2020 年 60 巻 12 号 p. 2755-2764
X-ray transmission imaging with X-ray diffractometry and time-resolved tomography with three-dimensional X-ray diffraction microscopy have been used to observe solidification and transformation in carbon steel and other Fe-based alloys. The imaging techniques showed a massive-like transformation, in which multiple austenite grains were produced in a single δ grain through a solid–solid transformation. The critical velocity from the diffusion-controlled growth of the γ phase to the massive-like transformation was as low as 5 µm/s. X-ray imaging indicated that the δ phase transforms massively to the γ phase in the conventional solidification processes, such as continuous casting. The massive-like transformation and multiple γ grains that were produced in the transformation were related to the subsequent microstructure evolution and casting defect formation. Solidification model including the massive-like transformation is expected to improve our understanding the solidification and the related phenomena in the peritectic steel systems.
The peritectic solidification has been key to understanding casting defect formation, such as the unevenness of solidifying shells and hot tears in continuous casting of steels. The unevenness of solidifying shell exhibits a maximum at a carbon content of 0.14 mass%.1,2) The unevenness and the hot tear, which result from depression caused by volume changes through the peritectic solidification, have been studied extensively.1,2,3,4,5,6,7,8,9,10,11,12,13,14,15) Studies on casting defect formation in hypoperitectic steels have been based on the peritectic reaction/transformation, which is a well-known solidification manner in multiphase and multicomponent alloys. In the peritectic reaction/transformation of the Fe–C system, the δ phase (ferrite, body-centered cubic) grows as a high-temperature phase, and the γ phase (austenite, face-centered cubic) is produced through a reaction between the δ and liquid phases. As a result of the reaction, the δ phase is covered with γ phase. The multiphase solidification has been accepted over an extended period. Thus, most studies focused on the mechanism of casting defect formation based on the peritectic reaction/transformation. Separately from solidification, an undesirable coarsening of γ grains has been recognized during subsequent cooling.16,17,18,19) The rapid coarsening of the γ phase and the peritectic solidification have been considered separately until recently.
An X-ray imaging study reported different solidification modes in the peritectic systems.20) One mode is that two phases grow layer by layer in the Sn–Cd peritectic system. The other is that the δ phase transforms massively to the γ phase in the Fe–C peritectic system. The findings initiated further developments of in-situ observation techniques to observe the solidification phenomena in Fe–C and Fe–Cr–Ni alloys in-situ.21) This paper reviews the time-resolved and in-situ observations of solidification in steels and discusses a massive-like transformation as a normal transformation mode from the δ to the γ phase in steel.
Pioneer X-ray imaging work22) on steel solidification was performed to observe the solidification front in an Fe–Si alloy by X-ray topography. A high-temperature laser-scanning confocal microscopy was also used to observe the solidification and related phenomena in steel.23,24,25,26,27,28) Since around 2000, third-generation synchrotron radiation facilities, such as SPring-8, Hyogo, Japan, allows us to use a brilliant and monochromatized hard X-ray, which is suitable for observing solidification in metallic alloys in-situ by transmission imaging. Thanks to the developments, the imaging techniques have been widely used to observe the microstructure evolution in metallic alloys in-situ. Here, the authors briefly review the imaging techniques that are used to observe solidification and transformation in metallic alloys and steels.
In the early stage, solidification in Sn, Al and Mg alloys was observed in-situ by X-ray transmission imaging (two-dimensional imaging).29,30,31,32,33) Later, the further-developed apparatus allowed for the observation of Cu, Ni, and Fe alloy solidification above 1300 K.21,34,35,36,37,38) Transmission imaging was combined with X-ray diffraction (XRD) measurements. Figure 1(a) shows a typical setup for transmission imaging with XRD. Transmission images were observed by a beam monitor (pixel size: 1 μm × 1 μm to 5 μm × 5 μm, frame rate: up to 500 fps). XRD images were observed by a panel-type detector to identify the crystal structure. The technique was extended to observe the semi-solid deformation in steel36,39,40) and to perform in-situ X-ray fluorescence analysis to measure solute partitioning at the solid–liquid interface.41,42) The advantage of transmission imaging is to observe the microstructure evolution with relatively high temporal and spatial resolutions. Thus, the technique was used to observe dendritic growth and the massive-like transformation in steels.
