The Impact of Residual Stress from Rail Straightening on Welded Joint Quality
Where is straightening residual stress concentrated in rails and what causes it?
Straightening residual stress is mainly concentrated on both sides of the rail head, rail base edges, and the transition zone between the rail web and head. Hot-rolled rails have natural bending, and cold straightening forces them straight through external pressure, creating residual tensile stress in compressed areas and residual compressive stress in stretched areas. The rail head and base undergo the largest deformation during straightening, leading to the most significant stress concentration. These stresses persist internally and become potential crack triggers.
What fatal consequences result from the superposition of residual stress and welding thermal stress?
Welding generates intense thermal stress, with metal near the weld experiencing severe thermal expansion and contraction. When welding thermal stress overlaps with straightening residual tensile stress, local stress exceeds the steel's yield strength and even tensile strength. This over-stress state initiates cracks in the weld or heat-affected zone, especially cold cracks that appear hours or days after welding. Cracks severely weaken joint strength and easily cause rail breakage during train operation, endangering traffic safety.
How do Chinese and international standards differ in controlling straightening residual stress?
Chinese standards require uniform residual stress distribution after straightening, with surface residual tensile stress on the rail head not exceeding specified limits, meeting domestic welding needs. International standards such as UIC 60 and EN 13674 impose stricter controls on high-speed rails, limiting not only stress values but also requiring stress distribution detection via ultrasonic or blind hole methods. Some international rails undergo online tempering to actively eliminate residual stress, ensuring stress stability during welding. This difference stems from the high reliability requirements of high-speed line weld joints.
What methods are commonly used to eliminate straightening residual stress before rail delivery?
The most common method is online low-temperature tempering, which uses controlled heating to relax the internal metal structure and release residual stress. For high-precision rails, natural aging is adopted-storing rails for a period to allow slow stress relaxation, though this is time-consuming. Additionally, optimizing the straightening process with multi-roll progressive straightening to reduce single deformation can minimize residual stress at the source.
How to mitigate risks from rail residual stress during on-site welding?
Preheat rail ends to the specified temperature before welding to reduce temperature differences and lower thermal stress peaks. Strictly follow welding procedures to control welding speed and interpass temperature, avoiding excessive thermal stress. Conduct post-weld heat treatment (normalizing or tempering) after welding to eliminate the superposition of welding thermal stress and residual stress. Meanwhile, avoid welding at locations with severe straightening deformation to avoid risks from the source.