THE INFLUENCE OF MICROSTRUCTURE OF MOTTLED CAST IRON FOR MILL ROLLS ON ITS PROPERTIES. Janusz KRAWCZYK, Jerzy PACYNA

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1 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, Krakow, Poland 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 K Ic. 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 K Ic, 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 1

2 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 GJSHV300(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 2

3 Table 1. Chemical composition (weight %) of the investigated GJS-HV300(SiNiCr2-3) cast iron C Mn Si P S Cr Ni Mo Mg Cu AL Ti V As Nb Fe 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 K IC 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 3

4 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 of obtaining of bainitic structure only in the matrix of cast iron, the heat treatment results in decrease of its hardness in relation to as-delivered state (from 524 HV30 to 400 HV30 in case of heat treatment in variant I and to 440 HV30 in case of heat treatment in variant II). It is a result of lower content of ledeburitic cementite (with adjoined hypereutectoid cementite) in the microstructure of samples after heat treatment in relation to as-delivered state. However, in each case of heat treatment applied the hardness is above 300 HV, so it is high enough to use this material for mill rolls production. One may also notice that raising the cooling rate within austenite 4

5 range for a given matrix results in increase of hardness. It is caused by the differences in microstructure (especially in volumetric balance between ledeburitic cementite and graphite). Heat treatment also affected the impact resistance of investigated cast iron. The samples were broken using Charpy vintage impact test machine with initial potential energy of 300 J. Test was performed at room temperature. The same impact resistance characterized cast iron in as-delivered state and after second variant of heat treatment (impact energy KV=3.3 J). While, in case of cast iron after variant I of heat treatment impact energy KV increased to 4 J. It indicates a beneficial influence of ledeburitic cementite content decrease and increase of its fragmentation degree on impact strength (in case of bainitic matrix). Whereas, heat treatment significantly increased stress intensity factor K IC (from ca. 24 MPa*m 1/2 in case of as-delivered state to ca. 30 MPa*m 1/2 in case of variant I of heat treatment and to ca. 28 MPa*m 1/2 in case of variant II of heat treatment). The samples used had dimensions of 15x30x150 mm with a notch roof 2 mm wide and 13 mm deep at flank surface and with V dihedral angle 120. Notch bottom was V- shaped with angle of 90. Notch bottom radius was 0.25 mm. The samples contained an about 2 mm long fatigue pre-crack. So prepared samples were subjected to threepoint bending test. In each sample there was a plane strain scheme maintained, and this is why determined K Q equals K IC. It can be observed that higher resistance to crack propagation characterizes cast iron that was cooled at lower rate within austenite range. One should connect it with the fraction of ledeburitic cementite the more of such cementite the lower is the resistance to crack propagation. Analogically one may state that increase of graphite amount results in increase of resistance to crack propagation. However, it should be noted that in case of present research the decrease of ledeburite fraction in microstructure was connected with increase of graphite amount. Similarly as in case of impact strength and K IC testing the bending strength R g of investigated cast iron in case of applied heat treatment was greater than in asdelivered state (there is an increase of R g from about 698 MPa in case of asdelivered state to 1003 MPa in case of variant I of heat treatment and to 891 MPa in case of variant II of heat treatment). A static bending test was conducted with three point load (the sample was subjected to load with force imposed in the middle point of length). Cylindrical samples for static bending test were 110 mm long and had diameter d 0 =10 mm. Working length was (support spacing) l=100 mm. It is easy to find that decrease of ledeburitic cementite amount and increase of fragmentation degree of its precipitates leads to increase bending strength. Due to roll geometry, which the samples were cut out from for mechanical testing, it was not possible to comply with the rule of their collection each time from the places characterized by the same crystallization conditions. Due to above, it was decided to collect the sample for tensile test in as-delivered condition from the place located between outer and inner area along roll s radius (R m =327 MPa). Test was performed on samples with following dimensions: diameter d 0 =10 mm, length L 0 =50 mm and cross section of S 0 =78,5 mm 2. In case of each of analysed heat treatment variants one sample was collected from the vicinity of the surface, while the second from the part of the roll the closest to its axis. This is the reason why only in case of as-delivered state it was possible to determine the distribution of test results. Whereas in case of samples after heat treatment a single result for inner (in case of variant I of heat treatment MPa, in case of variant II of heat treatment 351 MPa) and outer part of the roll (in case of variant I of heat treatment MPa, in 5

6 case of variant II of heat treatment 571 MPa) was obtained. Because the samples for the as-delivered state were breaking within elastic strain range it was not possible to determine offset yield strength R p0.2. However, it was possible to determine the R p0.2 for samples after heat treatment collected from the outer part of the roll (in case of variant I of heat treatment 570 MPa, in case of variant II of heat treatment 520 MPa). This is the reason why only for these samples the R m parameter was determined in accordance with guidelines from the standard (samples breakage occurred within the range of plastic strain). After analysis of obtained results of mechanical strength one may state that the area from which the sample was collected has significant influence on this parameter. Samples collected from the vicinity of roll s surface are characterized by higher mechanical strength than the ones collected from the area closest to the axis of the roll. One may observe that determined tensile strength after heat treatment was higher than R m determined for as-delivered state. The best mechanical properties characterized investigated cast iron after variant I of heat treatment i.e. a variant of heat treatment that was most beneficial from impact resistance K IC and R g point of view. Also the offset yield strength is the highest in case of variant I of heat treatment i.e. cooling at low rate within austenite range (12 C/h). Therefore it is allowed to state that, irrespectively to area of sample collection, decreased amount and refinement of ledeburitic cementite results in increase of tensile strength in case of bainitic matrix. 5. CONCLUSIONS Research carried out in present study allowed to formulate the following conclusions: Heat treatment allows to change the microstructure and properties of investigated cast iron within wide range. Decrease of hardness occurs as a result of heat treatment even if the matrix is 100% bainitic. However, hardness obtained in result of heat treatment is sufficient for the investigated cast iron to be used for mill rolls. In case of bainitic matrix the lowering of ledeburite cementite fraction and its fragmentation results in increase of impact strength of investigated cast iron. Decrease of ledeburitic cementite fraction in connection with increased amount of graphite results in increased fracture toughness. In result of applied heat treatment an increase of bending strength of investigated cast iron is obtained. Decrease of ledeburitic cementite amount, increase of both its fragmentation degree and the fraction of graphite is accompanied by the increase of flexural strength. Changes in crystallization conditions (outer and inner part of the roll) affect cast iron mechanical properties. Heat treatments applied in this study increase mechanical strength of investigated cast iron. In case of bainitic matrix, a decrease of ledeburitic cementite fraction and especially refinement of its precipitates, irrespectively to increase of graphite fraction, results in the increase of mechanical properties of investigated cast iron. ACKNOWLEDGEMENTS The authors would like to thank Adam Kokosza, Dariusz Tyrka and Artur Szopiński for help in this research. 6

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