In addition to the bending moment caused by the self weight of the cylinder and the weight of the concrete, the mixing cylinder also bears the torque caused by the rotary movement of the mixing cylinder and the circumferential and axial force of the concrete in the mixing cylinder, which is complex . Under the action of cyclic alternating load, the initial micro crack may form in the area with large stress, and then develop into macro crack, and then the macro crack will continue to expand, resulting in the cracking of weld bead or base metal. The main reasons affecting the initial microcracks of the raceway are as follows:
(1) Stress concentration at raceway weld
Cause: when the raceway is welded with the cylinder, stress concentration often occurs at the weld. In addition, during the use of the mixing drum, the concentrated load at the raceway and supporting wheel and the overall deformation of the raceway section will also cause the stress concentration at the welding joint.
Countermeasures: polish the weld with concentrated stress and re weld. For the weld or base metal with macro cracks, punch holes to cut off the crack source and re weld.
(2) Residual stress at raceway weld
原因：滚道与筒体刚度相差悬殊，一般情况下，搅拌筒筒体厚度为6mm 的钢板，滚道的径向厚度在60mm 左右，筒体和滚道厚度相差很大，在焊接过程中，滚道的热膨胀受到限制，并且在冷却过程中的收缩也受到限制，而筒体受限程度远小于滚道，最终影响到焊接应力的大小及分布不均，导致残余应力的产生。
Reason: the stiffness of the raceway is quite different from that of the barrel. Generally, the radial thickness of the raceway is about 60mm for the steel plate with a thickness of 6mm of the mixing barrel. The thickness of the barrel and the raceway is very different. During the welding process, the thermal expansion of the raceway is limited, and the shrinkage during the cooling process is also limited, and the degree of limitation of the barrel is much less than that of the raceway, Finally, it affects the size and uneven distribution of welding stress, resulting in the generation of residual stress.
Countermeasures: adjust the welding sequence, reduce the single heat input, knock the components with a copper hammer during the welding process, and adjust, homogenize and eliminate the residual stress by means of vibration aging. In addition, the weld bead shall be locally polished to reduce welding defects.
(3) Stiffness change of raceway and cylinder
Reason: the stiffness of raceway and cylinder decreases with the use of vehicles, especially for large-scale mixer. The weakening of stiffness will accelerate the fatigue failure of weld bead and lead to the cracking of weld bead or base metal.
Countermeasures: weld reinforcing plates inside the contact between the raceway and the cylinder to improve the overall stiffness of the area, so as to enhance the fatigue failure resistance of the welded structure.
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