Microstructure and hardness of Stellite 21 deposited on hot forging die using direct metal deposition technology

The Stellite 21 superalloy has excellent creep resistance and mechanical strength at elevated temperatures, is resistant to wear and thermal fatigue, and has good weldability. Therefore, it is frequently used as a surface-hardened metal for hot forging dies. In this study, the Stellite 21 superalloy was deposited on an SKD61 matrix material using direct metal deposition technology. The temperature field of the deposition region during hot forging was predicted using the finite-element method. The mcrostructure and hardness of the Stellite 21 before and after service were evaluated. Various methods were employed for these evaluations, such as microhardness tests, optical microscopy, scanning electron microscopy, energydispersive X-ray spectroscopy, and X-ray diffraction (XRD) analysis. The phase transformation of the Stellite 21 from gamma-Co to epsilon-Co before and after service was investigated using XRD. The results indicated that the hardness of the Stellite 21 after service was increased owing to precipitation strengthening by carbides and an increase in epsilon-Co.