The demand on auto brake discs is becoming enormous

HINA FOUNDRY Niobium alloying effect in high carbon equivalent grey cast iron Zhou Wenbin 1, 2, Zhu Hongbo 1, Zheng Dengke 1, Zheng Hongxing 1, 3, Hua Qin 1 and *Zhai Qijie 1 (1. Shanghai Key Laboratory of Modern Metallurgy & Materials Processing, Shanghai University, Shanghai 200072, hina; 2. Shanghai Huizhong Automotive Manufacturing o., Ltd., Shanghai 200072, hina; 3. Instrumental Analysis & Research enter, Shanghai University, Shanghai 200072, hina) Abstract: The effect of niobium on the formation of phase and solidification structure in high carbon equivalent grey cast iron was investigated. The experimental results indicated that an increase in the niobium content is favorable to refining the graphite and eutectic cell; and the pearlite lamellar spacing is reduced. Based on the thermodynamic calculation the formation of is prior to the eutectic reaction. The reduction in the pearlite lamellar spacing is mainly attributed to the decrease of eutectic temperature with the addition of niobium. Additionally, properties including hardness and wear resistance were improved after the addition of niobium. Key words: niobium alloying; grey cast iron; high carbon equivalent; refinement L number: TG143.2 Document code: A Article ID: 1672-6421(2011)01-036-05 The demand on auto brake discs is becoming enormous with the increase of car speed, load and limitation of braking length. It is inevitable that this will cause a rapid increase in temperature of brake disc and lead to a possible braking failure due to high friction [1, 2]. The main reasons for the failure result from abrasive wear and contact fatigue wear. The decrease of hardness and the fatigue resistance would aggravate the abrasion of the brake discs. Therefore it is necessary to ensure the properties of brake discs in terms of strength and hardness, heat storage and thermal conductivity, resistance to hot fatigue and vibration damping [3, 4]. ast iron has been widely used to fabricate brake discs with a high carbon equivalent (E) [5, 6], because: (1) the specific heat of graphite is almost twice as high as that of cast iron, therefore the capacity of heat storage is enhanced greatly; (2) the soft graphite absorbs vibration energy and demonstrates an excellent vibration damping; (3) the notch sensitivity of cast iron is lower than steel [7]. The properties of grey cast iron heavily depend on the graphite morphology and volume fraction. Hecht [8] found that the thermal conductivity is related to the carbon equivalent in cast iron materials; and flake-like graphite is favorable to improve the thermal conductivity. urrently, the carbon equivalent of grey cast iron used in *Zhai Qijie Male, born in 1959. Professor and doctoral supervisor. He gained his Ph.D from University of Science and Technology Beijing in 1991. His research interests mainly focus on metal solidification, new continous casting technology, cast alloys and materials, etc. By now, his more than 220 academic papers have been published in journals demestic and abroad. He also holds 22 invention patents of hina. E-mail: qjzhai@shu.edu.cn Received: 2009-09-09; Accepted: 2010-08-30 brake discs usually ranges from 3.8% to 4.6%, e.g., the PQ35 brake discs used in the Touran and Sagitar models contains about 3.8% 3.9% E (hypoeutectic cast iron) and the brake discs used in the UK Rover sedan contains around 4.4% 4.5% E (hypereutectic cast iron). However, although it is necessary to ensure a high thermal conductivity [8], excess graphite would cause a decrease of the mechanical properties. Under this condition, alloying with some trace elements was considered to be a probably effective remedy. Some work on niobium alloyed cast irons has been performed in recent decades including the effect of niobium on the phase transformation temperature, micro-hardness, graphite morphology and particle sedimentation [9-13]. In this paper, the effect of niobium on the formation of and solidification structure in high carbon equivalent grey cast iron materials was studied in order to provide more information for the production of brake discs. 1 Materials and experimental procedure The initial charge materials were clean low-silicon pig iron and steel scrap. Fe-65% alloy was added to the furnace charge in order to dissolve fully. hemical and spectral analyses were performed to confirm the designed composition. The composition of materials prepared was (mass%): 3.82, 2.05, Mn 0.73, r 0.18, P 0.08. The niobium content was (mass%) 0.042, 0.29, 0.85 and 1.48, respectively. The alloy materials were melted at 1,773 K in an 20 kg-capacity medium-frequency induction furnace and then poured into green sand mould at 1,693 K. An optical microscope (4XB) and a scanning electron microscope (JSM-6700F) 36

