THE INFLUENCE OF MICROSTRUCTURE OF MOTTLED CAST THE INFLUENCE OF MICROSTRUCTURE OF MOTTLED CAST IRON...

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Transcript of THE INFLUENCE OF MICROSTRUCTURE OF MOTTLED CAST THE INFLUENCE OF MICROSTRUCTURE OF MOTTLED CAST IRON...

  • METAL 2009 19. – 21. 5. 2009, Hradec nad Moravicí

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    THE INFLUENCE OF MICROSTRUCTURE OF MOTTLED CAST IRON

    FOR MILL ROLLS ON ITS PROPERTIES

    Janusz KRAWCZYK, Jerzy PACYNA

    AGH University of Science and Technology Faculty of Metals Engineering and Industrial Computer Science

    Department of Physical and Powder Metallurgy A. Mickiewicza Av. 30, 30-059 Krakow, Poland

    E-mail: jkrawczy@metal.agh.edu.pl, jkrawczy@ruczaj.pl

    ABSTRACT

    Mottled cast iron are widely used for mill rolls due to its content of both ledeburite and graphite in the microstructure. However, improper microstructure morphology of the cast iron may foster cracking or faulty fretting of mill rolls that had been made of such material. The purpose of this paper was to determine the influence of heat treatment induced changes in microstructure of mottled chromium-nickel cast iron on its properties. Heat treatment was conducted in order to obtain various morphology of ledeburite and graphite precipitates in bainitic matrix. Heat treatment consisted of under-annealing normalizing. The differences in morphology of transformed ledeburite and graphite were obtained by change of cooling rate within the range of austenite existence. Increase of cooling rate within this range resulted in the increase of transformed ledeburite content and decrease of graphite content in microstructure of cast iron. Bainitic matrix was obtained on the way of suitable cooling rate within the range of phase transformations existence. This cooling rate was matched on the basis of CCT diagram for this cast iron. Morphology of above mentioned precipitates were described using stereological parameters. Changes in microstructure affected hardness, impact strength, tensile and bending strength as well as stress intensity factor KIc. Increase of transformed ledeburite content along with decrease of graphite content in microstructure of investigated cast iron resulted in increase of hardness with decrease of impact strength, stress intensity factor KIc, tensile and bending strength as well as yield point. Thus, it was determined, to what extent is the pursuit of higher hardness (in order to lower tribological wear) creating a risk of roll fracture. Test results obtained in this paper shall allow to design an optimal microstructure of mill rolls. 1. INTRODUCTION

    Mill rolls are among the most expensive tools used in plastic working processes and must meet a number of criteria that will allow them to be admitted to work in the industry. Material from which the most frequently a roll for hot rolling is made of is cast iron, which is characterized by good operating properties. Cast iron rolls due to its fracture toughness and tribological properties are most often used in last rolling stands, where the least dynamic loads are present and the most important is surface quality of rolled product. The most important role in the microstructure of cast irons used for mill rolls is played by carbides. Most frequently it is ledeburitic cementite. Improper morphology of ledeburitic cementite may lead to adverse mechanisms of rolls wear (KRAWCZYK 2007, KRAWCZYK 2008a, KRAWCZYK KARAWAT SZCZYGIEŁ LATAŁA 2006, KRAWCZYK NOWAK ŻABA KAWECKI 2007, PACYNA KOKOSZA KRAWCZYK 2001, KRAWCZYK PACYNA 2006, KRAWCZYK PACYNA

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    FRONCZEK JUSZCZAK 2007, KRAWCZYK PACYNA SZCZYGIEŁ LATAŁA 2006, PACYNA KRAWCZYK 2005) e.g. roll collar break off, roll neck wrench off, etc. (KRAWCZYK 2008b, KRAWCZYK PACYNA 2007, KRAWCZYK PACYNA GRABOWSKI SIKORA 2008, KRAWCZYK PACYNA KĄC JUSZCZAK LIWOCH GRABOWSKI 2007, KRAWCZYK PACYNA LIWOCH GRABOWSKI JUSZCZAK 2007, KRAWCZYK TERCZYŃSKI SIKORA 2008). Similar problems may be a result of presence of phosphide eutectic or hypereutectoid cementite precipitated in form of net along grain boundaries of former austenite. By the heat treatment the microstructure of matrix and the morphology of carbide precipitates may be modified (KRAWCZYK PACYNA ZAJĄC 2001, PACYNA KRAWCZYK 2001, PACYNA KRAWCZYK ZAJĄC 2002), and thus affect cast iron properties (KRAWCZYK PACYNA KOKOSZA 2004, PACYNA KRAWCZYK ZAJĄC 2002, ZAJĄC 2001). However, such a research should be previously carried out in laboratory renge. The objective of this work was to determine the influence of microstructure changes on GJS-HV300 (SiNiCr2-3) cast iron properties. Obtained results shall be used for design of heat treatment of mill rolls in order to receive an optimal microstructure for operating parameters of that type of tools.

