In this study, the effect of double austenitization on microstructure and toughness of AISI M2 high speed steel was investigated. For double austenitization treatment, the specimens, which are hardened initially at 1220°C and quenched in air, were hardened for a second time in the temperature range 1150 – 1050°C. For comparison purposes, another set of specimens is austenitized singly in the temperature range 1150 – 1050°C. Tempering process was carried out between 500 – 640°C. A double austenitization causes a fine carbide precipitation in the matrix, having sizes in the range of 0.10 ± 0.05 μm and volume fractions of between 1 and 6%. It is shown that a double austenitization treatment causes a decrease in fracture toughness (KIc), when compared with single austenitized ones. The reason for the lower KIc values of double austenitized specimens are attributed to these fine carbide precipitates: It is suggested that they limit the plastic deformation capability of the matrix and yield lower fracture toughness values.
In this study, the relation of crack initiation and propagation to microstructure was shown in an AISI M2 high-speed steel using Vickers hardness indentation. Observations were carried out on the cracks generated from the corners of the square pyramid indentation in both the as-quenched, the quenched and tempered specimens. It is shown that cracks are nucleated from the corners of the indentation by either cleavage of the carbide or by decohesion between the carbide and the matrix, depending on the size of the carbide. A nucleated crack propagates by tearing of the martensitic matrix between the cracked carbides ahead of the indentation corner. It is shown that the crack resistance of the M2 steel is determined by the toughness of the matrix rather than the size and distribution of the carbides in the matrix. Beziehung zwischen Gefiige und brucheinleilenden Siellen in einem AISI·M2·Schnellarbeilsslahl anhand von Vickers·Eindriicken. In dieser Arbeit wird der Zusammenhang zwischen RiBeinleitung bzw. -fortschritt und dem GefUge in einem AISI-M2-Schnellarbeitsstahl anhand von Vickersharteeindrucken gezeigt. Es wurden solche Risse beobachtet, die von den Ecken des quadratischen Pyramideneindruckes ausgehen. Diese Untersuchungen wurden sowohl im abgeschreckten Zustand als auch am verquteten Stahl durchgefUhrt. Es wird gezeigt, daB die RiBkeimbildung von den Ecken der Pyramideneindrucke ausgeht, und zwar entweder durch Carbidspaltung oder durch Dekohasion von Carbid und Matrix, je nach GroBe der Carbide. Verantwortlich fUr den RiBfortschritt ist das AufreiBen der martensitischen Matrix zwischen den gespaltenen Carbiden vor der Ecke des Eindruckes. Es wird gezeigt. daB der RiBwiderstand des AISI-M2-Stahls eher von der Zahiqkeit der Matrix bestimmt wird als von der GroBe und Verteilung der Carbide in der Matrix.
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