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Article

Effects of Chemical Composition and Solidification Rate on the Solidification Behavior of High-Cr White Irons

by
Hee Young Son
1,2,
In Yong Jung
3,
Baig Gyu Choi
3,
Jong Ho Shin
4,
Chang Yong Jo
1,* and
Je Hyun Lee
1,*
1
Department of Advanced Materials, Changwon National University, 20 Changwondaehak-ro, Changwon-si 51140, Republic of Korea
2
KPCM, 249 Wachonseo-gil, Wachon-myoen, Gyeongsan-si 38412, Republic of Korea
3
High Temperature Materials Department, Korea Institute of Materials Science, 797 Changwondae-ro, Changwon-si 51508, Republic of Korea
4
Doosan Enerbility, 22 Doosan volvo-ro, Seongsan-gu, Changwon-si 51711, Republic of Korea
*
Authors to whom correspondence should be addressed.
Metals 2024, 14(3), 276; https://doi.org/10.3390/met14030276
Submission received: 29 January 2024 / Revised: 14 February 2024 / Accepted: 22 February 2024 / Published: 27 February 2024
Browse Figures
Versions Notes

Abstract

:
The effects of chemical composition and solidification rate on the solidification behavior of high-Cr white irons were investigated through directional solidification. Increasing the solidification rate in hypoeutectic alloys caused finer dendrite-arm spacing, as expected. The eutectic structure, which formed in the interdendritic region, was comprised of M7C3 and austenite; however, secondary dendrite arms of hypoeutectic alloys contained a few M7C3 particles that solidified prior to the eutectic structure. The transition from cellular to dendritic solidification occurred at a solidification rate between 50 µm/s and 100 µm/s in a near-eutectic alloy. In the near-eutectic alloy with cellular solidification, a directionally arrayed in-situ composite of M7C3/austenite formed within the cell. Speckle-like features appeared in the intercellular region due to M23C6 carbide precipitation during subsequent cooling after freezing. Like dendrite-arm spacing in hypoeutectic alloys, the inter-speckle spacing and the inter-fiber spacing became finer with an increasing solidification rate in the cellular solidification range.

1. Introduction

High-Cr white irons, which are applied to the wear-resisting components, typically have a microstructure that consists of a martensitic or austenitic matrix and an M7C3 carbide, which depends on the section size and chemical composition [1,2,3]. The phase fractions of the as-cast microstructure are dependent on chemical composition, especially C and Cr [2,3]. It is well known that the solidification of hypoeutectic high-Cr white irons begins with the formation of primary austenite dendrites, followed by the M7C3/austenite eutectic reaction [1,2,3,4]. During heat treatment of the alloys, the matrix becomes mainly martensitic, and M23C6 carbide precipitation occurs [2,3]. The excellent wear-resistance of high-Cr white irons is attributed to the hard martensitic matrix and these carbides.
Not only does the chemical composition have a strong influence on the cast microstructure of metallic alloys, but so do the casting variables. The morphology and size of the existing phases of the alloys are closely related to casting variables. For instance, the dendrite-arm spacing and size of existing phases in the cast alloys are governed by the solidification rate and thermal gradient during solidification [5]. It is well known that the dendrite morphology and heat-treatment cycle of the alloy are closely related to the mechanical properties [5].
As mentioned in a previous study [6], the casting variable in conventional casting is the pouring temperature during solidification, and the resulting microstructure is dependent upon many factors, such as alloy modification, semi-solid treatment, current pulse treatment, heat treatment, and inoculation [7,8,9,10,11]. Many studies to improve mechanical properties and corrosion resistance by the refinement of primary carbide of hypereutectic high-Cr white irons [7,8]. In conventional casting studies, it is not easy to measure the solidification rate or thermal gradient at the solid/liquid interface. Thus, the control of solidification variables during conventional casting of a given chemical composition is not available, except for the pouring temperature. However, directional solidification (DS) allows for control of the solidification variables by regulating the withdrawal rate from the furnace and thermal gradient at the solid/liquid interface (GL). The control of solidification variables in high-Cr white irons can develop various microstructures of a certain composition [6]. As reported earlier [2,3], variations in chemical compositions in high-Cr white iron also generate various fractions of primary austenite and eutectic structure in conventional castings.
Based on previous results [2,3,6], it is possible to find the relationship between microstructural features, directional solidification casting variables, and chemical compositions of high-Cr white irons. Ye et al. [12] studied the microstructure and corrosion wear properties of directionally solidified high-Cr iron. As reported earlier [2,3,6,7,8,9,10,11,12], microstructural features in hypereutectic high-Cr irons play important roles in the properties of hypereutectic high-Cr irons. Not only hypereutectic high-Cr irons but also hypoeutectic high-Cr irons are widely applied to wear-resisting components [1]. Therefore, in the present study, high-Cr white irons with four different hypoeutectic chemical compositions were directionally solidified at various solidification rates to find the relationship between the solidification rate and microstructural evolution. The development of microstructural features with various solidification rates for the four different hypoeutectic high-Cr white irons was analyzed.

