Effect of Mg on inclusions, microstructure, and mechanical properties of hot work die steel melted under pressurization

The influence of magnesium (Mg) treatment on the inclusions, microstructure, and mechanical properties of hot work die steel (5CrNiMo), a typical low alloy hot working die steel, was investigated in this paper. This steel is commonly used in the manufacturing of large and medium-sized forging dies, as well as various engineering components and mechanical parts. The experimental results show that the Mg treatment has a momentous influence on the refinement of the secondary dendritic arm spacing of as-cast steel samples. The secondary dendrite arm spacing decreased from 66.7 μm to 39.2 μm with the Mg content increased from 0.0007 wt% to 0.18 wt%. Furthermore, Mg treatment also contributed to the refinement of the interlamellar spacing of pearlite. The investigation of the quenched microstructure revealed that the Mg treatment led to a reduction in grain size, and the extent of refinement was higher with higher Mg content under the experimental conditions. The tensile properties of the steel exhibited a remarkable improvement due to the refining effect of Mg treatment. In detail, the addition of Mg can increase the tensile strength of the experimental steel from 1,043 MPa to 1,240 MPa and the impact energy from 10.8 J to 15.2 J, with the Mg content increasing from o to 0.069 wt%. A small amount of Mg played a beneficial role in enhancing the strength, plasticity, and toughness of the steel samples. However, excessive Mg content had a detrimental effect on these properties, leading to a decline in strength, plasticity, and toughness.