Projection Welded Joint With Mn Gradient for Delayed Fracture Resistance
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Solution Overview
Problem
Existing projection welding techniques for high-strength coated steel sheets and nuts improve peeling strength but compromise delayed fracture resistance due to high hydrogen embrittlement sensitivity of the steel sheet microstructure at the interface.
Innovation Solution
A projection welded joint is formed by using a high-strength coated steel sheet with a softened surface layer and optimizing the welding conditions, including controlling the content of solid solution Mn, to create a nugget or strong joint interface in the central portion of the weld, reducing hydrogen embrittlement sensitivity at the interface.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If projection welding is performed to improve peeling strength, then peeling strength is improved, but delayed fracture resistance decreases due to high hydrogen embrittlement sensitivity
Solution Approach 1:
The weld interface is segmented into multiple regions (first region within 50μm from outermost periphery, second region at central position, third region within 50μm from innermost periphery, fourth region at central position in thickness direction). By controlling Mn content differently in each region, the patent creates a gradient structure that reduces hydrogen embrittlement sensitivity at the interface while maintaining peeling strength.
Solution Approach 2:
The patent applies local quality by making different parts of the steel sheet have different Mn content characteristics. Specifically, the surface layer in the first and third regions has lower solid solution Mn content (40% or less of the central region content) compared to the second and fourth regions. This local variation in composition reduces hydrogen embrittlement sensitivity at the weld interface where it is most critical.
2Strength
If welding conditions are optimized to improve peeling strength, then peeling strength is improved, but hydrogen embrittlement sensitivity increases
Solution Approach 1:
The patent changes the chemical composition parameter (Mn content) in specific regions of the steel sheet. By controlling the solid solution Mn content in the surface layer to be 40% or less of the Mn content in the central region (within 50μm from the interface), the patent reduces hydrogen embrittlement sensitivity while maintaining adequate peeling strength through the overall high-strength steel sheet properties.
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 results in a projection welded joint with enhanced peeling strength and delayed fracture resistance, effectively addressing the limitations of previous techniques.
Implementation Method 1
projection welding
Implementation Method 2
resistance welding
Data Source
AI summary
A welded joint formed by performing projection welding on a high-strength coated steel sheet and a nut, includes a content of solid solution Mn in a first region in a surface layer of the steel sheet on an outermost periphery of a weld is 40% or less of a content of solid solution Mn in a second region in a central region in a thickness direction of the steel sheet on the outermost periphery of the weld, and a content of solid solution Mn in a third region in the surface layer of the steel sheet on an innermost periphery of the weld is 40% or less of a content of solid solution Mn in a fourth region in the central region in the thickness direction of the steel sheet on the innermost periphery of the weld.