2. Experimental Procedure

High-Cr irons with four distinct chemical compositions were prepared by master alloy-making and remelting, followed by pouring into Y-blocks as previously reported [2,3]. The chemical compositions of the alloys are provided in Table 1. Phase prediction was conducted using commercial software Thermo-Calc (Thermo-Calc 2019b) based on the Steels/Fe-Alloys (V.9.1) database DB TCFE9 to understand the phase evolution during cooling. Additionally, JMatPro (Version 12) was used to predict the phase diagram of the alloys based on Thermotech Fe data.
Specimens for directional solidification were machined from the Y-blocks to 4.7 mm diameter rods by electric wire discharging machining (EDM). The EDM-machined rods were mechanically polished with abrasive paper to remove any recast and oxidized layers at the surface. The rod was inserted into an alumina tube (ID 5 mm × OD 8 mm × L 700 mm) and then installed in a Bridgman-type directional solidification furnace (DS furnace) by clamping the bottom with a cold finger. The principle of the facility is the DS furnace, which is applied to fabricate eutectic composites [7].
The specimen in the DS furnace was heated to 1600 °C for melting and soaked for 30 min for stabilization. Each specimen was then directionally solidified at a constant thermal gradient of 10 K/mm and various withdrawal rates of 5, 25, 50, and 100 µm/s.
For optical microscopy (OM: Nikon/ECLIPSE MA200, Tokyo, Japan) and scanning electron microscopy (SEM: JEOL/JSM-IT500LV, Tokyo, Japan), conventionally cast and DS specimens were metallographically prepared and etched with Vilella’s reagent consisting of 45 mL glycerol, 15 mL nitric acid, and 30 mL hydrochloric acid. Microscopic observations were carried out both on the sections parallel and transverse to the solidified direction. Specimens for transmission electron microscopy (TEM: JEOL, Tokyo, Japan, JEM-2100F) were prepared by mechanical polishing to a thickness of 60 μm, followed by twin-jet polishing (Struers, Champigny sur Marne Cedex, France, TenuPol-5). The solution for twin-jet thinning was 10% perchloric acid. The existing phases were identified by TEM-selected diffraction pattern (SADP) analysis.

3. Results and Discussions

3.1. Phase Prediction of the Conventionally Cast Alloys

Figure 1 shows the microstructure of the alloys in the as-cast condition. As the carbon content increases, the fraction of primary austenite dendrite decreases, while that of the eutectic structure increases, which is expected. Alloys 2124, 2127, and 2427 contain large-volume primary austenite dendrites, whereas a little dendrite appears in alloy 2827. Based on the volume of primary austenite dendrites, 2124, 2127, and 2427 were classified into hypoeutectic alloys, whereas 2827 was classified as a near-eutectic alloy. The Thermo-Calc prediction of the pseudo-binary phase diagram in Figure 2a shows that alloy 2827 is in the hypereutectic composition range, even though alloy 2827 has some fraction of primary austenite dendrite in the conventional casting. However, the JMatPro prediction in Figure 2b shows that alloys 2127 and 2427 are obviously in the hypoeutectic range, and alloy 2827 is in the near-eutectic range. Moreover, the Thermo-Calc prediction shows that the solidification of 2127 begins with delta-ferrite formation, but the JMatPro prediction displays that the leading phase of solidification is austenite, like the other hypoeutectic alloys. The microstructural features of the alloys in Figure 1 and JMatPro phase prediction in Figure 2 display the same results.
Generally, it is known that the solidification of hypoeutectic cast iron begins with austenite formation from the liquid followed by a eutectic reaction in the interdendritic regions [1,13,14]. The eutectic reactions are graphite with austenite in gray irons and carbide with austenite in white irons [1,13]. It is known that the eutectic carbide in high-Cr white iron is M7C3 [1,13]. The gray eutectic areas in Figure 1 are composed of M7C3 and austenite (or martensite). Increasing C and Cr contents decreased the volume of primary austenite dendrite drastically, as displayed in Figure 1, but increased that of the eutectic structure, similar to that reported in previous in super-high-chrome white irons [13]. M23C6 carbide, which is a precipitation product found during heat treatment or post-cooling after solidification, may also exist in high-Cr white irons [1,15,16]. The carbides in the alloys will be addressed in directionally solidified materials.

3.2. Microstructure of the Directionally Solidified Alloys

Figure 1 shows that conventional casting produces an equiaxed microstructure with a random orientation of existing phases. Pouring temperature is a convenient casting parameter to control in the conventional casting process for a given configuration or mold, making it difficult to study the effect of casting or solidification variables. Directional solidification, which was developed for basic solidification studies and directional arrays, is useful for studying solidification behavior by controlling the cooling rate (or solidification rate) or thermal gradient at the solid/liquid interface [5]. In the present study, the solidification behavior of the alloys was studied by directional solidification at various solidification rates at a constant thermal gradient of 10 K/mm.
As mentioned above, the alloys 2124, 2127 and 2427 have a large volume of primary austenite dendrite in conventional castings, and the predicted phase diagram in Figure 2 also shows that the alloys are in the hypoeutectic composition range. However, the alloy 2827 is in the near-eutectic composition range of the hypoeutectic side. Thus, the directionally solidified alloys were classified into hypoeutectic alloys and a near-eutectic alloy based on microstructural analysis and JMatPro prediction.

