Effects of Inclusion Particles on the Microstructure and Mechanical Properties of High Strength Austempered Ductile Iron

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1 Materials Transactions, Vol. 44, No. 5 (2003) pp. 995 to 1003 #2003 The Japan Institute of Metals EXPRESS REGULAR ARTICLE Effects of Inclusion Particles on the Microstructure and Mechanical Properties of High Strength Austempered Ductile Iron Chang-Zhin Wu* 1 and Teng-Shih Shih* 2 Department of Mechanical Engineering, National Central University, Chung-Li, Taiwan 32054, R.O.China Effects of inclusion particles on the microstructure and mechanical properties of high strength austempered ductile iron (ADI) were investigated in this study. Inclusion particles, especially when their sizes are less than 5 mm, were mostly found in intercellular regions. Whether an inclusion particle can induce the formation of acicular ferrite depends on Mn segregation. In intercellular region, acicular ferrite was hard to form in the vicinity of inclusion particles due to (1) serious Mn segregation, and/or (2) the Mg-enriched inclusions here in halo-like. Consequently the surrounding austenite remained to be blocky type after austempering treatment. The fatigue life cycles of ADIs were affected by the particle counts and the microstructure. Increasing the count of fine inclusions along with the effect of Mn segregation deteriorated the fatigue life and elongation of high strength ADIs. (Received December 24, 2002; Accepted March 17, 2003) Keywords: austempered ductile iron, inclusion, segregation, martensite 1. Introduction Inoculation and spheroidization are important steps in producing ductile iron and have influential effects on the performance of castings. Most chemical compositions in inoculants and spheroidizers are highly reactive metals, such as Mg, Al, Si, Ti and Ca. These strongly oxidizing elements tend to combine with dissolved oxygen, sulfur or other finely divided oxides suspended in the molten iron and form inclusion particles. 1 3) The role of inclusion particles in the performance of mechanical properties has been a controversial subject. Shim et al. 4) demonstrated that inclusion particles dispersed in the austempering of medium carbon steels provided potential sites for the heterogeneous nucleation of ferrite during the austenite-ferrite transformation. In Shih s study, 5) higher concentration of sulfur caused more inclusion particles in matrix, which induced ferrite nucleation within austenite grains; that is, acicular ferrite was promoted by the presence of more inclusion particles. However, inclusion particles were thought by many researchers to be detrimental to mechanical properties of steels, because they acted as the sites for forming micro-voids resulting in cleavage cracks in the process of tensile, cyclic loading or impact testing. 3,6,7) Despite the controversy, the above studies acknowledged the correlations of inclusion particles and the microstructures in commercial steels. In fact, the mechanical properties of ADI are also affected by its matrix microstructure. Previous studies on ADI focused more on the discussion of austenitizing, austempering, alloy addition and nodularity/nodule count. The effects of inclusion particles on the matrix microstructure, however, are rarely discussed. So the aim of this study is to elucidate the influence of inclusion particles on the microstructure and mechanical properties of ADI. In fact, the high strength unalloyed ADIs with different contents of Mn and Si are the main focus of our experiment. * 1 Graduate Student, National Central University. * 2 Corresponding author: t330001@cc.ncu.edu.tw 2. Experimental Procedure Base irons with varying contents of Mn and Si were given in Table 1. All molten iron was treated in a ladle (50 kg) by the sandwich method, using 1.4 mass% spheroidizers (46% Si, 3.0% Ca and 5.5% Mg) and 0.3 mass% inoculants (49.9% Si, 1.2% Ca, 1.87% Al and 1.07% Ba). Residual magnesium content was controlled within the range of %. The molten iron