Experimental Evaluation of Tool Steel Fracture Toughness Using Circumferentially Notched and Precracked Tension Bar Specimen

Abstract

Dies and tools used in hot metal forming (extrusion, forging, rolling, etc.) are exposed to elevated temperatures and high contact pressures. As the automotive industry increasingly moves toward use of advanced high-strength steels, requirements on the wear and fatigue properties of tools and dies are constantly growing. This requires high-strength high-hardness materials where reliable determination of fracture toughness is crucial. However, for proper selection, optimization, and heat treatment of tool steel, fracture toughness data and especially its scatter are very important and need to be precisely determined and supported by other material properties. Therefore, the aim of the presented research work was to investigate the suitability of Circumferentially Notched and Precracked Tension Bar specimen (CNPTB) for evaluating fracture toughness of tool steels and to correlate fracture toughness properties of hot work tool steel to the local variation in microstructure, microhardness, and composition. Results of the work show that CNPTB specimen allows simultaneously assessment of basic properties such as hardness, fracture toughness, compressive strength, etc., which can be directly correlated to the vacuum heat treatment parameters and microstructure. Furthermore, presence of any weak point, either in a form of non-metallic inclusions, large undissolved eutectic carbide clusters, and bands, or their combination, especially if located in the region of positive segregation and close to the fatigue crack tip, will lead to considerable reduction in hot work tool steel fracture toughness.