Effects of hot isostatic pressing and direct tempering on the mechanical properties of additively manufactured carbon-bearing high strength Co-Ni steel

초록

The effect of post treatments on the microstructure evolution and mechanical properties of high CoNi steel manufactured by laser powder bed fusion were investigated in this study. Hot isostatic pressing (HIP) and direct tempering were applied for tailoring microstructural characteristics to get the highest strength-elongation combination. The effective grain sizes were measured as 1.46 +/- 1.03 mu m in as-built (AB), 1.68 +/- 1.37 mu m in direct-tempered (DT), and 3.45 +/- 2.40 mu m in HIP/tempered (HIPT) materials. The size and type of carbides (round M23C6 and needle M2C-type carbides) were changed with different post-treatments. The ultimate tensile strengths (UTSs) of as-built, direct aged, and HIP followed by tempering samples were 2054 MPa, 2041 MPa, and 1889 MPa, respectively. According to the post treatment conditions, the amount of uniform plastic strain was shown in the order of AB > HIPT > DT. It was found that the retained austenite did not change to reverted austenite even when single and double tempering was performed. From the perspective of microstructural characteristics, strengthening mechanism and deformation behavior were also discussed.

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