3.2.1. Hypoeutectic Alloys with a Large Volume of Primary Austenite

Directional solidification for the alloys was carried out with solidification rates of 5, 25, 50, and 100 µm/s and at a constant thermal gradient of the solid/liquid interface of 10 K/mm. As expected, primary austenite dendrite led the solidification, and final freezing occurred with the formation of an M7C3/austenite eutectic structure in the interdendritic regions, both in conventional casting and directional solidification. Figure 3 and Figure 4 display directionally solidified micrographs at each solidification rate for alloys 2124, 2127, and 2427. As indicated in Figure 2, the alloys have hypoeutectic compositions far below the eutectic composition. Thus, a directional array of dendrites along the solidified direction is obvious at all solidification rates for the alloys, as shown in Figure 3. However, a random orientation of dendrites appeared at the solidification rate of 100 µm/s. The existence of the randomly oriented dendrites means that the solidification condition is similar to that of equiaxed grains. The conventional equiaxed casting cooling rate (C) of the alloys might be calculated from the thermal gradient (GL) and solidification rate (VI) [17,18,19,20,21].
C = GL VI
where GL: 10 K/mm, VI: 100 µm/s, and pouring temperature 1600 °C.
Even though it includes several experimental errors, the cooling rate of conventional casting for the alloys is calculated to be 10 °C/s. The relatively faster solidification rate of 100 µm/s might cause more radiation cooling rather than conduction cooling of the cold finger.
In the section transverse to the solidified direction, well-developed secondary dendrite arms appeared clearly in Figure 4. The dendrite spacing becomes finer with increasing solidification rate in all alloys. The secondary dendrite arms have fine branches (or tertiary arms) to compensate heat conservation and constitutional supercooling at relatively high solidification rates, as shown in Figure 4. The results are similar to the solidification behavior of Ni-based superalloys, which is generally accepted [5]. The well-aligned dendrite structure was caused by the preferred solidification of austenite, which means that the leading phase is austenite. The M7C3/austenite eutectic structure formed in the interdendritic regions where the final freezing occurred. The results show good agreement with Thermo-Calc predictions of solidification sequence [2,3], and those of JMatPro. The alloys have similar solidification behavior except for the difference in the amount of primary austenite dendrite.

3.2.2. Directional Solidification of a Near-Eutectic Alloy

As mentioned earlier, hypoeutectic alloys 2124, 2127 and 2427 contain a large amount of primary austenite dendrite that led to the solidification and formation of an M7C3/austenite eutectic structure in the interdendritic regions, as shown in Figure 4. On the other hand, the conventional castings of alloy 2827 contain a small amount of primary austenite dendrite, as shown in Figure 1c.
However, directional solidification with solidification variables may generate various microstructural features. The directionally solidified alloy 2827 shows that the microstructure varies with solidification rate, as displayed in Figure 5. A directional array of M7C3 carbide and austenite (referred to as “in-situ composite” below) along the solidified direction is obvious at and below the solidification rate of 50 µm/s in Figure 5a–c, while primary austenite dendrite appears at the solidification rate of 100 µm/s in Figure 5d. The existence of randomly oriented dendrites at the solidification rate of 100 µm/s in alloy 2827 was also caused by the same cooling rate calculated above (10 °C/s).
Figure 6a–c shows SEM micrographs of the directional array of in-situ composites at and below the solidification rate of 50 µm/s, whereas Figure 6d shows the coexistence of dendrite and an M7C3/austenite eutectic structure at a solidification rate of 100 µm/s. The results are very similar to those of the solidification behavior of eutectic alloys at various solidification rates [7]. In the present study, it also displays an in-situ composite structure at a relatively low solidification rate of primary austenite dendrite and an M7C3/austenite eutectic structure at a high solidification rate, as shown in Figure 6.
The inter-fiber spacing in the in-situ composite becomes finer with increasing solidification rate at and below 50 µm/s, as shown in Table 2. The transition of solidification behavior, which is similar to that in the 4.25 wt% C eutectic alloy [7], was observed between 50 µm/s and 100 µm/s, as seen in Figure 5 and Figure 6. The micrographs transverse to the solidified direction in Figure 6 also demonstrate this transition, which is expected to be from cellular to dendritic solidification. The cellular solidification may have both relatively earlier and later freezing areas, like the existence of interdendritic regions and dendrite in dendritic solidification. The speckles (brown or gray) seen in Figure 7a–c transverse to the solidified direction are relatively later freezing areas during cellular solidification. Furthermore, the distribution and size of the speckles become uniform and finer with increasing solidification rate, as seen in Figure 7a–c. Table 3 shows the measured inter-speckle spacing in alloy 2827 that becomes finer with increasing solidification rate at and below 50 µm/s.