Typical setups to observe steel solidification and transformation from the δ to the γ phase. (a) Transmission imaging (2D observation) with X-ray diffraction and (b) time-resolved tomography (3D + time observation, 4D-CT) with X-ray diffraction.
Recently, time-resolved computed tomography (three-dimensional, 3D + time, referred as to 4D-CT) has been developed to observe dendrite evolution in Al–Cu alloys (pink X-ray beam, rotation of 0.1 rps)43,44) and Al–Mg–Si–Y2O3 alloys.45) 4D-CT was also used to observe the semi-solid deformation and transgranular liquation cracking in Al–Cu alloys.46,47) From an image-processing perspective, time-interlaced model-based iterative reconstruction was developed to improve the temporal resolution.48) A filtering technique with a phase field model was proposed to improve the image quality in 4D-CT even at a relatively high temporal resolution.49) The filtering enables dendrite observation in steel.49) XRD measurements were combined with 4D-CT to observe the microstructure and crystallographic orientation simultaneously in steel.49,50) In addition, 4D-CT and 3DXRD (three-dimensional X-ray diffraction microscopy51,52,53)) were performed to study the semisolid deformation in Al–Cu alloys.54) Thus, 4D-CT is now recognized as a 3D tool to observe the solidification phenomena in steel. Figure 1(b) shows a 4D-CT and XRD setup. Projection images for 3D reconstruction were observed by a beam monitor (typical frame rate for steel: 100 fps). X-ray diffraction spots were observed by a panel-type detector (pixel size: 100 μm × 100 μm, frame rate: 30 fps). The crystallographic orientation was analyzed from the XRD spot positions on the panel-type detector and the sample rotation angle. The typical temporal resolution was 4 s for steel solidification.49) This technique was used to observe the distribution of crystallographic orientation before/after the massive-like transformation.
A study25) using a high-temperature confocal scanning laser microscopy indicated that the δ phase did not transform massively to the γ phase in carbon steels (0.16 C, 1.08 Mn, 0.21 Si in mass%) at a cooling rate of less than 1 K/s, and the δ phase transformed to the γ phase massively at a cooling rate of 1 K/s. Similar results were obtained in previous works.23,24,26,27) Studies using X-ray imaging techniques, which are presented in this paper, show that the massive-like transformation was always selected even at lower cooling rates. Moreover, the massive-like transformation was observed at higher carbon contents (i.e., 0.45 mass% C). The authors consider no contradiction between the studies that use the two different techniques. The discrepancy arises mainly from the terminology and the difference in wave length used by the techniques. To avoid confusion, this paper defines the transformation modes here.
Figure 2 shows a schematic illustration of the transformation modes from the δ to the γ phase in the Fe–C binary alloys. In the peritectic transformation and peritectic reaction, which were proposed in previous works,55,56) γ phase growth is controlled by carbon diffusion in the γ and liquid phases, respectively. The diffusion-controlled mode includes continuous growth of the γ phase from a crystallographic perspective. In this paper, this mode including the continuous growth is defined as a diffusion-controlled peritectic transformation.
Definition of transformation modes in Fe–C binary alloys. (a) Diffusion-controlled peritectic transformation and (b) massive-like transformation. The former, including the peritectic reaction,55) is defined as a crystallographically-continuous growth of γ phase into the δ phase. The latter is defined as the formation of multiple γ grains in a single δ grain. The γ – liquid interface is partially indicated by red lines to compare the shape of γ phase in the two different modes. (Online version in color.)
