Effect of Sampling Position on Microstructure and Creep Properties of a Modified 9Cr-1Mo Steel Manufactured by Laser Powder Bed Fusion

Effect of sampling position on microstructure and creep properties of modified 9Cr-1Mo steel cylinder, with a dimension of 15 mm in diameter and 90 mm in height, manufactured by laser powder bed fusion (LPBF) was investigated. The LPBF process provides a complex microstructure to modified 9Cr-1Mo steel consisting of δ-ferrite, martensite, and retained austenite. The area fraction of these phases depended on the sampling position. In other words, the area fraction of martensite increased at the upper side and edge parts of the cylinder owing to the position-dependent thermal history. The Vickers hardness increased with the area fraction of martensite. Small punch creep (SPC) tests were performed at 923 K using disk-type specimens with a diameter of 3 mm sampled from the upper-center, middle-center, middle-edge, and lower-center parts of the cylinder to evaluate the local creep strength. However, no marked sampling position dependence on the creep strength was observed. This indicates that the inhomogeneous microstructure of modified 9Cr-1Mo steel manufactured by LPBF hardly influences the creep properties. Therefore, it was suggested that the allowable tensile strength of complex-shaped LPBF-produced components can be determined using creep data obtained from 6–10 mm size creep specimens.