Experimental Evaluation of Tool and High-Speed Steel Properties Using Multi-Functional KIc-Test Specimen

Significance statement

 With the introduction of cylindrical specimen with a notch and fatigue pre-crack in the notch root (KIc-test specimen) a method which allows the determination of fracture toughness of brittle metallic materials have been developed. The method is based on the fatigue pre-crack formation under rotary bending mode prior to the final heat treatment and fracture of heat treated sample under tensile load. By measuring the diameter of the quickly fractured area and knowing the maximum tensile load at fracture, fracture toughness of the material can then be calculated. Axially symmetric geometry of the specimen allows the study of the heat treatment and microstructure effect on the properties of tool steels, mainly fracture toughness and hardness. Non-standard method for measuring fracture toughness allows the formation of tempering diagrams, which allow determination of the optimal heat treatment parameters for a specific application of tool steel. In addition, KIc-test specimens allow determination of multiple tool steel properties, including fracture toughness, hardness, compressive and bending strength, thermal conductivity, machinability and grindability, nitridability, hardenability and wear resistance. Furthermore, since all properties are determined using the same master specimen correlation between different mechanical and technological properties can be investigated and correlated to the microstructure.       

 

Experimental Evaluation of Tool and High-Speed Steel Properties Using Multi-Functional KIc-Test Specimen

Journal Reference

Steel Research International, Volume 84, Issue 12, pages 1294–1301, December 2013.

Bojan Podgornik* and Vojteh Leskovšek
Institute of Metals and Technology, Ljubljana, Slovenia.

ABSTRACT

Normally, hardness and fracture toughness are used to determine the influence of vacuum-heat-treatment parameters and to optimize it for the specific operating conditions of the tool. However, there are also other tool properties which are equally important and need to be taken into consideration. To determine such a wide range of properties different test procedures and different test specimens are required since none of the standard tests alone is capable of providing relevant properties completely. Currently the best overall appraisal of tool and high-speed steel applicability seems to be a combination of fracture toughness, bending, or compression testing and in specific cases of impact or small-punch creep test. This paper clearly demonstrates usability of a single KIc-test specimen for determination of a wide range of properties being important for tool and high-speed steels, and their vacuum-heat-treatment optimization. In the case of ESR AISI M2 high-speed steel low tempering and austenitizing temperatures are preferable in terms of ductility and toughness, high tempering and lower austenitizing temperatures in terms of bending strength and high-speed machinability, while high austenitizing and mid tempering temperatures in terms of hardness, compression strength and grindabillity.

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