Lap Fillet Weld Toe Geometry for High-Strength Steel Fatigue
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Solution Overview
Problem
The fatigue strength of lap fillet arc welded joints is significantly reduced when the tensile strength of the base steel sheet exceeds 950 MPa, primarily due to stress concentration on soft ferrite grains, leading to early fatigue cracks, and using high-strength welding wires can cause hydrogen embrittlement cracks, increasing manufacturing costs.
Innovation Solution
A lap fillet arc welded joint with a weld metal that has a Vickers hardness of 400 HV or less, a gentle toe angle between 0° and 30°, and a controlled ratio of concave portions with ferrite grains larger than 10 μm, which suppresses stress concentration and reduces the occurrence of hydrogen embrittlement cracks.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high-strength welding wire is used to maintain weld metal strength when base steel tensile strength exceeds 950 MPa, then weld metal strength is improved, but hydrogen embrittlement cracks occur and manufacturing cost increases
Solution Approach 1:
The invention changes the chemical composition parameters of the welding wire, specifically controlling Ceq to 0.35% or less and adding boron (0.0005-0.05%) to achieve the desired hardness and strength properties without hydrogen embrittlement. This parameter optimization allows using lower strength welding wire while maintaining joint strength.
Solution Approach 2:
The invention creates a composite microstructure in the weld metal consisting of fine-grained ferrite and martensite by controlling alloying elements (B, Ti, Nb). This composite microstructure provides both the required strength and hardness while avoiding hydrogen embrittlement, resolving the contradiction between strength and reliability.
2Strength
If high-strength welding wire is used to maintain weld metal strength, then weld metal strength is improved, but manufacturing cost increases
Solution Approach 1:
The invention optimizes welding wire composition parameters (Ceq≤0.35%, adding B, Ti, Nb) to achieve high strength with lower alloy content, reducing material cost. The controlled composition allows using less expensive welding wire while maintaining required mechanical properties.
3Strength
If weld metal hardness is increased to prevent soft ferrite grain stress concentration, then fatigue strength is improved, but hydrogen embrittlement resistance deteriorates
Solution Approach 1:
The invention precisely controls weld metal hardness to 330-450 HV through Ceq control (≤0.35%) and specific alloying (B, Ti, Nb), achieving the optimal balance between fatigue strength improvement and hydrogen embrittlement prevention.
Solution Approach 2:
The invention creates a composite microstructure of fine-grained ferrite and martensite that provides enhanced fatigue strength through grain refinement and phase distribution, while the controlled hardness level prevents hydrogen embrittlement, resolving the contradiction between fatigue strength and reliability.
Data Source
AI summary
A lap fillet arc welded joint includes: a first steel sheet and a second steel sheet which are overlapped each other, the first steel sheet and the second steel sheet each having a tensile strength of 950 MPa or more; and a weld metal which extends along a corner formed by an upper surface of the first steel sheet and an end surface of the second steel sheet. When: a toe angle of the weld metal is defined as β; the total number of concave portions present on the surface of the weld metal included a range of 0.4 mm or less from a fusion boundary is defined as NA; and the number of concave portions in contact with ferrite grains having a maximum grain size of 10 μm or more is defined as NB, the weld metal satisfies the following conditional expressions (1) and (2) at the same time.0°<β<30° (1)NB/NA≤0.70 (2)(Here, NA is 20 or more).