Cr‐containing structural steels with quench‐polish‐quench (QPQ)‐treated surfaces are noticed having higher grinding cracking tendency compared to plain carbon steels. Herein, the compound layers of QPQ‐treated shafts produced from 20Cr and 40Cr steels by performing XRD, SEM, EDS, and microindentation measurements are investigated. The differences between the compound layers on the shafts with and without cracks are compared in morphology and element distribution. It is revealed that the basic reason of grinding cracking is the imperfect compound layer with serious porosities. The formation of serious porosities is mainly due to the inhibition of the outward diffusion of the nitrogen molecules from the nitride layer. Stronger nitride‐forming element Cr is considered to strengthen this inhibition.