The massive-like transformation is defined as a solid–solid transformation, which produces multiple γ grains in a single δ grain and means that the γ phase does not grow continuously. Concurrent nucleation events of the γ phase occur in a single δ grain.37,57,58,59) If the massive-like transformation occurs in the δ + liquid state, γ dendrites thicken because of γ-phase growth into the liquid phase. The morphological change in the massive-like transformation is similar to that in the diffusion-controlled peritectic transformation if the grain boundary formation is ignored, as shown in Fig. 2. In observations using laser-scanning microscopy,23,24,25,26,28) the grain structure of γ phase was not identified. The expression “massive” is rather qualitative. The critical point to identify the massive-like transformation is the formation of multiple γ grains in a δ grain by concurrent nucleation events.
Figure 3 shows solidification radiography in the 0.45 mass%C steel (0.45C, 0.6Mn and 0.3Si in mass%). As shown in Fig. 3(a), transformation from the δ to the γ phase in the δ + liquid state was observed at a cooling rate of 0.17 K/s.20) The γ phase grew along δ dendrites, and γ dendrites started to grow ahead of δ dendrite tips after the γ phase reached the δ dendrite tips. Because the transformation occurred in the mushy region, the transformation is categorized into a peritectic-like mode defined in an our early work.20) The transformation observed in the work20) is categorized into the massive-like transformation defined in this paper, because multiple γ grains formed in the δ dendrite arms.37)
Transmission images of a massive-like transformation from the δ to the γ phase in 0.45%C steel.20) (a) Cooling rate of 0.17 K/s and (b) 0.83 K/s. X-ray energy was 21 keV.
Figure 3(b) shows solidification at a cooling rate of 0.83 K/s. The δ phase was cooled below the peritectic temperature without γ-phase nucleation. The δ phase transformed into the γ phase in the solid state, and the transformation in the observation area was completed within 1 s. Because the volume shrinkage by transformation from δ to γ caused local melt flow, grooves were produced (white region). Many dark regions in the γ phase indicated that multiple γ grains were formed and strain was induced in the γ grains. The transformation occurred at 100 K below the liquidus temperature and the δ phase was placed in the single γ-phase region in the phase diagram. Thus, solute partitioning between the δ and γ phases was not required from a thermodynamics perspective. The interstitial carbon atoms can be partitioned at the interface because of a high diffusivity. This transformation was classified into the massive-like transformation.
High-speed radiography with a frame rate of 250 fps was performed to observe interfacial motion between the δ and γ phases during the massive-like transformation in 0.3C steel (0.3%C, 0.6Mn and 0.3Si in mass%) at a cooling rate of 0.83 K/s,60) as shown in Fig. 4. The interface between the δ and γ phases moved from the right to the left in the undercooled δ phase. The shape of the δ/γ interface was not planar and the moving velocity fluctuated. The moving velocity ranged from 0 to 170 mm/s.60) The fluctuation was related to nucleation events of the γ phase in the massive-like transformation.
Snapshots of fast radiography and tracking of δ–γ interface in massive-like transformation in 0.3C steel (0.3 C, 0.6 Mn, 0.3 Si in mass%) at a cooling rate of 0.83 K/s.60) (a) δ phase immediately before transformation at 0 s, (b) δ + γ phases at 0.16 s with tracked interfaces at 0.016, 0.020, and 0.024 s and (c) γ phase at 0.228 s. Frame rate was 250 fps.
An undercooling as large as 100 K of the δ phase from the peritectic temperature suggested a difficulty of γ-phase formation in the δ + liquid state and in the δ phase.20,37,60) Thus, the interface between the δ and liquid phases was not the preferred γ-nucleation site. However, nucleation events of the γ phase in the δ phase occurred concurrently during the massive-like transformation. The two nucleation tendencies appeared to be far apart. Concurrent nucleation was analyzed by using a phase field model.57,58,61) The energy barrier for nucleation of the γ phase in a homogeneous δ phase was more than three orders of magnitude greater than that for homogeneous nucleation of the δ phase. Thus, nucleation of the γ phase in the δ grains hardly occurs. Nucleation of the γ phase on a δ phase grain boundary (δ–δ interface) reduces the energy barrier, which increases the nucleation rate. Nucleation of the γ phase at the interface between the δ and γ phases halves the undercooling that is needed with a smaller critical radius. The study58) demonstrated that once nucleation of the γ phase is initiated, γ-phase nucleation is promoted by doubling the driving force. The results obtained in the studies explained the experimentally observed phenomena during the massive-like transformation.