3.3. Solidification Behavior of the Alloys

Based on the microstructural observations discussed above, the solidification behavior of the alloys at different solidification rates has been categorized in Table 4. In hypoeutectic alloys, which are significantly below the eutectic composition, dendritic solidification was observed at relatively low solidification rates (−50 µm/s), with dendrite-arm spacing decreasing as the solidification rate increased. However, at higher solidification rates (100 µm/s), the dendrite orientation became random. On the other hand, a transition from cellular to dendritic solidification was observed in a near-eutectic alloy (slightly away from the eutectic composition), occurring at a solidification rate between 50 µm/s and 100 µm/s in alloy 2827.
The solidification behavior in directional solidification, specifically in hypoeutectic alloys with a negative slope of the liquidus mL and solidus mS, is influenced by various factors [17,18,19,20,21], including the thermal gradient at liquid GL (Kmm/s), solidification rate VI (mms−1), melting range ΔT (K), and diffusivity in liquid DL (mm2/s). Charlmer and Mclean proposed conditions for solidification behaviors in Equations (2) to (4) [17,18,19,20,21] for the planar solidification:
GL/VI ≥ −mLCo(1 − k)/kDL
=ΔT/DL
or for the cellular and dendritic solidification:
GL/VI < ΔT/DL
In Equation (2), the condition for planar front solidification is presented, while Equation (4) represents the condition for cellular or dendritic solidification. In Equations (3) and (4), the GL value comes from the furnace and has a constant value of 10 K/mm in the present study. The VI values range from 5 µm/s to 100 µm/s for the solidification rate, and each alloy has its own values of ΔT and DL, which depend upon the alloy composition. Therefore, ΔT and DL are constant values for each alloy, regardless of solidification rate. The ΔT value for each alloy can be measured using DSC thermal analysis or calculated using Thermo-Calc or JMatPro simulations. The calculated solidification data are listed in Table 5, but unfortunately, the DSC measurement was not successful.
For example, dendritic solidification occurred at all solidification rates in hypoeutectic alloys 2124, 2127 and 2427, which have a sufficient wide range of ΔT melting, as listed in Table 5. Thus, the solidification conditions satisfied Equation (3). Cellular solidification occurred at and below 50 µm/s solidification rates in alloy 2827, and a transition from cellular to dendritic solidification appeared at solidification rates between 50 and 100 µm/s. This may be attributed to the small melting range ΔT of the alloy compared to the other hypoeutectic alloys that have a large volume of primary dendrite, such as 2124, 2127 and 2427, which have a wide melting range, as shown in Table 5.

3.4. Existing Microstructure

3.4.1. In-Situ Composite

As mentioned previously, an in-situ composite with a directional array along the solidified direction formed in the near-eutectic alloy 2827 under specific conditions. The formation of the in-situ composite is related to a combination of the melting range and solidification rate, which are in turn linked to the alloy composition and solidification variables. The in-situ composite was observed only under cellular solidification conditions in the alloy, which had a large volume of eutectic structure in conventional castings. However, the hypoeutectic alloys 2124, 2127 and 2427 did not exhibit in-situ composite formation due to their relatively wide melting ranges, as shown in Table 5. Consequently, only dendritic solidification occurred in these alloys, as listed in Table 4.
The micrographs of the in-situ composite in alloy 2827 display well-aligned M7C3/austenite along the solidified direction, as shown in Figure 6a–c. It is expected that the in-situ composite formed due to the competitive growth between M7C3 carbide and austenite within the cell. DSQ experiments revealed the competitive growth of M7C3 and austenite at cellular solidification condition of 50 µm/s in Figure 8a, whereas primary austenite dendrite leads the solidification at dendritic solidification condition of 100 µm/s in Figure 8b.

3.4.2. Speckles in a Near-Eutectic Alloy

As depicted in Figure 7, speckles are observed on the section transverse to the solidified direction, specifically in specimens where cellular solidification occurs. In the case of alloy 2827, the speckles appear at solidification rates at or below 50 µm/s, whereas dendrites appear above 100 µm/s. SEM microstructural observation of the speckles in Figure 9 reveals that they have similar constituents to those of the normal area but have many small, white particles and angular M7C3 carbides. On the other hand, the normal area has a few small, white particles and bigger angular M7C3 carbides. The size difference of M7C3 carbides indicates the time of solidification—bigger particles formed earlier in the normal area [Figure 9c] than smaller ones in the speckles [Figure 9b]. Therefore, the speckles are formed in the inter-cellular regions of the final freezing area during solidification.
The identification of the different phases present in the material was conducted by transmission electron microscopy (TEM) and selected-area diffraction pattern analysis (SADP). As previously mentioned, the big angular particles were confirmed to be M7C3 carbide (HCP) [22], while the small, white particles were identified as M23C6 carbide, which precipitated during solid-state cooling (similar to destabilization [15]), as illustrated in Figure 10 and Figure 11.
The speckle-like feature in Figure 7 and Figure 9 was attributed to the precipitation of M23C6 carbide among the big M7C3 particles.
The precipitation of M23C6 carbide in the speckle area was caused by the relatively high concentration of solute elements in the matrix austenite, as reported in reference [12]. The in-situ composite primarily solidified within the cells, which led to the rejection of solute elements to the remaining liquid in the vicinity, then, the rejected elements concentrated in the speckles where the final freezing occurred. Consequently, the matrix austenite in the speckle area became supersaturated. During the subsequent slow cooling after directional solidification, the austenite matrix became destabilized, resulting in the precipitation of secondary carbide M23C6 in the speckle area, as observed in Figure 9b. The white speckles depicted in Figure 9a were attributed to the destabilization of the supersaturated austenite that resulted in precipitation of M23C6 [15,16,23,24,25,26,27,28,29]. Even though several studies reported that precipitation of M23C6 carbide in high-Cr iron is attributed to the destabilization of austenite [15,16] or the reaction or transition from M7C3 carbide at high temperatures [15]. The precipitation of M23C6 carbides in the present study originated from destabilization in the supersaturated region (speckle region) during a relatively short subsequent solid-state cooling stage. Few M23C6 particles were observed in the inter-speckle regions where solidification occurred prior to speckle regions.
The EDS X-ray mapping results displayed in Figure 12 show concentration differences in speckle areas. As mentioned above, the speckle area has a higher concentration of Cr and C (the concentrations of Cr and C re too high to find the difference among the areas, but a combined mapping of Cr and C in Figure 12b allows us to note the high concentration of both elements).

