Effect of Shot Peening on Residual Stress and Microstructure of H08Mn2Si Cladding 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, investigates the effect of shot peening on the residual stress state and microstructural characteristics of H08Mn2Si weld overlay layers. Funded by the National Science and Technology Support Program (2011BAF11B07; 2011BAC10B05), the research was published in China Surface Engineering. The work addresses a critical issue in military equipment remanufacturing: optimizing the post-weld treatment of cladding layers to enhance fatigue resistance and service life of armored vehicle components.

Core Technical Content

Research Background and Material Selection

H08Mn2Si is a low-carbon manganese-silicon welding wire widely used in structural welding and overlay applications in the defense industry. The Mn-Si composition provides good weldability, adequate strength, and resistance to cold cracking. In the context of armored vehicle component repair, H08Mn2Si overlay layers are applied to restore worn surfaces on tracks, road wheels, and structural components subjected to high-impact loading.

The fundamental challenge is that weld overlay layers inherently contain high tensile residual stresses (typically 200–400 MPa) due to differential thermal contraction between the overlay and the base material. These tensile stresses significantly reduce the fatigue life and susceptibility to stress corrosion cracking of the overlay layer. Shot peening is employed as a post-weld treatment to introduce beneficial compressive residual stresses.

Shot Peening Parameters Investigated

The researchers systematically varied shot peening parameters to determine their influence on the residual stress profile and microstructure:

Residual Stress Analysis

Residual stresses were measured using X-ray diffraction (XRD) with the sin²ψ method at multiple depths from the surface:

The results demonstrate that increasing shot diameter and coverage systematically increases both the magnitude and depth of compressive residual stresses. The optimal parameter combination for the H08Mn2Si overlay layer was identified as 0.5 mm shot diameter with 200% coverage, producing surface compressive stresses of -480 MPa extending to a depth of 100 μm.

Microstructural Effects

The shot peening process also induces microstructural changes in the near-surface region of the overlay layer:

• Work hardening: Surface hardness increased from 220 HV₀.₁ (as-welded) to 320–380 HV₀.₁ after peening, due to dislocation density increase and grain refinement.
• Grain refinement: The grain size in the peened surface layer reduced from 15–20 μm (as-welded) to 3–5 μm, creating a refined surface layer that enhances fatigue resistance.
• Dislocation density: Estimated to increase from approximately 10¹² m⁻² (as-welded) to 10¹⁵ m⁻² (peened), providing significant strain hardening.
• Phase stability: No adverse phase transformations were observed; the ferrite-pearlite microstructure of the H08Mn2Si overlay remained stable under the cold deformation induced by peening.

Fatigue Performance Improvement

The combined effect of compressive residual stresses and work hardening was quantified through rotating bending fatigue tests:

Engineering Practice Integration

The research has direct applications in the remanufacturing of armored vehicle components, where fatigue resistance is critical for operational reliability. The identified optimal peening parameters can be incorporated into standard repair procedures, ensuring consistent quality and performance.

The findings also provide guidance for process parameter selection in industrial shot peening operations:

1. Parameter selection hierarchy: Shot diameter has the greatest influence on stress depth; coverage affects stress magnitude; shot velocity provides fine-tuning of stress level.
2. Quality control: Almen strip testing should be used to calibrate peening intensity, targeting an arc height of 0.3–0.5 mm for the identified parameters.
3. Process sequencing: Shot peening should be applied after all machining operations to avoid removing the beneficial compressive stress layer.
4. Inspection requirements: Post-peening inspection should include surface roughness measurement (Ra ≤ 12.5 μm), residual stress verification by XRD on critical components, and hardness testing to confirm work hardening.

Study Insights and Implications

This research demonstrates the powerful synergy between weld overlay and shot peening as a combined surface engineering strategy. The H08Mn2Si overlay layer, while providing adequate wear resistance, benefits substantially from the compressive stress introduction and work hardening achieved through shot peening. The improvement in fatigue limit of 75–85% is particularly significant for military applications where component reliability under cyclic loading is paramount.

The study also highlights an important consideration: the depth of the compressive stress layer must be adequate relative to the expected crack initiation depth in service. For the H08Mn2Si overlay with 0.5 mm shot at 200% coverage, the 100 μm compression depth is sufficient for most fatigue crack initiation scenarios in armored vehicle applications, where surface cracks typically initiate from surface roughness features or micro-defects within the first 50–80 μm.

The systematic investigation of multiple parameters and the quantitative correlation between peening intensity, residual stress profile, microstructural evolution, and fatigue performance establishes a comprehensive technical framework that can be applied to other weld overlay systems requiring enhanced fatigue resistance. This framework should serve as a reference for process development in any application where weld cladding and post-weld surface treatment are combined to achieve superior performance in demanding service environments.