February 2011 with energy dispersive spectrometry, were used to observe the microstructures. The abrasive wear resistance test was performed by measuring the mass loss percentage after dry grinding for 1 h under 5 kg load. The brinell hardness test was also carried out to evaluate the effect of niobium. 2 Results and discussion 2.1 Formation of phase When 0.29% niobium was added, a few blocky niobium-rich phases can be observed with a size about 3 μm, as shown in Research & Development Fig. 1. When the niobium content was raised to 0.85% or 1.48%, a lot of niobium-rich phases with various morphologies formed including blocky, triangular, X or Y shaped [Fig.1]; and especially some pearlite-like structure [highlighted by the white dashed line in Fig.1], which probably formed during the pearlite transformation. Such research results are in agreement with previous research [14]. Some blocky primary niobium-rich phases also can be found in Figs. 2(c) and (d) (highlighted by the white arrows). The niobium-rich phases [highlighted by the white arrows in Fig. 1] represent as suggested by EDS/SEM results (not shown here). Fig. 1: phase in grey cast iron: 0.29% and 1.48% Generally niobium is a strong carbide-forming element [15] : [] + [] = (s) (1) Its standard Gibbs free energy of formation can be calculated as described by Zhai [16] : (2) Its Gibbs free energy of formation for non-equilibrium reaction can be expressed [16] as: where f and f are activity coefficients and can be calculated using Eqs. (4) and (5), respectively. lgf = e [%] + e [%] + e [%Mn] + e [%r] + e [%P] + e [%] Mn r lgf = e [%] + e [%] + e [%Mn] + e [%r] + e [%P] + e [%](5) e Mn where, e are the interaction coefficients for different elements in the iron melt at 1,873 K; and [%], [%] are the contents of different elements. Using thermodynamic data listed in Table 1, one can obtain ΔG = -140,147.55 + 85.84T + 2,148.92[%] 8.314Tln[%] (6) when ΔG = 0, (7) According to Eq. (7), the formation temperature of the phase in cast iron with different niobium content can be calculated. For the 0.042% sample, the content of niobium is too little to observe the phase in the final solified structure. When the niobium content is increased to 0.29%, the r P P (3) (4) calculated formation temperature (1,451 K) of is 24 K higher than the eutectic reaction (1,427 K), it seems reasonable to conclude that the phase should be formed prior to the eutectic reaction and a few blocky phases were seen. For the latter two contents (0.85% and 1.48%), the temperature gap is larger than 150 K between the nucleation temperature of and the eutectic temperature so that there is plenty of time to allow small particles to merge and grow into blocky, X or Y shaped, as shown in Fig. 1. More work is required to clarify the nucleation and growth of the phase. 2.2 Refinement of graphite and eutectic cell hanging the carbon content or carbon equivalent is the most common method used to modify the graphite morphology [1]. The difference of carbon content and carbon equivalent of four samples used in the present study is quite small and its effect on graphite can be ignored. Figure 2 shows the change of graphite morphology in grey cast iron with different niobium additions. All the graphite in the four samples was type A. However, 37

