Abstract: A certain drive crank arm broke during operation. The cause of the arm fracture were analyzed by macroscopic observation, microscopic analysis, chemical composition analysis, and mechanical property testing. The results show that the material of the crank arm was changed from 35CrMo steel to 42CrMo steel, and improper dehydrogenation treatment of the material resulted in the diffusion of hydrogen elements into the material, ultimately led to hydrogen embrittlement fracture of the crank arm. Under the condition of hardness ranging from 269 to 331 HBW, both materials could be treated without dehydrogenation, and their mechanical properties were not affected. When the hardness was 353~381 HBW, 35CrMo steel needs to undergo dehydrogenation treatment within 4 hours, and 42CrMo steel needs to undergo dehydrogenation treatment within 3 hours.