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  • MK_mk17799 - 16.4.03/druckhaus kthen

    L. L. Mishnaevsky Ir. et al.: Micromechanisms and modelling of crack initiation and growth in tool steels

    Leon L. Mishnaevsky Jr.a, Nils Lippmannb, Siegfried Schmauderaa Staatliche Materialprfungsanstalt (MPA), University of Stuttgart, Stuttgart, Germanyb Robert Bosch GmbH, Stuttgart, Germany

    Micromechanisms and modelling of crackinitiation and growth in tool steels:role of primary carbidesDedicated to Professor Dr. Otmar Vhringer on the occasion of his 65th birthday

    Micromechanisms of damage initiation and crack growth inhigh speed and cold work steels are investigated using scan-ning electron microscopy in situ experiments. The role ofprimary carbides in initiation and growth of cracks in toolsteels is clarified. It is shown that initial microcracks in thesteels are formed in primary carbides and then join together.A hierarchical finite element model of damage initiation,which included a macroscopic model of the deformation ofthe specimen under real experimental conditions and a me-somechanical model of damage in real microstructures ofsteels, was developed. Using the hierarchical model, theconditions of local failure in the steels have been obtained.

    In order to study the effect of carbide arrangement on frac-ture, numerical simulations of fracture in steels with differ-ent ideal carbide arrangements were carried out and com-pared with each other. It is found that the heterogeneousarrangements of primary carbides can lead to strong devia-tions of a crack from the mode I path and, therefore, to asignificant increase of the fracture energy of the steels.

    Keywords: Mesomechanics; Finite elements; Fracture; Da-mage; Tool steels

    Mikromechanismen und Modellierung derRissinitiierung und -ausbreitung in Werkzeugsthlen:Rolle von Primrkarbiden

    Mikromechanismen der Schdigungsinitiierung und -evo-lution in Schnellarbeits- und Kaltarbeitssthlen werden un-ter Verwendung von ln-situ-Rasterelektronenmikroskopie-Biegeversuchen untersucht. Die Rolle der Primrkarbidebei der Schdigungsinitiierung und beim Risswachstum inWerkzeugssthlen wird geklrt. Es wurde gezeigt, dass sichdie Anfangsmikrorisse in den Primrkarbiden bilden unddann zusammenwachsen. Ein hierarchisches Finite-Ele-

    mente- Modell der Schdigungsinitiierung, das auch dasreale Mikrogefge des Stahls bercksichtigt, wurde ent-wickelt. Mit dem Mesomodell wurden die lokalen Bruchbe-dingungen bestimmt. Um den Einfluss der Primrkarbid-anordnungen auf das Bruchverhalten der Sthle zuuntersuchen, wurden numerische Simulationen des Bruchsin Sthlen mit unterschiedlichen idealen Karbidanordnun-

    Z. Metallkd. 94 (2003) 6 earl Hanser Verlag, Mnchen

    gen durchgefhrt und die Ergebnisse miteinander vergli-chen. Es wurde gefunden, dass die heterogenen Karbid-verteilungen in den Sthlen die starken Risspfad-abweichungen von dem Mode-I-Risspfad verursachen undsomit zu signifikant hheren Bruchenergien fhren.

    1. Introduction

    The improvement of service properties of tool steels pre-sents an important source of increasing the efficiency ofmetalworking industry. In order to develop a numericalmodel of damage or fracture in the steel, which shouldserve to predict the lifetime, or to improve the properties,one needs to know the mechanisms of damage and fracturein the steels [1-4]. The direct in situ observation of thefracture mechanisms of the steels under a microscope isquite difficult as compared with the case of more ductile .materials, since the material fails abruptly. Then, not onlyqualitative parameters of fracture (like its mechanisms) butalso quantitative ones (like critical damage parameters) areof interest.

    The purpose of this work was to study the mechanismsand conditions of damage initiation and growth in the toolsteels both qualitatively and quantitatively. The work in-cludes the following steps: Scanning electron microscopy (SEM) in situ experi-

    ments on 3-point bending of specimens with inclinednotches.

    Finite element (FE) simulation of the deformation of thespecimens on macro- and mesolevel, taking into ac-count the real microstructure of the steels observed inthe SEM -experiments.

    Numerical analysis of the effect of the arrangement ofprimary carbides in the tool steels on the fracture beha-VIOr.