3.4.3. Microstructural Evolution in Hypoeutectic Alloys

The initial stage of solidification in hypoeutectic alloys involves the formation of austenite. In the case of alloys 2124, 2127, and 2427, primary austenite dendrites formed under all solidification conditions, while a little primary austenite dendrite only appears at high solidification rates (100 µm/s) in alloy 2827. Figure 13 shows representative micrographs (taken transverse to the solidification direction) of each alloy, which reveal that the microstructure of the alloy consists of primary austenite dendrites and an M7C3/austenite eutectic structure, as expected. As shown in Figure 2, austenite is the leading solidification phase in hypoeutectic alloys. The JMatPro calculated eutectic range was 1290–1285 °C (and 1294–1285 °C with Thermo-Calc) for these hypoeutectic alloys, as shown in Table 5. Attempts to distinguish the formation of particles in the secondary arm from the M7C3 in the eutectic structure using differential scanning calorimetry (DSC) were not successful due to continuous formation and high temperature. However, a few M7C3 carbide particles were observed to be embedded in every secondary dendrite arm, with the particles located away from the primary austenite dendrite (the “Cross”-shaped center of the dendrite) and indicated by white arrows. The location of the M7C3 particles in the middle of the secondary arm provides indirect evidence of their formation time. The M7C3 particles were enveloped by austenite, which solidified prior to the eutectic (M7C3 /austenite) structure of final freezing, and the growth of M7C3 on the secondary arms was restricted by the solid austenite, as shown in Figure 13.
Similar to the speckles displayed in Figure 7 and Figure 9, tiny white M7C3 particles were observed in the interdendritic regions, especially among the secondary arms, as indicated by white circles. Furthermore, the interdendritic regions near the austenite dendrite have a similar feature to the speckles in the cellular solidification of a near-eutectic alloy. These regions (indicated by circles in Figure 13) contain many small, white particles of M23C6, which are believed to be caused by the rejection of solute elements during the progress of austenite dendrite solidification. X-ray mapping of C and Cr in Figure 14 displays the segregation of the elements in the interdendritic region and M7C3 carbides that embedded in the secondary dendrite arms. The solute elements became concentrated in the region of final freezing close to the dendrites. The segregated solutes, especially Cr and C, in the austenite of the eutectic structure destabilized during the subsequent solid-state cooling after solidification. The destabilization of austenite released M23C6 in the vicinity of dendrite arms, as indicated by the circles in Figure 13.
Therefore, the microstructural evolution during the whole cooling process (dendritic solidification and subsequent solid-state cooling) of the hypoeutectic alloys is schematically illustrated in Figure 15.

4. Conclusions

This directional solidification study conducted on high-Cr white irons has led to the following findings:
  • The cooling rate of conventional casting in hypoeutectic alloys (including a near-eutectic alloy) was around 10 °C/s, which was calculated by the casting parameters in directional solidification. Using the value that was similar in the four hypoeutectic experimental alloys, it would be useful to calculate the solidification duration in actual casting as well as in casting design.
  • Speckle-like features appeared in a near-eutectic alloy at and below 50 µm/s. The size and inter-speckle spacing became smaller with increasing solidification rate in the cellular solidification range. This would have originated from the solidification variables even in cellular solidification conditions.
  • The speckles were attributed to the segregation and destabilizing during subsequent cooling after solidification, which caused the precipitation of M23C6, which is a phenomenon similar to one found during the destabilization of the alloys.
  • A near-eutectic alloy displayed a directionally arrayed in-situ composite along the solidified direction. The inter-fiber spacing became finer with an increasing solidification rate within the cellular solidification condition.
  • A few M7C3 carbide particles were embedded in the secondary dendrite arms of hypoeutectic alloys. These carbide particles formed prior to the M7C3/austenite eutectic structure. Therefore, several M7C3 carbide particles may form during the M7C3/austenite eutectic reaction, as well as prior to the eutectic reaction.

5. Future Work

A mechanical property evaluation of the directionally solidified alloys after heat treatment will be carried out in the future. Furthermore, phase evolution during thermal treatment will also be studied.

Author Contributions

Conceptualization: C.Y.J. and J.H.L.; Mothology: C.Y.J., J.H.L., H.Y.S., I.Y.J. and B.G.C.; Software: B.G.C. and J.H.S.; Validation: H.Y.S. and I.Y.J.; Fromal analysis: B.G.C. and H.Y.S.; Investigation H.Y.S. and J.H.L.; Data curation: H.Y.S. and J.H.L.; Writing: C.Y.J., J.H.L. and H.Y.S.; Supervision: J.H.L.; Project administration: J.H.L.; Funding acquisition: J.H.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by Changwon National University 2023–2024.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Conflicts of Interest

Author Jong Ho Shin was employed by the company Doosan Enerbility. Author Hee Young Son was employed by the company KPCM. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

