High-Carbon Bolt Composition for Quench Crack Resistance
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Solution Overview
Problem
High-strength bolts made of high-carbon steel are prone to quench cracking and delayed fracture due to uneven carbon concentration gradients and increased martensite transformation temperatures, which can lead to brittle failure under static stress.
Innovation Solution
A high-strength bolt with a tempered martensite structure, comprising specific ranges of carbon, silicon, chromium, manganese, and molybdenum, and controlled impurities, where the carbon concentration gradient between the surface and inner parts is minimized to reduce martensite transformation temperature differences, thereby reducing quench cracking susceptibility and enhancing delayed fracture resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high-carbon steel is used to increase bolt strength, then tensile strength exceeds 1200 MPa, but quench cracking susceptibility increases significantly
Solution Approach 1:
The invention optimizes the carbon content parameter to a specific range (0.35-0.50 mass%) rather than using high-carbon steel (>0.60% C), and combines it with controlled amounts of alloying elements (Si: 1.00-2.50%, Cr: 0.50-2.00%, Mn: 0.10-1.00%, Mo: 0.10-0.50%) to achieve the desired strength while reducing quench cracking susceptibility through modified martensite transformation characteristics
Solution Approach 2:
The invention creates a composite steel material system by combining carbon with multiple alloying elements (Si, Cr, Mn, Mo) in specific proportions, forming a complex steel composition that achieves high strength through synergistic effects while suppressing quench cracking through modified transformation kinetics and microstructure
2Strength
If high-carbon steel with tempered martensite structure is used, then strength is increased, but delayed fracture resistance deteriorates due to hydrogen embrittlement
Solution Approach 1:
The invention reduces carbon content from high-carbon levels (>0.60%) to a moderate range (0.35-0.50 mass%), which decreases the formation of cementite and reduces hydrogen trapping sites, thereby improving delayed fracture resistance while maintaining strength through alloying element optimization
3Reliability
If carbon concentration gradient is reduced to minimize martensite transformation temperature difference, then quench cracking susceptibility decreases, but manufacturing control complexity increases
Solution Approach 1:
The invention modifies the chemical composition parameters (C: 0.35-0.50%, Si: 1.00-2.50%, Cr: 0.50-2.00%, Mn: 0.10-1.00%, Mo: 0.10-0.50%) to inherently reduce the martensite transformation temperature gradient during quenching, which simplifies manufacturing control by eliminating the need for complex carbon potential management while still achieving low quench cracking susceptibility
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 effectively decreases quench cracking susceptibility and improves delayed fracture resistance by ensuring even martensite transformation across the bolt, maintaining high strength and toughness while preventing crack generation and hydrogen embrittlement.
Implementation Method 1
decaburization is suppressed by silicon, chromium, manganese, and molybdenum
Implementation Method 2
a high-strength bolt of high-carbon steel having a tempered martensite structure
Implementation Method 3
High-strength bolts are generally increased in strength by quenching and tempering
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.