    2. Micromechanisms of damage initiation in toolsteels

    The mechanisms of local failure and critical values for fail-ure of the constituents of the steel have been determined.The constitutive law and elastic constants of the steel con-

  • MK_mk17799 - 16.4.03/druckhaus kthen

    L. L. Mishnaevsky Ir. et aI.: Micromechanisms and modelling of crack initiation and growth in tool steels

    Fig. 1. 3-point bending specimen: (a) scheme and (b) sideview of loading device.

    stituents are already known from literature and from ourprevious investigations [3, 4, 6]. The analysis of the me-chanisms of damage initiation in the tool steels includesSEM in situ experiments and FE simulation of the deforma-tion of the specimens on macro- and meso level. The SEMin situ observation of the damage initiation seeks to c1arifythe micromechanisms of damage initiation, whereas thehierarchical finite element model (macro- and mesomodel)is applied to determine the failure conditions for steel con-stituents using the real loading conditions and real micro-structures of the steel.

    In order to c1arify the mechanisms of damage initiationand growth in the steels, aseries of SEM in situ experimentswas carried out. 3-point bending specimens with an inclinednotch, as described in [5], were used in these tests. Thesespecimens allow to observe the micro- and mesoprocessesof local deformation and failure of carbides and the matrixof steels during loading of macroscopic specimens in theSEM. The shape of the specimens is shown schematicallyin Fig. 1a. A photograph of the specimen under loading isgiven in Fig. 1b. The advantage of the specimen with the in-clined notch is that the most probable location of first mi-crocrack initiation in the specimen notch can be simply pre-dicted (which is not the case in the conventional 3-pointbending specimens). Therefore, one can observe this loca-tion with high magnification during loading and identifyvery exactly the load and the point in time at which the firstmicrocracks form.

    Specimens made from the cold work steelX155CrVMo12-1 (in further text denoted as KA) and thehigh speed steel HS6-5-2 (denoted as HS) have been used.In the experiments, the specimens with different orienta-tions of primary carbide layers were studied. Since the toolsteels are produced in the form of round sampIes and be-cause they were subject to hot reduction after austenitiza-tion and quenching, they are anisotropic: the carbide layersare oriented typically along the axis of the cylinder (this isthe direction of hot reduction). Therefore, the followingdesignation of the specimen orientation was used: L - thedirection along the carbide layers, R - radial direction inthe workpiece, C - the direction along the workpiece axis.In the experiments, specimens with orientations CL (thespecimen is oriented along the carbide layers; the observedarea is oriented along to the ingot axis), LC (the specimenis oriented along the round ingot axis) and CR (the speci-men is oriented along the carbide layers; the observed areais oriented normally to the ingot axis) have been used. Thespecimens have been subjected to the heat treatment (hard-ening at 1070C in vacuum and tempering 2 h at 510 0c),and then polished with the use of the diamond pastes tillthe roughness Rz of the surface of the specimens does notexceed 311m. The notch region of the specimens was etched

    2

    Table 1. Critical forces in the tests.

    Type of Force at which a firstForce at which thethe

    microcrack was observedspecimen failed (N)specimen

    in the specimen (N)

    KALC

    95,52,37.5155,85,160KACR

    50,55,37.595,95,70HSCR

    45,50,5095,80,95HSLC

    50,72.52, 127200, 190, 195

    with 3 and 10 % HN03 until the carbides were c1early seenon the surface.

    The force-displacement curves were recorded during thetests. The loading was carried out in small steps, with a rateof loading of about 1 mmJs. The places in the specimennotch where microcrack initiation was expected have beenobserved through SEM during the tests. It was observed thatthe first microcracks formed only in the primary carbides,and not in the "matrix" of the steel. Also, no microcrackalong the carbide/matrix interface was observed in the tests.The forces at which the failure of primary carbides was ob-served in each specimen are given in Table 1. Fig. 2 showsSEM micrographs of a typical primary carbide in the notchregion of steels before and after failure. Since the picturewere taken frontal and the specimens used were with theinclined notch, the magnifications of micrographs in x-and y-directions in Fig. 2 are different. In some carbides,multiple cracking was observed as weil (see Fig. 3).

    E5-o

    40pm

    Fig.2. Carbide grains before (a, c) and after failure (b, d).

    Z. Metallkd. 94 (2003) 6

  • MK_mk17799 - 16.4.03/druckhaus kthen

    L. L. Mishnaevsky Ir. et al.: Micromechanisms and modelling of crack initiation and growth in tool steels

    Ea-o~

    40l1mFig. 3. Multiple cracks in a primary carbide.

    Fig.4. Microcracks (shown by arrows) in primary carbides.

    Generally, the course of failure of the specimens was asfolIows:1. Formation of a microcrack at some carbide.

    2. Formation of several microcracks at many carbides atdifferent locations of the observed area (in so doing,the microcracks are formed rather at larger carbides atsome distance from the boundary of the specimen, thanin more strained macroscopically areas in the vicinityof the lower boundary of the specimen; the local fIuc-tuations of stresses caused