UDK 669.131.6:536.421.4	ISSN 1580-2949
Professional article/Strokovni članek	MTAEC9, 46(4)389(2012)
NUMERICAL AND EXPERIMENTAL ANALYSES OF THE CHEMICAL HETEROGENEITY OF A SOLIDIFYING HEAVY DUCTILE-CAST-IRON ROLLER
NUMERIČNA IN EKSPERIMENTALNA ANALIZA KEMIJSKE HETEROGENOSTI STRJEVANJA LITEGA TEŽKO GNETLJIVEGA
ŽELEZNEGA VALJA
Karel Stransky1, Frantisek Kavicka1, Bohumil Sekanina1, Jana Dobrovska2,
Vasilij Gontarev3
1Bmo University of Technology, Technicka 2, 616 69 Brno, Czech Republic 2VSB TU Ostrava, 17.listopadu 15, 708 33 Ostrava, Czech Republic 3University of Ljubljana, Aškerčeva 12, 1000 Ljubljana, Slovenia stransky@fme.vutbr.cz
Prejem rokopisa - received: 2011-10-17; sprejem za objavo - accepted for publication: 2012-02-24
The quality of the working rollers used for rolling rails is determined by the chemical and structural compositions of the material of the rollers and the production technology. It is necessary to cast rollers with significantly improved utility properties, i.e., mainly a high wear resistance and optimal mechanical and structural properties. It is, therefore, necessary to find and ensure the optimal relationships between the matrix structure and the resulting values of the mechanical properties of the rollers in order to maximize their life time. The requirements introduced here cannot be ensured without a knowledge of the kinetics of the solidification. Therefore, numerical and experimental investigations of the temperature field of the solidifying roller were conducted. The kinetics of the solidification has a measurable and non-negligible influence on the chemical and structural heterogeneity of the investigated type of ductile cast-iron. Linking to the results of the model of the temperature field of the cast rollers, an original methodology was developed for the measurement of chemical micro-heterogeneity. The structure of this cast-iron is created by a large amount of the transition form of graphite and a small amount of globular graphite, and also the lamellar graphite and cementite, whereas the structure of the metal matrix is perlitic. The volume amounts of the structural components were determined using a quantitative metallographic analysis, according to which the places for the analysis of the element composition using X-ray energy-dispersive spectral micro-analysis were selected. The chemical and structural heterogeneity of the cast roller is, therefore, a significant function of the method of melting, modification and inoculation and the successive procedures of risering, casting and crystallization after cooling.
Keywords: spheroidal graphite cast iron, roller, solidification, chemical and structural heterogeneity, methodology for measurement
Kakovost delovnih valjev za valjanje tirnic je določena s kemijsko in strukturno sestavo materiala za valje in s tehnologijo proizvodnje. Potrebno je ulivati valje z znatno povečanimi koristnimi lastnostmi, kot so obrabna odpornost in optimalne mehanske in strukturne lastnosti. Tako je treba ugotoviti in zagotoviti optimalne odnose med strukturo matrice in rezultirajočimi vrednostmi mehanskih lastnosti valjev za maksimiranje trajnostne dobe. Zahteve, ki so tu predstavljene, ne morejo biti zagotovljene brez znanja kinetike strjevanja. Zato je bila vpeljana numerična in eksperimentalna raziskava temperaturnega polja valja pri strjevanju. Kinetika strjevanja ima merljiv in ne brezpomemben vpliv na kemijsko in strukturno heterogenost raziskovanega gnetljivega litega železa. Glede na rezultate modela temperaturnega polja ulitih valjev je bila razvita originalna metodologija za merjenje kemijske mikroheterogenosti. Struktura tega ulitega železa je bila ustvarjena z veliko količino prehodnih oblik grafita in manjših količin kroglastega grafita in cementita, medtem ko je struktura kovinske osnove perlitna. Volumenska količina strukturnih komponent je bila določena s kvantitativno metalografsko analizo, po kateri so bila izbrana mesta za rentgensko analizo sestave ter spektrografsko mikroanalizo. Kemijska in strukturna heterogenost ulitih valjev je pomembna funkcija pri taljenju in modificiranju ter za uspešno ulivanje in kristalizacijo po ohlajevanju.
