Resistance Spot Weld Nugget Tempering for Higher Cross Tension Strength
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Solution Overview
Problem
Resistance spot welding of high strength steel sheets with tensile strength of 780 MPa or more faces a reduction in cross tension strength (CTS), leading to brittle fracture and interface failure due to hardening and stress concentration at the edge of the nugget, which existing techniques fail to adequately address.
Innovation Solution
The solution involves forming a resistance spot welded joint with a specific chemical composition of the high strength steel sheets and a controlled microstructure in the edge portion of the nugget, where the hardness is reduced to improve toughness, and a post-tempering heat treatment process to ensure the nugget has a tempered martensite microstructure and sufficient carbide particle density, thereby preventing brittle fracture.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the edge portion of the nugget is hardened by quenching, then the strength of the nugget increases, but brittle fracture occurs leading to interface failure
Solution Approach 1:
The invention changes the physical parameters of the edge portion by controlling the average grain size to be 5 μm or less and specifying the microstructure type (tempered martensite, bainite, or ferrite). These parameter changes reduce the hardness and increase the toughness of the edge portion, preventing brittle fracture while maintaining adequate strength
Solution Approach 2:
The invention creates local quality differentiation between the edge portion and center portion of the nugget. The edge portion has a refined microstructure with average grain size of 5 μm or less, while the center portion has a different microstructure. This local differentiation prevents stress concentration at the edge, eliminating brittle fracture while preserving overall nugget 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
This approach enhances the cross tension strength of the weld by improving the toughness of the edge portion of the nugget, shifting the failure mode from brittle to ductile, thereby increasing the joint strength and preventing interface failure.
Implementation Method 1
resistance spot welding is mainly used to join components
Implementation Method 2
a melted and solidified portion (nugget) in a joint portion and the heat-affected zone that are formed when resistance welding steel sheets having a specific chemical composition are resistance-welded have a microstructure including tempered martensite or tempered bainite as a main phase
Data Source
AI summary
A resistance spot welded joint is formed by resistance-spot-welding two or more steel sheets including at least one high strength steel sheet. The high strength steel sheet has a chemical composition containing, in mass %, C: 0.05 to 0.6%, Si: 0.1 to 2.0%, Mn: 1.5 to 4.0%, and P: 0.10% or less, with the balance being Fe and incidental impurities. The hardness of a softest nugget region is 90% or less of the hardness of the heat-affected zone. The softest nugget region has a microstructure including tempered martensite. The average number density of carbide particles having a diameter of 100 nm or more in the softest nugget region is 10 or more per 5 μm2 in a cross section of the steel sheets.