Abstract: The longitudinal cracks on the blind hole step of 40Cr steel planetary carrier was analyzed by means of macroscopic crack analysis,chemical composition analysis,metallographic examination and energy spectrum analysis.The results show that the longitudinal cracks on the blind hole step of 40Cr steel planetary carrier are quenching cracks.The reason is that the shape of the planetary carrier makes the cooling of the inner and outer martensite transformation non-isochronous,it leads more structure stress at the blind hole step of the planetary carrier,and when the local tangential tensile stress is greater than the fracture strength of martensite,resulting in quenching cracks.