Fillet Welded Joint Hardness Gradient for Fatigue Strength
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Solution Overview
Problem
High-strength steel plates with tensile strength of 980 MPa or more pose challenges in achieving sufficient weld fatigue strength due to hardness differences between the weld metal and heat-affected zone, leading to potential low temperature cracking and reduced fatigue strength in fillet welded joints.
Innovation Solution
A fillet welded joint is achieved by using a high-strength steel plate with a carbon equivalent between 0.36 and 0.60 and a welding wire with a carbon equivalent between 0.50 and 0.80, ensuring specific Vickers hardness relationships between the weld metal and heat-affected zone to enhance fatigue strength while preventing low temperature cracking.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If welding wire with high carbon equivalent is used to increase weld metal hardness, then the fatigue strength is improved, but low temperature cracking occurs due to hydrogen embrittlement
Solution Approach 1:
The patent resolves this contradiction by optimizing the carbon equivalent parameters of both base material (0.36-0.60) and welding wire (0.50-0.80) within specific ranges, rather than simply increasing them. This balanced parameter control achieves sufficient weld metal hardness for fatigue strength while preventing excessive hardening that would cause hydrogen embrittlement and low temperature cracking.
Solution Approach 2:
The patent promotes homogeneity in the hardness distribution across the welded joint by establishing specific relationships between weld metal hardness, heat-affected zone hardness, and base material hardness. This homogeneous hardness distribution prevents stress concentration and reduces the risk of low temperature cracking while maintaining fatigue strength.
2Strength
If the hardness of weld metal is made higher than the heat-affected zone to improve fatigue strength, then the fatigue strength is improved, but the hardness difference causes stress concentration and reduces reliability
Solution Approach 1:
The patent applies local quality by creating a controlled hardness gradient across the welded joint zones. The weld metal hardness is made higher than the heat-affected zone to improve fatigue strength at the weld toe, while the heat-affected zone hardness is maintained higher than the base material to provide a gradual transition, thereby reducing stress concentration.
Solution Approach 2:
The patent resolves the stress concentration issue by precisely controlling the hardness parameters of each zone through carbon equivalent management. The specific carbon equivalent ranges and their ratios enable optimization of the hardness distribution, balancing fatigue strength improvement with stress concentration reduction.
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 approach results in a fillet welded joint with improved fatigue strength and suppressed low temperature cracking, maintaining the structural integrity of high-strength steel plates in vehicle chassis applications.
Implementation Method 1
performing fillet welding by using a welding wire
Implementation Method 2
the weld metal has an average carbon equivalent of 0.45 or more and 0.65 or less, and higher than the carbon equivalent of the base material
Implementation Method 3
the average value HVhaz of the Vickers hardness of the heat affected zone in a position of 0.1 mm or more and 0.3 mm or less from the boundary to the heat affected zone side satisfies HVhaz≤350
Data Source
AI summary
In a fillet welded joint a base material tensile strength is 980 MPa or more, a carbon equivalent is 0.36 or more and 0.60 or less, a tensile strength [MPa] is 1950 times or more of the carbon equivalent [wt %], a weld metal average carbon equivalent is 0.45 or more and 0.65 or less, and at a prescribed position below a surface of a weld toe, a Vickers hardness HVbond at a boundary between the weld metal and a heat affected zone, an average value HVwmt of the Vickers hardness of the weld metal in a position 0.1-mm or more and 0.3-mm or less to the weld metal side of the boundary, and an average value HVhaz of the Vickers hardness of the heat affected zone in a position 0.1-mm or more and 0.3-mm or less to the heat affected zone side of the boundary satisfy HVbond≤HVwmt, HVbond≥HVhaz-50, and HVhaz≤350.