Rail Bottom Composition and Cooling for Crack-Resistant Rails
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Solution Overview
Problem
Existing rail technologies fail to effectively prevent cracks and breakage from occurring at the rail bottom due to tensile stress, despite efforts to control microstructure and strengthen the rail.
Innovation Solution
A rail with a specific chemical composition and controlled cooling process, including a 0.2% proof stress, residual stress, and fatigue crack propagation rate, achieved through precise control of chemical elements and accelerated cooling rates.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the rail bottom is strengthened by controlling microstructure and chemical composition, then the strength and hardness increase, but the rail becomes more susceptible to crack propagation due to increased brittleness
Solution Approach 1:
The patent applies different chemical compositions and microstructures to different parts of the rail. The rail bottom has a specific composition (C: 0.65-1.40%, Si: 0.05-2.00%, Mn: 0.05-2.00%) optimized for strength, while the head has different composition and microstructure optimized for wear resistance. This local differentiation allows the bottom to achieve high strength without compromising overall rail reliability.
Solution Approach 2:
The patent creates a composite microstructure within the rail bottom consisting of pearlite colonies with controlled grain sizes (1-15 μm) and specific phase distributions. This composite structure at the micro level provides both the required strength and crack propagation resistance by combining hard pearlitic regions with more ductile inter-colony regions.
2Strength
If accelerated cooling is applied to the rail bottom to increase pearlite hardness, then the surface hardness increases to HB 320 or more, but residual tensile stress increases which promotes crack occurrence
Solution Approach 1:
The patent optimizes the cooling rate parameter within a specific range (1-5 °C/s) to achieve the desired balance. By controlling the cooling rate to fall within this window, the rail bottom achieves sufficient hardness (HB 320+) while limiting residual tensile stress development. The specific chemical composition parameters (C, Si, Mn ranges) are also optimized to work synergistically with the cooling rate to achieve both hardness and stress control.
3Strength
If higher carbon content is used to increase proof stress and prevent breakage, then the rail bottom strength increases, but the crack propagation rate increases due to formation of brittle pro-eutectoid cementite
Solution Approach 1:
The patent optimizes the carbon content parameter to fall within the range of 0.65-1.40%, which is higher than conventional rails but carefully controlled to avoid excessive carbon that would cause pro-eutectoid cementite formation. This optimized carbon level, combined with specific Si and Mn content ranges, achieves the required proof stress (>500 MPa) while maintaining acceptable crack propagation resistance.
Solution Approach 2:
The patent creates a composite microstructure with controlled pearlite colony sizes and phase distributions that compensates for the higher carbon content. The fine-grained pearlitic structure with controlled morphology reduces the harmful effects of high carbon by distributing the cementite in a finer, less brittle configuration, thereby maintaining both strength and fatigue resistance.
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 suppresses crack occurrence and propagation at the rail bottom, enhancing the rail's resistance to breakage under mechanical loads.
Implementation Method 1
a rail in which, while the rail head is subjected to accelerated cooling from the austenite region after rail rolling, the rail bottom surface is subjected to accelerated cooling between 800 °C and 450 °C at a cooling rate of 1 °C/s to 5 °C/s, so that the pearlite average hardness at the rail bottom becomes HB 320 or more
Implementation Method 2
the rail bottom surface is subjected to accelerated cooling between 800 °C and 450 °C at a cooling rate of 1 °C/s to 5 °C/s
Data Source
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AI summary
A rail that can suppress breakage from the rail bottom is provided. The rail includes a chemical composition containing, by mass%, C: 0.60 % or more and less than 0.90 %, Si: 0.10 % or more and 1.20 % or less, Mn: 0.10 % or more and 1.50 % or less, Cr: 0.05 % or more and 2.00 % or less, Al: 0.0002 % or more and 0.005 % or less, P: 0 % or more and 0.035 % or less, S: 0 % or more and 0.020 % or less, V: 0 % or more and 0.30 % or less, Cu: 0 % or more and 1.0 % or less, Ni: 0 % or more and 1.0 % or less, Nb: 0 % or more and 0.05 % or less, Mo: 0 % or more and 0.5 % or less, B: 0 % or more and 0.0050 % or less, Ti: 0 % or more and 0.01 % or less, Mg: 0 % or more and 0.01 % or less, Ca: 0 % or more and 0.02 % or less, W: 0 % or more and 0.10 % or less, Sb: 0 % or more and 0.05 % or less, Sn: 0 % or more and 0.05 % or less, and Co: 0 % or more and 1.0 % or less, with the balance being Fe and incidental impurities. The 0.2 % proof stress at the rail bottom center is greater than 500 MPa and less than 1100 MPa, the residual stress in the longitudinal direction at the rail bottom center is less than 200 MPa, and the fatigue crack propagation rate is 5.0 × 10-8 m/cycle or less when a stress intensity factor range ΔK at the rail bottom center is 15 MPa m1/2.