References

  1. Davis, J.R. Metallurgy and Properties of High-Alloy White Irons. In ASM Specialty Handbook Cast Irons, 1st ed.; ASM International: Novelty, OH, USA, 1996; pp. 111–122. [Google Scholar]
  2. Oh, J.S.; Song, Y.G.; Choi, B.G.; Bhamornsut, C.; Nakkuntod, R.; Jo, C.Y.; Lee, J.H. Effect of dendrite fraction on the M23C6 precipitation behavior and the mechanical properties of high-Cr white irons. Metals 2021, 11, 1576. [Google Scholar] [CrossRef]
  3. Song, Y.G.; Oh, J.S.; Choi, B.G.; Jo, C.Y.; Lee, J.H. Effects of primarily solidified dendrite and thermal treatments on the M23C6 precipitation behavior of high-Cr irons. Metals 2021, 11, 1690. [Google Scholar] [CrossRef]
  4. Smith, W.F. Structure and Properties of Engineering Alloys, 1st ed.; Brown, J.V., Maisel, J.W., Eds.; McGraw-Hill: New York, NY, USA, 1981; pp. 322–323. [Google Scholar]
  5. Reed, R. The Superalloys Fundamentals and Applications; Cambridge University Press: Cambridge, UK, 2006; pp. 139–200. [Google Scholar]
  6. Jang, D.W.; Shin, J.H.; Kim, I.S.; Jung, I.Y.; Jo, C.Y.; Lee, J.H. Effect of Solidification Variables on the Tensile Property of 2.8 wt% C–26 wt% Cr White Iron. Metals 2022, 12, 1416. [Google Scholar] [CrossRef]
  7. Trepczynstka-tent, M.; Boronski, D.; Mackowiak, P. Mechanical Properties and microstructure of directionally solidified Fe-4.25%C eutectic alloy. Mater. Sci. Eng. A 2021, 822, 1441644. [Google Scholar]
  8. Gong, L.; Fu, H.; Zhi, X. Corrosion Wear of Hypereutectic high Chrome Cast Iron. Metals 2023, 13, 308. [Google Scholar] [CrossRef]
  9. Jain, A.S.; Mustafa, M.I.; Sazili, M.I.I.M.; Chang, H.; Zhang, M.X. Effects of destabilization and temperaring on microstructure and mechanical properties of a hypereutectic high-chrome cast iron. J. Mater. Sci. 2022, 57, 15581–15597. [Google Scholar] [CrossRef]
  10. Zhi, X.; Xing, J.; Fu, H.; Gao, Y. Effect of titanium on the as-cast microstructure of hypereutectic high chrome cast iron. Mater. Chacter. 2008, 59, 1221–1226. [Google Scholar] [CrossRef]
  11. Prayogo, R.S.; Stetyana, L.D. The Improvement of Wear and Impact Resistance of High Chrome White Cast Iron for Crusher. J. Mater. Process. Charact. 2020, 1, 96–102. [Google Scholar]
  12. Ye, X.; Jiang, Y.; Li, Z.; Zhou, R.; Oi, T. Microstructure and corrosion wear properties of directionally solidified high chrome cast iron Cr28. Cast Technol. 2006, 27, 935–938. [Google Scholar]
  13. Dogan, Q.N.; Hawk, J.A.; Laird, G., II. Solidification structure and abrasion resistance of high chrome white irons. Metall. Trans. A 1997, 28, 1315–1328. [Google Scholar] [CrossRef]
  14. Tang, X.H.; Chung, R.; Pang, C.J.; Li, D.Y.; Hinckley, B.; Dolman, K. Microstructure of high (45 wt%) chrome cast irons and their resistance to wear and corrosion. Wear 2011, 271, 1426–1431. [Google Scholar] [CrossRef]
  15. Wienmoon, A.; Chairuangsri, T.; Pierce, J.T.H. A microstructural study of destabilized 30 wt%Cr-2.3 wt%C high-Cr cast iron. ISIJ 2004, 44, 396–403. [Google Scholar] [CrossRef]
  16. Kibble, K.A.; Pearce, J.T.H. Influence of heat treatment on the microstructure and hardness of 19% high-Chrome cast irons. Cast Met. 1993, 6, 9–15. [Google Scholar] [CrossRef]
  17. McLean, M. Directionally Solidified Materials for High Temperature Service. Met. Soc. 1983, 11–54. [Google Scholar]
  18. Tiller, W.A.; Jackson, K.A.; Rutter, J.W.; Charlmer, B. He Redistribution of Solute Atoms during the Solidification of Metals. Acta Metall. 1953, 1, 428. [Google Scholar] [CrossRef]
  19. Maritus, U.M. Solidification of metals. Progrerss Met. Phys. 1954, 5, 279–309. [Google Scholar] [CrossRef]
  20. Flemings, M.C. Solidification Processings; McGraw-Hill: New York, NY, USA, 1974. [Google Scholar]
  21. Lu, L.; Soda, H.; McLean, A. Microstructure and mechanical properties of Fe–Cr–C eutectic composites. Mater. Sci. Eng. A 2003, 347, 214–222. [Google Scholar] [CrossRef]
  22. Ma, S.; Xing, J.; He, Y.; Li, Y.; Huang, Z.; Liu, G.; Geng, Q. Microstructure and crystallography of M7C3 carbide in chrome cast iron. Mater. Chem. Phys. 2015, 11, 65. [Google Scholar] [CrossRef]
  23. Thrope, W.R.; Chicco, B. Development of a low melting point white cast iron for composite alloy manufacture. Mate. Forum 1985, 16A, 1541–1549. [Google Scholar]
  24. Tabrett, C.P. Microstructure-Property Relationships in High Chrome White Irons. Ph.D. Thesis, University of South Australia, Adelaide, Australia, 1997. [Google Scholar]
  25. Pearce, J.T.H. Structural characterization of high chrome cast irons. In Proceedings of the International Conference on Solidification Science and Processing: Outlook for the 21st Century, Bangalore, India, 18–21 February 2001; pp. 241–247. [Google Scholar]
