Projection Welded Joint Hardness Gradient for Peeling Strength
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Solution Overview
Problem
Conventional projection welding methods for high strength steel sheets face challenges in maintaining sufficient peeling strength due to embrittlement, particularly at the joint between the steel sheet and nut, which is exacerbated by the increased hardness and carbon content in these materials.
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, specifically region b, thereby reducing stress concentration and residual stress.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high strength steel sheets with increased carbon content are used, then the tensile 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 local quality by creating a specific hardness ratio relationship between region a (joint center) and region b (boundary region). The hardness of region a is controlled to be 0.95 times or less the hardness of region b, making the boundary region harder and more resistant to embrittlement while maintaining overall joint strength. This local differentiation of hardness properties resolves the contradiction between high tensile strength and sufficient peeling strength.
Solution Approach 2:
The patent employs parameter changes by controlling the hardness values of different regions through specific welding parameters and heat treatment conditions. The maximum hardness in region a is limited to 550 HV or less, while region b maintains a higher hardness, creating a controlled gradient that prevents embrittlement propagation. This parameter control enables high strength steel sheets to achieve both high tensile strength and adequate peeling strength.
2Strength
If the hardness of the joint interface is increased to improve strength, then the tensile strength is improved, but stress concentration increases and peeling strength decreases
Solution Approach 1:
The patent applies local quality by creating a specific hardness ratio relationship between region a (joint center) and region b (boundary region). The hardness of region a is controlled to be 0.95 times or less the hardness of region b, making the boundary region harder and more resistant to embrittlement while maintaining overall joint strength. This local differentiation of hardness properties resolves the contradiction between high tensile strength and sufficient peeling strength.
Solution Approach 2:
The patent employs beforehand cushioning by creating a harder boundary region (region b) that acts as a cushion against stress concentration and embrittlement propagation. The increased hardness in region b, compared to region a, provides a protective effect that prevents stress concentration from compromising the joint's peeling strength, thus resolving the contradiction between joint strength and stress concentration.
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 the peeling strength of the welded joint by alleviating stress concentration and improving toughness, even in high strength steel sheets with tensile strengths of 980 MPa or higher.
Implementation Method 1
projection welding a steel sheet and a nut having a projection to each other via the projection
Implementation Method 2
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, 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, and a second subsequent-current application step in which current is applied at a current value I3 (kA) satisfying Equation (5) below for a welding time t3 (ms) satisfying Equation (6) below
Data Source
Figure 1
Figure 2
Figure 3A~3B
AI summary
Provided are a projection welded joint with improved peeling strength, 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 Hvb (HV) of region b and the base metal hardness Hvm (HV) of the steel sheet satisfy Hvb/Hvm ≤ 0.90. Region b 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 between the steel sheet and the nut: 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 point located inside the steel sheet at a depth of 0.50 mm in the sheet thickness direction from point B being defined as point C, and a point located inside the steel sheet at a depth of 0.50 mm in the sheet thickness direction from point A being defined as point D, an internal region of a rectangle formed by connecting points A, B, C, and D is defined as region b.