades, to deposit wear-resistant layers onto base components. The CT scanning methodology enabled three-dimensional visualization of internal porosity, lack of fusion, cracks, and other volumetric discontinuities that are difficult or impossible to detect with conventional surface inspection methods such as magnetic particle testing (MT) or liquid penetrant testing (PT).

Key Findings on Defect Distribution

The stark difference between EL-600 HB (1.5%) and EL-500 HB (7.2%) is striking and warrants careful interpretation. The higher defect concentration in EL-500 HB likely relates to its composition, which may promote greater gas evolution during solidification or create larger thermal gradients due to differing thermal conductivity of the deposited alloy. From a metallurgical standpoint, wires with higher carbon or sulfur content tend to produce more porosity due to gas trapping during rapid solidification of the weld pool.

Impact of Wire Type on Defect Concentration

The study demonstrates that the wire composition and melting behavior fundamentally influence defect nucleation and growth. Wires designed for higher hardness often contain carbide-forming elements in greater quantities, which can alter the solidification sequence and create conditions favorable to micro-porosity formation. The thermal cycling characteristics of different alloy compositions also play a role — alloys with lower thermal conductivity retain heat longer, potentially leading to wider molten pools and greater susceptibility to turbulence-induced porosity.

Connection to Engineering Practice

In the context of machine part regeneration, such as restoration of pump impellers, valve seats, and crusher components, the internal quality of hardfacing deposits is critical. A coating with 7.2% defect area may appear sound on the surface but could suffer catastrophic spalling under cyclic loading. The CT-based assessment methodology described in this paper offers a compelling alternative to destructive cross-sectional examination, particularly for high-value components where sampling is not feasible.

Practical Implications for Quality Control

1. CT scanning should be considered as a qualification tool during hardfacing procedure qualification, providing baseline defect data that can inform acceptance criteria.
2. Wire selection should be guided not only by hardness requirements but also by defect susceptibility — a wire with marginally lower hardness but significantly fewer defects may offer superior service life.
3. The defect percentage data can be correlated with service performance to establish empirical acceptance thresholds for specific applications.

Key Questions and Reflections

The 1.5% defect level achieved with EL-600 HB raises the question of whether further optimization of welding parameters — such as reduced travel speed, increased shielding gas flow, or pulsed current application — could push this below 1.0%. Additionally, the study focuses on qualitative and quantitative defect assessment but does not extensively discuss the mechanical consequence of these defects on fatigue life or wear resistance. In practice, a coating with 1.5% porosity may still exhibit acceptable performance if the pores are small and well-distributed, whereas concentrated clusters could serve as stress concentrators.

The use of CT for hardfacing evaluation represents a significant advancement over traditional radiographic testing (RT), which provides only two-dimensional projections. The volumetric information obtained from CT enables more accurate defect sizing and characterization, which is essential for remaining life assessment of refurbished components. However, the cost and accessibility of CT scanning remain barriers to widespread industrial adoption, and the study does not address the economic viability of CT-based quality assurance for high-volume production scenarios.

Study Insights and Implications

This work reinforces the importance of volumetric NDE in hardfacing quality assurance and provides quantitative defect benchmarks that can be used to compare wire grades and welding procedures. The clear correlation between wire type and defect concentration underscores the need for comprehensive consumable qualification that extends beyond hardness and wear rate measurements to include internal soundness assessment. For engineers responsible for specifying hardfacing procedures in critical applications, this literature provides valuable data to support wire selection decisions and to justify investment in advanced NDE capabilities.