was poured into green sand molds to get 25.4 mm Y-block castings and meanwhile cast in a copper mold for chilled samples. Tensile bars (gauge length: 35 mm, gauge diameter: 8.75 mm) and un-notched impact specimens (10 mm 10 mm 55 mm) were cut from the legs of Y- block castings. Preheated at 823 K for 900 s, the specimens were austenitized at 1173 K for 1.5 h and eventually subjected to 573 K and 593 K for 2 h for austempering treatment. Ultimate tensile strength and elongation test were conducted with tensile bars. Their values were determined from the average of 2 tests. Un-notched impact values were obtained from the average of three tests at room temperature by Charpy impact testing machine. A PC-based Image Analysis system was used to detect the particle count in chilled samples. The same method was also applied to measure the particle count (<20 mm) and nodule count (>20 mm) of ADI. Then, the inclusion particles in chilled and ADI specimens (electrolytic etched by 2% picral 8) ) were investigated by using scanning electron microscopy (SEM) equipped with an energy dispersive X-ray (EDX) to analyze Table 1 Chemical composition of the base irons used in this study. Chilled samples C(%) Si(%) Mn(%) S(%) Mg(%) Particle count (N/countmm 2 ) Chill ( ) Chill ( ) Chill ( ) Chill ( ) Chill ( )

2 996 C.-Z. Wu and T.-S. Shih their constituents, segregation of Mn and Si, or nearby matrix microstructure. Electron probe microanalyzer (EPMA) with line scan or mapping photo system was also adopted to detect the segregation of alloying elements. 3. Results and Discussion 3.1 Analyses of inclusion particles Size and constituent of inclusion particles In Skaland s 9) study on chilled samples, the size of inclusion particles was reported as around mm diameter, and there were few particles more than 20 mm. Itofuji 10) also indicated that most inclusion particles in chilled matrix were with the size of about mm. Our results regarding the inclusion particles in chilled samples corresponded to these previous studies. OM and SEM observations show that most particles in chilled samples were about the size of mm, and the size over 20 mm was rarely seen. Further detected by EDX for chemical compositions, the inclusion particles observed in chilled samples here exhibited as the following types: (a) Ca, Si, S, Mn; (b) Al, Si, S, Ti; (c) Si, S, Cl; (d) Mg, Al, Si, S, [see Figs. 1(a) (d)]. According to Onsoien et al., 11) inclusion particles acted as heterogeneous nucleation sites for forming graphites during solidification, and the particles composed of Ca, Mg, Si, S and O were the primary nucleation sites for forming nodule graphite in ductile iron. Several researchers 12 14) also pointed out the nuclei of nodule graphite consisted of (Mg, Ca) sulfide, MgO and (Mg, Si, Al) N. The constituents of inclusion particles in chilled samples were very similar to those found in the nuclei of nodule graphite. So it was suggested that some inclusion particles emerged at/near the center of nodule graphite and thought to be nuclei for graphite. Some that failed to grow into graphite might exist as particles in the matrix. In the present study, inclusion particles with the size less than 20 mm were defined as pseudo-graphite (5 20 mm) and inclusions (<5 mm), while those over 20 mm were considered nodule graphite. Figure 2(a) shows the SEM observation on a chilled sample associated with O, Mg, S and Si mapping photos, Figs. 2(b) (e). These inclusion particles are mostly rich in oxygen, being oxide-type particles, Figs. 2(a) and (b). Some of these oxide-type particles contain Mg and/or S, Figs. 2(c) and (d). The particle count containing Mg is about N/ countmm 2 (N means number here), which corresponds to nodule count of irons, Figs. 2(c) and (f). Apparently, oxidetype particle containing Mg formed nodule graphite after solidification. According to Itofuji s Site Theory, Mg gas Fig. 1 Constituents of (a) Ca, Si, S, Mn; (b) Al, Si, S, Ti; (c) Si, S, Cl; (d) Mg, Al, Si, S analyzed on inclusion particles of chilled samples.

