Zhuojin Pipe Fitting Co., LtdEffect of Shot Peening on Residual Stress and Microstructure of H08Mn2Si Surfacing Layer
Literature Overview
This study, published in Chinese Surface Engineering (2013, Vol. 26, No. 4), investigates the effects of pneumatic shot peening on the residual stress distribution, microstructure, hardness, and surface roughness of H08Mn2Si surfacing layers. The research was conducted at the Key Laboratory of Remanufacturing Technology, Academy of Armored Force Engineering, and was supported by the National Science and Technology Support Program. The work provides valuable quantitative data on the depth and character of the peened layer, which is essential for optimizing shot peening parameters in surface engineering applications.
Material and Experimental Setup
H08Mn2Si is a low-carbon manganese-silicon welding wire commonly used for surfacing applications where a combination of wear resistance, toughness, and corrosion resistance is required. The surfacing layer produced with this filler metal typically consists of a ferritic matrix with dispersed carbide particles, providing good mechanical properties but limited inherent resistance to contact fatigue.
The shot peening was performed using a pneumatic shot peening machine. The experimental methodology included:
| Characterization Method | Purpose |
|---|---|
| X-ray diffraction (XRD) | Residual stress measurement at various depths |
| Metallographic examination | Microstructure analysis of the peened layer |
| Micro-hardness testing | Hardness profile through the peened depth |
| 3D surface profiler | Surface roughness characterization |
Residual Stress Analysis
The XRD measurements revealed that shot peening introduced a compressive residual stress layer approximately 500 μm deep into the H08Mn2Si surfacing layer. The residual stress distribution followed a characteristic pattern:
- Near the surface, the compressive stress reached its maximum value, providing the greatest benefit against surface-initiated fatigue cracks.
- With increasing depth, the compressive stress gradually decreased, transitioning through zero to a tensile stress region below the peened zone.
- The tensile stress region is a consequence of volume conservation during plastic deformation; the material displaced outward during peening must be balanced by tensile stress at greater depths.
The 500 μm depth of the compressive stress layer is significant because it exceeds the typical crack initiation depth for surface fatigue in many engineering applications. This means that cracks initiating at the surface would encounter compressive stress, which retards crack propagation and significantly extends fatigue life.
Microstructure and Hardness Changes
Metallographic analysis revealed that the ferritic microstructure of the H08Mn2Si surfacing layer did not undergo phase transformation as a result of shot peening. This is an important finding because it indicates that the strengthening mechanism is purely mechanical—work hardening and dislocation accumulation—rather than metallurgical transformation.
The micro-hardness profile showed a characteristic pattern of initial increase followed by decrease with depth. This distribution is consistent with the residual stress profile: the maximum plastic deformation occurs near the surface, producing the highest dislocation density and work hardening, while deformation decreases with depth.
| Depth from Surface | Approximate Hardness Trend | Residual Stress |
|---|---|---|
| 0–100 μm | Maximum hardness (work hardened) | Maximum compressive stress |
| 100–300 μm | Gradual hardness decrease | Compressive stress decreasing |
| 300–500 μm | Approaching base hardness | Near-zero stress |
| >500 μm | Base material hardness | Tensile stress |
The surface roughness increased to varying degrees after peening, which is an expected consequence of the plastic deformation process. The increase in roughness must be considered in applications where surface finish is critical, such as hydraulic seals or bearing surfaces.
Engineering Practice and Study Insights
The findings of this study have direct implications for the design of shot peening processes in surface engineering. The 500 μm depth of the effective peened layer provides a quantitative benchmark for evaluating peening effectiveness and for selecting appropriate shot size and velocity.
Several practical considerations emerge:
- The absence of phase transformation means that the strengthening effect is limited by the work-hardening capacity of the ferritic matrix. For higher-strength requirements, alternative surfacing materials with higher hardenability or pre-hardened structures may be more suitable.
- The increase in surface roughness after peening may require a subsequent finishing operation for applications with strict surface finish requirements.
- The tensile stress region below the peened layer should be evaluated for its potential to initiate subsurface fatigue cracks, particularly in applications with high cyclic loading.
The methodology of combining XRD residual stress measurement with microstructural and hardness analysis provides a comprehensive characterization of the peened layer and serves as a model for similar studies on other surfacing materials and peening conditions.