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The polycrystalline cubic boron nitride (PCBN) milling tool can be used in the mold industry to replace cemented carbide tools to improve machining efficiency and quality. It is necessary to study the tool wear and failure mechanism to increase machining efficiency and extend tool life. Cr12MoV is used to analyze the failure form of PCBN tools in the interrupted cutting of hardened steels at low and high speed conditions in milling experiments. Experimental results show that the failure forms of PCBN tools include chipping and flank wear at low speed, and the failure modes at high speed are flank wear, the surface spalling of the rake face, and the fatigue failure on the flank face. The failure mechanism of different failure forms is analyzed by observing the surface morphology of the tool and using the theory of fracture mechanics. The results show that a high cutting speed should be selected to avoid the early damage of low speed and achieve better application of PCBN tools. At high cutting speed, tool failure is mainly caused by mechanical wear, diffusion wear, and oxidation wear. Moreover, a fatigue crack will occur at the cutting edge on the chamfered tool under thermal−mechanical coupling because of the intergranular fracture of the CBN grain and binder. A large area of accumulated fatigue damage may appear due to the influence of alternating mechanical stress and thermal stress. Finally, the control method to avoid tool failure is presented.