Resistance Spot Welded Joint with Toughened Nugget Edge
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Resistance spot welding of high strength steel sheets with tensile strength of 780 MPa or more faces challenges in maintaining cross tension strength (CTS) and is prone to delayed fracture due to hardening of the nugget edge and introduction of hydrogen, with existing techniques failing to effectively control microstructure and temperature during welding.
Innovation Solution
A resistance spot welding method involving controlled temperature history during welding to form a duplex microstructure of ferrite and martensite in the nugget edge, combined with tempering processes to relax stress concentration and reduce hydrogen entry, ensuring a nugget edge region with at least 1% ferrite and specific hardness ratios.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the tensile strength of the base material is increased to 780 MPa or more, then the joint strength in shear direction increases, but the mode of failure changes from ductile fracture to brittle fracture
Solution Approach 1:
The patent creates a bimodal microstructure where the nugget center contains hard martensite for strength while the nugget edge contains tempered martensite for ductility. This local quality differentiation ensures that failure occurs in the tougher tempered region rather than propagating through the entire nugget, thereby maintaining reliability while achieving high tensile shear strength.
Solution Approach 2:
The patent applies preliminary action by performing a tempering process after the primary welding operation. This secondary heating treatment transforms the brittle as-quenched martensite into tempered martensite before the weld is subjected to service loads, thereby preventing brittle fracture and improving reliability in advance.
2Strength
If secondary energization is performed to improve microstructure, then the cross tension strength increases, but the temperature control during secondary energization is insufficient
Solution Approach 1:
The patent implements feedback control by monitoring the temperature history during secondary energization and adjusting the heating parameters accordingly. The tempering process is controlled to achieve a maximum temperature that ensures complete transformation to tempered martensite without excessive heating, thereby achieving precise temperature control that enables both improved CTS and reliable microstructure formation.
3Ease of manufacture
If hydrogen is introduced into the weld metal during welding, then the welding process becomes simpler, but delayed fracture occurs
Solution Approach 1:
The patent converts the harmful effect of hydrogen introduction into a beneficial outcome. By performing tempering after welding, the patent transforms the as-quenched martensite structure (which is susceptible to hydrogen embrittlement) into tempered martensite with improved toughness and reduced hydrogen sensitivity. This converts the initial vulnerability caused by hydrogen into a strengthened structure that resists delayed fracture.
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 method enhances joint strength and delayed fracture resistance by improving the toughness of the nugget edge and reducing stress concentration, thereby stabilizing the cross tension strength and preventing brittle fracture.
Implementation Method 1
resistance spot welding is mainly used to join components
Implementation Method 2
thermal shrinkage in a cooling process during welding causes tensile residual stress in martensite that is a hard microstructure
Implementation Method 3
a melted and solidified portion (nugget) in a joint portion and the heat-affected zone that are formed when resistance welding steel sheets having a specific chemical composition are resistance-welded have a microstructure including tempered martensite or tempered bainite as a main phase
Data Source
AI summary
A resistance spot welded joint is formed by resistance-spot-welding a plurality of steel sheets including at least one high strength steel sheet. The high strength steel sheet has a specific chemical composition, and the microstructure of a nugget edge region includes ferrite at an area fraction of 1% or more with respect to the total area of the nugget edge region. The hardness Hv of a softest portion of the nugget edge region and the hardness Hvm of a central portion of the nugget satisfy the relation 0.90×Hvm>Hv, and the hardness Hvh of a HAZ softened region and the hardness Hvm of the central portion of the nugget satisfy the relation 0.90×Hvm>Hvh.