Bolt Composition for Hydrogen Embrittlement Resistance
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Solution Overview
Problem
High-strength bolts with high tensile strength face challenges in achieving both excellent hydrogen embrittlement resistance and cost-effective production, as high alloy content leads to increased hardenability and production costs, and existing solutions either compromise on strength or require extensive heat treatments that can decrease hydrogen embrittlement resistance.
Innovation Solution
A bolt composition with a balanced chemical composition of C, Si, Mn, Cr, and B, optimized to achieve high tensile strength (1000-1300 MPa) without excessive hardenability, using the formula 4.9 ≤ 10C + Si + 2Mn + Cr + 4Mo + 5V ≤ 6.1 and Mn/Cr ≤ 0.55, along with a tempered martensite single-phase structure and sufficient dissolved Cr, to enhance cold workability and hydrogen embrittlement resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If alloying elements such as Mo and V are contained in large amounts to enhance hydrogen embrittlement resistance, then hydrogen embrittlement resistance characteristics are improved, but hardenability increases causing formation of hard microstructure (bainite) that reduces cold workability
Solution Approach 1:
The patent optimizes the chemical composition parameters by limiting Mo to 0.005-0.05% and V to 0.005-0.05%, combined with specific amounts of C (0.20-0.40%), Si (0.10-0.50%), Mn (0.30-2.00%), Cr (0.01-1.50%), and B (0.0005-0.0100%). This parameter optimization achieves the right balance between hardenability and cold workability, allowing the steel to form a tempered martensite structure that is both strong and workable.
Solution Approach 2:
The patent creates a composite microstructure consisting of tempered martensite as the primary phase with controlled amounts of other phases. This composite structure at the microstructural level provides both the strength and ductility needed, while the specific alloy composition creates a composite effect that enhances both hydrogen embrittlement resistance and cold workability simultaneously.
2Reliability
If alloying elements such as Mo and V are contained in large amounts to enhance hydrogen embrittlement resistance, then hydrogen embrittlement resistance characteristics are improved, but production cost increases
Solution Approach 1:
The patent replaces expensive alloying elements (Mo and V limited to 0.005-0.05% each) with more cost-effective alternatives. By using affordable elements like C (0.20-0.40%), Si (0.10-0.50%), Mn (0.30-2.00%), and Cr (0.01-1.50%) in optimized combinations, the patent achieves the same hydrogen embrittlement resistance at lower material cost, making the steel more economically viable.
Solution Approach 2:
The patent changes the compositional parameters to use cost-effective alloying strategies. By optimizing the ratios and amounts of common, inexpensive alloying elements rather than relying on expensive Mo and V, the patent reduces material costs while maintaining or improving hydrogen embrittlement resistance through the synergistic effects of the optimized composition.
3Ease of manufacture
If alloying elements are reduced to suppress bainite formation and simplify heat treatment, then production cost decreases and heat treatment complexity is reduced, but bolt strength and hydrogen embrittlement resistance characteristics decrease
Solution Approach 1:
The patent maintains sufficient hardenability by optimizing the combined effect of multiple alloying elements: C (0.20-0.40%), Si (0.10-0.50%), Mn (0.30-2.00%), Cr (0.01-1.50%), and small amounts of Mo (0.005-0.05%) and V (0.005-0.05%). This parameter optimization ensures bainite formation is suppressed during hot rolling, allowing simple heat treatment while achieving high bolt strength (1000-1300 MPa tensile strength) through the formation of tempered martensite.
4Strength
If boron is added to increase hardenability and strength, then bolt strength is improved, but hydrogen embrittlement resistance characteristics decrease when Mn content is high and Cr content is low
Solution Approach 1:
The patent optimizes the interaction between B (0.0005-0.0100%), Mn (0.30-2.00%), and Cr (0.01-1.50%) by specifying that when B is present, the Mn/Cr ratio should be controlled. This parameter control ensures that boron's hardening effect is maximized while maintaining hydrogen embrittlement resistance. The optimized composition creates a synergistic effect where boron enhances strength without compromising reliability, provided the Mn and Cr levels are appropriately balanced.
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 enables high-strength bolts with excellent hydrogen embrittlement resistance characteristics while reducing production costs by minimizing the need for extended softening heat treatments, maintaining high tensile strength and improved cold workability.
Implementation Method 1
a tempered martensite single-phase structure
Implementation Method 2
subjected to a heat treatment for softening for a predetermined period of time or less before the wire drawing and the cold forging
Data Source
Figure 1
Figure 2~3
AI summary
A bolt is provided that has high strength and excellent hydrogen embrittlement resistance characteristics. A bolt according to an embodiment of the present invention consists of, in mass%, C: 0.32 to 0.39%, Si: 0.15% or less, Mn: 0.40 to 0.65%, P: 0.020% or less, S: 0.020% or less, Cr: 0.85 to 1.25%, Al: 0.005 to 0.060%, Ti: 0.010 to 0.050%, B: 0.0010 to 0.0030%, N: 0.0015 to 0.0080%, O: 0.0015% or less, Mo: 0 to 0.05%, V: 0 to 0.05%, Cu: 0 to 0.50%, Ni: 0 to 0.30%, and Nb: 0 to 0.05%, with the balance being Fe and impurities. The bolt satisfies Formula (1) and Formula (2), and has a tensile strength of 1000 to 1300 MPa and satisfies Formula (3). 4.9≤10C+Si+2Mn+Cr+4Mo+5V≤6.1 Mn/Cr≤0.55 dissolved Cr/Cr≥0.70