Fillet Welded Joint Hardness Balancing for Fatigue and Crack Resistance
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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 developed using a high-strength steel plate with a carbon equivalent between 0.36 and 0.60, and a welding wire with a carbon equivalent of 0.50 to 0.80, ensuring Vickers hardness relationships that balance weld metal and heat-affected zone hardness to enhance fatigue strength while preventing low temperature cracking.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Weight of moving object
If high strength steel plate with tensile strength of 980 MPa or more is used to reduce vehicle body weight, then weight reduction is achieved, but weld fatigue strength is significantly reduced due to hardness difference between heat-affected zone and weld metal
Solution Approach 1:
The invention changes the chemical composition parameters of the steel plate (carbon equivalent between 0.36-0.60, specific ranges of C, Si, Mn, Al, and other alloying elements) to control the hardness of the heat-affected zone and weld metal, ensuring their hardness difference does not exceed 50 HV, thereby maintaining weld fatigue strength while using high-strength steel plates for weight reduction
Solution Approach 2:
The invention applies local quality control by specifically managing the chemical composition and hardness characteristics in different zones of the welded joint (base material, heat-affected zone, and weld metal), ensuring that each zone has appropriate properties to prevent hardness differences that would lead to fatigue failure
2Strength
If high strength steel plate is used to increase member strength, then tensile strength is improved, but low temperature cracking occurs due to high hardness of heat-affected zone
Solution Approach 1:
The invention modifies the chemical composition parameters (carbon equivalent, alloying element contents) to control the hardenability and final hardness of the heat-affected zone, ensuring it remains below 350 HV to prevent low temperature cracking while maintaining the required tensile strength of 980 MPa or more
Solution Approach 2:
The invention converts the potential harm of high carbon equivalent (which could cause cracking) into a benefit by carefully selecting the range (0.36-0.60) and combining it with specific alloying element ratios, which enables achieving high tensile strength while controlling heat-affected zone hardness through optimized composition rather than just reducing carbon content
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 a high tensile strength ratio and preventing hydrogen embrittlement, thus ensuring excellent durability for vehicle chassis applications.
Implementation Method 1
a fillet welded joint obtained by overlapping a part of two sheets of base material and performing fillet welding
Implementation Method 2
an area of a ferrite micro-structure in a heat affected zone of the welded joint is from 20 to 100% of the area of the heat affected zone, and the balance comprises at least one of a bainite, a martensite, a pearlite and a residual austenite micro-structure
Data Source
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AI summary
The present invention relates to a fillet welded joint and a manufacturing method thereof, and addresses the problem of providing a fillet welded joint having excellent weld fatigue strength while suppressing low temperature cracking. In a fillet welded joint according to the present invention, a tensile strength of a base material 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%], an average carbon equivalent of weld metal 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.