Projection Weld Joint Hardness Control
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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 welding process involving current application and non-current periods 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 implementing a multi-stage current application process with different current values and timing. A first current value is applied during welding, followed by a second current value (lower than the first) applied after a non-current period. This temporal and parametric variation in current application softens the joint interface, reducing embrittlement and improving peeling strength while maintaining the high strength properties of the steel sheet.
Solution Approach 2:
The patent employs periodic action through the multi-stage current application process. The welding process includes alternating periods of current application and non-current periods. Specifically, after the initial welding current is applied, a non-current period occurs, followed by a subsequent current application at a reduced value. This periodic alternation allows for controlled heating and cooling cycles that prevent excessive hardening and embrittlement of the joint interface.
2Strength
If high strength steel sheets are used, then the strength is improved, but the susceptibility to hydrogen embrittlement and delayed fracture increases
Solution Approach 1:
The patent uses parameter changes to mitigate hydrogen embrittlement by implementing a two-stage current application with different magnitudes. The first current value completes the welding, followed by a non-current period, then a second current value (lower than the first) is applied. This parametric variation controls the thermal history of the joint, preventing excessive hardening that would increase susceptibility to hydrogen embrittlement and delayed fracture.
Solution Approach 2:
The periodic alternation between current application and non-current periods creates controlled thermal cycles. The non-current period allows for cooling and stress relief, while the subsequent lower current value provides additional heating that tempers the joint without causing excessive hardening. This periodic action reduces residual stress and hardness in the heat-affected zone, thereby reducing susceptibility to hydrogen embrittlement and delayed fracture.
3Device complexity
If conventional projection welding is used, then the welding process is simple, but the joint interface remains hard and stress concentration occurs
Solution Approach 1:
The patent applies periodic action by implementing a multi-stage current process with alternating current application and non-current periods. After the initial welding current is applied, a non-current period occurs, followed by a subsequent current application at a reduced value. This periodic variation in current application softens the joint interface, improving toughness and reducing stress concentration while adding only moderate complexity to the welding process.
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 embrittlement.
Implementation Method 1
projection welded to each other via the projection
Implementation Method 2
projection welding
Implementation Method 3
current is applied at a current value I1 (kA) for a welding time t1 (ms) to form a joint
Implementation Method 4
current is applied at a current value I2 (kA) for a welding time t2 (ms)
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 has a steel sheet and a nut, in which the steel sheet has a predetermined chemical composition, and the minimum hardness Hva (HV) of region a and the base metal hardness Hvm (HV) of the steel sheet satisfies Hva/Hvm≤0.90. Region a is defined below based on a cross-section including a center axis of the nut and passing through a center of a joint between the steel sheet and the nut, of the projection welded joint: with an end point on an inner peripheral side of the nut in contact with the steel sheet being defined as point A, and an end point on an outer peripheral side of the nut in contact with the steel sheet being defined as point B, a length of a line segment connecting points A and B being defined as 2L, a point located on the line segment at a distance of L/2 from point B toward point A being defined as point C1, a point located on a line formed by a surface of the steel sheet continuous with the line segment, at a distance of L from point B, being defined as point C2, points located inside the steel sheet at a depth of 0.80 mm in a sheet thickness direction from points C2 and C1 being defined as points C3 and C4, respectively, an internal region of a rectangle formed by connecting points C1, C2, C3, and C4 is defined as region a.