Study of the effect of deep cryogenic treatment on the gradient diffusion bonding of WC-20Co cemented carbide and 42CrMo4 steel

The construction of intermediate layers in the diffusion bonding of cemented carbide and steel is an effective way to mitigate the difference in thermal expansion coefficients and reduce the induced residual stress (RS). However, additional pre- or post-treatment is crucial to further release the RS generated during bonding and enhance the performance of the joints. In this study, annealing and deep cryogenic treatment (DCT) are introduced in the diffusion bonding of WC-20 and 42CrMo4. The bending flexural strength increases from 702 to 1060 MPa (increased by 51.0 %) after pre-DCT but decreases to 324 MPa (decreased by 53.8 %) after annealing. The changes in mechanical properties are attributed to the evolution of RS and microstructure. It is revealed that DCT before bonding alters the RS from compressive -269 MPa to tensile 1188 MPa, which enhances the resistance to external compressive loads during bending. Although annealing reduces the original harmful compressive RS to -173 MPa, the coarsening of the Fe3C under long-term thermal exposure results in embrittlement and premature fracture, thereby weakening the bending strength. Furthermore, the compressive residual stress of -154 MPa in the joint with post-DCT indicates that DCT facilitates the transfer of residual stress from the interior to the surface.