Microstructural Homogenization and Mechanical Property Enhancement of High-Manganese Cast Steel through a Hot Forging Process

This study investigated the effects of a hot forging process on homogenizing the microstructure and enhancing the mechanical properties of high manganese steel (Fe-24M3n-3Cr-0.7Si-0.5C-0.4Cu wt%) cast specimens. Cast specimens exhibited coarse-grained microstructures with average grain sizes about 900 μm, significant elemental segregation of Mn and Cr in interdendritic regions, and M23C6 carbide precipitation, which persisted even after solution heat treatment at 1000 oC for five hours. Hot forging conducted at temperatures ranging from 1200 oC to 900 oC with a reduction ratio of 2.8 effectively refined the grain structure to approximately 100 μm, reducing the grain size by more than 80%, and developed an equiaxed grain structure. Electron probe microanalysis revealed substantial mitigation of Mn and Cr segregation after forging, particularly in specimen surface regions where the elemental distribution became nearly homogeneous. Transmission electron microscopy confirmed that most M23C6 carbides present in the as-cast material were dissolved during hot forging, whereas in cast specimens they remained even after prolonged solution treatment due to sluggish diffusion in segregated regions. While hardness values showed negligible differences between the cast (180.2 HV) and forged specimens (177.5 HV), tensile properties demonstrated remarkable improvement in the latter. Yield strength, tensile strength, and uniform elongation increased from 395 MPa, 608 MPa, and 41.1% in cast specimens to 409 MPa, 787 MPa, and 63.9% in forged specimens, respectively. The brittle intergranular fracture mode observed in the cast specimens was transformed to ductile dimple fracture in the forged specimens. This substantial enhancement is attributed to the combined effects of grain refinement, reduced elemental segregation, and carbide dissolution, which collectively minimize strain localization during tensile loading. These findings demonstrate that hot forging effectively homogenizes the microstructure of high manganese steel castings. This approach is thus suitable for manufacturing complex-shaped components in cryogenic applications.