Effect of austenite grain size on isothermal bainite transformation in low carbon microalloyed steel

Regulation of austenite grain growth

Figure 2 shows austenite grain growth behaviour as a function of maximum heating temperature and holding time. In the microalloyed steel, austenite grain growth is suppressed at lower temperatures (1000–1100°C), presumably because of pinning effect of grain boundaries by microalloy carbides or nitrides such as Nb(CN).13 At higher temperatures, some grains begin to grow abnormally at the expense of the fine surrounding grains. Therefore, the average grain size abruptly increases, as shown in Fig. 2, and austenite grain coarsening temperature of tested steel can be determined between 1100 and 1150°C. When specimens are subjected to different maximum heating temperatures of 1000 and 1200°C, different austenite grain sizes can be obtained, as shown in Fig. 3. Fine average grain size is ∼20 μm, while coarse average grain size is ∼260 μm. The proposed method of selecting two widely varying grain sizes is to conveniently study the effect of austenite grain size on isothermal bainite transformation.

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Figure 2

Regulation of austenite grain growth with maximum heating temperature and holding time

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Figure 3

Optical micrographs showing prior austenite grain boundaries of specimens with different maximum heating temperatures

Morphology of isothermal bainite microstructure

Figure 4 shows optical microstructures of the fine and coarse austenite grain size specimens after isothermal holding at various transformation temperatures for different times. When isothermal holding time is ∼5 s, the microstructure obtained is complete lath martensite, indicating that the incubation time of isothermal bainite transformation is more than 5 s. In the fine austenite grain size specimen, a small amount of bainite forms with very few or without carbide precipitation at 580°C for 100 s (Fig. 4a). This kind of bainite morphology is B-I type in the definition by Ohtani et al.14 However, such microstructure cannot be observed in the coarse austenite grain size specimen until holding time prolongs to 150 s (Fig. 4d). As transformation temperature lowers, the sheaves of bainitic ferrite start to form, as well as martensite/austenite constituents and carbides occur between those sheaves, as shown in Fig. 4b and e, which is B-II type bainite in the classification system of Ohtani et al. In a coarse austenite grain, bainite transformation has an obvious character of two stage nucleation. The first stage is the primary plates nucleating at the boundaries and growing towards austenite grain inside. The second stage is the secondary plates nucleating, attaching to the primary plates and growing towards the other direction.15 When isothermal temperature further lowers to 520°C for 100 s, the distribution of martensite/austenite constituents and carbides becomes more dispersive, and the amount of martensite transformed decreases sharply, but the morphology of bainitic ferrite does not show obvious change (see Fig. 4c and f).

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Figure 4

Optical micrographs showing isothermal bainite microstructures: coarse austenite grain size specimens transformed a at 580°C for 100 s, b at 560°C for 50 s, c at 520°C for 100 s and fine austenite grain size specimens transformed d at 580°C for 150 s, e at 560°C for 50 s and f at 520°C for 100 s (B: bainite, M: martensite, M/A: martenite/austenite constituent)

Figure 5 shows TEM images of the fine and coarse austenite grain size specimens transformed at 540°C for 100 s. The microstructure consists of martensite and bainite in the fine austenite grain size specimen (Fig. 5a), and prior austenite grain boundary is retained perfectly as indicated by an arrow. Bainite transformation is confined to the prior austenite grain due to shear transformation mechanism.16 Figure 5b shows magnified bainite microstructure corresponding to the square grid region in Fig. 5a. There are high densities of dislocations and some sub-boundaries inside the bainite grain, and many small martensite/austenite constituents form on the sub-boundaries to accommodate supersaturated carbon. Bainitic ferrite plate and lath-like martensite/austenite constituent can be simultaneously observed in fine and coarse austenite grain size specimens (see Fig. 5c and d). It is noted that the width of bainitic ferrite formed in different austenite grain sizes is approximately in the same range of 0·5–1·2 μm, but the length of bainitic ferrite in fine austenite grain size specimens is often shorter than that in coarse austenite grain size specimens, as shown in Fig. 4. According to the model of the bainitic ferrite lath width expressed by equation (1),17 the width size is a function of temperature, austenite strength and driving force for bainitic ferrite formation under paraequilibrium. Thus, austenite grain size has no direct influence on the width of bainite lath (1) where W_B is the width of bainitic ferrite lath, ΔG^γ→α is the driving force for bainitic ferrite formation, S_γ is the strength of austenite and T is the absolute temperature. A lath-like martensite/austenite constituent seems to be transforming to bainite by partitioning carbon into retained austenite as indicated by a circle (Fig. 5c). A kind of thin film structure precipitates present between the bainitic ferrite plates as indicated by arrows (Fig. 5d), which may be also composed of martensite or retained austenite according to the results of Zhong et al.18 The relatively large bulk shaped martensite/austenite constituents in the coarse austenite grain size specimen are shown in Fig. 5e. The magnified martensite/austenite constituent corresponding to the square grid region in Fig. 5e presents an intricate microtwin structure because of high carbon concentration (Fig. 5f). Some neighbouring bainite microstructure having obviously different orientations may be due to their formation at different periods of holding process.

