Iron Oxide Film Bolt Structure for Delayed Fracture and Torque Stability
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Solution Overview
Problem
High-strength bolts, particularly those made of high-carbon steel with silicon, face issues with variable fastening axial force due to friction losses and are prone to delayed fracture, which is exacerbated by surface treatments intended to prevent galling and stabilize fastening properties.
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, 1.5-5.0% Mo, and <0.03% P+S, coated with an iron-based oxide film of 3-20 μm thickness, where Fe3O4 is predominant on the surface and Fe2SiO4 on the base material, reducing friction and enhancing affinity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a surface treatment such as chromium plating or phosphoric acid film is applied to high-carbon steel bolts, then delayed fracture resistance is improved, but friction loss increases and fastening axial force becomes difficult to manage
Solution Approach 1:
The invention changes the surface treatment parameters by specifying an iron-based oxide film with controlled thickness (3-20 μm) and composition (Fe3O4 and Fe2SiO4 in specific ratios), transforming the surface properties to achieve both low friction and high delayed fracture resistance
Solution Approach 2:
The surface film is designed as a composite structure containing multiple iron-based oxides (Fe3O4 and Fe2SiO4) in specific proportions, creating a multi-functional coating that simultaneously provides lubrication and corrosion resistance
2Strength
If tightening torque is increased to secure fastening axial force, then fastening strength is improved, but friction loss increases and fastening axial force becomes difficult to control
Solution Approach 1:
The invention converts the naturally formed iron-based oxide film, which would normally be considered a surface defect or scale, into a beneficial lubricating layer that reduces friction and enables precise fastening force control
3Strength
If high-carbon steel with silicon is used to improve tensile strength and delayed fracture resistance, then material strength is improved, but fastening axial force varies greatly with tightening torque
Solution Approach 1:
The invention applies a localized surface treatment (iron-based oxide film) specifically on the bolt surface, creating a region with different friction properties while maintaining the bulk material's high strength characteristics
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 consistent fastening axial force and excellent delayed fracture resistance by minimizing friction and preventing film defects, while maintaining high tensile strength and fatigue resistance.
Implementation Method 1
a fastening axial force that varies little with respect to tightening torque, thanks to a low friction coefficient
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.
Implementation Method 3
an iron based oxide film with a film thickness of 3 μm or greater and 20 μm or less on a surface thereof. The iron based oxide film contains Fe3O4 in a greater amount on a surface side of the bolt than on a bolt base material side of the bolt and contains Fe2SiO4 in a greater amount on the bolt base material side of the bolt than on the surface side of the bolt
Data Source
Figure 1
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.