Atomic-scale study on the mechanism of formation of reverted austenite and the behavior of Mo in a low carbon low alloy system

The mechanism of formation of reverted austenite in a low carbon low alloy system was comprehensively studied by 3D atom probe tomography and Thermo-Calc simulation. It was observed that low C-content provided the condition for nucleation of reverted austenite at lath boundaries, while the alloying elements, Mn, Ni and Cu led to the growth of reverted austenite. Mo provided diverse impact on mechanical properties of steel at high temperature. (1) The diffusional direction of Mo atoms was controlled by the partitioning of Mo; (2) MoC co-segregation layers, nano MoC clusters, nano Mo clusters and MoC precipitates were formed at two-phase interfaces; (3) Mo formed nano MoC clusters and MoC precipitates at lath boundaries or in the matrix; (4) Mo promoted the formation of Nb carbides and was dissolved in carbides to produce (NbxMo1-x)C precipitates. High resolution transmission electron microscopy images of (NbxMo1-x)C precipitates showed that they had face-centered cubic structure and the lattice constant was in the range of 0.440–0.445 nm. Nano Cu precipitates were observed in reverted austenite. MoC co-segregation layers, nano MoC clusters, nano Mo clusters and MoC precipitates formed at γ/α interfaces effectively enhanced the grain boundary strength. Nano MoC clusters, MoC precipitates and (NbxMo1-x)C precipitates in the matrix effectively pinned dislocations and prevented movement of dislocations. The combined effect led to good fire resistance and heat resistance properties of studied steel.