  26. Pearce, J.T.H. Examination of M7C3 carbides in high chrome cast irons using thin foil electron microscopy. J. Mater. Sci. Lett. 1983, 2, 428–432. [Google Scholar] [CrossRef]
  27. Pearce, J.T.H. Wear Abrasion Resisting Materials. Ph.D. Thesis, University of Aston in Birmingham, Birmingham, UK, 1982. [Google Scholar]
  28. Tabrett, C.P.; Sare, I.R.; Gomashchi, M.R. Microstructure-property relationships in high chrome white iron alloys. Int. Mater. Rev. 1996, 41, 59–80. [Google Scholar] [CrossRef]
  29. Dupin, P.; Saverna, J.; Schissler, J.M. Structural study of high chrome white cast iron. AFS Trans. 1982, 90, 711–718. [Google Scholar]
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Figure 1. Optical micrographs of the conventionally cast alloys: (a) 2127 (b) 2427 (c) 2827 (d) 2124.
Figure 1. Optical micrographs of the conventionally cast alloys: (a) 2127 (b) 2427 (c) 2827 (d) 2124.
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Metals 14 00276 g002aMetals 14 00276 g002b
Figure 2. Prediction of pseudo-binary phase diagram at constant Cr content of the alloys: (a) Thermo-Calc; (b) JMatPro.
Figure 2. Prediction of pseudo-binary phase diagram at constant Cr content of the alloys: (a) Thermo-Calc; (b) JMatPro.
Metals 14 00276 g002aMetals 14 00276 g002b
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Figure 3. Micrographs parallel to the solidified direction at each solidification rate. Arrow shows solidified direction: (a) 2124 at at 5 µm/s; (b) 2124 at 25 µm/s; (c) 2124 at 50 µm/s; (d) 2124 at 100 µm/s; (e) 2127 at 5 µm/s; (f) 2127 at 25 µm/s; (g) 2127 at 50 µm/s; (h) 2127 at 100 µm/s; (i) 2427 at 5 µm/s; (j) 2427 at 25 µm/s; (k) 2427 at 50 µm/s; (l) 2427 at 100 µm/s.
Figure 3. Micrographs parallel to the solidified direction at each solidification rate. Arrow shows solidified direction: (a) 2124 at at 5 µm/s; (b) 2124 at 25 µm/s; (c) 2124 at 50 µm/s; (d) 2124 at 100 µm/s; (e) 2127 at 5 µm/s; (f) 2127 at 25 µm/s; (g) 2127 at 50 µm/s; (h) 2127 at 100 µm/s; (i) 2427 at 5 µm/s; (j) 2427 at 25 µm/s; (k) 2427 at 50 µm/s; (l) 2427 at 100 µm/s.
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Figure 4. Micrographs transverse to the solidified direction at each solidification rate: (a) 2124 at 5 µm/s; (b) 2124 at 25 µm/s; (c) 2124 at 50 µm/s; (d) 2124 at 100 µm/s; (e) 2127 at 5 µm/s; (f) 2127 at 25 µm/s; (g) 2127 at 50 µm/s; (h) 2127 at 100µm/s; (i) 2427 at 5 µm/s; (j) 2427 at 25 µm/s; (k) 2427 at 50 µm/s; (l) 2427 at 100 µm/s.
Figure 4. Micrographs transverse to the solidified direction at each solidification rate: (a) 2124 at 5 µm/s; (b) 2124 at 25 µm/s; (c) 2124 at 50 µm/s; (d) 2124 at 100 µm/s; (e) 2127 at 5 µm/s; (f) 2127 at 25 µm/s; (g) 2127 at 50 µm/s; (h) 2127 at 100µm/s; (i) 2427 at 5 µm/s; (j) 2427 at 25 µm/s; (k) 2427 at 50 µm/s; (l) 2427 at 100 µm/s.
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Metals 14 00276 g005aMetals 14 00276 g005b
Figure 5. Micrographs of alloy 2827 along the solidified direction at various solidification rates. Arrow shows solidified direction: (a) 5 µm/s; (b) 25 µm/s; (c) 50 µm/s; (d) 100 µm/s.
Figure 5. Micrographs of alloy 2827 along the solidified direction at various solidification rates. Arrow shows solidified direction: (a) 5 µm/s; (b) 25 µm/s; (c) 50 µm/s; (d) 100 µm/s.
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Figure 6. SEM micrographs of directionally solidified alloy 2827 showing in-situ composite at the solidification rates below 50 µm/s. Arrow indicates the solidified direction: (a) 5 µm/s; (b) 25 µm/s; (c) 50 µm/s; (d) 100 µm/s.
Figure 6. SEM micrographs of directionally solidified alloy 2827 showing in-situ composite at the solidification rates below 50 µm/s. Arrow indicates the solidified direction: (a) 5 µm/s; (b) 25 µm/s; (c) 50 µm/s; (d) 100 µm/s.
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Figure 7. Micrographs of alloy 2827 transverse to the solidified direction at various solidification rates: (a) 5 µm/s; (b) 25 µm/s; (c) 50 µm/s; (d) 100 µm/s.
Figure 7. Micrographs of alloy 2827 transverse to the solidified direction at various solidification rates: (a) 5 µm/s; (b) 25 µm/s; (c) 50 µm/s; (d) 100 µm/s.
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Figure 8. DSQ micrographs of alloy 2827: (a) 25 µm/s; (b) 100 µm/s.
Figure 8. DSQ micrographs of alloy 2827: (a) 25 µm/s; (b) 100 µm/s.
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Figure 9. SEM micrographs transverse to the solidified direction of a near-eutectic alloy solidified at 25 µm/s: (a) SEM micrograph of speckle; (b) ‘A’ area in (a); (c) ‘B’ area in (a).
