Microstructure and mechanical properties of large-sized pre-hardened plastic die steel 718

The microstructure and mechanical properties of pre-hardened 718 plastic die steel with a cross-section size of 695 mm×1360 mm were studied by rockwell hardness test, tensile test, impact test, OM, SEM and TEM. The results show that the microstructure of the 718 bloom is tempered sorbite at the edge, mixed microstructure of tempered bainite and upper bainite at 1/4 position, fine lamellar pearlite at the core. The hardness of the experimental steel is uniform relatively, the maximum hardness difference of the head cross-section is 2.9 HRC, the tail is 2.3 HRC. The edge has the highest impact toughness, the larger deviation of impact toughness is resulted by microstructure segregation at 1/4 position, and the pearlite microstructure makes the impact properties decrease seriously at the core. The reason for the formation of different microstructure and the influence for mechanical properties of different microstructure are disscussed.