A selection of the massive-like transformation was confirmed in 1-mm-thick bulk specimens.62) Undercooling for nucleation of the γ phase was smaller compared with undercooling in 100-μm-thick specimens. Thus, the specimen dimension influences undercooling for γ-phase nucleation, but a massive-like transformation was still selected and undercooling was required for nucleation of the γ phase even in the bulk specimens. Cross sections of 5 mm × 1 mm in the bulk specimens were of the same order of the δ grain size at the surface region of casting. Time-resolved tomography (4D-CT) confirmed the massive-like transformation in 0.8–1-mm diameter, 5-mm-long rod-shaped specimens. Therefore, the massive-like transformation is not induced by the size effect and normally occurs in conventional solidification even if the transformation temperature changes.
The massive-like transformation was confirmed in Fe–18mass% Cr–Ni alloys (8, 11, 14 and 20 mass% Ni).59) According to the equilibrium phase diagram (Fig. 5), the γ phase is the primary phase at 14 mass% Ni and 20 mass% Ni and γ-phase solidification is expected at 20 mass% Ni. As shown in Fig. 4(a), solidification was initiated by nucleation of the δ phase and fine γ grains formed by the massive-like transformation at 8 and 11 mass% Ni. The selection was essentially the same as that observed in the Fe–C alloys. δ-phase nucleation as a metastable phase was preferably selected even at 14 mass% Ni and 20 mass% Ni, as shown in Fig. 5(a). In subsequent cooling, the γ phase was produced through the massive-like transformation, as shown in Fig. 5(b). Metastable δ-phase nucleation triggers the massive-like transformation. The results indicate that massive-like transformation is generally expected in the Fe-based alloys, in which δ and γ phases compete from a thermodynamic perspective. The nucleation temperatures of the δ phase at 20 mass% Ni exceeded the liquidus temperatures of the δ phase, which was estimated by extrapolating the liquidus line. The inconsistency requires further studies on phase equilibrium.
(a) Nucleation temperatures of δ and γ phases and (b) temperatures of massive-like transformation.59) Data are plotted on the Fe-rich portion of the pseudo-binary phase diagram of (Fe–18 mass% Cr)–(Ni–18 mass% Cr). The dashed line in the δ + γ region indicates the T0 curve. Closed circle and open rectangles in (a) correspond to δ- and γ-phase nucleation, respectively.
As mentioned in this section, the massive-like transformation was selected when the δ phase was sufficiently undercooled below the peritectic temperature. Thus, the massive-like transformation in Figs. 3 and 4 is a transient phenomenon. For example, the massive-like transformation occurs when the δ phase in the surface region of casting is undercooled. Two possible transformation modes occur after the transformation reaches steady state. One is that the massive-like transformation is still selected in the steady state. The other is that the mode is switched to the diffusion-controlled peritectic transformation.
It is critical to confirm that the γ phase transforms into the δ phase massively or the γ phase grows in the diffusion-controlled mode. Figure 6 shows radiography snapshots during the unidirectional solidification of 0.3C steel (0.3 C, 0.6 Mn and 0.3 Si in mass%) at a pulling rate of 50 μm/s.34,38,63) The γ phase was intended to exist behind the liquid and δ phases before pulling down (out of the observation area). The vertical axis in the specimen was defined to track the interfaces, as shown in Fig. 6.
