Effect of cobalt-based coating microstructure on the thermal fatigue performance of AISI H13 hot work die steel

Abstract

The Co-based coating by laser deposition could improve the thermal-fatigue (TF) resistance of hot working dies but the effect of technology on properties remained to be studied further. This work investigated the influence of the microstructure and γ/ε transformation on the TF life of the Co-based coating of the AISI H13 hot work die steel. Direct laser deposition (DLD) layers were prepared on the steel surface using modified Co-based alloys, which increased the contents of Cr, W, and Ni. The single and duplex layers were made under precise conditions, and the difference in the TF test life was significant. Meanwhile, the hardness of the duplex layer coating was much higher than that of the single one in the TF process consistently. Various detection methods were adopted to characterize the Co-based coating structures and crack propagation during the TF test. The microstructure of the two deposition coatings uniformly included the γ-Co phase and M23C6 crystalline phase. Moreover, after the TF experiment, the γ/ε transformation only occurred in the single layer as observed by transmission electron microscopy and electron backscattered diffraction. This indicated that the dilution of the base metal element distribution played a key role in reducing the stability of the γ-Co phase, which resulted in relatively low stacking fault (SF) energies, micro-hardness, a high fraction of the precipitate, and an early crack generation of the coating. This research was beneficial for understanding the microstructure, grain growth, γ/ε transformation, and crack propagation behavior in the DLD layers and TF test.