Why is the control of residual stress critical in continuous welded rail (CWR) welding procedures?

Controlling residual stress is critical in continuous welded rail (CWR) welding procedures because uncontrolled residual stress can lead to track buckling in hot weather or rail breaks in cold weather. Continuous welded rail is created by welding together sections of rail, often hundreds of feet long, to eliminate joints. These joints are sources of weakness and require frequent maintenance. However, the welding process introduces residual stresses into the rail. Residual stresses are stresses that remain in a material after the original cause of the stresses has been removed. In the case of CWR welding, these stresses are primarily tensile (pulling) near the weld and compressive (pushing) away from the weld. If the rail is not properly stressed during the welding process to achieve a target neutral temperature (the temperature at which the rail is neither in tension nor compression), then high tensile stresses can be present in the rail during cold weather. These tensile stresses can combine with stresses from train loads and temperature changes, potentially leading to rail breaks. Conversely, in hot weather, high compressive stresses can cause the rail to buckle, especially if the track is not properly supported by the ballast and fasteners. Buckling is a sudden and dangerous lateral deflection of the track. Therefore, CWR welding procedures include steps to control residual stresses, such as heating the rail before welding and carefully monitoring the welding process, to ensure that the rail is properly stressed and stable over a wide range of temperatures. The goal is to achieve a balanced stress state that minimizes the risk of both rail breaks and track buckling.