Projection Welded Joint Hardness Control for Peeling Strength
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Solution Overview
Problem
Conventional projection welding methods for high strength steel sheets face issues with decreased peeling strength and increased susceptibility to delayed fracture due to embrittlement and hydrogen absorption, particularly when anti-rust coatings are present.
Innovation Solution
A projection welded joint is achieved by controlling the chemical composition of the steel sheet and nut, with specific hardness ratios and a multi-step current application process to soften the joint interface, reducing stress concentration and residual stress.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high strength steel sheets with higher carbon content are used, then the strength of the steel sheet is improved, but the joint becomes more prone to embrittlement and peeling strength decreases
Solution Approach 1:
The patent applies parameter changes by controlling the carbon content within a specific range (0.15-0.40%) rather than using high carbon content, and by adjusting the hardness ratio parameter (Hvc/Hvm) to 0.75 or less through multi-step current application. This resolves the contradiction by finding an optimal parameter range that maintains steel sheet strength while preventing joint embrittlement and ensuring sufficient peeling strength.
2Strength
If high strength steel sheets are used, then the strength of the steel sheet is improved, but the susceptibility to hydrogen embrittlement and delayed fracture increases
Solution Approach 1:
The patent changes parameters by controlling carbon content (0.15-0.40%) and applying multi-step current with specific hardness ratios to reduce joint hardness. This softening effect reduces hydrogen embrittlement susceptibility and delayed fracture while maintaining the high strength properties of the steel sheet through controlled composition and processing.
3Ease of manufacture
If conventional projection welding is applied to high strength steel sheets, then the welding process is simple, but peeling strength decreases and delayed fracture occurs
Solution Approach 1:
The patent segments the welding process into multiple steps with different current values and durations, creating distinct heating and softening phases. This multi-step approach improves joint reliability by controlling hardness distribution while maintaining reasonable manufacturing simplicity through automated process control.
Solution Approach 2:
The patent employs periodic action through alternating current application and interruption cycles, where current is applied in specific patterns (first current value for first time, second current value for second time) to achieve controlled softening. This periodic current application resolves the contradiction by improving joint strength through controlled thermal cycles while keeping the process manageable.
4Reliability
If the joint interface is softened to improve peeling strength, then the toughness of the joint is improved, but the strength of the joint may be reduced
Solution Approach 1:
The patent applies local quality by creating a specific hardness distribution where the joint interface region (region c) has controlled hardness (Hvc/Hvm ≤ 0.75) to improve peeling strength and toughness, while the base metal maintains its original high strength properties. This localized hardness control resolves the contradiction by softening only where needed for peeling resistance while preserving overall joint 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 method enhances peeling strength and delays fracture resistance by ensuring the joint interface is sufficiently softened, improving toughness and reducing stress concentration.
Implementation Method 1
a non-current application step in which a non-current period having a non-welding time tc1 (ms) satisfying Equation (2) below is provided, a first subsequent-current application step in which current is applied at a current value I2 (kA) satisfying Equation (3) below for a welding time t2 (ms) satisfying Equation (4) below
Implementation Method 2
projection welding has a main current application step in which current is applied at a current value I1 (kA) for a welding time t1 (ms) to form a joint between the steel sheet and the nut
Data Source
Figure 1
Figure 2
Figure 3A~3B
AI summary
Provided are a projection welded joint with improved peeling strength and delayed fracture resistance, and a method for producing the same. The projection welded joint of this disclosure has a steel sheet and a nut, in which the steel sheet has a predetermined chemical composition, and the minimum hardness Hvc (HV) of region c and the base metal hardness Hvm (HV) of the steel sheet satisfy Hvc/Hvm ≤ 0.85. Region c includes a center axis of the nut of the projection welded joint and is defined below based on a cross-section passing through a center of a joint: with an end point on an inner peripheral side of the nut in contact with the steel sheet being defined as point A, an end point on an outer peripheral side of the nut in contact with the steel sheet being defined as point B, a sheet thickness of the steel sheet at the cross section being defined as t (mm), a point located inside the steel sheet at a depth of 0.20 mm in the sheet thickness direction from point A being defined as point A1, a point located inside the steel sheet at a depth of 2/3t in the sheet thickness direction from point A being defined as point A2, a point located inside the steel sheet at a depth of 0.20 mm in the sheet thickness direction from point B being defined as point B1, and a point located inside the steel sheet at a depth of 2/3t in the sheet thickness direction from point B being defined as point B2, an internal region of a rectangle formed by connecting points A1, B1, B2, and A2 is defined as region c.