Flash-Butt Welded Rail Joints With Controlled HAZ Softening
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Solution Overview
Problem
Existing techniques for flash butt welding of rails fail to adequately address the increased fatigue damage resistance and breakage resistance required for welded rails, particularly in severe track environments with high loading freight cars.
Innovation Solution
A welded rail with a specific chemical composition and controlled heat-affected zone (HAZ) width, where the HAZ width is limited to 60 mm or less, and the total number of intersections of pro-eutectoid cementite structures in a defined evaluation region is 26 or less, to enhance fatigue damage resistance and breakage resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If flash butt welding is performed to join rails, then automation and welding speed are improved, but the hardness of the heat affected zone decreases causing softening
Solution Approach 1:
The patent applies parameter changes by controlling the cooling rate of the heat affected zone to be 1 to 10°C/sec, and by adjusting the chemical composition parameters (C: 0.7-1.0%, Si: 0.10-2.00%, Mn: 0.10-2.00%, Cr: 0.10-1.50%, P: 0.0250% or less, S: 0.0250% or less) to prevent softening while maintaining welding productivity
Solution Approach 2:
The patent applies local quality by attaching a patch with different material properties (higher Si content: 2.00-4.00%, higher Mn content: 1.00-3.00%) to the rail end surface within 50mm from the end, creating a localized region that prevents softening in the HAZ during flash butt welding
2Strength
If the hardness of HAZ is reduced to improve wear resistance, then wear resistance is improved, but unevenness occurs in the welded joint portion
Solution Approach 1:
The patent controls the cooling rate parameter (1 to 10°C/sec) and chemical composition parameters to maintain HAZ hardness within 20-50 HRc, preventing both excessive softening and unevenness in the welded joint portion
Solution Approach 2:
The patch creates a localized quality improvement at the rail end surface, ensuring uniform hardness distribution in the HAZ while maintaining adequate wear resistance through controlled composition and cooling
3Strength
If cooling rate is increased to suppress pro-eutectoid cementite formation, then breakage resistance is improved, but fatigue damage resistance may be affected
Solution Approach 1:
The patent optimizes the cooling rate parameter to the range of 1 to 10°C/sec, which is fast enough to suppress pro-eutectoid cementite formation (improving breakage resistance) but controlled to maintain adequate fatigue damage resistance by preventing excessive hardness and brittleness
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 proposed solution significantly improves the fatigue damage resistance and breakage resistance of the welded joint portion, leading to a substantial increase in the service life of the rail.
Implementation Method 1
Flash butt welding is a technique in which rail end surfaces are melted by heating, and then the melted surfaces are brought into pressure contact with each other to join the rails to each other
Implementation Method 2
austenitization and pearlitic transformation of the metallographic structure of the rail portion accompanied by heating to the A1 point or more during welding, and partial austenitization of the metallographic structure of the rail portion and decomposition of the pearlite structure accompanied by heating to the vicinity of the A1 point occur
Implementation Method 3
any one or both of a rail head portion and a base portion heated to a range of 800 to 900° C. in a two-phase state in which an austenite phase and a cementite phase are mixed are accelerated and cooled from a temperature range of 750° C. or higher at a cooling rate of 1 to 10° C./sec
Data Source
AI summary
A welded rail having excellent fatigue damage resistance and breakage resistance of a welded joint portion according to an aspect of the present invention includes: a plurality of rail portions; and a welded joint portion joining the rail portions, in which a HAZ width (W) is 60 mm or less, and when an interval between a most softened portion and a welding center measured along a longitudinal direction is defined as WX and a region where the distance from the welding center is 0.6 WX to 0.7 WX and the depth from a top portion outer surface is 2 to 5 mm is defined as a pro-eutectoid cementite structure evaluation region, in the pro-eutectoid cementite structure evaluation region, a total number of intersections (N) of a pro-eutectoid cementite structure intersecting a cross line including two line segments having a length of 100 μm parallel to the longitudinal direction and the vertical direction is 26 or less.