Residual Stress Control Technology and Weld Durability Improvement of Rail Welded Joints
What are the distribution characteristics of residual stress in rail welded joints?
The distribution characteristics of residual stress in rail welded joints are obvious inhomogeneity, with bidirectional tensile stress peaks in the weld zone and heat-affected zone. During welding, the metal in the weld zone undergoes rapid melting and solidification with an extremely large temperature gradient: the temperature at the weld center can reach above 1500℃, while the temperature of the base metal is only room temperature. This temperature difference causes the metal in the weld zone to be constrained by the base metal when it shrinks during cooling, generating tensile stress. In the longitudinal direction (length direction) of the rail, the peak longitudinal residual tensile stress at the weld center can reach 80%-90% of the material's yield strength, gradually attenuates to both sides of the base metal, and basically returns to zero stress state beyond 50mm. In the transverse direction (width direction), the peak transverse tensile stress in the heat-affected zone is about 60%-70% of the yield strength, mainly concentrated in the area 10-20mm on both sides of the weld. In addition, there are differences in the residual stress distribution between the rail head and rail bottom: the rail head has a higher residual stress peak due to faster cooling speed, which is a high-incidence area for joint cracking.
What are the core welding process optimization measures for residual stress control of rail welded joints?
The core welding process optimization measures for residual stress control of rail welded joints are adopting the process of preheating + multi-layer multi-pass welding + segmented welding to reduce the temperature gradient during welding. Preheating is the key step: preheat the rail joint to 200-250℃ before welding to reduce the temperature difference between the weld and the base metal, and reduce the constraint stress during cooling shrinkage. Excessively low preheating temperature has no obvious effect, while excessively high temperature will lead to coarse grains. The multi-layer multi-pass welding process divides the weld into 3-5 layers for welding. After each layer is welded, it needs to be cooled to 150-200℃ before welding the next layer, avoiding excessive heat concentration in single-layer welding, reducing the temperature gradient. At the same time, the stress of multi-layer welds can offset each other, reducing the residual stress peak. The segmented welding process adopts a symmetrical segmented method, such as segmented welding from the weld center to both sides, to distribute heat evenly and avoid stress unbalance caused by unilateral heat concentration. The optimized welding process can reduce the residual stress peak by 30%-40%, greatly improving the stability of the joint.
What are the key post-weld heat treatment process points for residual stress control of rail welded joints?
The key post-weld heat treatment process points for residual stress control of rail welded joints are adopting the composite treatment process of stress relief annealing + local tempering to eliminate or reduce residual stress. Stress relief annealing is the core step: heat the entire welded joint to 550-600℃, keep it warm for 2-3 hours, then cool it slowly to room temperature with the furnace, and control the cooling rate within 50℃/h. This process can make the microstructure inside the joint undergo recovery and recrystallization, release residual stress, and reduce the peak longitudinal residual tensile stress to below 30% of the yield strength. The local tempering process is aimed at the heat-affected zone of the rail head: heat the rail head to 400-450℃, keep it warm for 1 hour, further reduce the stress peak of the rail head, and improve fatigue resistance. During heat treatment, the heating rate and cooling rate must be strictly controlled to avoid new stress caused by excessive temperature changes. For rail joints used in high-speed railways, ultrasonic impact treatment is also required: mechanical impact is used to produce plastic deformation on the weld surface, offset part of the tensile stress, and form a beneficial compressive stress layer.
What is the influence of different welding methods on the residual stress of rail joints?
The influence of different welding methods on the residual stress of rail joints is significantly different, mainly depending on the energy density and heating rate of the welding heat source. Flash butt welding is a commonly used method for on-site rail welding. Its heat source has high energy density and fast heating rate, resulting in a large temperature gradient in the weld zone and a high residual stress peak: the peak longitudinal tensile stress can reach about 85% of the yield strength. However, it has high welding efficiency and is suitable for on-site welding of ordinary-speed railways and heavy-haul railways. Thermite welding has a low heat source energy density and slow heating rate, with a relatively small temperature gradient and a low residual stress peak: the peak longitudinal tensile stress is about 60% of the yield strength. However, the weld strength is low, which is suitable for emergency repair and small-radius curve sections. Gas pressure welding uses gas flame as the heat source, with uniform heating and small temperature gradient, resulting in the lowest residual stress peak: the peak longitudinal tensile stress is only 40%-50% of the yield strength, and the weld quality is stable, suitable for rail welding of high-speed railways. Laser welding has extremely high energy density, small heating range and narrow heat-affected zone, with more uniform residual stress distribution. However, the equipment cost is high, and it is currently mainly used for factory welding of rails.
What are the detection methods and acceptance standards for the residual stress of rail welded joints?
The detection methods for the residual stress of rail welded joints mainly include blind hole method, X-ray diffraction method and ultrasonic method. The blind hole method is a commonly used on-site detection method, and the acceptance standards must comply with TB/T 1632-2014 rail welding standards. The detection steps of the blind hole method are: drill a blind hole with a diameter of 1-2mm on the joint surface, measure the strain change before and after drilling, calculate the residual stress value through the stress-strain formula, with a detection accuracy of ±10MPa, suitable for rapid on-site detection. The X-ray diffraction method is a non-destructive detection method, which calculates residual stress by measuring the displacement of crystal diffraction peaks. It has high detection accuracy and is suitable for precise laboratory detection, but it is limited by detection equipment and difficult to use on-site. The ultrasonic method detects residual stress by using the wave velocity change of ultrasonic waves under stress, which can realize non-contact detection and is suitable for large-area rapid scanning. The acceptance standard stipulates that the peak longitudinal residual tensile stress of rail welded joints for high-speed railways ≤150MPa, for heavy-haul railways ≤200MPa, and for ordinary-speed railways ≤250MPa; the residual stress distribution must be uniform without obvious stress concentration. The sampling ratio is 3 joints per 100 joints. If one is unqualified, double sampling shall be conducted; if it is still unqualified, the welding operation shall be suspended and the process parameters shall be checked.