Transmission images during unidirectional solidification in Fe–0.3C–0.6Mn–0.3Si.34,38) Pulling rate was 50 μm/s. Arrows on the left (transmission images) indicate γ-phase growth front. Pixel size for transmission image was 2.5 μm × 2.5 μm. Frame rate of transmission images was 1 fps. X-ray energy was 21 keV.
Dendritic growth of the δ phase was followed by transformation from the δ to the γ phase as 80 s. Many dark spots, where the Bragg condition was satisfied, were observed within the γ region. Because X-ray beam divergence at the beamline BL20B2 was 1.5 mrad in the vertical direction and 0.06 mrad in the horizontal direction,64) a number of spots indicated that multiple γ grains with different crystallographic orientations were produced at the transformation front. Thus, the massive-like transformation was selected even at a growth rate of 50 μm/s. In addition to the multiple γ grains that were produced through the massive-like transformation, a coarse γ grain, which had a front at 500 μm from the tip, appeared at 100 s. The γ grain size was as large as 1 mm. The similar γ grain morphology existed in specimens that were quenched during unidirectional solidification.65,66,67) Thus, the X-ray imaging confirmed how the configuration of γ grains evolved during unidirectional solidification.
Figure 7 shows the δ dendrite tip position on the axis in Fig. 4(a). The fronts of fine γ grains (front of the massive-like transformation) and of coarse γ grains are also plotted in Fig. 5. δ dendrite growth, the growth of fine γ grains and the growth of coarse γ grains nearly reached steady state after 100 s. The observations prove that the massive-like transformation is selected even in steady state growth during unidirectional solidification. In previous work,34,38) a critical growth velocity from the diffusion-controlled peritectic solidification to the massive-like transformation was as low as 5 μm/s. In conventional continuous casting, the growth velocity was several mm/s at the surface and approximately 1 mm/s at 4 mm from the surface.68) Thus, the massive-like transformation was selected dominantly in conventional solidification.
Positions of liquid–δ interface, δ–γ (fine grains) interface and γ (fine grains)–γ (coarse grain) interface during unidirectional solidification at a pulling rate of 50 μm/s.63) (Online version in color.)
Carbon and manganese profiles in the diffusion-controlled growth of the γ phase was analyzed to estimate the driving force for γ-phase nucleation ahead of the planar γ interface.34,38) The relationships between the temperature and concentration are drawn on the binary phase diagram. Figure 8(a) shows the manganese profile at a growth velocity of 0.5 μm/s for a temperature gradient of 2 K/mm. The positions, z = 0 mm and z = 2 mm, are the distances from the planar γ interface. The diffusion layer thickness, 2D/V, is approximately 40 μm, because of lower substitute element diffusivities, and so constitutional undercooling always exists ahead of the γ front. The argument is based on the partition local equilibrium mode (P–LE),69,70,71) in which the chemical potential of substitutional manganese is continuous at the δ–γ interface in the transformation. As shown in Fig. 6(a), the Negligible−Partition Local Equilibrium mode (NP–LE)69,70,71) can also be selected in the massive-like transformation. Because the driving force in the NP–LE is larger than that in the P–LE mode, the driving force for γ-phase nucleation arises near the δ–γ interface for both cases. The previous work shows the discussion also reaches the same conclusions for substitutional silicon. The carbon profiles on the binary phase diagram are shown in Fig. 8(b). The high diffusivity of interstitial carbon atoms tends to suppress constitutional undercooling. However, constitutional undercooling is generated at a growth rate of 5 μm/s (the critical growth rate from the diffusion-controlled transformation to the massive-like transformation). The estimations indicate that the driving force for γ-phase nucleation exists ahead of the γ interface during unidirectional solidification. As mentioned previously, the interface between the δ and γ phases is a preferred nucleation site for another γ grain.57,58,61) The barrier for nucleation of the γ phase leads to an undercooling and consequently promotes the massive-like transformation.
