Carburized Component Fatigue Strength via Shot Peening
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Solution Overview
Problem
Current methods for enhancing fatigue strength in high-strength steel carburized components, particularly in the low to medium cycle region, face challenges such as embrittlement and increased manufacturing costs, and fail to effectively address the initiation of fatigue cracks due to surface roughness and residual stress distribution.
Innovation Solution
Optimizing the carbon concentration distribution in the hardened layer and implementing a two-stage shot peening treatment with specific media hardness and particle sizes to achieve compressive residual stress and reduce surface roughness, thereby improving toughness and fatigue strength.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If carburizing and quenching is performed to achieve high hardness, then surface hardness is improved, but embrittlement occurs due to high-carbon martensite structure
Solution Approach 1:
The patent applies parameter changes by controlling the carbon concentration in the surface layer to be 0.30-0.70% (optimal: 0.40-0.60%) rather than the conventional higher carbon content. This parameter optimization prevents the formation of excessive high-carbon martensite, thereby avoiding embrittlement while maintaining adequate surface hardness. The controlled carbon concentration ensures the martensite structure remains strong but not overly brittle.
Solution Approach 2:
The patent implements local quality by creating a gradient structure where the surface layer has optimized carbon concentration (0.30-0.70%) different from the core material. This local optimization allows the surface to have the desired hardness while the overall component maintains better toughness. The shot peening treatment further enhances this by creating compressive residual stress locally at the surface without affecting the core properties.
2Reliability
If shot peening treatment is applied to provide compressive residual stress, then fatigue strength is improved, but surface roughness increases leading to crack initiation
Solution Approach 1:
The patent applies periodic action through two-stage shot peening treatment. The first stage uses larger shot particles (0.6-1.0mm) to create initial compressive residual stress and surface work hardening. The second stage uses smaller shot particles (0.1-0.5mm) to smooth the surface and reduce roughness. This sequential, periodic application of different shot sizes achieves both compressive stress and surface smoothing, resolving the contradiction between fatigue strength improvement and surface roughness control.
3Strength
If carbon concentration in surface layer is increased to improve hardness, then surface strength is improved, but low cycle fatigue strength deteriorates due to brittle fracture
Solution Approach 1:
The patent optimizes the carbon concentration parameter to a specific range (0.30-0.70%, optimal: 0.40-0.60%) that balances surface strength and low cycle fatigue resistance. This parameter optimization prevents the surface layer from becoming too brittle while maintaining adequate hardness. The controlled carbon level ensures the material can withstand impulsive loading without brittle fracture, improving low cycle fatigue strength while preserving surface strength.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The approach significantly enhances fatigue strength in the low to medium cycle region by suppressing brittle fractures and ensuring compressive residual stress distribution, making the components suitable for impulsive loading conditions without increasing manufacturing costs.
Implementation Method 1
a method of performing a shot peening treatment after a surface hardening treatment such as carburizing and quenching to provide compressive residual stress on the component's surface
Implementation Method 2
In order to enhance the fatigue strength of components, generally, 'carburizing and quenching' is often used as a surface hardening treatment
Implementation Method 3
the carbon concentration of the hardened portion of the surface reaches about 0.8% by mass%, and the micro structure after quenching becomes a high-carbon martensite structure
Data Source
Figure 1
AI summary
A carburized component has improved fatigue strength in a "low to medium cycle region", wherein base steel is a steel having a chemical composition containing, by mass%, C: 0.15-0.25%, Si: 0.03-0.50%, Mn: more than 0.60% and not more than 1.5%, P≤0.015%, S: 0.006-0.030%, Cr: 0.05-2.0%, Al≤0.10%, N≤0.03%, and O≤0.0020%, and optionally at least one element selected from Mo, Cu, Ni, B, Ti, Nb and V, the balance being Fe and impurities, wherein a surface hardened layer portion satisfies following conditions of (a) an average carbon concentration in the region from the outermost surface to a point of 0.2 mm depth: by mass%, 0.35-0.60%, (b) surface roughness Rz≤15 µm, and (c) σr(0) ≤-800 MPa, σr(100) ≤-800MPa, and residual stress intensity index Ir≥80000. The residual stress intensity index Ir is calculated by [Ir = ∫|σr(y)|dy], where y µm is the depth from the outermost surface and σr(y) is the residual stress for the points from the outermost surface to a depth of 100 µm. Here, the integration interval, that is, the range of y is 0 to 100 (µm).