Blind Fastener Heat Treatment for Hardness and Elongation
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing blind fasteners, such as blind nuts and rivets, face challenges in achieving high surface hardness and sufficient elongation, leading to potential cracking during expansion and deformation, as existing surface hardening treatments like nitriding and austempering do not adequately balance hardness and ductility.
Innovation Solution
A heat treatment method involving carburization and austempering of cold-rolled low carbon steel blind fasteners, where the carbon concentration is increased at the surface to 0.3-0.5% through sustained heating and isothermal maintenance, hardening the surface while maintaining inner softness for elongation, is employed.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If nitriding treatment is performed to harden the surface, then surface hardness is improved, but elongation cannot be sufficiently increased and cracking occurs during deformation
Solution Approach 1:
The patent applies differential heat treatment to create distinct microstructures in different regions of the sleeve. The surface layer undergoes carburization followed by quenching to form hard martensite (HRC 58-65), while the base material is austempered to form ductile bainite. This local quality differentiation allows the surface to provide hardness and wear resistance while the core provides elongation and crack resistance.
Solution Approach 2:
The patent creates a composite microstructure within the single steel component. By combining carburization (adding carbon to surface) with austempering (isothermal transformation), the sleeve develops a composite structure of martensite surface layer over bainitic core, achieving properties of both hard and ductile materials in one component.
2Reliability
If austempering is performed to increase elongation, then ductility is improved, but surface hardness cannot be sufficiently increased
Solution Approach 1:
The patent performs carburization as a preliminary action before austempering. By pre-enriching the surface with carbon through carburization, the subsequent austempering process creates a carbon-enriched bainitic structure at the surface that achieves both high elongation and adequate hardness, while the core material maintains its ductility.
3Strength
If the sleeve is made hard to obtain high fastening strength, then fastening strength is improved, but the sleeve may crack when expanded and deformed
Solution Approach 1:
The patent creates a gradient microstructure where the surface layer has high hardness (HRC 58-65 martensite) for fastening strength and bite into the substrate, while the core material has high ductility (bainite structure) to accommodate expansion and deformation without cracking. This local quality differentiation resolves the contradiction between hardness for strength and ductility for crack resistance.
4Strength
If carbon content is increased to improve hardness, then surface hardness is improved, but elongation is reduced
Solution Approach 1:
The patent applies carbon enrichment locally to the surface through carburization, achieving high carbon content (0.3-0.5% C) only in the surface layer for hardness, while the core material maintains low carbon content for ductility and elongation. This spatial differentiation of carbon content resolves the contradiction between hardness and elongation.
Solution Approach 2:
The patent creates a composite material structure with a carbon-enriched surface layer (high carbon for hardness) over a low-carbon core (low carbon for elongation). This composite structure allows the component to exhibit both high surface hardness and sufficient overall elongation.
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 method effectively enhances the surface hardness of blind fasteners to prevent cracking and ensure strong fastening while maintaining suitable elongation, allowing for secure attachment without buckling or cracking.
Implementation Method 1
the carbon concentration at the surface of the hollow sleeve is increased by performing carburization to a depth of approximately 0.025 mm during the sustained heating
Implementation Method 2
subsequent quenching and isothermal maintenance of the sleeve in an austempering process
Implementation Method 3
subsequent quenching and isothermal maintenance of the sleeve in an austempering process, wherein the carbon percentage (CP) value of the carburized depth of the sleeve is from 0.3 to 0.5%
Data Source
Figure 1~2
Figure 3~4(c)
Figure 5
AI summary
The method for the heat treatment of a blind fastener (1) having a hollow sleeve (3) includes austempering. Austempering includes a step in which the hollow sleeve (3) of cold-rolled low carbon steel with a carbon percentage of 0.08 to 0.13% is heated and maintained from 800°C to 950°C during carburization, and a subsequent step in which isothermal maintenance is performed from 320°C to 500°C. The resulting carbon percentage (CP) value is from 0.3 to 0.5%. Even in the case of a low carbon steel blind fastener, austempering can harden the surface of the sleeve and soften the inner portion other than the surface for suitable deformability in order to provide a blind fastener having a sleeve with high surface hardness and sufficient elongation.