Projection Welded Nut Joint Hardness Control for Fracture Resistance
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Solution Overview
Problem
Existing projection welded joints between high strength steel sheets and nuts face challenges in achieving satisfactory peeling strength and delayed fracture resistance, with previous methods either improving peeling strength at the expense of delayed fracture resistance or vice versa.
Innovation Solution
A projection welded joint is formed by controlling the hardness distribution across the weld metal zone, ensuring a hardened region on both the steel sheet and nut sides extends continuously for at least 500 µm, with the nut side having higher maximum hardness in a martensite microstructure, and specific chemical compositions and welding conditions are used to enhance both peeling strength and delayed fracture resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If welding conditions are controlled to improve peeling strength (as in PTL 1), then peeling strength increases, but delayed fracture resistance deteriorates
Solution Approach 1:
The patent applies local quality by creating distinct microstructure zones with different properties at specific locations. The heat-affected zone on the steel sheet side is engineered to have a fine martensite microstructure with specific hardness characteristics, while other zones have different microstructures. This localized control of microstructure and hardness distribution allows the weld joint to simultaneously achieve high peeling strength through the hardened zone and good delayed fracture resistance through the fine martensite structure that resists hydrogen embrittlement.
Solution Approach 2:
The patent employs parameter changes by precisely controlling chemical composition parameters (C: 0.15-0.35%, Si: 0.01-1.65%, Mn: 1.00-3.40%, Al: 0.01-0.10%, Ti: 0.005-0.050%, Nb: 0.005-0.050%, B: 0.0005-0.0050%) and welding process parameters to achieve the desired microstructure. By adjusting these parameters, the patent creates a fine martensite microstructure in the heat-affected zone that provides both the hardness needed for peeling strength and the microstructural refinement needed for delayed fracture resistance.
2Strength
If area of high hardness region is increased to improve peeling strength (as in PTL 2), then peeling strength increases, but delayed fracture resistance deteriorates
Solution Approach 1:
The patent creates a localized fine martensite microstructure specifically in the heat-affected zone on the steel sheet side, rather than uniformly increasing hardness throughout the weld joint. This localized microstructural control allows the high hardness region to provide peeling strength while the fine martensite structure in this specific zone resists hydrogen embrittlement, thereby maintaining delayed fracture resistance despite the presence of hardened regions.
Solution Approach 2:
The patent creates a composite microstructure within the weld joint, consisting of different microstructural zones including a fine martensite region in the heat-affected zone on the steel sheet side, and other microstructures in different zones. This composite microstructure allows different regions to fulfill different functions: the fine martensite region provides both hardness for peeling strength and microstructural refinement for delayed fracture resistance, while other zones provide complementary properties.
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 solution achieves excellent peeling strength, as evidenced by a peeling strength of 9.0 kN or higher, and stable delayed fracture resistance, with the nut and steel sheet remaining intact after immersion in hydrochloric acid for 100 hours under load, demonstrating improved joint integrity.
Implementation Method 1
projection welding
Implementation Method 2
projection welding
Implementation Method 3
weld heat affected zone on the steel sheet side... formed of a fine martensite microstructure
Data Source
AI summary
A projection welded joint is formed by joining a steel sheet having a tensile strength of 980 MPa or higher and a nut by performing projection welding. In a hardness distribution from the steel sheet to the nut through a weld metal zone, and a hardened region exists on each of the steel sheet side and the nut side. A hardness of each hardened region is higher than that of the base metal on the corresponding side, and each hardened region extends continuously for a distance of 500 µm or more. The maximum hardness NHmax on the nut side is higher than the maximum hardness SHmax on the steel sheet side. A region where the maximum hardness SHmax on the steel sheet side exists is formed of a martensite microstructure having a prior austenite grain size of 20 µm or less.