Effects of heat treatments on the microstructure and tensile properties of IN738 superalloy with high carbon content fabricated via laser powder bed fusion

Heat treatment is a key process governing the final microstructure and mechanical properties of precipitation-hardened superalloys. However, traditional heat treatments are typically designed for cast or wrought superalloys. Therefore, this study investigates the effects of hot isostatic pressing (HIP) and a two-step heat treatment (THT) process that combines HIP with solution heat treatment (SHT) process. Heat treatments were conducted on a crack-free high carbon content IN738 alloy (IN738–0.3 C) fabricated via laser powder bed fusion (LPBF). HIP significantly reduced the porosity and residual stress of the LPBF IN738–0.3C samples. The HIP samples exhibited a large number of well-developed cuboidal γ′ precipitates (∼800 nm) and a few fine near-spherical γ′ precipitates (∼40 nm), whereas the THT samples had uniformly distributed γ′ precipitates with an average size of ∼80 nm. Therefore, the HIP and THT methods have potential to improve the properties of additive manufactured (AM) IN738 alloy. A superior combination of yield strength (1050 MPa), ultimate tensile strength (1295 MPa), and elongation (16.5 %) was achieved through THT. Additionally, the study revealed the detailed relationship between the microstructure and the mechanical properties. These results can serve as a reference for exploring suitable heat treatments for other AM precipitation-hardened Ni-based superalloys.