Current status of the characteristics and control of primary carbides in H13 steel

The H13 steel is widely used in hot rolling, hot extrusion, and hot forging because of its remarkable fatigue resistance performance, hot strength, and impact toughness. To further extend the service life of the steel, the production of high-purity die steel is vital. Carbide-forming elements with a mass fraction of about 8%, including Cr, Mo, and V, are contained in H13 steel to prevent the coarsening of austenite or strengthen the steel through secondary hardening. However, owing to the presence of these elements, primary carbides are easily to be found in H13 steel because of the segregation of carbide-forming elements during solidification. The presence of large primary carbides is one of the main causes of the fatigue crack of H13 steel. Therefore, it is of great significance to systematically study the behavior of primary carbides in H13 steel. The excellent material properties of H13 steel were firstly affirmed in combination with the composition characteristics, and then the relationship between the primary carbides and the service life of H13 steel were summarized. The service life of H13 steel reduces with an increase in the primary carbide size. The hazard of primary carbide on the H13 surface steel is much greater than that inside the steel. The characteristics of primary carbides in H13 steel were further systematically discussed, including two-dimensional (2D) and three-dimensional (3D) morphologies, thermal stability and precipitation mechanism. A great difference exits between the 2D and 3D characteristics of primary carbides. In the 2D analyses, the density of the primary carbides appears high, while their size is small. The actual characteristics of primary carbides can be obtained only by 3D observation. Four primary carbide control methods in H13 steel are compared, including composition optimization, cooling rate control, Mg treatment and rare earth treatment. Composition optimization and cooling rate control have been successfully applied in the actual production process. Rare earth treatment may be an effective control method with Chinese characteristics. The relevant discussion and research work in this article can play a certain enlightening role in the rational control of the primary carbides in H13 steel.