3 Effects of Inclusion Particles on the Microstructure and Mechanical Properties of High Strength Austempered Ductile Iron 997 Fig. 2 (a) SEM micrograph of the chilled sample associated with (b e) O, Mg, S and Si mapping photos; and (f) the nodule count of ADI specimen under the same magnification. bubbles are considered as a kind of free surface in liquid for graphite precipitation. Graphite forms into a sphere shape within the Mg gas bubble. When spheroidal graphite nucleates and grows in the Mg gas bubble, Mg is often located at the site between spheroidal graphite and the matrix structure in halo-like. 15,16) Experimental observation also indicated that most inclusion particles (mainly <5 mm) were located in the last solidification area, that is, intercellular region, and only some were randomly distributed throughout the matrix Effects of Mn and Si Table 1 also reveals the effects of Mn and Si on the amount of inclusion particles in chilled samples. When carbon was controlled at % and silicon at %, the total particle count decreased with the increasing Mn content ( %). But when manganese was %, increasing Si content from 2.45% to 3.10% contrarily enhanced the total particle count. For instance, Chill 3 (low Si and high Mn) exhibited the lowest particle count (931 N/countmm 2 ) because there were more coarse particles and fewer fine ones in its matrix. Nevertheless, the total particle counts of Chill4 and Chill5 (high Si) increased due to a great number of fine particles distributed in matrix (1476 and 1650 N/ countmm 2 ). ADI specimens also revealed similar results. The mean graphite nodules of ADIs are around N/ countmm 2. The mean particle counts of ADI specimens, from ADI1 to ADI5, are 190, 198, 136, 258 and 288 N/ countmm 2, respectively. Obviously, high particle count

4 998 C.-Z. Wu and T.-S. Shih was obtained in ADI4 and ADI5 (high Si), while low particle count was in ADI3 (low Si and high Mn). 3.2 Effect of inclusion particles on the microstructure of ADI Inclusion particles within eutectic cell Ausferrite transformation occurs during austempered treatment of ADIs. In Stage I transformation, acicular ferrite () initially nucleates and grows in the vicinity of graphite and along the grain boundary of prior austenite. With the growth of ferrite, carbon diffuses to the nearby austenite and forms carbon-enriched stabilized austenite ( hc ). When austenite () is saturated with carbon, ferrite stops growing and the Stage I reaction is completed. In Stage II transformation, carbon-enriched stabilized austenite ( hc ) decomposes to ferrite () and carbide. 17) The addition of Mn is to ensure sufficient hardenability when specimens are austempered. Otherwise, pearlite will form during cooling from the austenitizing to the austempering accompanied with adverse effects on the mechanical properties. But Mn also tends to segregate in intercellular region and form a barrier to hinder carbon diffusion, thus retarding the transformation of ausferrite. As a result, the stage I reaction in the intercellular area may not be completed before the stage II transformation starts in the eutectic cell. As mentioned, most inclusion particles were observed in intercellular regions, and only some were randomly distributed in areas near eutectic cells. But the inclusion particles existing at different locations produced varying matrix microstructure. Manganese tends to segregate to intercellular region. The colony within the eutectic cell has the abundant Si and depleted Mn, which is beneficial for ausferrite transformation. So acicular ferrite formed around pseudographite (5 20 mm) located within the eutectic cell a similar effect as nodule graphite (see Fig. 3). But if the size of inclusion particles is less than 5 mm, their effect of inclusions on the matrix microstructure nearby the inclusions is complicated. We explored this effect and observed several samples. For areas near eutectic cells, the Mg-deficient inclusions were likely to be surrounded by ferrite rings, as shown in Figs. 4(a) and (b). The Mg mapping photo confirmed the existence of Mg-deficient inclusion, Fig. 3 Acicular ferrite seen around a pseudo-graphite (5 20 mm) located within the eutectic cell, as heat-treated ADI4 (3.46%C; 2.90%Si; 0.50%Mn). Fig. 4(a), and as a result, a ferrite ring was clearly seen to surround the inclusion, Fig. 4(b), after austempering process. Figure 5(a), by contrast, shows the mapping of a Mg-enriched inclusion within eutectic cell. The corresponding microstructure in Fig. 5(b) presents the formation of ferrite laths instead of a ferrite ring around the inclusion. As reported by Itofuji, 18) a Mg-halo was observed as a layer surrounding most graphite nodules and some inclusion particles. The current experiment indicates that the detected Mg-halo may exist at the interface of inclusion and matrix, and it may hinder the diffusion of carbon in austenite matrix nearby the inclusion into interface during ausferrite transformation. Carbon indeed diffuses via a preferential path, such as prior austenite grain boundary, and forms ferrite laths. If there is no detected Mg-halo, carbon may quickly diffuse into interface and consequently causes the emergence of ferrite rings, Figs. 4(a) and (b) Inclusion particles in intercellular region In the colony of intercellular regions, all specimens showed the existence of blocky or island-like austenite. This can be ascribed to Mn segregation to intercellular regions and retarding the transformation of ausferrite. Due to similar compositions in ADI1 and ADI2 (low Si and low Mn), and Fig. 4 (a) Mapping of a Mg-deficient inclusion (<5 mm) within eutectic cell, and (b) a ferrite ring surrounding a Mg-deficient inclusion, as heat-treated ADI2 (3.46%C; 2.44%Si; 0.30%Mn).

