Analysis of a New High-Toughness Ultra-high-Strength Martensitic Steel by Transmission Electron Microscopy and Atom Probe Tomography

The microstructure of a new martensitic high-strength steel (Fe-0.40C-3.81Ni-1.31Cr-1.50Si-0.75Mn-0.52Mo-0.51Cu-0.30V) with high fracture toughness is characterized by transmission electron microscopy and atom probe tomography (APT). MC, M 6 C, and M 23 C 6 precipitates form inside the martensitic lath matrix. The fracture toughness is insensitive to the dissolution of M 23 C 6 precipitates at austenitizing temperatures above 1164 K (891 °C). APT reveals that solute segregation at the prior austenite grain boundaries (PAGB) is not uniform, with C, Mo, Si, Ni, and/or P enrichment varying at different areas of the PAGB. Si depletion is detected in the same area as the highest C enrichment. Carbon also segregates at lath boundaries. Segregation of C indicates the presence of retained austenite films at both PAGB and lath boundaries. Regions enriched in C up to 10 pct were found within the laths; however, no regions were enriched to the level expected of cementite or ε-carbide. The observed C distribution and high fracture toughness indicates that the tempering behavior is significantly different than that observed in 300M steel. The effect of Si, Ni, and Cu on the formation and stabilization of the regions of C enrichment and retained austenite require further study, as it may be key to the increased toughness.