High Strength Bolt Delayed Fracture Resistance
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Solution Overview
Problem
High strength bolts made from steel materials like chromium steel and chrome molybdenum steel face significant risks of hydrogen embrittlement and delayed fracture, limiting their tensile strength to below 1150 MPa due to inadequate resistance, and existing methods to improve this resistance either fail to provide substantial improvements or increase manufacturing costs.
Innovation Solution
A high strength bolt with a tensile strength of 1400 MPa or more is achieved by carefully controlling the chemical composition of the steel, including carbon, silicon, manganese, molybdenum, and other alloying elements, and through specific heat treatment processes such as quenching and tempering, along with thread rolling and surface microstructure optimization to enhance hydrogen trapping and crack propagation resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the tensile strength of high strength bolts is increased to 1400 MPa or higher, then the strength and weight reduction benefits are improved, but the risk of hydrogen embrittlement and delayed fracture increases significantly
Solution Approach 1:
The patent applies parameter changes by precisely controlling the chemical composition parameters (C: 0.20-0.35%, Si: 0.05-0.50%, Mn: 0.10-2.00%, Mo: 0.05-0.60%, B: 0.0005-0.0100%) and heat treatment parameters (quenching temperature 800-950°C, tempering temperature 150-500°C) to achieve a balance between high tensile strength (1400 MPa or higher) and delayed fracture resistance, resolving the contradiction through optimized parameter combinations
Solution Approach 2:
The patent creates a composite microstructure consisting of martensite as the primary phase with dispersed carbide precipitates, combining the high strength characteristics of martensite with the hydrogen trapping capability of carbides, thereby achieving both high tensile strength and improved delayed fracture resistance
2Reliability
If the microstructure is transformed to a bainite structure to improve delayed fracture resistance, then the delayed fracture resistance is improved, but the strength drops below the required 1400 MPa level
Solution Approach 1:
Instead of using bainite transformation to improve delayed fracture resistance (which reduces strength), the patent inverts the approach by using martensite transformation (which provides high strength) combined with carbide precipitation (which provides hydrogen trapping), thereby achieving improved delayed fracture resistance while maintaining tensile strength of 1400 MPa or higher
3Reliability
If conventional methods are used to improve delayed fracture resistance, then some improvement is achieved, but the manufacturing costs increase significantly
Solution Approach 1:
The patent optimizes the composition parameters within specific ranges (particularly using moderate amounts of Si: 0.05-0.50% and Mn: 0.10-2.00% as cost-effective alloying elements) and heat treatment parameters to achieve improved delayed fracture resistance without requiring expensive additional alloying or complex multi-step heat treatment processes, thereby controlling manufacturing costs
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 significantly improves hydrogen embrittlement resistance, enabling safe use of high strength bolts in automotive and civil engineering applications, reducing weight, increasing efficiency, and enhancing safety while maintaining economic viability.
Implementation Method 1
the steel materials exceed 1300 MPa in tensile strength, the risk of hydrogen embrittlement... are quenched and tempered
Implementation Method 2
are quenched and tempered
Implementation Method 3
diffuse and distribute single or composite precipitates of oxides, carbides, and nitrides into the steel for trapping the hydrogen
Implementation Method 4
transform the microstructure to a bainite structure
Data Source
AI summary
The present invention provides a high strength bolt excellent in delayed fracture resistance able to advantageously prevent hydrogen embrittlement as represented by the delayed fracture phenomenon occurring along with an increase in strength and causing a particular problem, and a method of production of the same, containing, by mass%, C: 0.2 to 0.6%, Si: 0.05 to 0.5%, Mn: 0.1 to 2%, Mo: 0.5 to 6%, and Al: 0.005 to 0.5%, having a tensile strength of 1400 MPa or more, and having a compressive residual stress of the surface layer of the thread root of 10 to 90% of the tensile strength. Further, a surface layer part of the thread root from the surface down to at least 50 µm has pre-austenite grains with an aspect ratio of the axial direction and radial direction of 2 or more and that part has a hardness of Hv 460 or more. Further, the method of production comprises using the steel material having the above ingredients to shape the bolt head and shaft, then heat the bolt to 900 to 1100°C, quench it, temper it by a 580°C or higher temperature, then thread roll it.