    2. TEST MATERIAL

    Test material was a chromium-nickel mottled nodular cast iron GJS- HV300(SiNiCr2-3). Chemical composition of investigated cast iron is presented in Table 1. The microstructure in as-delivered condition is presented in Figure 1. a) b)

    c) d)

    Fig. 1. Microstructure of investigated cast iron (as-delivered condition). Etched with 2% nital

  • METAL 2009 19. – 21. 5. 2009, Hradec nad Moravicí

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    Table 1. Chemical composition (weight %) of the investigated GJS-HV300(SiNiCr2-3) cast iron

    C Mn Si P S Cr Ni Mo

    3.2 0.52 2.24 0.07 0.012 0.53 3.49 0.56

    Mg Cu AL Ti V As Nb Fe

    0.047 0.20 0.008 0.014 0.020 0.002 0.024 bal.

    One may see that, it is mottled cast iron with pearlite-bainitic matrix (with upper bainite). It is characterized by high content of ledeburitic cementite (with adjoined hypereutectoid cementite), which creates a continuous net. Test material was sampled from cast roll delivered by the manufacturer. The samples were collected in a way that allowed collecting them from the places where the conditions of crystallization would be possibly the same. In case of notched impact strength test samples the notch was made along the plane perpendicular to roll’s axis. While in case of KIC samples the notch was made parallel to roll’s axis, along its radius.

    3. HEAT TREATMENT

    Modification of investigated cast iron microstructure was carried out by heat treatment. Two variants of heat treatment were performed diversified by cooling rate within the range of austenite presence. Austenitizing temperature was 950°C. The temperature was matched on the basis of research presented in studies (ZAJĄC 2001, WASYLEWICZ 2002, WAŻNY 2005), as well as so to comply with technological potential of the manufacturers of mill rolls and to ensure the highest possible solubility of ledeburitic cementite (with adjoined hypereutectoid cementite) in austenitic matrix. First variant of heat treatment was characterized by low cooling rate (12°C/h) within the range of austenite presence (to the temperature of 700°C). Further cooling was carried so to ensure bainitic matrix (216°C/h). Second variant of heat treatment was characterized by high cooling rate (45°C/h) within the range of austenite presence (to the temperature of 700°C). Further cooling was carried so to ensure bainitic matrix (216°C/h). The applied heat treatments resulted in distinct changes of investigated cast iron microstructure. The microstructures after corresponding variants of heat treatment are presented in Figure 2. The heat treatment, irrespective of the variant applied, caused a clearly visible fragmentation of ledeburitic cementite precipitations and distinct reduction of its content in the volume of the cast iron. One may also notice the increase of graphite precipitations size and increase of its content in the volume of the cast iron. It indicates that during heat treatment of cast irons containing graphite a part of carbon would always diffuse towards these precipitates in result of which the matrix shall be carbon impoverished and the amount of ledeburitic cementite shall be reduced. Selection of cooling rate during heat treatment within the range of eutectoid transformation on the basis of CCT diagram allowed to receive a desired (expected) bainitic structure in the matrix of investigated cast iron. It can be noticed that fraction of lower bainite increases (from 15 to 18%), while of upper bainite decreases (from 63 to 53%) with the increase of cooling rate within austenitic range. The differences of cooling rates within austenitic range have also a significant influence on the amount of ledeburitic cementite (with adjoined hypereutectoid cementite). One may observe that increase of cooling rate results in increase of amount of ledeburitic

  • METAL 2009 19. – 21. 5. 2009, Hradec nad Moravicí

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    cementite (with adjoined hypereutectoid cementite) in microstructure of investigated cast iron. While, in as-delivered state there was about 39% of this carbide phase present in the microstructure of cast iron, after heat treatment its amount decreased to about 16% in case of variant I and about 25% in case of variant II. Explanation of the above differences between the two amounts of ledeburitic cementite requires the analysis of changes in graphite content. It was found that decrease of cooling rate within austenite range leads to the increase of graphite amount in matrix of investigated cast iron. While, in as-delivered state there was about 3% of graphite present in the microstructure of cast iron, after heat treatment its amount increased to about 6% in case of variant I and about 4% in case of variant II. a) b)

    c) d)

    Fig. 2. Microstructure of investigated cast iron after heat treatment: a,b) variant I, c,d) variant II. Etched with 2% nital

    4. RESEARCH RESULTS OF MECHANICAL PROPERTIES AND ITS

    DISCUSSION

    In spite