5 Effects of Inclusion Particles on the Microstructure and Mechanical Properties of High Strength Austempered Ductile Iron 999 Fig. 5 (a) Mapping of a Mg-enriched inclusion (<5 mm) within eutectic cell, and (b) acicular ferrite around a Mg-enriched inclusion, as heat-treated ADI2 (3.46%C; 2.44%Si; 0.30%Mn). Table 2 Segregations of Mn and Si in ADI2, ADI3 and ADI4 measured by EDX. Alloying ADI2 ADI3 ADI4 element (low Si and low Mn) (low Si and high Mn) (high Si and high Mn) Location Si% Mn% Si(S.R.-1) :1 0: :00 0:07 0: :02 0: Mn(S.R.-1) 0: :27 0:35 0: :19 0: :28 S.R represents segregation ratio, where locations 1 and 5 are near the graphite and location 3 is in intercellular region. ADI4 and ADI5 (high Si and high Mn), the segregation of Mn and Si was further measured from ADIs 2, 3 and 4 by EDX and depicted in Table 2. Each specimen measured three times, each time on five locations between two graphite nodules. The spacing between two nodules was converted to be dimensionless distance. The distance between two nodules was about 160 mm. Detailed method can refer to Shih et al. 19) EDX analyses show that Mn segregates to intercellular regions, whereas Si tends to be rich in areas near graphite nodule. The segregation of ADI3 was especially serious because of low Si but high Mn. The segregation of Mn in ADI4 was favorably offset by the increase of Si addition. The intercellular regions usually contain the highest Mn but lowest Si content, so the resultant microstructure is greatly affected by Mn segregation. Two inclusion particles exist in intercellular region showing different degree of Mn segregation, Fig, 6(a). The one on the left has 3 mm in size showing a severe Mn segregation and remaining the blocky austenite on nearby matrix. The other has 6 7 mm in size showing less degree of Mn segregation and forming acicular ferrite in the surrounding area, Fig. 6(b). Also, if the colony in intercellular region is not highly containing Mn, such as samples of low Si and low Mn or high Si and high Mn, the Mg-deficient inclusions [Fig. 7(a)] Fig. 6 (a) Line scanning of Mn and Si and (b) larger blocky austenite emerged in intercellular region, as heat-treated ADI3 (3.47%C; 2.45%Si; 0.48%Mn).

6 1000 C.-Z. Wu and T.-S. Shih Fig. 7 (a) EDX analysis of a Mg-deficient inclusion in intercellular region, and (b) few martensite formed due to the formation of acicular ferrite around a Mg-deficient inclusion, as heat-treated ADI4 (3.46%C; 2.90%Si; 0.50%Mn). Fig. 8 (a) EDX analysis of a Mg-enriched inclusion in intercellular region, and (b) transformed martensite seen near a Mg-enriched inclusion, as heattreated ADI4 (3.46%C; 2.90%Si; 0.50%Mn). function as the site for the formation of ferrite ring and/or coarse ferrite plates. This would reduce the size of blocky austenite nearby the inclusion and therefore greatly decreased the chance for forming martensite in intercellular region, Fig. 7(b). But if the inclusion was rich in Mg, [Fig. 8(a)] which might be halo-like, the nearby matrix would remain the blocky type austenite and increased the chance inducing the transformation of martensite, Fig. 8(b). If the colony in intercellular region was highly rich in Mn, the chance for remaining blocky austenite would be promoted whether the halo-like Mg existed at the interface of inclusion and matrix or not. 3.3 Mechanical properties of high strength ADI Characteristic of high strength ADI For a high strength ADI, slim and long ferrite plates densely distributed over the matrix is the desired microstructure. Figure 9 shows the typical morphology of ferrite plate developed from the high strength unalloyed ADI2. String-type austenite exists within ferrite plate accompanied with carbide precipitates along the interface of austenite and ferrite plate. Austenite has multi-slip systems during deformation and can execute a substantial work hardening. Stringtype austenite located within a ferrite plate toughens the Fig. 9 Typical morphology of ferrite plate developed from high strength ADI (: ferrite, : austenite), as heat-treated ADI2 (3.46%C; 2.44%Si; 0.30%Mn). ferrite phase during deformation. Meanwhile, slip systems interact within blocky austenite, when the matrix of ADI is subjected to stress. After the interaction persists a period of time, blocky austenite may transform into martensite and/or carbon may diffuse and precipitates to form carbide and ferrite lath within blocky austenite. Wu et al. 20) used

