CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure of Surface Nanostructured Layer Produced by Pre-Compressive Rolling of Overlay Layer
Fundamental Principles and Processing Overview
Surface nanostructuring through plastic deformation is a well-established technique for enhancing the mechanical and tribological properties of metallic surfaces. When applied to weld overlay layers, this technique offers a unique opportunity to combine the beneficial alloying effects of the overlay material with the mechanical advantages of a nanostructured surface. The study examines the microstructural evolution of a surface nanostructured layer produced by pre-compressive rolling of an overlay layer deposited on a carbon steel substrate.
The pre-compressive rolling process involves subjecting the overlay layer to severe plastic deformation through multi-directional rolling at ambient temperature. The rolling is performed with a series of passes at decreasing reduction ratios, typically starting at 30% and progressively reducing to 5%, which ensures that the plastic deformation is concentrated in the near-surface region without causing cracking or delamination. The resulting surface nanostructured layer exhibits grain sizes below 100 nm in the most severely deformed region, transitioning to microstructures with grain sizes of 1–10 μm in the subsurface region.
Microstructural Evolution and Characterization
The microstructural evolution of the nanostructured layer was characterized using transmission electron microscopy (TEM), X-ray diffraction (XRD), and electron backscatter diffraction (EBSD). The results reveal a complex gradient microstructure that can be divided into three distinct regions based on the depth from the surface.
| Region | Depth from Surface | Grain Size | Dislocation Density | Microstructure Type |
|---|---|---|---|---|
| Surface layer | 0–2 μm | 20–80 nm | 10¹⁵–10¹⁶ m⁻² | Nanostructured equiaxed grains |
| Transition layer | 2–10 μm | 100 nm–2 μm | 10¹⁴–10¹⁵ m⁻² | Mixed nano-grains and micro-grains |
| Subsurface layer | 10–50 μm | 2–10 μm | 10¹²–10¹³ m⁻² | Deformed micro-grains with high dislocation density |
| Bulk overlay layer | >50 μm | 10–50 μm | 10¹¹–10¹² m⁻² | Original weld microstructure |
The surface nanostructured layer exhibits a high density of nanoscale grains with a preferred orientation that develops during the rolling process. The XRD patterns show significant peak broadening, which is indicative of severe lattice distortion and high dislocation density. The EBSD analysis reveals a high fraction of high-angle grain boundaries (HAGBs) in the surface layer, with the fraction of low-angle grain boundaries (LAGBs) decreasing with increasing deformation.
Dislocation Structures and Grain Boundary Character
TEM analysis of the surface layer reveals a complex dislocation structure consisting of dislocation cells, dislocation walls, and dislocation tangles. The dislocation cell size decreases from approximately 200 nm in the transition layer to below 50 nm in the surface layer. The dislocation walls are composed of arrays of edge dislocations with Burgers vectors oriented parallel to the rolling direction, which is consistent with the slip systems activated during plastic deformation.
The grain boundary character distribution analysis shows that the surface layer is dominated by random high-angle grain boundaries, with a small fraction of special boundaries (Σ3, Σ5, and Σ7). The presence of these special boundaries is attributed to the dynamic recovery processes that occur during rolling, where the stored energy drives grain boundary migration and recrystallization-like processes at the nanoscale.
Mechanical and Tribological Performance
The surface nanostructured layer produced by pre-compressive rolling exhibits significantly enhanced mechanical and tribological properties compared to the as-welded overlay layer. The hardness increase is primarily attributed to the Hall-Petch effect, where the grain refinement leads to increased dislocation pile-up at grain boundaries and enhanced yield strength.
| Property | As-Welded Overlay | After Pre-Compressive Rolling | Improvement |
|---|---|---|---|
| Surface hardness (HV0.025) | 580 | 950 | 64% increase |
| Surface roughness Ra (μm) | 1.2 | 0.3 | 75% reduction |
| Wear volume loss (mm³/N·m) | 4.5 × 10⁻⁶ | 1.2 × 10⁻⁶ | 73% reduction |
| Friction coefficient | 0.65 | 0.42 | 35% reduction |
| Compressive residual stress (MPa) | -50 | -420 | 840% increase in magnitude |
| Fatigue life (cycles, 10⁷) | 2.1 × 10⁵ | 8.5 × 10⁵ | 305% improvement |
The compressive residual stress introduced by the rolling process is a particularly important factor in the performance enhancement. The residual stress profile shows a peak compressive stress of -420 MPa at the surface, which decays to near-zero at a depth of 50 μm. This compressive stress field effectively counters the tensile stresses that develop during service, thereby delaying crack initiation and propagation.
Wear Mechanism Analysis
Wear testing was conducted using a ball-on-disc tribometer under dry sliding conditions against SiC counterfaces. The wear volume loss decreased by 73% after pre-compressive rolling, which is attributed to the combined effects of increased hardness, reduced surface roughness, and compressive residual stress. The wear mechanism analysis using scanning electron microscopy (SEM) reveals that the as-welded overlay layer exhibits a combination of adhesive and abrasive wear, while the nanostructured surface shows predominantly abrasive wear with significantly reduced material transfer.
The wear track analysis shows that the nanostructured surface layer acts as a protective barrier that prevents the penetration of wear debris into the bulk material. The high density of nanoscale grains provides a large number of grain boundaries that impede dislocation motion and crack propagation, thereby enhancing the wear resistance of the surface.
Process Parameters and Optimization
The effectiveness of the pre-compressive rolling process is highly dependent on the rolling parameters. The study systematically investigated the effects of rolling reduction, number of passes, rolling speed, and rolling direction on the microstructure and properties of the nanostructured layer.
| Parameter | Range Investigated | Optimal Value | Effect on Nanostructuring |
|---|---|---|---|
| Total reduction (%) | 30–90 | 75 | Higher reduction produces finer grains |
| Number of passes | 1–10 | 6 | More passes improve uniformity |
| Rolling speed (mm/s) | 10–200 | 50 | Higher speed reduces thermal effects |
| Rolling direction change | 0°–90° | 90° | Cross-rolling improves isotropy |
| Inter-pass annealing | None to 200°C | None | Annealing reduces nanostructuring |
The optimal rolling protocol consists of six passes with alternating rolling directions (0° and 90°), a total reduction of 75%, and a rolling speed of 50 mm/s. This protocol produces a surface nanostructured layer with a uniform grain size of 30–50 nm, a compressive residual stress of -400 MPa, and a surface hardness of 950 HV0.025.
Study Insights and Reflections
The study provides compelling evidence that surface nanostructuring through pre-compressive rolling is a highly effective approach for enhancing the performance of weld overlay layers. The combination of grain refinement, compressive residual stress, and surface roughness reduction creates a synergistic effect that significantly improves the wear resistance and fatigue life of the overlay. This is particularly relevant for applications in the oil and gas, mining, and power generation industries, where overlay layers are subjected to severe wear and fatigue conditions.
From a practical standpoint, the pre-compressive rolling process is relatively simple and cost-effective compared to other surface modification techniques such as shot peening or laser texturing. The process can be performed on existing overlay layers without requiring additional equipment or materials, making it an attractive option for retrofitting and maintenance applications. However, the process must be carefully controlled to avoid excessive deformation that could lead to cracking or delamination, particularly in overlay layers with high hardness or low ductility.
The study also raises important questions about the long-term stability of the nanostructured layer under service conditions. The nanostructured grains are thermodynamically unstable and will tend to coarsen at elevated temperatures. For applications involving temperatures above 400°C, the nanostructured layer may not be stable, and alternative surface modification techniques may be more appropriate.