Zhuojin Pipe Fitting Co., LtdNanocrystalline Surface Layer on Hardfacing Deposits: Friction and Wear Performance
Literature Overview
This study by Baderma, Meng Fanjun, Sun Xiaofeng, and Qiu Ji from the Academy of Armored Force Engineering, published in the Tribology journal (Vol. 34, No. 2, 2014, pp. 120–126), investigates the friction and wear properties of a nanocrystalline layer prepared on hardfacing repair deposits using pre-pressure rolling technology. Supported by the National Natural Science Foundation of China (Grants 51105376 and 51005245), this work bridges the gap between surface nanocrystallization technology and practical tribological performance.
Core Technical Content
Surface Nanocrystallization Process
The authors employed a pre-pressure rolling technique to produce a nanocrystalline layer on the surface of hardfacing repair deposits. This mechanical surface treatment approach is attractive for industrial applications because it requires no additional materials and can be performed in-situ on large components.
| Parameter | Value |
|---|---|
| Nanocrystalline layer thickness | Approximately 10 μm |
| Grain size in nanocrystalline layer | Less than 100 nm |
| Average grain size at surface | Approximately 10 nm |
| Hardness improvement (surface vs. base) | Approximately 3 times |
| Friction coefficient reduction | Approximately 10% |
| Wear volume reduction | 25–30% |
Microstructural Characterization
Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) analyses confirmed the formation of a well-defined nanocrystalline layer. The grain size distribution shows a gradient from approximately 10 nm at the surface to larger sizes approaching the base hardfacing layer, indicating progressive grain refinement with increasing plastic deformation depth.
Tribological Performance
Using a CETR-3 multifunctional friction and wear tester under dry sliding conditions, the following results were obtained:
- The friction coefficient decreased by approximately 10% compared to the untreated hardfacing layer.
- The wear volume loss was reduced by 25–30%.
- The wear mechanism transitioned from a combination of abrasive and adhesive wear (in the original hardfacing layer) to predominantly abrasive wear (in the nanocrystalline-treated layer).
Mechanism Analysis
The improvement in tribological performance is attributed to two primary factors:
- Increased hardness: The Hall-Petch relationship dictates that grain refinement leads to increased hardness. The approximately 3-fold hardness increase at the surface significantly resists micro-ploughing and micro-cutting by abrasive particles.
- Reduced plasticity: The nanocrystalline structure exhibits lower plasticity compared to the coarse-grained base material. This reduces the tendency for adhesive wear by limiting plastic deformation at asperity contacts.
The shift in wear mechanism from abrasive-plus-adhesive to predominantly abrasive wear is significant. Adhesive wear is typically more severe because it involves material transfer and can lead to rapid surface degradation. By suppressing adhesive wear through nanocrystallization, the overall wear resistance is substantially improved.
Engineering Practice Integration
For engineers working on surface hardening of pipe components, pump impellers, and valve seats, this research offers several practical implications:
- Post-weld surface treatment: Applying pre-pressure rolling after hardfacing can provide an additional layer of wear protection without re-welding.
- Applicability to large components: Unlike thermal spray or plasma deposition, mechanical surface nanocrystallization can be applied to large, complex geometries using portable equipment.
- Complementary approach: The nanocrystalline layer works synergistically with the hardfacing deposit, combining the bulk properties of the weld overlay with the surface properties of the nanocrystalline treatment.
Process Parameter Recommendations
| Parameter | Recommended Range | Rationale |
|---|---|---|
| Rolling pressure | 50–200 MPa | Sufficient plastic deformation without surface cracking |
| Number of passes | 3–8 | Progressive grain refinement |
| Roller diameter | 20–50 mm | Appropriate contact geometry |
| Surface roughness target | Ra < 0.4 μm | Smooth surface for reduced friction |
Study Insights
This paper demonstrates that mechanical surface nanocrystallization is a viable and effective method for enhancing the tribological performance of hardfacing deposits. The approximately 3-fold hardness increase at the surface is remarkable and suggests that even a thin nanocrystalline layer can significantly influence the wear behavior of the entire component. From a practical standpoint, this approach is particularly attractive for repair applications where the hardfacing deposit has already been applied and additional surface refinement is desired. The transition from mixed wear to predominantly abrasive wear is a key finding, as it suggests that the nanocrystalline layer effectively eliminates adhesive wear, which is often the more damaging mechanism in industrial applications. Engineers should consider incorporating surface nanocrystallization into their hardfacing process specifications for critical wear applications.