7 Effects of Inclusion Particles on the Microstructure and Mechanical Properties of High Strength Austempered Ductile Iron 1001 ultrasonic vibration to induce micro-jet impacting the ADI specimens. For unalloyed ADIs, martensitic transformation occurred in the initial period of impact. Therefore, martensitic transformation in most blocky austenite and strain hardening of austenite in most string-type austenite both led the unalloyed ADIs to develop high strength. The ADI specimens used in this study are all unalloyed ADIs, so a similar phenomenon can be observed. Figures 10(a) and (b) show the microstructure in the intercellular Fig. 10 Observed microstructure of (a) as heat-treated ADI3 (3.47%C; 2.45%Si; 0.48%Mn), and (b) sample after tensile loading and observing area near the main fracture surface (M: transformed martensite region; arrows indicated: precipitated carbides). region of ADI3 before and after tensile loading, respectively. Apparently carbides developed at different areas of matrix after subjecting to tensile loading: at the interface of ferrite plate and austenite plate and at area within blocky austenite Fig. 10(b). During deformation, carbide, which originally exists at the interface of ferrite plate and austenite, becomes coarsen along with the string-type austenite consuming its size; that is, the dissipation of string-type austenite is the source for growing carbide particle, (see Fig. 9). Figure 10(b) also shows the coexistence of martensite and precipitated carbide, as indicated by an arrow. Indeed, few carbide particles can be observed within blocky austenite before deformation, Fig. 10(a). Again, if the intercellular region is rich in Mn and has Mg-enriched inclusion particles, the blocky austenite would extend its size and promoted martensite forming in the central region. During deformation, martensite may be induced within the colony of small size blocky austenite, but the original existed martensite may transform to precipitate carbide, as shown in Fig. 10(b). The dispersion hardening of fine carbide precipitates further enhances the strength of unalloyed ADIs. With the existence of martensite regardless originating from austempering or induced by deformation, elongation and impact energy would be deteriorated Effect of Mn segregation Experimental results show that all specimens possessed excellent ultimate tensile strength ( MPa) but moderate elongation ( %) and impact energy ( J). This was ascribed to the low austempering temperature K associated with unalloyed ADIs, which enabled matrix of ADI to present a microstructure characterized with the clustering of dense acicular ferrite and lesser amount of blocky austenite in the matrix. The tensile strength of ADI specimens was close; however, their elongation and impact energy varied with different Mn and Si contents, as listed in Table 3. ADI2 and ADI4 obviously presented better elongation (4.8% and 4.9%) and impact energy (68.0 J and 87.8 J), but ADI3 was poorer (3.7% and 57.3 J) at 593 K. The lower elongation and impact energy of ADI 3 was caused by Mn segregation, which fostered the increase of blocky austenite in intercellular region. When the blocky austenite increased its size, it also raised the chance for forming martensite after austempering treatment. The possibility for forming stress-induced martensite was also promoted, thus hampering the ductility of ADI Effect of inclusion particles Apart from austempering temperature and Mn segregation, the sizes of inclusion particles also affected the mechanical Table 3 Mechanical properties of ADI specimens subjected to low austempering temperatures 573 K and 593 K for 2 h. Specimens UTS Elongation Impact energy Nodule count Nodularity Particle count (MPa) (%) (J) (N/countmm 2 ) (%) (N/countmm 2 ) ADI1 (2.42 Si, 0.25 Mn) 1410/ / / ( ) ADI2 (2.44 Si, 0.30 Mn) 1435/ / / ( ) ADI3 (2.45 Si, 0.50 Mn) 1379/ / / ( ) ADI4 (2.90 Si, 0.50 Mn) 1451/ / / ( ) ADI5 (3.10 Si, 0.48 Mn) 1438/ / / ( ) Particle count includes pseudo-graphite (5 20 mm) and inclusion (<5 mm).

