Design of advanced tempering diagrams of tool steels using multifunctional CNPTB specimen

Properties of tool steels depend on the balanced chemical composition and the processing route, but mainly on the heat-treatment process, which defines final microstructure. Traditionally, trade-off between high hardness and sufficient toughness is required. On the other hand, vacuum heat treatment allows optimization of the tool steel microstructure, which satisfies ever greater demands on tool properties, particularly in respect of greater fracture toughness while maintaining high hardness. In this respect hardness and fracture toughness are the most common properties used for optimizing vacuum heat treatment of tool steels. However, beside hardness and toughness there are also other tool steel properties, which depending on the application are becoming important as we move toward more and more complex tools. These include creep and wear resistance, compressive and bending strength as well as machinability. Although all these properties can be determined using standard test methods, each one requires specific and often quite unique and demanding test specimens. Furthermore, different geometries of standard specimens mean different heat treatment conditions, which make it practically impossible to directly correlate properties of tool steels after heat treatment. On the other hand, circumferentially notched and fatigue-precracked tensile bar (CNPTB) specimen has been found as the best alternative, which after fracture toughness measurement can be used to determine many other properties. The advantage of the CNPTB specimen lies in the radial symmetry, which makes the specimen particularly suitable for studying the influence of tool steel properties and to optimize its heat treatment.

The aim of this research work was to determine the applicability of a single CNPTB test specimen for determination of a wide range of hot work tool steel properties and their correlations, including hardness, fracture toughness, compressive and bending strength, work hardening exponent, wear resistance and machinability. Furthermore, work is intended to show the potential of CNPTB test specimen for designing advanced tempering diagrams and optimizing vacuum heat treatment of hot work tool steels depending on the application.