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Figure 5

Images (TEM) showing fine microstructures at isothermal temperature of 540°C for 100 s of a–c fine austenite grain size specimen and d–f coarse austenite grain size specimen (B: bainite, M: martensite, BF: bainite ferrite, M/A: martenite/austenite constituent)

Effect of austenite grain size on nucleation of isothermal bainite transformation

It is a certainty that the nucleation of sheaves of bainite normally occurs at the austenite grain boundaries, and they propagate towards the grain interiors by the nucleation and growth of individual subunits.19 Austenite grain size has a significant influence on the morphology characteristics of bainite through controlling the number of nucleation sites. Adopting the equiaxed and spherical geometry for undeformed austenite grain, the effective nucleation area of the grain boundaries per unit volume S_v0 (mm⁻¹) can be expressed as equation (2).20 Here, d_γ is the austenite grain size. In the present case, the S_v0 value varies from about 8 to 100 mm⁻¹ when decreasing the austenite grain size from about 260 to 20 μm. In particular, there is a large increase in effective grain boundary area for smaller austenite grain sizes. As a result, lowering the austenite grain size leads to the occurrence of more amounts of bainite transformed and the apparent refinement of transformation products under the same isothermal condition (2) For example, several nucleation sites in a coarse austenite grain form simultaneously at the boundary, and hard impingement events occur between bainite sheaves inside the grain, as shown in Fig. 6a, and the corresponding schematic diagram is presented in Fig. 6b. Owing to small austenite grain size, the early soft impingement events between bainite and prior grain boundaries always take place,21 and the larger proportion of austenite grain boundary area promoting primary nucleation makes higher volume fraction of bainite transformed, as shown in Fig. 6c.

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Figure 6

Effect of austenite grain size on morphology of bainite sheaves at 560°C for 150 s

Effect of austenite grain size on kinetics of isothermal bainite transformation

Figures 7 and 8 show changes in the volume fraction transformed at the isothermal temperatures of 560 and 540°C respectively. At a given isothermal condition, the volume fraction transformed in fine austenite grain size is distinctly higher than that in coarse austenite grain size, and the kinetics of bainite transformation is retarded in coarse austenite grain size. That is because the grain boundaries as the optimal sites for the nucleation of bainite ferrite promote bainite transformation as stated above. It is also implied that the rate of bainite transformation mainly depends on the nucleation rate for this tested steel. Therefore, this result is consistent with the conclusions proposed by Umemoto et al.4 and Matsuzaki and Bhadeshia.5 They also claimed that bainite formation was accelerated as the grain size decreased in the high carbon (∼1 wt-C) steel. However, the morphologies of bainite between the present steel and their studied steels have some discrepancy mainly because of different carbon contents. 7 7,22

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Figure 7

Isothermal transformation kinetics of different austenite grain sizes at 560°C

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Figure 8

Isothermal transformation kinetics of different austenite grain sizes at 540°C

However, when the fraction transformed reaches to ∼80 at 540°C within 150 s, the rate of transformation slows down apparently, and a ‘transformation stasis’ phenomenon exhibits. Although the data seems to be not enough to explain the ‘transformation stasis’ phenomenon, the effect of austenite grain size on the volume faction transformed basically disappears when holding for 150 s, and the volume fraction transformed in coarse austenite grain size specimens is slightly higher than that in fine austenite grain size specimens when holding for 300 s, which strongly prove the existence of this phenomenon. In the late stage of transformation, untransformed austenite is enriched by carbon with the formation of bainitic ferrite, and the driving force of ferrite transformation is decreased. When the remaining austenite carbon content reaches the T₀ curve, which is the locus where austenite and ferrite have the same free energy, the ‘transformation stasis’ phenomenon occurs. 16 19 16,19,21 As shown in Fig. 9, before quenching at 540°C for 150 s, the small untransformed austenite grain and the large block shaped untransformed austenite present in the fine austenite grain size and coarse austenite grain size microstructures respectively. Those untransformed austenite have nearly the same free energy to ferrite, so they can exist for a long time. There are two popular mechanisms to explain the ‘transformation stasis’ phenomenon: the inhibiting effect of Si on the carbides precipitation and the solute drag effect of alloying elements on the interphase migration.19,23^–27 The first mechanism is responsible for such a phenomenon in the Si added steels (about 1·5–3 wt-Si content).23^–25 However, the alloys used in the present study contain only 0·3 wt-Si, which does not have enough effect on the precipitation kinetics of cementite. Because of many alloying elements, such as Nb and Mo, added in the steel, those elements can segregate at the phase interface in solid solution and form solute drag effect to retard the interface movement.26^–28 Thus, it can be proposed that solute drag effect inhibiting growth of ferrite by Nb and Mo segregation at the austenite/ferrite interphase boundary is mainly responsible for the ‘transformation stasis’ in the present steel.

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Figure 9

Images (SEM) showing isothermal bainite microstructure of different austenite grain sizes at 540°C for 150 s (B: bainite, M: martensite)

The effect of austenite grain size on the bainite start transformation C curves is presented in Fig. 10. It is obviously revealed that the coarse austenite grain size can increase the incubation time of bainite transformation, even at the nose temperature of C curves. However, it seems to have no influence on the nose temperature of C curves, which is ∼534°C. This is consistent with the results of Morozov et al.29 They showed that with decreasing the austenite grain size from 115 to 20 μm, the incubation time of upper bainite transformation was remarkably reduced in a carbon eutectoid steel.

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Figure 10

Bainite start transformation C curves of different austenite grain sizes