Effect Of Chill Cooling Treatment Of Weld Toe Dressed By TIG On Residual Stress And Fatigue Strength Of Welded Joints

With the rapid development of high-speed railway in China, bogies have all been changed from the steel castings to the welded frameworks. With the increase of speed, the number of damaged components for the passenger trains is also increased. The fatigue failure of welded joints has become more apparent. Bogie is one of the most important parts for the vehicle structure. During the high-speed running of the trains, welded frames of bogies withstand the strong vibration and the large impact loads. The fatigue performance and structure reliability directly affect the safe driving of trains. While, the welded joints are the weak links of structure. The merits for the fatigue performance of joints also restrict the safety life of the bogies. Therefore, deep studying the process methods for improving the fatigue strength of welded joints is very necessary.The fatigue strength of structures depends on the stress concentration of structures and the distribution of residual stress to a large extent. Reducing the stress concentration or adjusting the field of residual stress can improve the fatigue strength. Traditional TIG dressing process at the weld toes can improve the transition angles of welds, reduce the stress concentration, and increase the fatigue strength of welded joints and structures. However, TIG dressing process cannot change the nature and distribution of the residual stress field fundamentally. Thus, the extent of improving the fatigue strength by TIG dressing process is very limited. In this paper, after the TIG dressing process, a cold source at a certain temperature is located at the weld toes and the chilling treatment is then performed. This method can improve the forming of the weld toes, obtain the smooth transitions from welds to the base metals, and decrease the stress concentration. On the other hand, it can also adjust the residual stress distribution through the chilling treatment and increase the fatigue strength further.The numerical simulation is performed using the software named ABAQUS. The residual stress distribution is simulated for three cases which are the welded state, the state of TIG dressing at the weld toes, and the state of the chilling treatment after TIG dressing process, respectively. The effect of the chilling treatment after TIG dressing on the residual stress at the weld toes is analyzed. Through the calculation result, it can be seen that for the bogies of high-speed trains, the residually tensile stresses in the vertical and transverse directions can be eliminated through the chilling treatment at 450℃after TIG dressing process for Q345-C butt joints and T-angle joints. When the chilling temperature is 550℃or 650℃, which is higher than 450℃, the residual stresses in the vertical and transverse directions are the compression stresses. The distribution of the biaxial compression stress is conducive to the improvement of the fatigue strength.The stress tests of the weld toes for these three states are performed using the blind hole method. The test results state that the residual stresses of weld toes are the tensile stresses for the welded state and the state of TIG dressing. The chilling treatment is performed at 350℃, 450℃,550℃, and 650℃, respectively. With the increase of the chilling temperature, the tensile stresses are decreased and are changed to the compressive stresses. The critical temperature of this change is 450℃. Test results are consistent with the results of numerical simulation.The fatigue tests for the specimens of three states are performed using three-point bending method. Test results show that the fatigue strength of T-joints can be increased in a certain degree through TIG remelting at the weld toes when the chilling treatment is performed at 550℃. The fatigue strengths of joints can be increased by a factor of 61% and 294% when TIG remelting and the chilling treatment are performed at the weld toes, respectively. The fatigue strength of joints can be improved greatly by the chilling treatment after performing TIG remelting at the weld toes. The impact test shows that when the temperature of chilling treatment is higher than 650℃, the impact energy values of gaps at the weld toe samples are decreased to a certain extent. In order to prevent the brittle tendency of joints, the upper limit temperature of chilling treatment is 650℃.The effect of the chilling treatment on the mechanical properties is studied by stretching, bending, impact, hardness, and metallographic tests, respectively. From the test results, it can be seen that the strength, toughness and other mechanical properties of weld joints at the chilling temperature of this experiment are not decreased.