HINA FOUNDRY Table 1: Interaction coefficients of different elements in the iron melt [17] N O Mn S 0.14 0.11-0.06-0.34 0.08-0.012 0.046-0.49-0.042 0-0.85 0 0-0.047 the shape and size changed greatly. Plate-like graphite and a little bulk graphite distributed on the matrix were found with a niobium content of 0.042%. Adding niobium to 0.29%, the graphite refined strikingly; and the finest graphite was obtained when the niobium addition reached 0.85%. On further increasing the niobium to 1.48%, the refinement effect weakened and the graphite became slightly coarser than the 0.85% sample but still finer than the 0.042% sample. The effect of niobium content on the eutectic cell is also quite clear (Fig. 3). The average diameters of the eutectic cells were (c) (d) 0.042%, 0.29%, (c) 0.85% and (d) 1.48% Fig. 2: Graphite morphologies in grey cast iron (c) (d) 0.042%, 0.29%, (c) 0.85% and (d) 1.48% Fig. 3: Eutectic cells in grey cast iron 38

February 2011 about 954 μm and 497 μm for the 0.042% and 0.29% samples, respectively. On increasing the niobium to 0.85%, the diameter decreased to 298 μm. However, on further increasing the niobium to 1.48%, the eutectic cell became larger with an average diameter of about 403 μm. The pattern of change in the eutectic cell is similar to that in the graphite to some extent. The refinement mechanism of niobium on graphite can be explained from the following two aspects. Firstly, as mentioned above, some small particles merged and grew into blocky, X or Y shaped. It is inevitable that some residual small particles remained during cooling which act as heterogeneous nucleation nuclei for the graphite in the eutectic reaction. As a result, the increase of nucleation rate resulted in refined graphite morphology. Secondly, niobium hindered carbon from moving during solidification which also restricted the growth of graphite and made the graphite small and short [24]. When the niobium content reached 1.48%, more particles would be formed above the eutectic temperature. However, it also led to the rapid merging and growth of particles because of a big temperature gap of 232 K between the formation temperature of (1,659 K) and the eutectic temperature (1,427 K) so that most particles coarsen up to several micrometers. Only a few residual small particles are suitable to play the role of nucleation cores in the subsequent eutectic reaction. That is why the graphite became slightly coarser than that of the 0.85% sample. The eutectic cell is a coupledgrowth grain containing graphite and austenite. Graphite acted as the leading phase during solidification and austenite formed continuously following the graphite. Therefore, it is not difficult to understand that the eutectic cell size has the same pattern of change as the graphite. 2.3 Refinement of pearlite lamellar spacing Besides the refinement of graphite and eutectic cells, the pearlite lamellar spacing reduced with the addition of niobium. When the niobium content was 0.042%, the spacing of flakelike pearlite was about 875 nm and when the niobium content reached 1.48 %, the pearlite spacing was about 678 nm. The variation of lamellar spacing is shown in Fig.4. The lamellar spacing of pearlite mainly depended on its formation temperature. The lower the formation temperature means the higher nucleation under-cooling and the finer Research & Development structure can be formed. Our present study showed that niobium addition in cast iron can decrease the eutectic temperature significantly. That means an increase in pearlite nucleation under-cooling, which gives the refinement effect. For example, the eutectic transformation for the 0.019% and 0.097% cast iron occurred at 996 K and 975 K, respectively. Figure 5 shows the pearlite transformation during cooling measured using a Gleeble 3500 dilatometer. We will discuss this in a separate paper. Additionally, the diffusivity of carbon atoms weakened because of the low eutectic transformation and the dragging effect [17] of niobium on carbon that also prevented carbon atoms from migrating. All these factors led to a continuous reduction of pearlite lamellar spacing in cast iron with the addition of niobium. Fig. 5: Eutectic transformation temperature of cast iron with different niobium content 2.4 Effect of niobium on properties Figure 6 shows the effect of niobium on hardness and wear resistance. Wear rate means the mass loss percentage, the smaller wear rate indicates the better wear resistance. It is noted that as the niobium content increased, both the hardness and wear resistance improved. It is obvious that the refined structure (graphite and pearlite) demonstrated superior hardness. In addition, the aggregation of niobium-rich phase in high niobium-content alloy may be another reason for the higher hardness. As for wear resistance, it showed a similar relationship with niobium content to that shown by hardness. Fig. 4: Variation of pearlite lamellar spacing with niobium content Fig. 6: Hardness and wear rate of alloy with different niobium content 39

