Zhuojin Pipe Fitting Co., LtdMicrostructure of Pre-Pressure Rolling Surface Nanocrystallized Layer on Surfacing Deposits
Literature Overview
The paper by Baderma, Meng Fanjun, Sun Xiaofeng, Liu Xiaoting, and Li Xin, published in the Journal of Thermal Analysis and Calorimetry in 2015 (Vol. 36, No. 1, pp. 173-177), investigates the microstructural evolution of surfacing deposits subjected to pre-pressure rolling surface nanocrystallization. The research was conducted at the Department of Equipment Remanufacturing Engineering, Academy of Armored Force Engineering, and the Beijing Special Vehicle Research Institute, with funding from the National Natural Science Foundation of China (Grant No. 51105376) and the Remanufacturing National Defense Science and Technology Key Laboratory Fund. The study uses medium manganese welding wire to create surfacing deposits on Cr-Ni alloy steel substrates, followed by pre-pressure rolling treatment to achieve surface nanocrystallization.
Surface Nanocrystallization Mechanism
Surface nanocrystallization through severe plastic deformation is a well-established approach to enhance the mechanical properties of metal surfaces. The pre-pressure rolling process applies controlled compressive stress to the surfacing deposit surface, inducing significant plastic deformation in the near-surface region. This plastic deformation refines the grain structure from the original dendritic microstructure of the surfacing deposit to nanoscale grains, dramatically increasing surface hardness and fatigue resistance.
The authors employed a combination of field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), and three-dimensional surface profilometry to characterize the microstructural evolution. These advanced characterization techniques provide complementary information: FE-SEM reveals surface morphology and shallow microstructural features, HR-TEM provides atomic-scale resolution of grain boundaries and dislocation structures, and surface profilometry quantifies the surface roughness improvement.
Microstructural Characterization Results
The microstructural characterization reveals a clear gradient structure in the rolled surface layer, which can be divided into three distinct zones based on depth from the surface:
| Depth from Surface | Grain Size | Microstructural Features | Deformation State |
|---|---|---|---|
| 0–10 μm (surface layer) | ~10 nm | Equiaxed grains with random orientation | Severe plastic deformation |
| 10–15 μm (transition layer) | ~100 nm | Equiaxed grains and cellular structures | Moderate plastic deformation |
| >15 μm (base layer) | Original dendritic | Unchanged surfacing microstructure | No deformation |
The surface layer, extending to approximately 10 μm from the surface, exhibits the most dramatic microstructural refinement. The average grain size in this zone is approximately 10 nm, which represents a reduction of three to four orders of magnitude compared to the original dendritic grain size of the surfacing deposit. The grains in this zone are equiaxed with random crystallographic orientation, indicating that the deformation has been sufficient to break down the original columnar dendritic structure and create a fully recrystallized nanocrystalline structure.
The transition layer, located at 10–15 μm from the surface, shows an intermediate grain size of approximately 100 nm. This zone contains both equiaxed nanocrystals and cellular structures of similar dimensions. The presence of cellular structures suggests that this zone is in a transitional state between severe deformation and moderate deformation, where dislocation cell formation has occurred but complete grain refinement has not yet been achieved.
The grain size in the nanocrystalline zone increases with distance from the surface, which is consistent with the expected stress gradient during the rolling process. The rolling pressure is highest at the surface and decreases with depth, resulting in progressively less severe plastic deformation and therefore less grain refinement.
Surface Roughness Improvement
One of the practical advantages of the pre-pressure rolling process is the significant improvement in surface roughness. The authors report that the surface roughness is markedly reduced after rolling treatment. This improvement is attributed to two mechanisms: the mechanical leveling effect of the rolling process, which compresses surface asperities and fills surface valleys, and the grain refinement effect, which creates a more uniform and homogeneous surface structure.
For surfacing deposits, surface roughness is often a concern because the as-welded surface is typically irregular and may contain spatter, slag inclusions, and geometric irregularities from the welding process. The pre-pressure rolling process not only refines the grain structure but also provides a finishing function that improves surface quality, potentially reducing or eliminating the need for subsequent grinding operations.
Hardness Enhancement and Mechanical Properties
The grain refinement achieved through pre-pressure rolling is expected to produce substantial hardness enhancement according to the Hall-Petch relationship. For a grain size reduction from the typical dendritic size of 50–200 μm to 10 nm, the theoretical hardness increase can be estimated to be in the range of 200–400 HV. This dramatic increase in surface hardness is directly attributable to the increased grain boundary area, which impedes dislocation motion and increases the resistance to plastic deformation.
The random orientation of the nanocrystalline grains is also significant from a mechanical properties perspective. Random orientation eliminates the preferential crystallographic texture that is typical of as-welded deposits, resulting in more isotropic mechanical properties at the surface. This isotropy is beneficial for applications where the loading direction is variable or unpredictable, such as in rotating machinery components and pressure vessel surfaces.
Engineering Applications and Remanufacturing Context
The research context of this paper is particularly relevant to the remanufacturing industry, which focuses on restoring worn or damaged components to a like-new condition. Surfacing followed by surface nanocrystallization offers a powerful combination for remanufacturing applications: the surfacing layer restores material loss due to wear or damage, while the nanocrystallization treatment enhances the surface properties beyond the original condition.
This approach is particularly attractive for high-value components such as turbine blades, bearing races, hydraulic cylinder barrels, and transmission gears, where the cost of replacement is high and the downtime associated with component failure is significant. The remanufacturing industry has been increasingly adopting surface engineering approaches as a sustainable alternative to component replacement, reducing material consumption and environmental impact.
Reflections on Process Integration
In my experience with surface engineering for critical components, the integration of surfacing and surface nanocrystallization represents a significant advance in process capability. The key challenge is to ensure proper bonding between the surfacing deposit and the substrate while achieving the desired microstructural refinement without introducing defects. The pre-pressure rolling approach addresses both concerns by providing a gentle but effective deformation mechanism that does not require the extreme temperatures or pressures associated with other severe plastic deformation techniques such as high-pressure torsion or equal-channel angular pressing.
The gradient microstructure produced by the rolling process is actually advantageous from an engineering perspective. The hard nanocrystalline surface layer provides wear and fatigue resistance, while the softer underlying layers provide toughness and resistance to crack initiation and propagation. This gradient design principle mimics the natural microstructures found in biological materials such as bone and nacre, which have evolved over millions of years to optimize the balance between hardness and toughness.
The characterization techniques employed in this study—particularly the combination of FE-SEM and HR-TEM—set a standard for microstructural analysis in surface engineering research. Engineers involved in surface engineering quality assurance should ensure that their inspection protocols include both surface and cross-sectional characterization to fully understand the microstructural state of treated components.