CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure of Surface Nanocrystalline Layer Formed by Pre-Compressive Stress Rolling on Cladding Deposits
Literature Overview
Published in the Journal of Heat Treatment of Materials in 2015, this research by scholars from the Armored Forces Engineering Academy and Beijing Special Vehicle Research Institute investigates a novel surface modification technique applied to cladding layers: pre-compressive stress rolling to achieve surface nanocrystallization. Supported by the National Natural Science Foundation of China and the Re-manufacturing National Defense Science and Technology Key Laboratory, this work represents the intersection of surface engineering, severe plastic deformation (SPD), and remanufacturing technology. The research addresses the challenge of enhancing the surface properties of existing cladding deposits through mechanical surface treatment rather than thermal or metallurgical modification.
Technical Principle
The core principle involves applying controlled compressive stress through roller-based plastic deformation of the cladding surface. The rolling process introduces high dislocation density, which through dynamic recovery and recrystallization mechanisms, progressively refines the grain structure to the nanocrystalline regime (grain size below 100 nm). The key advantage of this approach is that it modifies only the near-surface region (typically 20-100 μm depth) without altering the bulk microstructure or requiring high temperatures that could cause intermetallic compound formation at the cladding-base metal interface.
Experimental Methodology and Results
The study employed both experimental and numerical approaches to characterize the nanocrystallization process:
| Parameter | Initial Condition | After 1 Pass | After 3 Passes | After 5 Passes |
|---|---|---|---|---|
| Surface grain size (nm) | 5-15 μm | 200-500 | 80-150 | 30-80 |
| Surface hardness (HV) | 450-500 | 650-700 | 800-900 | 950-1050 |
| Compressive residual stress (MPa) | Near zero | -200 to -300 | -400 to -550 | -600 to -750 |
| Surface roughness Ra (μm) | 0.8-1.2 | 0.3-0.5 | 0.1-0.3 | 0.05-0.15 |
The microstructural evolution was characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), and electron backscatter diffraction (EBSD). The results showed a progressive transition from coarse martensitic or austenitic grains through a dislocation cell structure to an equiaxed nanocrystalline structure with random orientation.
Process Parameter Optimization
The rolling process parameters that influence nanocrystallization effectiveness include:
- Roller diameter: Smaller rollers (10-20 mm) produce higher strain rates and more effective grain refinement per pass.
- Rolling force: Forces in the range of 5-15 kN per roller produce optimal plastic strain without causing surface cracking.
- Rolling speed: Lower speeds (0.5-2 m/min) allow more uniform plastic deformation and reduce adiabatic heating effects.
- Number of passes: The relationship between passes and grain refinement follows a logarithmic trend, with diminishing returns after 5-7 passes.
- Substrate temperature: Rolling at ambient temperature produces the finest grains; preheating above 200°C activates dynamic recovery that counteracts grain refinement.
Engineering Significance
The surface nanocrystallization technique offers several advantages for cladding applications in military and industrial equipment:
- Remanufacturing capability: Existing cladding layers on worn components can be surface-modified without re-cladding, reducing material consumption and environmental impact.
- Combined property enhancement: The simultaneous improvement of hardness, compressive residual stress, and surface finish provides synergistic benefits for fatigue life and wear resistance.
- Compatibility with existing processes: The technique can be applied after conventional cladding operations without requiring specialized welding equipment.
- Quantifiable quality metrics: The surface hardness and residual stress values provide clear acceptance criteria for quality assurance.
Study Reflections
This research represents a sophisticated application of severe plastic deformation principles to practical cladding technology. The connection between surface nanocrystallization and improved tribological performance is well-established in the literature, but the specific application to cladding deposits—particularly those with complex microstructures resulting from welding solidification—adds significant technical depth. In pressure vessel and equipment remanufacturing applications, this technique could extend the service life of cladded components by 2-3 times without the cost and disruption of complete re-cladding. The challenge lies in maintaining consistent rolling parameters over large surface areas, which requires either automated roller systems or carefully planned manual procedures with frequent parameter verification. The integration of this surface modification with existing inspection protocols (UT for bond integrity, MT for surface defects, and hardness mapping) will be essential for industrial implementation.