Lap Fillet Weld Microstructure Control for 950 MPa 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 (0° to 30°), and a controlled ratio of concave portions with ferrite grains over 10 µm in size, ensuring NA ≥ 20 and NB/NA ≤ 0.70, to reduce stress concentration and prevent hydrogen embrittlement.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high-strength welding wires are used to maintain weld metal strength when base steel sheet tensile strength exceeds 950 MPa, then the weld metal strength is improved, but hydrogen embrittlement cracks occur and manufacturing costs increase
Solution Approach 1:
The invention changes the chemical composition parameters of the welding wire, specifically controlling Ceq (0.35-0.55%), Ti (0.05-1.0%), and B (0.0003-0.03%) to achieve optimal weld metal properties that balance strength and hydrogen embrittlement resistance without using high-strength welding wires
Solution Approach 2:
The invention creates a composite microstructure in the weld metal consisting of acicular ferrite and bainite phases through controlled alloying elements (Ti and B), which provides both the required strength and improved resistance to hydrogen embrittlement cracks
2Strength
If the tensile strength of the base steel sheet is increased to 950 MPa or more to reduce vehicle weight and improve collision safety, then the static strength is improved, but the fatigue strength of the welded joint is significantly reduced
Solution Approach 1:
The invention changes the chemical composition parameters of the welding wire, specifically controlling Ceq (0.35-0.55%), Ti (0.05-1.0%), and B (0.0003-0.03%) to achieve optimal weld metal properties that balance strength and hydrogen embrittlement resistance without using high-strength welding wires
Solution Approach 2:
The invention creates a composite microstructure in the weld metal consisting of acicular ferrite and bainite phases through controlled alloying elements (Ti and B), which provides both the required strength and improved resistance to hydrogen embrittlement cracks
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 enhances the fatigue strength of the lap fillet arc welded joint without using high-strength welding wires, preventing hydrogen embrittlement cracks and maintaining overall joint strength, while reducing manufacturing costs.
Implementation Method 1
stress concentration at the toe portion becomes very high when the shape of the toe portion of the weld metal is steep
Implementation Method 2
a Vickers hardness of the weld metal is 400 HV or less
Implementation Method 3
the fatigue strength of the lap fillet arc welded joint is lower than the fatigue strength of the steel sheet which is a base material of this joint
Implementation Method 4
it is possible to suppress stresses from concentrating at the toe portion when the shape of the toe portion of the weld metal is gentle
Data Source
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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° NB/NA≤0.70 (Here, NA is 20 or more.)