Materials technology Correlation of microstructure and failure initiating sites in an AISI M2 high- speed steel using Vickers indentation Bilgehan Ogel and Erdogan Tekin 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-Schnellar- beitsstahl 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. Dr. Bilgehan Ogel; Professor Dr. Erdogen Tekin, Middle East Technical University, Ankara, Turkey. A reformulation of the reciprocal of the parameter a gives: alloy and also it is very easy to calculate. The dimension (force per unit length) corresponds to energy per unit area and suggests a simple relationship to a physically signifi- cant property of the material which may be defined as crack resistance parameter. Nevertheless, Palmqvist's method is not widely used mainly for two reasons: i) the relationship between crack length and the techno- logical properties of the material (wear and shock re- sistance) is not clear; ii) the crack length is strongly dependent on surface treat- ment, by which precise and reproducible results are hard to obtain"). As far as gathered from the literature, this method has not been applied to high-speed steels yet, probably due to difficulties in producing corner cracks in those steels. In the present study it was considered that the microstructure of high-speed steels was particularly suitable for the ap- plication of this method. The microstructure of high-speed steels in the hardened condition consists of alloy carbides dispersed in a marten- sitic matrix. In the AISI M2 high-speed steel, the undis- solved alloy carbides are mainly M 6C and MC type; M 6C is a Wand Mo based carbide, whereas MC is a V based carbide. Due to their hard and brittle nature, these excess carbides provide wear resistance of the cutting tool. Car- bides in the matrix of high-speed steels arouse considerable interest, as it is generally believed that they are detrimental to toughness. There is sufficient evidence for the deterio- ration effect of carbides on mechanical properties such as bend strength7) and impact toughness"). The results of re- cent studies on static bend tests and impact tests of high- speed steels indicate that coarse carbides and severe band- ing cause a considerable decrease in toughness and de- forming capability of these steels. In this study, the aim was not to estimate the fracture toughness, but to relate the initiation and propagation of (2) (1) L = «-P. P w= -. L The advantage of this parameter is that its numerical value is independent of the load and of the hardness of the The simplicity and quickness of hardness testing has long attracted the researchers to correlate hardness with other mechanical properties of various materials. Among these properties tensile strength, ductility and fracture toughness may be mentioned. The earliest work in this field is on the relation between tensile strength and hardness. Today, fol- lowing the approaches of Dugdale 1 ), Shaw et al. 2 ) and Cahoon et al.:') who used the slip-line field theory, a good agreement between hardness and tensile properties of the materials can be obtained. Determination of ductility from the hardness impression was suggested by Boklen"). He proposed a model of the upward plastic flow around the indentation. Relating the height (h) of the pile-up wall near the indentation to work- hardening exponent of the material, a good correlation was shown between the h and the uniform elongation for many kinds of metals such as steels, aluminium and copper al- loys. Evaluation of fracture toughness of cemented carbide- cutting tools by Vickers indentation is a method developed by Palmqvisr'). It consists of applying a Vickers hardness indentor to a polished surface of the material using suffi- ciently high loads to cause cracking at the corners of the indentation. Based on empirical observations, the total length, L, of the cracks is linearly related to the load P: 624 steel research 61 (1990) No. 12