Oxide-Coated High-Strength Bolt for Stable Fastening Axial Force
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Solution Overview
Problem
High-strength bolts, particularly those made of high-carbon steel with silicon, face challenges in maintaining consistent fastening axial force due to varying friction levels and surface treatments that can lead to delayed fracture and reduced fatigue strength.
Innovation Solution
A high-carbon steel bolt with a composition of 0.50-0.65% C, 1.5-2.5% Si, 1.0-2.0% Cr, 0.2-1.0% Mn, and 1.5-5.0% Mo, coated with an iron-based oxide film of 3-20 μm thickness, formed through heat treatment to prevent defects and ensure reliable fastening axial force.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high-carbon steel with silicon is used to increase tensile strength and delayed fracture resistance, then strength and delayed fracture resistance are improved, but fastening axial force varies greatly with tightening torque due to friction variations
Solution Approach 1:
The invention changes the surface friction parameter by forming an iron-based oxide film with controlled thickness (3-20 μm) and composition (Fe, FeO, Fe3O4, Fe2O3). This modifies the surface properties to reduce friction coefficient and its variation, making fastening axial force less sensitive to tightening torque variations while maintaining the high strength properties of the silicon-containing steel material.
Solution Approach 2:
The iron-based oxide film acts as an intermediary layer between the high-carbon steel bolt surface and the mating fastened member. This intermediate oxide layer mediates the friction interaction, providing a stable friction characteristic that reduces the direct contact friction variations between the high-strength steel surfaces, thereby stabilizing the fastening axial force.
2Reliability
If surface treatment such as chromium plating or phosphoric acid film is applied to prevent galling and stabilize fastening, then fastening properties are improved, but hydrogen invasion occurs causing delayed fracture and surface roughness decreases fatigue strength
Solution Approach 1:
The invention uses an iron-based oxide film that can be formed directly on the bolt surface through controlled oxidation, replacing complex multi-layer plating systems. This simpler oxide film provides sufficient friction control and protection without creating the hydrogen trapping issues associated with phosphoric acid films or chromium plating, effectively using a simpler, shorter-process solution.
Solution Approach 2:
The invention converts the naturally occurring oxide layer on the steel surface, which would normally be considered a surface defect or contamination, into a beneficial functional layer. By controlling the oxide film thickness to 3-20 μm and composition, the naturally formed oxide that would otherwise be harmful is transformed into a protective layer that reduces friction and stabilizes fastening properties without inviting hydrogen invasion.
3Force
If tightening torque is increased to secure fastening axial force, then fastening axial force is improved, but friction loss increases and fastening axial force varies due to surface roughness and lubrication state
Solution Approach 1:
The invention changes the surface friction parameter by forming an iron-based oxide film with controlled thickness (3-20 μm) and composition (Fe, FeO, Fe3O4, Fe2O3). This modifies the surface properties to reduce friction coefficient and its variation, making fastening axial force less sensitive to tightening torque variations while maintaining the high strength properties of the silicon-containing steel material.
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 iron-based oxide film provides excellent delayed fracture resistance and consistent fastening axial force by minimizing friction and preventing film defects, enhancing both strength and reliability of the bolt.
Implementation Method 1
a friction coefficient of the bolt is changed and variation of a fastening axial force is generated
Implementation Method 2
The delayed fracture is a kind of environmental embrittlement that occurs by mutual interaction among material, environment, and stress. It is considered that the delayed fracture is due to embrittlement of material caused by hydrogen.
Data Source
AI summary
A bolt of the present invention has a composition comprising: 0.50 mass % or greater and 0.65 mass % or less of carbon (C), 1.5 mass % or greater and 2.5 mass % or less of silicon (Si), 1.0 mass % or greater and 2.0 mass % or less of chromium (Cr), 0.2 mass % or greater and 1.0 mass % or less of manganese (Mn), 1.5 mass % or greater and 5.0 mass % or less of molybdenum (Mo), wherein a total amount of phosphorous (P) and sulfur (S) as impurities is 0.03 mass % or less, the remaining is iron (Fe), and the bolt comprises an iron based oxide film with a film thickness of 3 μm or greater and 20 μm or less on the surface thereof. The bolt has excellent delayed fracture resistance and reliably provides a fastening axial force.