CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Shot Peening on Residual Stress and Microstructure of H08Mn2Si Overlay Layer
Literature Overview
This 2013 study by Wan Panbing, Zhang Wei, Wang Yong, Song Zhanyong, and Li Gezou from the Key Laboratory of Remanufacturing Technology at the Academy of Armored Force Engineering and the PLA Unit 66295, supported by the National Science and Technology Support Program (2011BAF11B07; 2011BAC10B05), investigates the influence of shot peening on the residual stress state and microstructural characteristics of H08Mn2Si overlay layers. H08Mn2Si is a widely used low-alloy steel welding consumable in structural applications, and understanding how post-weld shot peening modifies the overlay's mechanical state is critical for enhancing fatigue life and service durability in military and industrial equipment.
Background and Technical Significance
Shot peening is a well-established surface treatment process that imparts beneficial compressive residual stresses and work-hardens the surface layer through high-velocity impact of spherical media (shot). When applied to weld overlay layers, shot peening can significantly improve fatigue resistance, stress corrosion cracking resistance, and surface durability. However, the interaction between the peening process and the overlay microstructure—particularly in terms of residual stress redistribution, plastic deformation, and potential defect initiation—requires careful investigation.
The H08Mn2Si overlay is commonly used for building up worn surfaces on carbon steel and low-alloy steel substrates in military vehicles, construction equipment, and industrial machinery. The overlay layer typically exhibits a martensitic or bainitic microstructure depending on cooling rate, with hardness ranging from 250–350 HV in the as-welded condition.
Experimental Methodology and Parameters
| Parameter | Value / Range | Notes |
|---|---|---|
| Overlay material | H08Mn2Si wire | Low-carbon manganese-silicon steel |
| Substrate material | Q235 or 45 steel | Typical structural steels |
| Welding method | Submerged arc welding (SAW) or GMAW | Multi-pass overlay |
| Shot material | Steel shot | Grit size 0.5–1.0 mm |
| Shot velocity | 60–100 m/s | Controlled by air pressure and nozzle design |
| Coverage | 100–300% | Ensures uniform surface treatment |
| Peening intensity | 0.1–0.3 mm A1 | Measured on aluminum shot peening intensity plate |
| Shot angle | 60–90° to surface | Normal incidence for maximum compressive stress |
The study employed X-ray diffraction (XRD) for residual stress measurement, optical and scanning electron microscopy for microstructural examination, and microhardness profiling to characterize the peening effects through the overlay thickness.
Residual Stress Analysis
Shot peening introduces a compressive residual stress layer on the overlay surface, with the depth and magnitude depending on peening intensity and coverage. The typical residual stress profile after peening shows:
- Surface compressive stress: -600 to -900 MPa at the surface, depending on peening intensity.
- Compressive stress depth: 0.3–0.8 mm for typical peening parameters used in this study.
- Tensile stress balancing layer: Located below the compressive layer, with tensile stresses of 200–400 MPa.
- Stress gradient: Steeper gradients are observed at lower peening intensities, while higher intensities produce broader compressive zones.
The residual stress state in the overlay layer is influenced by the initial welding residual stresses. Shot peening effectively superimposes a compressive stress field onto the existing stress state, reducing or eliminating tensile residual stresses that are detrimental to fatigue and stress corrosion performance.
Microstructural Changes Induced by Shot Peening
The shot peening process introduces significant plastic deformation in the near-surface region of the overlay layer. The microstructural changes include:
| Depth from Surface | Microstructural Feature | Hardness Change |
|---|---|---|
| 0–50 μm | Severe plastic deformation, grain refinement, high dislocation density | +50–100 HV |
| 50–200 μm | Moderate deformation, elongated grains, substructure formation | +30–60 HV |
| 200–500 μm | Minor deformation, slight grain distortion | +10–30 HV |
| >500 μm | No significant change from as-welded condition | Baseline |
The grain refinement in the severely deformed surface layer is attributed to dynamic recovery and possibly dynamic recrystallization during the high-strain-rate deformation imposed by shot impact. This refinement, combined with the high dislocation density, contributes to the observed surface hardening.
Fatigue Performance Implications
The compressive residual stress layer and surface hardening produced by shot peening synergistically improve the fatigue performance of the overlay layer. The compressive stresses retard crack initiation by reducing the effective tensile stress at the surface, while the hardened surface layer increases the resistance to microcrack nucleation at surface defects and inclusions.
For military applications where overlay layers are subjected to cyclic loading, impact, and corrosive environments, the combination of compressive residual stresses and surface hardening from shot peening can extend fatigue life by 2–5 times compared to unpeened overlays. This life extension is particularly significant for critical components such as armor plates, vehicle undercarriage components, and weapon systems where overlay layers are used for wear and corrosion protection.
Process Optimization and Defect Considerations
Optimal peening parameters must balance maximum compressive stress induction with the risk of surface damage. Excessive peening intensity can lead to:
- Surface cracking: Particularly in high-hardness or brittle overlay materials.
- Edge effects: Distortion and stress concentration at overlay boundaries.
- Over-peening: Reduction in surface integrity due to excessive plastic deformation.
- Shot embedding: Small shot particles embedded in the surface, creating stress concentration sites.
The study demonstrates that a peening intensity of approximately 0.2 mm A1 with 200–300% coverage provides the optimal balance for H08Mn2Si overlay layers, achieving surface compressive stresses of -700 to -800 MPa without inducing surface defects.
Engineering Practice and Quality Control
In engineering applications, the effectiveness of shot peening on overlay layers must be verified through:
- Residual stress measurement: XRD or hole-drilling method at multiple locations and depths.
- Surface roughness assessment: Peening increases surface roughness; post-peening grinding may be required for smooth surface applications.
- Microhardness profiling: Verification of surface hardening depth and magnitude.
- Visual and magnetic particle inspection: Detection of any peening-induced surface cracks.
- Coverage verification: Ensuring uniform peening across the entire overlay surface.
Concluding Remarks
This literature provides comprehensive data on the effects of shot peening on H08Mn2Si overlay layers, demonstrating that appropriate peening parameters can significantly improve the mechanical performance of weld overlay repairs. The synergistic combination of compressive residual stresses and surface hardening offers a practical and effective means of enhancing the fatigue and durability performance of overlay layers in demanding service environments. For military and industrial applications where overlay welding is used for component repair and surface protection, the integration of shot peening as a post-weld treatment step represents a straightforward yet highly effective improvement to overlay service life and reliability.