Spot Welded Joint Hardness Control for HAZ Softening
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Solution Overview
Problem
High-strength steel sheets used in vehicle structures exhibit a softened region around spot welds due to heat-affected zones, which can lead to breaking under in-plane tensile stress, compromising collision resistance and shape accuracy.
Innovation Solution
A spot welded joint design that includes a hardness control region around the nugget, where the hardness difference is controlled to 80 HV or less, and tempering is applied to suppress breaking from the heat-affected zone softened portion, ensuring the maximum hardness is lower than the base metal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If resistance spot welding is performed on high-strength steel sheets with quenched structure, then welding joint is formed, but HAZ softened portion is created causing breaking under in-plane tensile stress
Solution Approach 1:
The invention applies tempering treatment to the HAZ softened portion to change its physical properties. By controlling the tempering parameters (temperature, time), the hardness of the HAZ region is adjusted to reduce the hardness difference with the base metal, thereby preventing strain concentration and breaking under in-plane tensile stress while maintaining the welding joint strength.
Solution Approach 2:
The invention converts the harmful effect of the HAZ softened portion (which causes breaking) into a beneficial outcome by applying tempering treatment. The tempering process transforms the soft, brittle HAZ region into a more ductile and resilient zone that can accommodate stress without breaking, thus turning the weakness into a stress-relief mechanism that enhances overall joint reliability.
2Strength
If high-strengthening of base metal is performed to improve collision resistance, then material strength increases, but HAZ softened portion still causes breaking
Solution Approach 1:
The invention applies local tempering treatment specifically to the HAZ softened portion rather than the entire base metal. This localized approach maintains the high strength of the base metal while selectively modifying only the vulnerable HAZ region to prevent breaking, thus achieving both high collision resistance and reliability without compromising overall material strength.
3Strength
If number of welding points is increased to improve deformation resistance, then collision energy absorption increases, but HAZ softened portion breaking risk increases
Solution Approach 1:
The invention applies tempering treatment to the HAZ softened portion before the component undergoes collision or service loading. This preliminary action pre-conditioned the vulnerable HAZ regions, reducing their hardness and preventing them from becoming breaking points when the component is subjected to collision forces, thus enabling the use of multiple welding points for improved deformation resistance without compromising reliability.
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 solution effectively prevents breaking from the softened region, enhancing collision resistance and maintaining shape accuracy, thereby improving the structural integrity of vehicle components.
Implementation Method 1
When resistance spot welding is performed, a nugget that welds steel sheets is formed, and a heat affected zone (hereinafter referred to as HAZ) is generated in the periphery of the nugget
Implementation Method 2
the quenched structure of the base metal is tempered by heat from resistance spot welding
Data Source
AI summary
This spot welded joint includes a first steel sheet having tensile strength of 1100 MPa or higher and hard martensitic structure as main structure, a second steel sheet stacked on the first steel sheet, a nugget having diameter D at an interface between the first and second steel sheets and formed between the first and second steel sheets, and a hardness control region occupying, in a cross section of the first steel sheet in sheet thickness direction that passes a nugget center, a region that is the first steel sheet in sheet thickness direction and is from 0.5×D to 1.0×D from the nugget center in sheet surface direction and in which difference between maximum value and minimum value in hardness in the region is 80 HV or less, and the maximum hardness value in the region is lower than the maximum hardness value of the first steel sheet.