Abstract: After 30CrMnSiA arm parts undergo machining and heat treatment, it is found that there are many cracks in the axis of the parts through magnetic particle inspection. In order to find out the cause accounting for the crack in the arm parts, in this work, macroscopic observation, metallographic structure analysis, scanning electron microscope and energy spectrum analysis, mechanical properties analysis, chemical composition analysis ect. were conducted and compared with that of the pristine one. The results show that the microstructure of the part is not uniform, resulting in large stress. The distribution direction of the abnormal banded structure is consistent with the crack propagation direction, and the cracks are located at the dimensional change of the cross section part. As the quenching cooling rates are different due to the variable rib sizes, there are high thermal stress at the cracks. Analysis results illustrate that all the cracks of the parts are quenching cracks. The comprehensive factors such as the uneven banded structure of the raw materials and the significantly varied shapes lead to the quenching cracks of the parts.