The massive-like transformation from the δ to the γ phase influences subsequent microstructure evolution, such as dendrite arm fragmentation and γ-grain coarsening. Figure 9 shows an example of the dendrite arm fragmentation that is induced by the massive-like transformation in 0.45C steel (0.45 C, 0.6 Mn, 0.3 Si in mass%).37) The δ dendrites were cooled at approximately 20 K below the peritectic temperature (after 152 s) and the temperature gradient was maintained. The massive-like transformation from the δ to the γ phase occurred at 346 s. The γ phase with a perturbed liquid–γ interface grew into the liquid phase rapidly because the liquid phase in equilibrium with the δ phase at temperatures below the peritectic temperature was undercooled below the liquidus temperature of the γ phase. Growth of the γ phase into the liquid phase or the shape change of the dendrite arms appears like the peritectic reaction/transformation, because δ dendrites appear to be covered with a γ phase. However, darks spots in the dendrite arms indicate that multiple γ grains were formed within the dendrite arms of the δ phase. Liquid film formation at the γ grain boundaries was also detected (blue arrows in Fig. 9). The γ grain boundaries migrated and the velocity was as fast as 1 μm/s. As a result of this coarsening, γ grains in the dendrite arms were isolated by the liquid phase, as shown in the image at 565 s. The fragmentation was not observed in continuously cooled specimens after the massive-like transformation.34,38,60,63,72)
Fragmentation of γ grains induced by massive-like δ–γ transformation in 0.45C steel (0.45 C, 0.6 Mn, 0.3 Si in mass%).37) Transformation occurred at 20 K below the peritectic temperature after 152 s. Liquid film produced at γ grain boundaries indicated by blue arrows. The X-ray energy and exposure time were 21 keV and 50 ms, respectively. (Online version in color.)
The fragmentation mechanism observed in the carbon steel is unique, compared with several other known mechanisms.30,73,74,75,76) One of the known mechanisms is temperature gradient zone melting74) and was observed by X-ray imaging.73,75) Another mechanism is solute transport because of melt flow in the mush.30,76) In both mechanisms, solute transport causes melting at the dendrite arm necks. The other mechanism is the shape instability mechanism,77) which operates during solidification in an undercooled melt. The mechanism of fragmentation that is induced by the massive-like transformation leads to multiple fragmentations. The fragmentation can be related to branched columnar grains,78) which was observed only in solidified steel structures.
Recently, a 4D-CT setup was used to perform 3DXRD (three-dimensional X-ray diffraction for measuring crystallographic orientations of solid grains in the mushy)54) and to measure the crystallographic orientation of dendrites.50) The technique was used to observe γ-grain coarsening after the massive-like transformation.63) Normal vectors of (111), (200), and (220) of γ grains after the massive-like transformation are plotted in stereo projections in Fig. 10(a). Because the specimen consisted of a single grain of δ phase before the massive-like transformation, multiple γ grains proved that massive-like transformation occurred. The scatter of each plane tended to decrease with an increase in time after the massive-like transformation.
(a) Crystallographic orientations of γ phase at 0–4 s and 316–320 s after transformation and (b) close-up view of rectangular region in (a).63) Time was designated to be zero when the massive-like transformation was detected. (Online version in color.)
Close-up views of the rectangular region in Fig. 10(a) are shown in Fig. 10(b). The “G1” intensity, which is indicated by yellow arrows in the (111) plane, decreased immediately after the massive-like transformation and vanished at 320 s. The “G1” grain vanished during coarsening. The “G2” intensities, which are indicated by green circles, spread widely after the massive-like transformation and converged gradually during coarsening. The change is attributed to strain release that is introduced by the volume change through the massive-like transformation. The observed phenomena in “G1” and “G2” are normally expected during coarsening. The “G3” intensities appeared during coarsening as shown in the (220) normal vectors. The creation of a new “G3” grain is not expected in conventional coarsening as controlled by the curvature effect. The results suggest that the γ-grain coarsening after the massive-like transformation is not simply explained by conventional coarsening. The grain evolution can be similar to recrystallization. Further studies are expected to understand γ-phase coarsening.