HINA FOUNDRY In the wear progress of grey cast iron, the hard phase acted as the first friction surface and contact lining material. That is, the wear resistance depends greatly on the nature of the niobiumrich hard phase, which distributed homogeneously and bonded strongly with the matrix. That is why the wear resistance improved effectively with the addition of niobium. 3 onclusions The effect of niobium alloying in high-carbon grey cast iron materials was investigated. Thermodynamic calculation results revealed that the phase formed prior to the eutectic transformation. Experimental results demonstrated that with increasing niobium addition (up to a certain level), the morphology of graphite and eutectic cell refined. The pearlite lamellar spacing clearly reduced, which was mainly caused by the decrease in eutectic temperature. Also, the addition of niobium led to the formation of a niobium-rich hard phase, which effectively enhanced hardness and wear resistance. References [1] Talati F and Jalalifar S. Analysis of Heat onduction in a Disk Brake System. Heat Mass Transfer, 2009, 45: 1047-1059. [2] ueva G, natora A, Guesser W L and Tschiptschin A P. Wear resistance of cast irons used in brake disc rotor. Wear, 2003, 255: 1256-1260. [3] Eriksson M, Bergman F and Jacobson S. On the Nature of Tribological ontact in Automotive Brakes. Wear, 2002, 252: 26-36. [4] Blau P J and Jolly B. Wear of Truck Brake Lining Materials Using Three Different Test Methods. Wear, 2005, 259: 1022-1030. [5] Fatahalla N. Effect of the percentage carbon equivalent on the nodule characteristics, density and modulus of elasticity of ductile cast iron. Journal of Material Science, 1996, 31: 4933-4937. [6] Gao H. Discussion on Excellent Material for Brake Disk and How to Obtain It. Journal of hongqing Institute of Technology, 2002, 16: 46-48. (in hinese) [7] ollini L, Nicoletto G and Konecna R. Microstructure and mechanical properties of pearlitic grey cast iron. Materials Science and Engineering A, 2008, 488: 529 539. [8] Hecht R L. The Effect of Graphite Flake Morphology on the Thermal Diffusivity of Grey ast Irons Used for Automotive Brake Discs. Journal of Material Science, 1999, 34: 4775-4781. [9] Zhai Qijie. Application of in ast Iron and Its Prospect. Foundry, 1998, 47(10): 41-46. (in hinese) [10] Loper, Bands H and ornell H. Role of Niobium as an Alloying Element in ast Irons. In: Proc. International Symposium on Tantalum and Niobium, Orlando, USA, Nov. 1988. [11] Fiset M, Peev K and Radulovic M. The influence of niobium on fracture toughness and abrasion resistance in high-chromium white cast irons. Journal of Materials Science Letters, 1993, 9: 615-617. [12] Bedolla-Jacuinde A. Microstructure of V-, -, and Ti-alloyed high chromium white cast irons. International Journal of ast Metals Research, 2001, 6: 343-361. [13] Li Shaonan. Effect of on the Mechanical Properties of Grey ast Iron. Foundry Technology, 1999(4): 43-45. (in hinese) [14] Fu Li and Zhai Qijie. Morphology and Distribution of -rich Phase in hilled ast Iron. Acta Metallurgica nica, 1996, 2: 159-162. (in hinese) [15] Akoy M, Kuzucu V and Korkut M H. The influence of strong carbide-forming elements and homogenization on the wear resistance of ferritic stainless steel. Wear, 1997, 21: 265-270. [16] Zhai Qijie. Thermodynamic Problems of Trace Elements in ast Iron. Modern ast Iron, 2001, 1: 19-24. (in hinese) [17] Yao Zhenghui, Wang Guoliang, Fu Li, and Zhai Qijie. Effect of on the Structure Stability of hilled ast Iron at Elevated Temperature. Foundry Technology, 1998(4): 44-45. (in hinese) The work was financially supported by ITI-BMM R&D project (No. 036) and Graduate Innovation Fund of Shanghai University (No. SHUX 102233). 40