Improving the fracture toughness of high-strength gear steel by tailoring cracking path

This work is dedicated to improving the fracture toughness of the high-strength gear steel 20CrNi4A. Through thermomechanical processing at different temperatures, the microstructures with prior austenite grain size (PAGS) of 16 μm and 64 μm were generated, respectively, and the one with smaller PAGS was associated with a distribution of Cr-rich undissolved carbide. The microstructures involved similar grain size and dislocation density of the martensite phase, and presented similar mechanical properties when assessed by uniaxial tension. However, the fracture toughness tests show that the sample with smaller PAGS exhibited a higher fracture toughness for crack initiation and crack growth. As revealed by X-ray microtomography, the crack extension was proceeded by joining with the voids in the near tip region. But the microstructures exhibited different cracking paths. In the sample with larger PAGS, the crack mainly propagated along the prior austenite grain boundaries as a result of the preferred damage nucleation at this region. In sample with smaller PAGS, the crack propagated through the prior austenite grains by coalescing the voids formed at the carbide particles, and more significant crack tip plasticity was associated, which contributed to a higher fracture toughness.