Figure 11 shows a schematic illustration of γ-grain coarsening after the massive-like transformation.63) The fine γ grains, in which strains are induced, are produced in a δ grain through the massive-like transformation. The gradual change in crystallographic orientation within a γ grain reflects strain and lattice defects. γ grains coarsened and/or vanished after the massive-like transformation. Simultaneously, induced strains were released and new γ grains were created. The process was not simply controlled by the curvature effect. The strains can influence coarsening kinetics. Thus, the massive-like transformation should be included to understand the coarsening kinetics in subsequent cooling after solidification.
Schematic illustration of coarsening of γ grains after massive-like transformation.63) (a) Single δ grain, (b) γ grains immediately after massive-like transformation and (c) γ grains during coarsening. Colors in the grains correspond to crystallographic orientation. (Online version in color.)
In the next stage, it is of interest to understand the massive-like transformation quantitatively and to build physical models for solidifying phenomena. Physical properties related to the solidification and the transformation will be required to perform quantitative analyses. 4D-CT will be used to measure volume changes at high temperatures. The number of voxels in 4D-CT is typically in the order of 106. Thus, the specimen volume is evaluated by counting the specimen voxels (solid and liquid phases).
Figure 12 shows the change in volume during cooling from the melt at a cooling rate of 0.33 K/s in 0.18C steel (0.18 C, 0.6 Mn, 0.3 Si in mass%).49) The liquid-phase volume decreased gradually because of thermal shrinkage. The linear expansion coefficient was estimated to be 42 × 10−6 K−1. Because the linear expansion coefficient calculated from the reported values ranged from 38 × 10−6 K−1 to 74 × 10−6 K−1,79) the value obtained by 4D-CT appears to be valid. According to a review of the density of pure iron,80) solidification shrinkage that was calculated from the densities of the liquid and δ phases ranges from 3.4% to 3.6% and the volume change because of the transformation from the δ to the γ phase was –0.57%. The solidification shrinkage observed by 4D-CT was 3.0%, which is smaller than the reported values. The volume change because of the transformation was –0.51%, which agrees with the reported value. Although the origin of discrepancy in solidification shrinkage should be examined furthermore, 4D-CT will be used to measure the volume change in the transformation.
Volume change of 0.18 mass%C steel during cooling from the melt.49) The volume was normalized by the volume immediately before the massive-like transformation. (Online version in color.)
The authors would like to stress that the volume change of the δ phase from the end of solidification to the massive-like transformation was as large as 2%, because the massive-like transformation occurred in the undercooled δ phase. The value of 2%, which is four times larger than that induced by the transformation, can influence stress/strain in the solidifying shell. Quantitative analyses based on the observed structure and the measured properties are expected to improve our understanding of the massive-like transformation and the formation of a solidifying shell.
As shown in the previous sections, transformation from the δ to the γ phase differs from the diffusion-controlled growth, which is normally termed the peritectic solidification. However, the massive-like transformation has not been explicitly included in modeling of solidification and casting defect formation, although the massive-like transformation has accepted as a transformation mode, as presented in a very recent review.81) Thus, it will be critical to improve our understanding of the transformation and casting defect formation from an industrial perspective.
Cracking formation in continuous casting of hypoperitectic steels has been discussed with the peritectic reaction.10,11,14) The strain that is induced by the peritectic reaction was introduced in a mathematical model.11) Here, we compare the strain that was induced by transformation from the δ to the γ phase. In the case of the diffusion-controlled peritectic transformation, volume shrinkage occurs nearly at the peritectic temperature and the shrinkage caused a solidifying shell uplift. In the massive-like transformation nearly at steady state, as shown in Fig. 6, the volume shrinkage is essentially the same as that caused by the diffusion-controlled peritectic transformation, because the δ–γ interface front is below the peritectic temperature but close to the peritectic temperature. If a massive-like transformation occurs in the undercooled δ phase, as shown in Figs. 3 and 4, the volume shrinks from the surface to the inside rapidly because the velocity of the δ–γ interface can be as large as 100 mm/s.59,60) Volume shrinkage requires only 1 ms for the solidifying shell with a thickness of 0.1 mm. The increase in strain rate increases the stress in the solidifying shell. Thus, some modifications may be required to estimate the induced stress and strain. For further discussions, we should understand how the transformation occurs in the surface region during continuous casting.
