TEM study on the vein-like grain boundary structures in nitrocarburized cool-work tool steels and its formation mechanism

Vein-like grain boundary (VLGB) structures commonly appear in the nitrided or nitrocarburized iron-based materials. Such VLGB structures are extremely bad for its mechanical properties. However, the formation mechanisms of the VLGB structures are still unclear. Here we employ transmission electron microscopy and electron probe X-ray microanalysis to reveal the detailed microstructures and the formation mechanism of the VLGB structure in nitrocarburized Cr12MoV cool-work tool steel at 540 °C and 580 °C. The VLGB structures entirely consist of orthorhombic (Cr,Fe)7C3 phase, with high density of micro-twins and stacking faults, and specific crystallography relationship between the (Cr,Fe)7C3-typed VLGB structure and matrix is: (101)α//(2¯4¯1¯)C + 7.5° and [1¯11]α//[1¯02]C. The formation of the (Cr,Fe)7C3-typed VLGB structures are closely associated with the C/Cr concentrations, elastic strain energy, low-energy interface and formation enthalpy of the VLGB structure. Similar interpretation is also carried out to illuminate the formation of the VLGB structures in the other nitrocarburized or nitrided alloying steels.