Figure 9. SEM micrographs transverse to the solidified direction of a near-eutectic alloy solidified at 25 µm/s: (a) SEM micrograph of speckle; (b) ‘A’ area in (a); (c) ‘B’ area in (a).
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Figure 10. TEM micrograph +and SAD P of M7C3. (a) BF image (b) SADP.
Figure 10. TEM micrograph +and SAD P of M7C3. (a) BF image (b) SADP.
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Metals 14 00276 g011
Figure 11. TEM BF image (a), and SADP of ‘1’ in (a) showing M23C6 (b).
Figure 11. TEM BF image (a), and SADP of ‘1’ in (a) showing M23C6 (b).
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Metals 14 00276 g012
Figure 12. SEM micrographs transverse to the solidified direction of a near-eutectic alloy solidified at 25 µm/s: (a) SEI of speckle; (b) C Kα + Cr Kα mapping; (c) C Kα mapping; (d) Cr Kα mapping.
Figure 12. SEM micrographs transverse to the solidified direction of a near-eutectic alloy solidified at 25 µm/s: (a) SEI of speckle; (b) C Kα + Cr Kα mapping; (c) C Kα mapping; (d) Cr Kα mapping.
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Figure 13. SEM micrograph (transverse to DS direction) of each alloy showing M7C3 embedded dendrite (arrow shows M7C3 in the secondary arm, circle indicates solute segregated region): (a) 2127 (at 50 µm/s); (b) 2427 (at 50 µm/s); (c) 2827 (at 100 µm/s).
Figure 13. SEM micrograph (transverse to DS direction) of each alloy showing M7C3 embedded dendrite (arrow shows M7C3 in the secondary arm, circle indicates solute segregated region): (a) 2127 (at 50 µm/s); (b) 2427 (at 50 µm/s); (c) 2827 (at 100 µm/s).
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Figure 14. Distribution of alloying elements in alloy 2127 solidified at 50 µm/s: (a) SEI; (b) C Kα (c) Cr Kα.
Figure 14. Distribution of alloying elements in alloy 2127 solidified at 50 µm/s: (a) SEI; (b) C Kα (c) Cr Kα.
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Metals 14 00276 g015
Figure 15. Scheme of microstructural evolution during solidification and subsequent solid-state cooling of hypoeutectic alloys.
Figure 15. Scheme of microstructural evolution during solidification and subsequent solid-state cooling of hypoeutectic alloys.
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Table 1. Chemical compositions of the specimens (wt%).
Table 1. Chemical compositions of the specimens (wt%).
AlloyCSiMnCrNiMoFe
21272.130.710.6727.000.890.86Bal
24272.430.680.7027.060.860.85Bal
28272.780.700.7027.340.870.85Bal
21242.120.660.6524.050.850.86Bal
Table 2. Measured inter-fiber spacing in alloy 2827 solidified at and below 50 µm/s.
Table 2. Measured inter-fiber spacing in alloy 2827 solidified at and below 50 µm/s.
solidification rate (µm/s)52550
inter-fiber spacing (µm)6.66.04.0
Table 3. Measured inter-speckle spacing in alloy 2827 solidified at and below 50 µm/s.
Table 3. Measured inter-speckle spacing in alloy 2827 solidified at and below 50 µm/s.
solidification rate (µm/s)52550
inter-speckle spacing (µm)27674.554.5
Table 4. Solidified feature of the alloys at each solidification rate.
Table 4. Solidified feature of the alloys at each solidification rate.
Solidification Rate
Alloy		5 µm/s25 µm/s50 µm/s100 µm/s
2127dendritedendritedendriterandom-oriented dendrite
2427dendritedendritedendriterandom-oriented dendrite
2827in-situ
composite		in-situ
composite		in-situ
composite		random-oriented dendrite
2124dendritedendritedendriterandom-oriented dendrite
Table 5. Thermo-Calc and JMatPro predictions on melting range of each phase (°C).
Table 5. Thermo-Calc and JMatPro predictions on melting range of each phase (°C).
AlloyTlTETEfΔT (JMatPro)
(Tl-Ts)		ΔT (Thermo-Calc)
(Tl-Ts)
21271320128712833741
24271307128912852221
2827129212901286615
21241328129012833942
where Tl: liquidus, Ts: solidus, TE: eutectic begins, TEf: eutectic finishes.
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MDPI and ACS Style

Son, H.Y.; Jung, I.Y.; Choi, B.G.; Shin, J.H.; Jo, C.Y.; Lee, J.H. Effects of Chemical Composition and Solidification Rate on the Solidification Behavior of High-Cr White Irons. Metals 2024, 14, 276. https://doi.org/10.3390/met14030276

AMA Style

Son HY, Jung IY, Choi BG, Shin JH, Jo CY, Lee JH. Effects of Chemical Composition and Solidification Rate on the Solidification Behavior of High-Cr White Irons. Metals. 2024; 14(3):276. https://doi.org/10.3390/met14030276

Chicago/Turabian Style

Son, Hee Young, In Yong Jung, Baig Gyu Choi, Jong Ho Shin, Chang Yong Jo, and Je Hyun Lee. 2024. "Effects of Chemical Composition and Solidification Rate on the Solidification Behavior of High-Cr White Irons" Metals 14, no. 3: 276. https://doi.org/10.3390/met14030276

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