Spot Welded Joint Microstructure for High Cross Tensile Strength
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Solution Overview
Problem
Spot welded joints formed with high strength steel sheets often suffer from insufficient toughness and achieving high cross tensile strength is challenging, especially when incorporating steel sheets with tensile strengths above 750 MPa.
Innovation Solution
The solution involves controlling the structure of both the nugget and the heat affected zone by maintaining the assembly at the Ms point or higher after main welding, and heating the heat affected zone within a specific temperature range (Ar3 point to Ar3 point + 400°C) for a controlled duration to suppress low load fractures and enhance martensite block widths, thereby improving cross tensile strength.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If temper current is supplied after main current to anneal the bonded zone and reduce hardness, then the toughness of the weld zone improves, but the nugget is softened and shear strength decreases
Solution Approach 1:
The patent extracts the harmful effect of excessive tempering by eliminating the need for separate temper current supply or post-weld heat treatment. Instead, it achieves the desired heat affected zone structure (lath martensite with controlled block width) through controlled main welding parameters and steel sheet composition (carbon equivalent control), thereby avoiding nugget softening while still achieving improved toughness
Solution Approach 2:
The patent applies preliminary action by pre-controlling the carbon equivalent of the steel sheet (0.20≤Ceq<0.55) before welding, which enables the heat affected zone to form the desired lath martensite structure during normal welding without requiring subsequent tempering. This preliminary composition control prevents the need for post-weld heat treatment that would soften the nugget
2Reliability
If separate heating means is used to temper the weld zone after welding, then the toughness of the heat affected zone improves, but the work procedure becomes complicated and special equipment is required
Solution Approach 1:
The patent merges the heat treatment function into the main welding process itself. By controlling the steel sheet carbon equivalent (0.20≤Ceq<0.55) and welding parameters, the main welding current simultaneously creates the nugget and produces the desired lath martensite structure in the heat affected zone. This eliminates the need for separate tempering equipment and procedures, resolving the contradiction between improved toughness and reduced complexity
Solution Approach 2:
The patent applies self-service by enabling the steel sheet composition (controlled carbon equivalent) to self-determine the heat affected zone microstructure during welding. The controlled composition causes the heat affected zone to automatically form the desired lath martensite structure with appropriate block width during the welding process itself, without requiring external tempering intervention
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 effectively enhances the cross tensile strength of spot welded joints containing high strength steel sheets by optimizing the martensite structure within the heat affected zone, leading to improved joint reliability and strength.
Implementation Method 1
spot welding a plurality of steel sheets
Implementation Method 2
the martensite transformation has to be made to substantially end before supplying the temper current
Data Source
Figure 1
Figure 2
Figure 3A
AI summary
A spot welded joint in which a high CTS is obtained even if including one or more high strength steel sheets and a method of welding of the same, that is, a spot welded joint 1 including a plurality of steel sheets, wherein one or more steel sheets is a tensile strength 750 to 2500 MPa high strength steel sheet and, at a cross-section in the sheet thickness direction of the steel sheets, when defining superposed surfaces of a high strength steel sheet S1 arranged at an outermost side and another steel sheet 1B as the "plane A" and defining a plane passing through a point of one-half of a distance between a nugget end position E of a high strength steel sheet S1 side on the line L1 in the sheet thickness direction and a crossing point O of the plane A and the line L1 and parallel to the plane A as the "plane B", at a square area SA of sides of 30 µm centered about a crossing point X of a line L2, which is separated by 250 µm to the heat affected zone side from a tangent at any position on a nugget end line NEL sandwiched between the plane A and plane B and parallel to that tangent, and a line L3 vertical to L2 included in a heat affected zone 4 having a high strength steel sheet S1 as a base material, an average value of widths of the blocks formed from lath martensite is 0.5 to 7.0 µn.