Resistance Spot Welding with Post-Tempering for Nugget Edge Toughness
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Solution Overview
Problem
Resistance spot welding of high-strength steel sheets with tensile strength of 780 MPa or more faces challenges in achieving stable joint strength due to embrittlement and segregation at the nugget edge, leading to decreased cross tension strength, particularly in medium Mn steel sheets.
Innovation Solution
A method involving a main current application step followed by a cooling process to solidify the nugget and transform austenite into martensite, and a subsequent tempering heat treatment step to reduce segregation and improve toughness, along with a post-segregation reduction heat treatment step to further enhance solidification and tempering effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If main current application is performed to form a nugget in high-strength steel sheets (tensile strength ≥780 MPa), then joint strength is improved, but embrittlement and segregation occur at the nugget edge leading to decreased cross tension strength
Solution Approach 1:
The welding process is segmented into multiple distinct phases: main current application to form the nugget, cooling phase to solidify and transform austenite to martensite, and post-tempering heat treatment phase to reduce embrittlement. This segmentation allows each phase to address specific requirements without compromising the others, resolving the contradiction between achieving strong joints and preventing nugget edge embrittlement
Solution Approach 2:
The welding process employs periodic action through cyclic heating and cooling phases. The main current application heats the material to form austenite, followed by cooling to transform to martensite, then subsequent heating during post-tempering to reduce embrittlement. This periodic thermal action enables the material to undergo controlled phase transformations that simultaneously achieve strength and toughness
2Productivity
If short-time cooling and postheating are performed as in conventional techniques, then productivity is improved, but complete martensitic transformation and tempered martensite structure cannot be ensured
Solution Approach 1:
The invention changes the thermal parameters of the welding process by extending the cooling time and implementing a controlled post-tempering heat treatment phase. Instead of short-time cooling followed by immediate postheating, the process uses sufficient cooling time to ensure complete martensitic transformation, then applies a lower temperature, longer duration heat treatment to achieve proper tempering. This parameter adjustment ensures microstructural precision while maintaining acceptable productivity
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 effectively improves cross tension strength by preventing embrittlement and segregation at the nugget edge, resulting in enhanced joint strength and stability, particularly in high-strength steel sheets like medium Mn steel.
Implementation Method 1
applying current while applying pressure... main current application step in which current application is performed
Implementation Method 2
cooling is performed for a cooling time... transform austenite into martensite
Implementation Method 3
post-tempering heat treatment step... tempering heat treatment step to reduce segregation and improve toughness
Data Source
Figure 1~3
AI summary
The present invention provides a resistance spot welding method. A resistance spot welding method according to the present invention in which a sheet combination of two or more overlapping steel sheets is sandwiched between a pair of electrodes and joined together by applying current while applying pressure, the method including a main current application step in which current application is performed with a current Iw (kA), and subsequently, a post-tempering heat treatment step in which after cooling is performed for a cooling time tct (ms) shown in formula (1) below, current application is performed with a current It (kA) shown in formula (2) below for a current application time tt (ms) shown in formula (3) below, wherein at least one steel sheet in the sheet combination has a composition containing 0.08 ≤ C ≤ 0.3 (% by mass), 0.1 ≤ Si ≤ 0.8 (% by mass), 2.5 ≤ Mn ≤ 10.0 (% by mass), and P ≤ 0.1 (% by mass), with the balance being Fe and unavoidable impurities: 800 ≤ tct ··· formula (1), 0.5 × Iw ≤ It ≤ Iw ··· formula (2), and 500 ≤ tt ··· formula (3).