Friction and Wear Performance of Nanocrystalline Layer on Overlay Weld Surface

Literature Overview

This research by Ba Dema, Meng Fanjun, Sun Xiaofeng, and Qiu Ji from the Department of Equipment Remanufacturing Engineering at the Academy of Armored Force Engineering, published in the Tribology Transactions (Chinese and English Edition) in 2014 (Vol. 34, No. 2, pp. 120-126), investigates the tribological performance of a nanocrystalline layer produced on the surface of an overlay weld repair layer using pre-pressure rolling technology. Funded by the National Natural Science Foundation of China (Grants 51105376 and 51005245), this work addresses the challenge of improving the surface integrity of overlay weldments through post-weld mechanical surface treatment, a technique of considerable interest in the remanufacturing of worn piping components, pump housings, and valve bodies.

Core Technical Approach

The study employs a two-stage methodology: first, a nanocrystalline layer is produced on the surface of an overlay weld repair layer using pre-pressure rolling (also known as shot peening or mechanical surface nanocrystallization), and second, the friction and wear performance of this modified surface is evaluated under dry sliding conditions using a CETR-3 multifunctional friction and wear testing machine. Microstructural characterization is performed using transmission electron microscopy (TEM) and scanning electron microscopy (SEM).

Microstructural Results

The surface nanocrystallization process produces a nanocrystalline layer approximately 10 μm in thickness, with grain sizes below 100 nm throughout the layer. Most remarkably, the outermost surface layer exhibits an average grain size of approximately 10 nm, representing an extreme degree of grain refinement. Nanoindentation testing reveals that the hardness of this nanocrystalline layer is approximately three times that of the original overlay weld layer. This dramatic hardness increase is consistent with the Hall-Petch relationship, which predicts that yield strength increases as grain size decreases, and is further enhanced by the introduction of dislocation pile-ups and grain boundary strengthening mechanisms.

Tribological Performance

The improvement in wear resistance is attributed to two synergistic factors. First, the threefold increase in surface hardness directly resists abrasive material removal by hard counterface asperities and third-body particles. Second, the fine grain structure reduces the plasticity of the surface layer, which suppresses the adhesive wear mechanism. Adhesive wear occurs when asperity contact leads to localized welding and subsequent tearing of material from the surface; the high hardness and low ductility of the nanocrystalline layer prevent the formation of these adhesive junctions.

The transition from a combined abrasive-adhesive wear mechanism to a purely abrasive mechanism is significant. In many industrial applications involving piping components, the counterface may contain hard oxide particles or debris from other worn components. A surface that resists abrasive wear while eliminating adhesive wear provides a more predictable and consistent wear life, which is critical for maintenance planning and reliability engineering.

Engineering Practice Implications

Applicability to Piping and Fitting Repair

For engineers involved in the repair and remanufacturing of worn piping components, this study offers a practical post-weld treatment option. Traditional overlay welding alone often leaves a surface with coarse microstructure and relatively high residual stresses, both of which contribute to premature wear failure. The application of pre-pressure rolling after overlay welding can significantly extend the service life of the repaired component without requiring additional welding passes or complex heat treatment cycles.

The 10 μm nanocrystalline layer thickness, while relatively thin, is sufficient to provide meaningful wear resistance improvement in many applications. However, engineers must consider the following practical limitations:

1. Load capacity: The nanocrystalline layer is a surface modification; the bulk of the overlay weld layer retains its original mechanical properties. Under high contact stress conditions, the thin nanocrystalline layer may be plastically deformed or removed, exposing the softer underlying material.
2. Fatigue considerations: The high residual compressive stresses introduced by the rolling process may be beneficial for fatigue resistance, but the extreme grain refinement could potentially reduce the material's resistance to crack initiation under cyclic loading.
3. Thermal stability: The nanocrystalline structure is metastable and may coarsen during subsequent thermal exposure. For piping components operating at elevated temperatures, the long-term stability of the nanocrystalline layer must be evaluated.

Process Integration

The pre-pressure rolling process can be integrated into a standard overlay welding repair procedure as follows:

• Step 1: Surface preparation and substrate cleaning
• Step 2: Overlay welding deposition to the required thickness
• Step 3: Post-weld heat treatment (if required by the weld specification)
• Step 4: Pre-pressure rolling to produce the nanocrystalline surface layer
• Step 5: Surface inspection and functional testing

This approach is particularly suitable for components where the overlay weld layer serves as a wear-resistant surface, such as pump impellers, valve seats, and pipeline fittings in abrasive service. The additional rolling step adds minimal processing time and equipment requirements compared to alternative surface modification techniques such as ion implantation or physical vapor deposition.

Key Insights and Reflections

The most compelling finding of this study is the quantitative improvement in wear volume (25-30% reduction) achieved through a relatively simple mechanical surface treatment. In the context of piping component repair, where the goal is to restore worn surfaces to functional condition at minimal cost, this approach represents an attractive option. The elimination of adhesive wear is particularly valuable in high-pressure piping systems where galling and seizure can lead to catastrophic failure.

However, the study's dry sliding wear test conditions may not fully represent the complex tribological environments encountered in actual piping service. Corrosive media, temperature fluctuations, and cyclic loading are absent from the test protocol. Future work should extend the tribological evaluation to include mixed lubrication conditions and corrosive wear scenarios that are more representative of real-world piping applications.

This literature demonstrates that post-weld mechanical surface treatment can significantly enhance the tribological performance of overlay weldments. For engineers involved in piping component remanufacturing, the combination of overlay welding for bulk wear resistance and surface nanocrystallization for enhanced surface integrity represents a practical and cost-effective strategy for extending component service life.