8 1002 C.-Z. Wu and T.-S. Shih properties of ADI. Sofue et al. 21) remarked that more small nodule graphite and less nonmetallic inclusion particles could improve fatigue and tensile strength. Shih et al. 19) also proved increasing smaller nodule graphite levitated Mn segregation and improved ductility and fatigue life. We found that a great number of pseudo-graphites (5 20 mm) distributed in the matrix of ADI4 and ADI5 and led to a better ductility. Similar to the function of nodule graphite, pseudographites were uniformly distributed in matrix and facilitated the formation of acicular ferrite, so a more homogeneous microstructure could be obtained. But if the inclusion particle size was less than 5 mm, in ADI specimens with similar chemical composition and nodule count, the increase of inclusions became critical or unfavorable to mechanical properties. The increase of inclusions (<5 mm) had a slight effect on tensile strength, but was detrimental to elongation, impact energy and fatigue life. For instance, when the count of fine inclusions in ADI2 (593 K/ 2 h) increased from 100 to 145 N/countmm 2, the tensile strength varied from 1352 to 1325 MPa, elongation from 5.2 to 3.8%, and impact energy from 75 to 52 J. High-cycle fatigue specimens were conducted in a rotary bending testing machine at a frequency of 2400 min 1 (40 Hz). Figure 11 indicates that for a given stress amplitude 290 MPa, increasing the count of fine inclusions (<5 mm) apparently decreases the life cycles of ADI2. The particle count less than 5 mm was around 100 N/countmm 2 for the tested bar of ADI2, developing 4: life cycles. However the life cycles declined along with the increase of the fine inclusions. When the count of fine inclusions increased to 145 N/countmm 2, the fatigue life declined to 1: cycles. Because most fine inclusions contained Mg compounds and was located in intercellular regions, they did not contribute to the nucleation of acicular ferrite. What s worse, inclusions might foster the propagation of cracks or cleavages in the stress concentrated Particle, N/count. mm Failure cycles (stress290mpa) : 1,200,000 cycles : 2,800,000 cycles : 4,800,000 cycles Nodule count : The Diameter of Particle (P/µm) Fig. 11 Variations of fatigue strength with different inclusion counts in ADI2 (3.46%C; 2.44%Si; 0.30%Mn). Fig. 12 Distribution of the fine inclusions in intercellular region of ADI3 (3.47%C; 2.45%Si; 0.48%Mn). area of the matrix. Figure 12 shows the distribution of the fine inclusions in intercellular region of ADI3. The plastic deformation brought about the effect of stress concentration around inclusions; as a result, the crack accelerated its speed along the inclusions. In fact, the mechanical properties of high strength ADI were chiefly affected by Mn segregation. But under similar chemical compositions, different particle counts of ADIs accounts for varying performance. If the particle count (especially pseudo-graphite) is too low, the matrix will increase the extent of blocky austenite but decrease the acicular ferrite or ferrite lath especially in the intercellular region. By contrast, if there are too many inclusion particles, the fine inclusions may foster the propagation of cracks or cleavages in stress concentration area. So an optimal particle count is important for mechanical properties of high strength ADIs with similar compositions. In this study of high strength unalloyed ADIs, the optimum particle count is about N/countmm 2 when the nodule count is N/ countmm Conclusion (1) Within the colony of eutectic cell, a Mg-deficient inclusion tended to be surrounded by ferrite ring, while the vicinity of a Mg-enriched inclusion formed acicular ferrite or ferrite lath. (2) Whether an inclusion particle in intercellular region could facilitate the growth of acicular ferrite mainly depended on Mn segregation. A Mg-deficient inclusion could promote the growth of acicular ferrite, but a Mgenriched inclusion failed to induce it. As a result, martensite was prone to emerge in intercellular region. (3) More pseudo-graphites within the colony of eutectic cells could contribute to the formation of acicular ferrite and a more uniform matrix microstructure, whereas too many inclusions (<5 mm) were detrimental to the ductility and life cycles of high strength unalloyed ADIs.

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