Tempered Martensite Bolt Composition for Delayed Fracture Resistance
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Solution Overview
Problem
High-strength bolts made from high-carbon steel are prone to quench cracking and delayed fracture due to carbon concentration gradients and martensite transformation disparities between the surface and inner parts during heat treatment, which can lead to defects that may not be detected by magnetic particle testing.
Innovation Solution
The solution involves minimizing the carbon concentration gradient between the surface and inner parts of the bolt by controlling the carbon concentration within specific ranges and adjusting the quenching atmosphere to inhibit decarburization, ensuring even martensite transformation and reducing quench cracking susceptibility while maintaining high tensile strength and delayed fracture resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high-carbon steel is used to increase bolt strength, then tensile strength is improved, but quench cracking susceptibility increases
Solution Approach 1:
The patent applies parameter changes by precisely controlling the carbon concentration gradient between the surface and inner part of the bolt. Specifically, it limits the carbon concentration at the surface to 0.20 mass% or less and at the inner part to 0.35 mass% or less, while maintaining an average carbon concentration of 0.30 mass% or more. This parameter control resolves the contradiction by preventing excessive carbon accumulation that causes quench cracking while maintaining sufficient overall strength.
Solution Approach 2:
The patent applies local quality by creating a controlled non-uniform carbon distribution within the bolt. The surface region has lower carbon concentration (0.20 mass% or less) to reduce quench cracking susceptibility, while the inner part has moderate carbon concentration (0.35 mass% or less) to maintain strength. This local differentiation resolves the contradiction between surface integrity and overall strength.
2Strength
If carbon concentration is increased to improve strength, then tensile strength is improved, but delayed fracture resistance deteriorates
Solution Approach 1:
The patent applies parameter changes by establishing specific carbon concentration thresholds: average carbon concentration of 0.30 mass% or more for strength, while limiting surface carbon to 0.20 mass% or less and inner part carbon to 0.35 mass% or less. This resolves the contradiction by preventing localized carbon excess that causes delayed fracture while maintaining sufficient average strength.
Solution Approach 2:
The patent applies local quality by controlling carbon distribution to prevent localized carbon accumulation at the surface and inner regions. The controlled non-uniform distribution (surface ≤0.20 mass%, inner part ≤0.35 mass%) prevents hydrogen embrittlement and delayed fracture while maintaining overall strength through adequate average carbon content.
3Strength
If martensite transformation occurs from surface side during quenching, then surface hardness is improved, but temperature difference between surface and inner part increases causing quench cracking
Solution Approach 1:
The patent applies parameter changes by controlling the carbon concentration gradient to minimize the temperature difference during martensite transformation. By limiting surface carbon to 0.20 mass% or less and inner part carbon to 0.35 mass% or less, the Ms point difference between surface and inner part is reduced, thereby minimizing thermal stress and quench cracking while maintaining sufficient surface hardness.
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 approach results in a high-strength bolt with reduced quench cracking susceptibility and excellent delayed fracture resistance, ensuring the bolt's integrity and performance by minimizing carbon concentration gradients and optimizing heat treatment conditions.
Implementation Method 1
High-strength bolts are generally increased in strength by quenching and tempering, and has a tempered martensite structure
Implementation Method 2
decarburization easily occurs with high-carbon steel at the time of quenching
Data Source
AI summary
A bolt of the present invention is a high-strength bolt of high-carbon steel having a tempered martensite structure, wherein the composition of the bolt comprises: 0.50 mass% or more and 0.65 mass% or less of carbon (C); 1.5 mass% or more and 2.5 mass% or less of silicon (Si); 1.0 mass% or more and 2.0 mass% or less of chromium (Cr); 0.2 mass% or more and 1.0 mass% or less of manganese (Mn); and 1.5 mass% or more and 5.0 mass% or less of molybdenum (Mo); a total content of impurities being phosphor (P) and sulfur (S) is 0.03 mass% or more; and the remaining is iron (Fe). Furthermore, the carbon concentration satisfies the following Formula (1): 0.75 ≦ X < 1... Formula (1) wherein, in Formula (1), X represents surface carbon concentration / inner carbon concentration. Therefore, the bolt of the present invention has low quench cracking susceptibility and excellent delayed fracture resistance, because an increase in temperature at which martensite transformation occurs (Ms point) on the surface side is held down.