Ključne besede: siva litina s kroglastim grafitom, valj, strjevanje, kemijska in strukturna heterogenost, metodologija meritev
1 INTRODUCTION
2 STRUCTURAL AND CHEMICAL HETEROGENEITY OF THE ROLLER
The kinetics of solidification has a non-negligible
influence on the chemical and structural heterogeneity of	The final mechanical properties of the rollers - of the
the cast-iron in question1. An original methodology for	spheroidal graphite cast-iron - are determined, not
the measurement of the micro-heterogeneity was	primarily by the chemical composition, but mainly by
developed, based on the results of the model of the	their structural and chemical heterogeneity, which occurs
temperature field of the cast rollers. The chemical and	during casting, crystallization and successive cooling of
structural heterogeneity of the cast roller has proven to	the material. Some defects occurring in this way can be
be a significant function of the method of melting, modi-	corrected by heat treatment; however, the quality of the
fication and inoculation and the successive procedures of	pouring structure is very important, especially with
risering, casting and crystallization after cooling.	graphite cast-iron.
The chemical composition is given in Table 1. The samples used for determining the structural and chemical heterogeneity were taken from the spheroidal graphite cast-iron roller with the chemical composition given in Table 1 from the upper part (Section III) and from the bottom part (Section I). Figure 1 shows how the four samples were taken, with the upper surfaces -marked "X" - being analyzed. In addition, they were specified as follows:
•	The upper part of the roller on the outer surface (^1 = 540 mm) marked TR1, TR2
•	The upper part of the roller on the inner surface (^4 = 400 mm) marked TR3, TR4
•	The lower part of the roller on the outer surface marked BR1, BR2
•	The lower part of the roller on the inner surface marked BR3, BR4
The actual analyses were conducted at a specialized workplace2,3. The structural analysis of the cast-iron samples was conducted using a Neophot 32 metallographic microscope and an Olympus digital camera. The measurements of the metallographic parameters of the graphite and the evaluation of the volume part of the structural components were conducted on the Olympus CUE4 image analyzer. The basic statistical parameters of the graphite particles and also the stereological estimate of certain other parameters2 were calculated from the data files.
The JSM-840 (JEOL) electron scanning microscope was used for determining the chemical heterogeneity together with the LINK AN 10/85S X-ray energy-dispersive microanalyzer. Based on the results of the metallographic analysis2, two pairs of graphite grains were selected on the ground surface of every sample in order for the line of measurement between the first pair to pass through the perlite (marked "a") and the cemen-tite (marked "b"). The third line was selected through two boundaries created by particles of lamellar graphite, where the measurement passes through the basic perlite (marked "c"). The individual elements (Si, P, Mo, Cr, Mn, Fe, Ni, and Cu) were analyzed by means of a point
Figure 1: The set-up of a vertically cast roller and sampling Slika 1: Pogled na navpično ulit valj in vzorčenje
X-ray microanalysis in a direct line with a step of 3 ^m 3'4. Figure 2 illustrates the material around the selected line of the measurement on sample TR1 through the scanning electron microscope and the X-ray energy-dispersive microanalyzer. Following the chemical (elemental) analysis, the samples were etched using 2 % nital in order to make the contamination traces visible and display the line of measured points, including their connection to the sample microstructure.
2.1 ^ttructui^ial heteiroge^ei^ty
Based on experience, it can be assumed that the structure of the material of the casting, spheroidized graphite cast iron (named "ductile cast-iron" for working purposes) will, besides the globular graphite that is characteristic of ductile cast-iron, also contain a mixture of a certain amount of transition forms of graphite and lamellar graphite found between the globular and lamellar graphite. Table 2 shows the results from the quantitative metallographic analyses of the structures of the samples in Figure 1. The procedure of the quantitative structural measurements, including the results, is described in detail in a special report2.
