Effect of Mo on Nano‐Precipitation Behavior and Microscopic Mechanical Characteristics of Ferrite

The undercooling austenite transformation, nano-precipitation behaviors, and microscopic mechanical properties of ferrite in Nb-Ti-Mo and Nb-Ti bearing steels have been studied by thermal simulation experiments, Vickers hardness, transmission electron microscopy and nanohardness indentation. The results show that Mo tends to promote the austenitebainite transformation and refine ferrite grain, nano-precipitates and sheet spacing. There are two different precipitation behaviors (interphase precipitation and random dispersive precipitation) in ferrite for the two tested steels. The EDS analysis indicates that these particles are (Nb, Ti)(C, N) for the size of more than 150 nm, (Nb, Ti)C and TiC for the size of less than 30 nm. Mo is not found in the precipitate particles of Nb-Ti-Mo steel. Nanoindentation results demonstrate that the values of nanohardness and Young's modulus are 4.13 and 249.3 GPa for Nb-Ti steel, 4.41 and 232.0 GPa for Nb-Ti-Mo steel, respectively. Mo could increase nanohardness of ferrite 280 MPa, and reduce Young's modulus of ferrite 17.3 GPa. Nb-Ti-Mo steel has a higher work hardening rate compared with Nb-Ti steel.