Projection Weld Joint Tempering for Delayed Fracture Resistance
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Solution Overview
Problem
Existing methods for manufacturing projection weld joints using high strength steel sheets with tensile strengths of 1.5 GPa or more do not adequately address the issue of delayed fracture resistance.
Innovation Solution
A method involving first energizing a steel sheet and a steel member to form joint portions through projection welding, followed by a cooling step to quench the joint portions, and a second energizing step to temper the end region of each joint portion, thereby reducing the hardness of the dangerous part and enhancing delayed fracture resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If projection welding is performed on high strength steel sheets with tensile strength of 1.5 GPa or more, then the strength of the vehicle structural member is improved, but the delayed fracture resistance of the joint portion deteriorates due to high hardness and hydrogen concentration
Solution Approach 1:
The invention changes the physical and chemical parameters of the joint portion through controlled cooling and tempering processes. By adjusting cooling rate and tempering temperature, the hardness of the joint portion is reduced from high levels (that cause delayed fracture) to optimized levels (350-550 Hv), while maintaining the high strength properties of the steel sheet
Solution Approach 2:
The invention applies different thermal treatments to different regions of the joint portion. The end region receives tempering treatment to reduce hardness and eliminate hydrogen, while other regions maintain their strength properties. This creates local quality differences that optimize both strength and delayed fracture resistance at specific locations
2Strength
If the joint portion is cooled rapidly to increase strength, then the tensile strength is improved, but the delayed fracture resistance deteriorates due to increased hardness and residual stress
Solution Approach 1:
The invention performs tempering treatment on the end region of the joint portion before the joint is subjected to service loads. This preliminary thermal treatment reduces hardness and eliminates hydrogen in advance, preventing delayed fracture from occurring during subsequent use of the structural member
Solution Approach 2:
The invention applies periodic thermal treatment through controlled cooling followed by tempering. This multi-stage thermal process first creates the desired microstructure through cooling, then relieves harmful effects through tempering, achieving both strength and delayed fracture resistance
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 proposed method effectively improves the delayed fracture resistance property of projection weld joints in high strength steel sheets, ensuring the joints can withstand stress without sudden brittle fracture.
Implementation Method 1
these projections are energized to locally heat a contact interface between each projection and the steel sheet, thereby causing melting and solidification or solid phase joining
Implementation Method 2
a cooling step of quenching each of the plurality of joint portions
Implementation Method 3
a second energizing step of further energizing the steel sheet and the steel member so as to temper the end region in each of the plurality of joint portions
Data Source
AI summary
A method for manufacturing a projection weld joint according to an aspect of the present invention includes: a first energizing step of energizing a steel sheet and a steel member so as to form a plurality of joint portions by projection welding a steel member to a first surface of a steel sheet that is a non-coated steel sheet or a zinc-type-coated steel sheet, in which the steel sheet has a tensile strength of 1.5 GPa or more, and the steel sheet has a Ceq of 0.30 mass % or more, and the manufacturing method further includes: a cooling step of quenching each of the plurality of joint portions; and a second energizing step of further energizing the steel sheet and the steel member so as to temper an end region in each of the plurality of joint portions.