Table 1: The chemical composition of ductile cast-iron B10 (w/%) Tabela 1: Kemiiska sestava sive litine B10, z globularnim grafitom (w/%)
Cast-iron B10
Element
Mn
Si
P
S
Cr
Ni
Mo
Mg
Cu
Content (%)
3 31
0 65
0 70
0.105
0 005
0.35
2 5Q
0 5Q
0 04
1 48
Table 2: The results from the quantitative measurements of the volume part of the components (in volume fractions, Tabela 2: Rezultati kvantitativnih meritev volumenskega dela komponent (volumenski deleži,
Sample
All graphite, incl . micro-shrinkages
Globular graphite
Transition forms of graphite and lamellar graphite
Pementite
Perlite
BR1
10 5 + 2 8
1 3
Q 2
2 2 + 1 7
87 3
BR3
13 0 + 5 4
1 1
11 Q
1 2 + 1 2
85 8
TR1
13 4 + 5 6
1 1
12 3
2 2 + 1 8
84 4
TR3
12 6 + 5 7
1 1
11 5
1 3 + 1 4
86 1
Figure 2: The structures in the area of sample TR1 selected for analysis: a) between two spheres of graphite through perlite, b) between the two spheres of graphite through cementite, c) through lamellar graphite
Slika 2: Strukture v obmo~ju vzorca TR1 za analizo: a) med dvema kroglama grafita v perlitu, b) med dvema kroglama grafita v cementitu, c) v lamelnem grafitu
2.2 Chemical he^e^ogene^^^
The distribution of the concentration of the eight elements, measured three times in one line for each of the samples (Figures 2a-c), was evaluated using statistics: the arithmetic mean of the concentration of the element in the selected interval Cst, the standard deviation an-i of the measured concentration, the minimum concentration Cmin and the maximum Cmax - measured each time in the selected interval of the sample (in mass fractions, w/%). Besides these basic concentration parameters, the segregation index of each measured element was determined and defined as Is = Cmax/Cst, i.e., as the quotient of the maximum concentration of elements measured on the given interval and the arithmetic mean of its concentration in the same interval. The tendency of the element to segregate - based on experience - is expressed in the values of the segregation indexes. A detailed description of the measurement procedure and the corresponding results can be found in the reports3,4.
Figure 3 illustrates the values of this parameter for individual elements and samples measured in one line across the lamellar graphite - c. Similarly, the segregation index values can be plotted for the area between the globular graphite in a line across perlite - a, and also in the area between the globular graphite in a line across cementite - b. Figure 4 shows the segregation index values for the elements calculated as the average of the values for all three areas - a, b, c. Furthermore, the graphs in Figures 3 and 4 also indicate that the segregation indexes of the highest values - from the set of measured elements - belong to phosphorus and molybdenum, and the lowest values refer to nickel and iron, which make up the matrix.
3 CONCLUSION
This article introduces an original methodology for the measurement of the chemical heterogeneity of cast-iron of the composition given in Table 1. The structure of this cast-iron is created by a large amount of the transition form of graphite and small amount of
Figure 3: The index of the segregation of elements for individual samples in the area of the transition graphite in the measured area across lamellar graphite
Slika 3: Indeks izcejanja elementov posameznih vzorcev v obmo~ju prehoda grafita v merjeno podro~je lamelnega grafita
Figure 4: The average index of segregation of the analyzed elements in individual samples
Slika 4: Povpre~ni indeks izcejanja analiziranih elementov v posameznih vzorcih
globular graphite and also lamellar graphite and cemen-tite, whereas the structure of the metal matrix is perlitic. The volume amounts of the structural components were determined using a quantitative metallographic analysis, which was simultaneously the basis for the selection of the places for the analysis of the element composition using an X-ray energy-dispersive spectral microanalysis. Inside the microstructure of four samples, taken from the outer and inner parts of the ring at the top and bottom of the roller (Figure 1), in a total of twelve areas, the linear concentration dispersion was measured on the eight elements which make up the basic constitution of the cast-iron: Si, P, Mo, Cr, Mn, Fe, Ni, and Cu. A statistical analysis was used to determine the distribution characteristics of the concentration of the individual elements, including the segregation indexes and the relationships among them. In this way it was possible to assess the influence of the changing kinetics of the temperature field on the resultant structural and chemical heterogeneity of the roller. The kinetics of the solidification of the cast-iron roller had a non-negligible influence on the chemical and structural heterogeneity of the investigated type of cast-iron. It is obvious that the chemical and structural heterogeneity of the cast roller is a significant
function of the method of melting, modification and inoculation and the successive procedures of risering, casting and crystallization after cooling.
Acknowledgements
This analysis was conducted using a program devised within the framework of the GACR projects No. 106/08/0606, 106/09/0940, 106/09/0969 and P107/11/ 1566.
4 REFERENCES
1F. Kavicka et al.: Numerical optimization of the method of cooling of a massive casting of ductile cast-iron. In: Book of Abstracts and CD ROM of the 13"" International Heat Transfer Conference, Sydney, Australia, August 2006, 27 2 J. Belko, K. Stransky, Structural analysis of cast-iron samples (in
Czech). Research report, VTUO Brno, 2005 3Z. Winkler, K. Stransky, Microheterogeneity of the composition ductile cast-iron samples alloyed with Mn, Cu, Ni and Mo (in Czech). Research report, VTUO Brno, 2005 4R. Kamensky, et al.: The influence of silicon on the thickness of the hardened surface layer of cast-iron castings (in Czech). Slevarenstvi, 16 (1968) 1, 35-39