The massive-like transformation induced γ grains that were isolated by the liquid film.37) The volume change in the transformation from the δ to the γ phase was –0.5%.49) The volume shrinkage was equivalent to a strain of only –0.17% for isotropic shrinkage. The value was much smaller than the critical strains for internal cracking formation.3,15) Local strain that is induced by the massive-like transformation is not linked directly to crack formation at the liquid film between the γ grains. Strains originated by various mechanisms,4,10,13,14) such as friction between the solidifying shell and mold, unevenness of the solidifying shell and static pressure, and should be considered. A challenge should include the massive-like transformation.
This paper reviews the transformation from the δ to the γ phase during/after solidification in carbon steel and stainless steel based on observations using X-ray imaging techniques.
Time-resolved transmission imaging with X-ray diffraction measurements and time-resolved tomography with X-ray diffraction measurements, in synchrotron radiation facilities, allowed us to observe solidification and transformation from the δ to the γ phase in steel in-situ. Observations of morphological evolution in-situ and changes in crystallographic orientation provide evidence for solidification and transformation.
In this paper, the massive-like transformation is defined as a transformation, in which multiple γ grains are produced in a single δ grain though solid–solid transformation. The peritectic reaction/transformation, in which the γ phase is produced through a reaction between δ and the liquid phase and grows continuously from a crystallographic perspective, is defined as the diffusion-controlled peritectic transformation.
When δ is undercooled below the peritectic temperature, massive-like transformation is selected in carbon steel and Fe–Cr–Ni alloys. In the Ni-rich Fe–Cr–Ni alloys in which γ solidification is expected in equilibrium, selection of a metastable δ phase results in the massive-like transformation. The selection of a δ phase triggers γ-phase solidification.
The massive-like transformation is also selected in unidirectional solidification at a growth rate of 50 μm/s. Multiple γ grains are produced behind the δ dendrite tips. The distance between the tip and front of the γ phase is less than 1 mm under a temperature gradient of 10 K/mm. Coarse γ grains are observed behind the fine γ grains. The critical velocity from the diffusion-controlled peritectic transformation to the massive-like transformation is as low as 5 μm/s. Thus, massive-like transformation will be selected in conventional solidification processes for steels.
The production of multiple γ grains influences the subsequent microstructure formation. The dendrite arm fragmentation is induced by γ grain-boundary melting when the temperature is kept constant or increases slightly. γ-grain coarsening can be influenced by the massive-like transformation. The X-ray imaging results show that massive-like transformation has to be included to consider solidification and casting defects in steel.
In-situ observations using synchrotron radiation X-rays were performed as long-term and general projects at the BL20B2 and BL20XU of SPring-8 (JASRI), Japan. The authors express our thanks to valuable discussions in a research group “Visualization of Solidification” of the 19th Committee on Steelmaking of JSPS in the initial stage of “massive-like transformation” research. The authors also acknowledge financial support from Heterogeneous Structure Control: Towards Innovative Development of Metallic Structural Materials of the Industry–Academia Collaborative R&D Program (JST) and a Grant-in-Aid for Scientific Research (S) (No. 17H06155) for observing solidification phenomena at SPring-8. The authors thank graduate students, who contributed to the studies, at Osaka University and Kyoto University. We thank Laura Kuhar, PhD, from Edanz Group (https://en-author-services.edanzgroup.com/ac) for editing a draft of this manuscript.
